Scripta Materialia 2017 Volume 134(Volume 134) pp:
Publication Date(Web):1 June 2017
DOI:10.1016/j.scriptamat.2017.02.044
Creep deformation is of concern in PbTe thermoelectric modules operating for years at high homologous temperatures while being subjected to internal or external stresses. The creep strain rates of hot-pressed PbTe are measured at 400 °C; a power-law is found, with a stress exponent n = 1.7 at low stresses (2–7 MPa) and n = 4.4 at high stresses (7–35 MPa). Creep rates measured at low stresses (4–6 MPa) between 350 and 500 °C provide a creep activation energy Q = 181 ± 18 kJ/mol. The creep parameters and the good hot ductility of PbTe are compared to those for cast Bi2Te3 reported in the literature.Download high-res image (287KB)Download full-size image
Co-reporter:Matthew P.B. Glazer, Junjie Wang, Jiung Cho, Jonathan D. Almer, John S. Okasinski, Paul V. Braun, David C. Dunand
Journal of Power Sources 2017 Volume 367(Volume 367) pp:
Publication Date(Web):1 November 2017
DOI:10.1016/j.jpowsour.2017.09.040
•Lithiation-induced strains quantified in a Ni3Sn2 inverse opal anode in operando.•Lithiation induces compressive average strains in Ni inverse opal scaffold.•Ni inverse opal scaffold strain distribution reversibly broadens upon lithiation.•Three measured volumes show similar cyclic strain averages and distributions.•Ni3Sn2 measured cyclic strains are similar to prior Si inverse opal anode studies.Volume changes associated with the (de)lithiation of a nanostructured Ni3Sn2 coated nickel inverse opal scaffold anode create mismatch stresses and strains between the Ni3Sn2 anode material and its mechanically supporting Ni scaffold. Using in operando synchrotron x-ray diffraction measurements, elastic strains in the Ni scaffold are determined during cyclic (dis)charging of the Ni3Sn2 anode. These strains are characterized using both the center position of the Ni diffraction peaks, to quantify the average strain, and the peak breadth, which describes the distribution of strain in the measured volume. Upon lithiation (half-cell discharging) or delithiation (half-cell charging), compressive strains and peak breadth linearly increase or decrease, respectively, with charge. The evolution of the average strains and peak breadths suggests that some irreversible plastic deformation and/or delamination occurs during cycling, which can result in capacity fade in the anode. The strain behavior associated with cycling of the Ni3Sn2 anode is similar to that observed in recent studies on a Ni inverse-opal supported amorphous Si anode and demonstrates that the (de)lithiation-induced deformation and damage mechanisms are likely equivalent in both anodes, even though the magnitude of mismatch strain in the Ni3Sn2 is lower due to the lower (de)lithiation-induced contraction/expansion.Download high-res image (286KB)Download full-size image
Co-reporter:Daniel J. Sauza, Peter J. Bocchini, David C. Dunand, David N. Seidman
Acta Materialia 2016 Volume 117() pp:135-145
Publication Date(Web):15 September 2016
DOI:10.1016/j.actamat.2016.07.014
Abstract
The effect of 2 at.% Ru addition on the elemental partitioning and microstructural evolution of a base Co-8.8Al-7.3W at.% superalloy, consisting of a γ-(fcc) matrix with γ’-(L12 structure) precipitates is studied using scanning electron microscopy and atom-probe tomography. Ruthenium partitions to the γ’-precipitates in the Co-9.4Al-7.5W-2.1Ru at.% alloy with a partitioning coefficient, = 1.27, after aging at 900 °C for 16 h, in contrast to the behavior observed in Ni-base superalloys and theoretically predicted for Co-base superalloys, for which Ru partitions preferentially to the γ-phase. The addition of ruthenium does not significantly affect the γ′ volume-fraction or the coarsening kinetics of the γ′ precipitates compared to the base ternary alloy. The addition of Ru also leads, however, to a rapid discontinuous transformation of (γ plus γ′), which initiates at the grain boundaries after 128 h aging at 900 °C; (γ plus γ′) is transformed into a lamellar phase mixture containing Co3W (D019), fcc solid-solution (γ), and Co(Al,W) (B2). After 256 h aging at 900 °C in the Ru-containing alloy, some grains have completely transformed, although regions of γ plus γ′ persist. The base ternary CoAlW alloy does not exhibit a discontinuous transformation and contains a (γ plus γ′) microstructure up to 1024 h of aging at 900 °C.
Porous NiTi-Nb containing a 3D array of orthogonally interconnected microchannels was created via a novel powder metallurgy process combining: (i) Mg ribbon scaffold construction, (ii) slip casting of NiTi + Nb powder blend within the scaffold, (iii) Mg scaffold vacuum evaporation, and (iv) NiTi + Nb liquid phase sintering. The later stage was achieved by creating small amounts of quasi-binary NiTi-Nb liquid eutectic, which wicked between NiTi particles and bonded them together while leaving 28 vol.% of residual pores. These hierarchical porous structures have a total porosity of 30–53%, an effective stiffness of 5–9 GPa, and a yield strength of 20–80 MPa. They exhibit the shape memory effect, with 3% strain recovery after 7% compressive deformation upon multiple load-unload cycles. Finite element modeling (FEM) is used to model the anisotropy of these structures, as well as to probe the strain distributions on a microscopic level. Mechanical anisotropy was present in FEM of all structures, though more pronounced in structures with high microchannel volume fraction. With mechanical properties between those of trabecular and cortical bone, these structures are of great interest for bone implant applications.
Co-reporter:Hoon-Hwe Cho, Matthew P.B. Glazer, Qian Xu, Heung Nam Han, David C. Dunand
Acta Materialia 2016 Volume 107() pp:289-297
Publication Date(Web):1 April 2016
DOI:10.1016/j.actamat.2016.01.064
Abstract
A volume expansion of up to ∼310% occurs upon the lithiation of silicon in Si-coated nickel inverse opal anodes, which causes (de)lithiation-induced mismatch stresses and strains between the Si and Ni during battery cyclical (dis)charging. These (de)lithiation-induced mismatch strains and stresses are modeled via sequentially coupled diffusion- and stress-based finite element (FE) analysis, which takes the mechanical contact between the Si and Ni phases into account, as well as the complex geometry and material properties of the Si-coated Ni inverse opal anode system. During lithiation, compressive strains up to 0.2% are developed in the Ni scaffold since the Si active layer expands. A rapid recovery of these lithiation-induced mismatch strains occurs during subsequent delithiation, though full recovery is not achieved. Strain histories upon multiple (de)lithiation cycles vary with the choice of various mechanical contact conditions employed between the two phases, since the mechanical contact properties determine how the contacted phases interact mechanically. The numerically predicted strains are compared with experimental strain data collected in operando using X-ray diffraction. The simulated strain histories agree with the measured data, enabling the possibility of predicting mechanical performance and eventual degradation using only numerical modeling. In particular, the FE model indicates that plastic deformation occurs first in the lithiated Si active layer, then in the Ni scaffold.
Metallurgical and Materials Transactions E 2016 Volume 3( Issue 1) pp:46-54
Publication Date(Web):2016 March
DOI:10.1007/s40553-016-0068-y
Nickel foams, consisting of 51 to 62 pct aligned, elongated pores surrounded by a network of Ni walls, were fabricated by reduction and sintering of directionally cast suspensions of nanometric NiO powders in water. Use of dispersant in the slurry considerably affected the foam morphology and microstructure at both the micro- and macro-scale, most likely by modifying ice solidification into dendrites (creating the aligned, elongated macro-pores) and NiO powder accumulation in the inter-dendritic space (creating the Ni walls with micro-pores). The mean width of the Ni walls, in foams solidified with and without dispersant, was 21 ± 5 and 75 ± 13 µm, respectively. Additionally, the foams with the dispersant showed less dense walls and rougher surfaces than those without the dispersant. Moreover, the fraction of closed pores present in the foam walls with the dispersant was higher than that of the samples without dispersant. We finally verified the potential energy application of the Ni foam produced in this study by carrying out a preliminary single-cell performance test with the Ni foam sample as the gas diffusion layer on the anode side of a polymer electrolyte membrane fuel cell.
Co-reporter:Matthew P. B. Glazer;Jiung Cho;Jonathan Almer;John Okasinski;Paul V. Braun;David C. Dun
Advanced Energy Materials 2015 Volume 5( Issue 14) pp:
Publication Date(Web):
DOI:10.1002/aenm.201500466
Elastic strains are measured in operando in a nanostructured silicon-coated nickel inverse opal scaffold anode, using X-ray diffraction to study the Si (de)lithiation-induced Ni strains. The volume expansion upon lithiation of the Si in the anode is constrained by the surrounding Ni scaffold, causing mismatch stresses and strains in the Si and Ni phases during cycling. The Ni strains are measured in operando during (dis)charge cycles, using diffraction peak position and peak broadness to describe the distribution of strain in the Ni. During lithiation, compressive strains in the Ni first increase linearly with charge, after which a gradually decreasing strain rate is observed as the maximum lithiation state is approached; upon delithiation a similar process occurs. In-plane average compressive strains on the order of 990 ± 40 με are measured in the Ni scaffold during lithiation, corresponding to compressive stresses of 215 ± 9 MPa. The decreasing strain rates and decreasing maximum and recovered strains suggest that plasticity in Ni and/or Si, as well as delamination between Ni and Si, may occur during cycling. Rate sensitivity in capacity is correlated with strain and a maximum Ni compressive stress of 230 ± 40 MPa is measured at the maximum state of lithiation.
Co-reporter:Peiqi Zheng, Nikole J. Kucza, Zilong Wang, Peter Müllner, David C. Dunand
Acta Materialia 2015 Volume 86() pp:95-101
Publication Date(Web):March 2015
DOI:10.1016/j.actamat.2014.12.005
Abstract
Ferromagnetic Ni–Mn–Ga shape memory alloys with large magnetic-field-induced strains are promising candidates for actuators. Here, we cast replicated Ni–Mn–Ga foams with 57 vol.% of 355–500 μm open pores, with and without directional solidification. The 10M martensitic phase was determined in all foam samples. Directionally solidified foam had a fiber texture, with <1 0 0> closely aligned with the solidification direction. In contrast, foams without directional solidification were more randomly textured. One directionally solidified foam showed a maximum magnetic-field-induced strain of 0.65%, which was twice the value displayed by other foams without directional solidification. This improvement is consistent with a reduction in incompatibility stresses between neighboring grains deforming by twinning, generated by a reduction in crystallographic misorientation in textured foam.
Co-reporter:Z.L. Wang, P. Zheng, Z.H. Nie, Y. Ren, Y.D. Wang, P. Müllner, D.C. Dunand
Acta Materialia 2015 Volume 99() pp:373-381
Publication Date(Web):15 October 2015
DOI:10.1016/j.actamat.2015.08.002
Abstract
The link between microstructure and mechanical properties is investigated for a superelastic Ni–Mn–Ga microwire with 226 μm diameter, created by solidification via the Taylor method. The wire, which consists of bamboo grains with tetragonal martensite matrix and coarse γ precipitates, exhibits fully reversible superelastic behavior up to 4% tensile strain. Upon multiple tensile load–unload cycles, reproducible stress fluctuations of ∼3 MPa are measured on the loading superelastic stress plateau of ∼50 MPa. During cycles at various temperatures spanning −70 to 55 °C, the plateau stress decreases from 58 to 48 MPa near linearly with increasing temperature. Based on in situ synchrotron X-ray diffraction measurements, we conclude that this superelastic behavior is due to reversible martensite variants reorientation (i.e., reversible twinning) with lattice rotation of ∼13°. The reproducible stress plateau fluctuations are assigned to reversible twinning at well-defined locations along the wire. The strain recovery during unloading is attributed to reverse twinning, driven by the internal stress generated on loading between the elastic γ precipitates and the twinning martensite matrix. The temperature dependence of the twining stress on loading is related to the change in tetragonality of the martensite, as measured by X-ray diffraction.
Co-reporter:Peiqi Zheng, Nikole J. Kucza, Charles L. Patrick, Peter Müllner, David C. Dunand
Journal of Alloys and Compounds 2015 Volume 624() pp:226-233
Publication Date(Web):5 March 2015
DOI:10.1016/j.jallcom.2014.11.067
•We prepared oligocrystalline Ni–Mn–Ga Taylor wires.•The superelasticity of oligocrystalline Ni–Mn–Ga Taylor wires was studied by tensile testing at ambient temperature.•We reported the magnetic-torque-induced bending phenomenon of oligocrystalline Ni–Mn–Ga Taylor wire.Ni–Mn–Ga wires with sub-millimeter diameter, either as individual wires or as part of 2D or 3D wire assemblies, are promising candidates for actuators, sensors, magnetic cooling systems and energy harvesting devices. Here, we report on Ni–Mn–Ga wires with 200–400 μm diameter fabricated by the Taylor method, followed by annealing at 1050 °C for 1 h to achieve grain growth. The mechanical behavior of these oligocrystalline wires with bamboo grains was studied by tensile tests at room temperature. Wires with martensitic structure exhibited a very low Young’s modulus of 5–8 GPa and superelastic behavior with twinning stresses of 22–30 MPa and maximum recoverable strain of 3.5%, and accumulated a plastic strain of ∼0.6% over 8 consecutive 0–50 MPa mechanical loading cycles. A 1 T rotating magnetic field caused the wire to bend to a curvature corresponding to a surface strain of 1.5%. During a full-field revolution, the wire deflected back and forth twice when the field direction was approximately perpendicular to the average wire direction. This effect was attributed to magnetic-torque-induced bending (MTIB) enabled by twinning.
Metallurgical and Materials Transactions A 2015 Volume 46( Issue 4) pp:1433-1436
Publication Date(Web):2015 April
DOI:10.1007/s11661-015-2781-z
NiTi wires were brazed together via liquid eutectic formation between NiTi and Nb powders deposited at the wire contact region. The brazed region shows proeutectic NiTi(Nb) in contact with the wires, sandwiching a NiTi-Nb eutectic structure, whose microhardness and stiffness, as characterized via nanoindentation, are higher than the NiTi wires, while also showing signs of high ductility. NiTi-Nb eutectic bonding may thus be a viable approach for producing shape-memory NiTi scaffolds brazed from stacked, woven, or braided wires.
Materials Science and Engineering: A 2015 Volume 634() pp:153-160
Publication Date(Web):14 May 2015
DOI:10.1016/j.msea.2014.12.088
NiTi porous structures with fully 3D interconnected microchannels were created by a powder-metallurgy method using steel wires as spaceholders. Prealloyed NiTi powders were near-fully densified by hot pressing within a high carbon steel wire scaffold, which was then electrochemically dissolved. This resulted in a regular 3D network of orthogonally interconnected microchannels with ellipsoidal cross-sections with 60% volume fraction. The measured elastic stiffness of 14 GPa compares well to porous and composite models, as well as finite element modeling despite varying geometry and deformation model assumptions. The structure, which is martensitic at room temperature, exhibits brittle fracture at a relatively low stress of 88 MPa due to a TiC interphase at all NiTi powder boundaries. The volume fraction, orientation, shape, and spatial distribution of the microchannels is fully controlled with this method. This makes the structure attractive for biomedical applications, specifically bone implants. The potential shape memory properties achievable through optimized processing would also make the structure effective for energy absorption or actuators.
Journal of Materials Processing Technology 2014 Volume 214(Issue 9) pp:1895-1899
Publication Date(Web):September 2014
DOI:10.1016/j.jmatprotec.2014.04.006
A process was developed for fabricating 3D fully interconnected microchannels in superelastic NiTi-Nb for bone implant applications by combining spaceholder powder metallurgy and liquid-phase sintering. Prealloyed NiTi powders were blended with 3.1 at.% Nb and cold-pressed around a 3D scaffold of carburized steel tubes acting as space-holders. The tubes were then electrochemically dissolved to form orthogonally interconnected microchannels with 400 μm diameter and ∼34% volume fraction. Finally, the powder preform was heated to 1185 °C to form a quasi-binary NiTi-Nb eutectic liquid, which liquid-phase-sintered the NiTi powders without filling the microchannels. The resulting continuously bonded matrix contains an additional 16% porosity, for a total structure porosity of ∼50%. NiTi-Nb micro-architectured structures have excellent potential as bone implant scaffolds due to the high versatility in channel size, fraction, and spatial arrangement. Fully interconnected 3D microchannels also increase fluid transport within the scaffold, assisting in nutrient delivery and waste transport to and from cells deep within the scaffold.
Metallurgical and Materials Transactions E 2014 Volume 1( Issue 4) pp:303-310
Publication Date(Web):2014 December
DOI:10.1007/s40553-014-0031-8
To investigate its application as a material for solid oxide fuel cell interconnects, oxidation rates of replicated E-Brite (Fe-26Cr-1Mo, wt pct) foams with 43 and 51 pct open porosity were measured in static laboratory air for up to 200 hours. Results correlate well with previously reported values for dense material when normalized by surface area. Area-specific resistance measurements, taken in the range of 823 K to 1073 K (550 °C to 800 °C) after 24 hours of oxidation at 1123 K (850 °C), yield activation energies in the range 69 to 82 kJ mol−1 for porous E-Brite that closely match dense E-Brite. Compressive creep properties, measured at 1073 K (800 °C) for pristine and oxidized porous E-Brite, show that pre-oxidation (10 hours at 1073 K (800 °C)) led to a ~100-fold decrease in creep rate. This is due to strengthening of the alloy foam by the formation of a continuous network of oxide, which coats the internal pore surface and reduces porosity by as much as 10 pct after 200 hours of oxidation at 1073 K (800 °C). Choking of the fenestrations between the pores, however, leads to an increase in closed porosity. Strengthening and pore filling effects should be taken into account in the design of the SOFC stack when using E-Brite as a porous interconnect material.
Equal-channel angular extrusion is used to consolidate a blend of amorphous Zr56.3Nb5.1Cu15.6Ni12.9Al10.0 and crystalline W powders into dense composites. Chemical dissolution of the crystalline phase results in amorphous foams with elongated pores, aligned at a 22–28° angle with respect to the extrusion direction, whose compressive properties are studied for various orientations. As the angle between the pore long direction and the applied stress direction increases from 0° to 68°, there is a significant decrease in loading stiffness and peak stress, as expected from predictive analytical models; however, the observed increase in stiffness and peak stress observed when the pores are oriented 90° to the direction of loading is not predicted by all of the models. Foams with pores aligned 24–68° to the direction of loading show increased plastic bending in individual walls and accumulation in microscopic damage without failure, leading to increased compressive ductility and absorbed energy over other orientations.
Co-reporter:Alix C. Deymier-Black, Anjali Singhal, Fang Yuan, Jonathan D. Almer, L. Catherine Brinson, David C. Dunand
Journal of the Mechanical Behavior of Biomedical Materials 2013 Volume 21() pp:17-31
Publication Date(Web):May 2013
DOI:10.1016/j.jmbbm.2013.01.016
Under long-term loading creep conditions, mineralized biological tissues like bone are expected to behave in a similar manner to synthetic composites where the creeping matrix sheds load to the elastic reinforcement as creep deformation progresses. To study this mechanism in biological composites, creep experiments were performed at 37 °C on bovine compact bone and dentin. Static compressive stresses were applied to the samples, while wide- and small-angle scattering patterns from high energy synchrotron X-rays were used to determine, respectively, the elastic strain in the hydroxyapatite (HAP) platelets and the strain in the mineralized collagen fibril, as a function of creep time. In these highly irradiated biological composites, the reinforcing hydroxyapatite platelets progressively transfer some of their stress back to the softer protein matrix during creep. While such behavior can be explained by damage at the interface between the two phases, it is not consistent with measurements of the apparent moduli – the ratio of applied stress to elastic HAP strain measured throughout the creep experiments by elastic unload/load segments – which remained constant throughout the experiment and thus indicated good HAP/protein bonding. A possible explanation is a combination of X-ray and load induced interfacial damage explaining the shedding of load from the HAP during long term creep, coupled with interfacial re-bonding of the load-disrupted reversible bonds upon unloading, explaining the unaffected elastic load partitioning during unload/load segments. This hypothesis is further supported by finite element modeling which shows results mirroring the experimental strain measurements when considering interfacial delamination and a compliant interstitial space at the ends of the HAP platelets.
Co-reporter:Anjali Singhal, Alix C. Deymier-Black, Jonathan D. Almer, David C. Dunand
Materials Science and Engineering: C 2013 Volume 33(Issue 3) pp:1467-1475
Publication Date(Web):1 April 2013
DOI:10.1016/j.msec.2012.12.069
Synchrotron X-ray diffraction is used to study in situ the evolution of phase strains during compressive creep deformation in bovine bone and dentin for a range of compressive stresses and irradiation rates, at ambient and body temperatures. In all cases, compressive strains in the collagen phase increase with increasing creep time (and concomitant irradiation), reflecting macroscopic deformation of the sample. By contrast, compressive elastic strains in the hydroxyapatite (HAP) phase, created upon initial application of compressive load on the sample, decrease with increasing time (and irradiation) for all conditions; this load shedding behavior is consistent with damage at the HAP–collagen interface due to the high irradiation doses (from ~ 100 to ~ 9,000 kGy). Both the HAP and fibril strain rates increase with applied compressive stress, temperature and irradiation rate, which is indicative of greater collagen molecular sliding at the HAP–collagen interface and greater intermolecular sliding (i.e., plastic deformation) within the collagen network. The temperature sensitivity confirms that testing at body temperature, rather than ambient temperature, is necessary to assess the in vivo behavior of bone and teeth. The characteristic pattern of HAP strain evolution with time differs quantitatively between bone and dentin, and may reflect their different structural organization.Highlights► First systematic study of varying creep stresses on bone and dentin at nanoscale. ► HAP in highly irradiated bone and dentin sheds load during creep at all stresses. ► This suggests HAP–collagen interfacial damage due to irradiation and applied stress. ► HAP and fibril strain rates increase with stress, temperature and irradiation. ► Temporal evolution of strains different in bone and dentin
Materials Science and Engineering: A 2013 Volume 585() pp:32-38
Publication Date(Web):15 November 2013
DOI:10.1016/j.msea.2013.06.086
Iron- and nickel-based sandwich structures with open porosity facings and closed-porosity cores were created by melt infiltration and powder metallurgy, respectively, for application as interconnects in high-temperature fuel cells. For E-Brite (Fe–Cr–Mo) sandwiches, open porosity faces were created by evaporation of NaCl particles mixed with the metallic powders, while closed porosity in the core resulted from partial sintering of the pure metallic powders. Sandwiches from J5 (Ni–Mo–Cr–Ti–Mn–Al–Y) were produced by infiltrating the liquid alloy into a sandwich scaffold of permanent Al2O3 hollow spheres in the core and leachable NaAlO2 particles in the facings. Mechanical properties of both sandwich types were measured in three-point bending and indicated similar modes of failure by face yielding. Stiffness measurements closely match model values for E-Brite sandwiches but are below expected values for J5 sandwiches. In the case of sandwich yield load, calculated values for E-Brite slightly underestimated experimental values, while J5 experimental performance was significantly overestimated.
Materials Science and Engineering: A 2013 Volume 565() pp:321-325
Publication Date(Web):10 March 2013
DOI:10.1016/j.msea.2012.12.052
In thermoelectric devices, Bi2Te3 must be able to operate under high temperature gradients for very long times (years) without appreciable deformation, while being subjected to stresses created by internal thermal mismatch and external forces (e.g., vibrations). The deformation rate of cast, coarse-grained Bi2Te3 was measured under uniaxial compressive stresses from 2 to 16 MPa at temperatures between 400 and 500 °C. At all temperatures, a power-law behavior is observed, with a stress exponent n=3–6 and an activation energy of ∼145 kJ/mol. Significant differences in creep rates are observed in duplicate experiments, which may be due to anisotropic creep resistance in coarse-grained specimens. Bi2Te3 is brittle at 400 and 450 °C but shows some compressive ductility at 500 °C, indicating that hot forging may be possible.
The effect of substituting 0.01 at.% Er for Sc in an Al–0.06Zr–0.06Sc–0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 °C and 8 h/425 °C) is assessed to determine the viability of dilute Al–Si–Zr–Sc–Er alloys for creep applications. Upon aging, coherent, 2–3 nm radius, L12-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 °C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 °C, the precipitates grow modestly to a radius of 5–10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al3Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses.
Journal of the Mechanical Behavior of Biomedical Materials 2012 Volume 5(Issue 1) pp:71-81
Publication Date(Web):January 2012
DOI:10.1016/j.jmbbm.2011.08.005
Various methods are used to investigate the variability in elastic properties across a population of deciduous bovine incisor root dentin samples spanning different animals, incisor types, and locations within teeth. First, measurements of elastic strains by high-energy synchrotron X-ray scattering during compressive loading of dentin specimens provided the effective modulus–the ratio of applied stress to elastic phase strain–for the two main phases of dentin (hydroxyapatite crystals and mineralized collagen fibrils), shedding light on load transfer operating at the nanoscale between collagen and mineral phases. Second, Young’s moduli were measured at the macroscale by ultrasonic time-of-flight measurements. Third, thermogravimetry quantified the volume fractions of hydroxyapatite, protein and water at the macroscale. Finally, micro-Computed Tomography determined spatial variations of the mineral at the sub-millimeter scale. Statistical comparison of the above properties reveals: (i) no significant differences for dentin samples taken from different animals or different incisor types but (ii) significant differences for samples taken from the cervical or apical root sections as well as from different locations between buccal and lingual edges.
Co-reporter:Marie E. Cox, Laszlo J. Kecskes, Suveen N. Mathaudhu, David C. Dunand
Materials Science and Engineering: A 2012 Volume 533() pp:124-127
Publication Date(Web):30 January 2012
DOI:10.1016/j.msea.2011.11.032
Warm equal channel angular extrusion is used to consolidate a blend of amorphous Hf44.5Cu27Ni13.5Ti5Al10 and crystalline W powders. Dissolution of the W phase results in ∼60% aligned, elongated pores within an amorphous Hf-based matrix exhibiting ductility in compression, but at lower strengths than similar amorphous Zr-based foams due to incomplete bonding between Hf-based powders.Highlights► Hf-based bulk metallic glass foams with aligned, elongated pores. ► Warm equal channel angular extrusion used to consolidate amorphous Hf-based powders and crystalline W space holders. ► Lower strengths than expected due to incomplete bonding of BMG powders.
Materials Science and Engineering: A 2012 Volume 558() pp:129-133
Publication Date(Web):15 December 2012
DOI:10.1016/j.msea.2012.07.099
Nickel-base superalloy foams were created by a casting replication technique where a lightly sintered preform of SrF2 was pressure infiltrated with molten IN792; after solidification of the composite, the salt phase was dissolved with HCl to create 65% open porosity. Room temperature yield strength and stiffness of the IN792 foam compared well with existing models. Monolithic and foam samples were tested under creep conditions at 750 and 850 °C at stresses ranging from 5 to 40 MPa for the foams and from 150 to 650 MPa for the monolithic alloy. Both exhibited power-law creep behavior at high stresses and a transition to viscous flow at lower stresses, which was modeled using equations for dislocation and diffusional creep.
The off-stoichiometric Ni2MnGa Heusler alloy is a magnetic shape-memory alloy capable of reversible magnetic-field-induced strains (MFIS). These are generated by twin boundaries moving under the influence of an internal stress produced by a magnetic field through the magnetocrystalline anisotropy. While MFIS are very large (up to 10%) for monocrystalline Ni-Mn-Ga, they are near zero (<0.01%) in fine-grained polycrystals due to incompatibilities during twinning of neighboring grains and the resulting internal geometrical constraints. By growing the grains and/or shrinking the sample, the grain size becomes comparable to one or more characteristic sample sizes (film thickness, wire or strut diameter, ribbon width, particle diameter, etc), and the grains become surrounded by free space. This reduces the incompatibilities between neighboring grains and can favor twinning and thus increase the MFIS. This approach was validated recently with very large MFIS (0.2-8%) measured in Ni-Mn-Ga fibers and foams with bamboo grains with dimensions similar to the fiber or strut diameters and in thin plates where grain diameters are comparable to plate thickness. Here, we review processing, micro- and macrostructure, and magneto-mechanical properties of (i) Ni-Mn-Ga powders, fibers, ribbons and films with one or more small dimension, which are amenable to the growth of bamboo grains leading to large MFIS, and (ii) “constructs” from these structural elements (e.g., mats, laminates, textiles, foams and composites). Various strategies are proposed to accentuate this geometric effect which enables large MFIS in polycrystalline Ni-Mn-Ga by matching grain and sample sizes.
A process was developed for fabricating arrays of micro-channels in shape-memory NiTi for bone implant applications, with a tailorable internal architecture expected to improve biomechanical compatibility and osseointegration. Ni–51.4 at.% Ti with 24–34 vol.% porosity was fabricated by electrochemical dissolution of parallel layers of steel wire meshes embedded within a NiTi matrix during hot pressing of NiTi powders. The resulting NiTi structures exhibit parallel layers of orthogonally interconnected micro-channels with 350–400 μm diameters that exactly replicate the steel meshes. When low-carbon steel wires are used, iron diffuses into the surrounding NiTi during the densification step, creating a Fe-enriched zone near the wires. For high-carbon steel wires, TiC forms at the steel/NiTi interface and inhibits iron diffusion but also depletes some titanium from the adjacent NiTi. In both cases, the NiTi regions near the micro-channels exhibit altered phase transformation characteristics. These NiTi structures with replicated networks of micro-channels have excellent potential as bone implants and scaffolds given: (i) the versatility in channel size, shape, fraction and spatial arrangement; (ii) their low stiffness (15–26 GPa), close to 12–17 GPa for cortical bone; (iii) their high compressive strength (420–600 MPa at 8–9% strain); and (iv) their excellent compressive strain recovery (91–94% of an applied strain of 6%) by a combination of elasticity, superelasticity and the shape-memory effect.
The aging behavior at 300 °C of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys is studied by local-electrode atom-probe tomography, transmission electron microscopy and microhardness measurements. The ternary alloys exhibit high number densities of coherent L12 precipitates at aging times up to 1536 h (64 days). In the Al–0.06Sc–0.02Gd alloy, the Al3(Sc1−xGdx) precipitates are always Sc-rich, displaying a small Gd concentration (x < 0.12) in the precipitates. In the Al–0.06Sc–0.02Yb alloy, the precipitates are initially Yb-rich, Al3(Yb1−xScx), with Sc diffusing subsequently to the precipitates, resulting in a core/shell structure and an overall Sc-rich composition, Al3(Sc1−xYbx). Gd and Yb, like other lanthanides but unlike the transition metals Zr and Ti, do not retard the coarsening kinetics compared with binary Al–Sc alloys. Additionally, the creep resistance of these alloys is greater than that of Al–Sc alloys. The coarsening kinetics and creep properties of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb alloys are compared with other Al–Sc-based alloys and with coarsening models for ternary alloys.
Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (∼550 μm) and small (∼80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (∼430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts.
An Al–6.3Li–0.07Sc–0.02Yb (at.%) alloy is subjected to a double-aging treatment to create nanoscale precipitates, which are studied by atom-probe tomography and transmission electron microscopy. After homogenization and quenching, Yb atoms form clusters exhibiting L12-like order. A first aging step at 325 °C leads to a doubling of microhardness as a result of the formation of coherent precipitates with an Al3Yb-rich core and an Al3Sc-rich shell. The core and shell both exhibit the L12 structure and both contain a large concentration of Li, which substitutes for up to 50% of the Sc or Yb atoms at their sublattice positions. These core/single-shell precipitates provide excellent resistance to overaging at 325 °C. Subsequent aging at 170 °C increases the microhardness by an additional 30%, through precipitation of a metastable δ′-Al3Li second shell on the core/single-shell precipitates, thereby forming a chemically and structurally complex core/double-shell structure. The metastable δ′-Al3Li phase is observed to form exclusively on pre-existing core/shell precipitates.
Titanium foams with aligned, elongated pores were created by directional freeze-casting of aqueous slurries of titanium powders, followed by ice sublimation and powder sintering. Increasing sintering times from 8 to 24 h and decreasing powder size from 20 to 10 μm resulted in improved densification within cell walls and decreased overall foam porosity, with a concomitant increase in compressive stiffness, yield strength and energy absorption. A simple model for foam stiffness and strength is in general agreement with experimental measurements of strength but overpredicts stiffness, probably because it does not take into account micro-plasticity occurring during measurements.
A model for creep threshold stresses in alloys strengthened by coherent, misfitting precipitates is developed for the case where the precipitate is not sheared, and where there are elastic interactions between a dislocation and the precipitate over which it climbs. Calculations of the particle stress field due to a positive stiffness and lattice parameter mismatch between precipitate and matrix predict that the mismatch forces help the dislocation climb/glide process over the precipitates but that they trap it at the departure side of the particle. This results in a true threshold stress, rather than a slowing of the kinetics of dislocation climb as in previous models, which is given by the applied stress necessary to free the dislocation by a glide mechanism. Model predictions and experiment are compared for precipitation-strengthened aluminum alloys containing nanosize Al3Sc, Al3(Sc, Li) and Al3(Sc, Yb) precipitates with various sizes and mismatches. In agreement with experimental creep results, the model predicts that the threshold stress increases nearly linearly with precipitate radius, and also with the magnitude of the precipitate/matrix lattice mismatch.
Ni–51.4 at.% Ti (Nitinol) containing 24 and 34 vol.% orthogonally interweaving micro-channels with 350–400 μm diameters was fabricated by a powder-metallurgy method. NiTi powder preforms containing steel wire meshes arranged in parallel layers were hot-pressed into NiTi/steel composites, from which the meshes were removed electrochemically, thereby producing micro-channels with tailorable morphology, fraction, and orientation. The compressive stiffness (15–35 GPa) and strength (420–780 MPa) can be controlled by the volume fraction and orientation of the micro-channels. Stiffness values are compared against analytical foam and composite models. A combination of elasticity, superelasticity, and the shape-memory effect allows for high strain recovery (93–95% of an applied compressive strain of 5%).
Ti–6Al–4V, with a network of elongated, open pores aligned along two perpendicular directions, is produced by a two-step replication process: (i) Ti–6Al–4V powder or foil preforms containing low-carbon steel wire meshes are densified by hot pressing under transformation superplasticity conditions; (ii) porosity is created by electrochemical dissolution of the low-carbon steel wires and the adjacent Fe-containing Ti–6Al–4V matrix. If high-carbon steel wires are used, Fe diffusion into Ti–6Al–4V is inhibited by a carbide layer forming at the wire/matrix interface, and pores exactly replicate the shape of the wires. Ti–6Al–4V with ∼19% and 34% porosity, without and with Fe–Ti interdiffusion respectively, shows low oxygen contamination and good compressive ductility. Strength and stiffness, as measured by compression testing and ultrasonic measurements, are compared with simple analytical models and numerical finite-element models.
Reticulated titanium scrolls are produced by printing titanium hydride lattices composed of two orthogonal layers of ink filaments, which are then rolled into cylinders and reduced to titanium upon partial vacuum sintering. The resulting three-dimensional titanium scrolls contain a hierarchical pore size distribution composed of macroporosity between patterned filaments and micropores within each filament. These reticulated architectures exhibit an attractive combination of stiffness, strength, and ductility when tested in uniaxial compression.
Materials Science and Engineering: C 2011 Volume 31(Issue 7) pp:1423-1428
Publication Date(Web):10 October 2011
DOI:10.1016/j.msec.2011.05.011
Stabilization of biological materials by freezing is widespread in the fields of medicine and biomaterials research and yet, in the case of hard biomaterials such as dentin, there is not a good understanding of how such treatments might affect the mechanical properties. The freezing and thawing may have a number of different effects on dentin including formation of cracks in the microstructure and denaturation of the collagen. Using high-energy synchrotron X-ray diffraction, the apparent moduli of bovine dentin samples were measured before and after various numbers of freeze–thaw cycles. It was determined that repeated freezing and thawing has no measurable effect on the hydroxyapatite or fibrillar apparent moduli up to 10 cycles. This confirms that the use of low temperature storage for stabilization of dentin is reasonable in cases where stiffness is a property of importance.Highlights► Studied the effect of freezing on the load transfer of HAP and fibrils in dentin. ► X-ray scattering measured HAP and fibril apparent moduli vs. freezing cycles. ► Apparent moduli did not vary significantly between 0 and 10 freeze thaw cycles. ► Residual strains imply no widespread cracking due to volumetric expansion of water. ► Dentin can be freeze-thawed with no significant effects on elastic properties.
Journal of Materials Engineering and Performance 2011 Volume 20( Issue 4-5) pp:511-516
Publication Date(Web):2011 July
DOI:10.1007/s11665-010-9827-6
Porous NiTi was produced by sintering pre-alloyed NiTi powders (with small Ni addition to form Ni-rich composition) with NaCl powders which are removed to create 40-60 vol.% macropores which are open to the surface, blocky in shape, and 100-400 μm in size. The microporosity present between the NiTi powders is infiltrated by an in situ created NiTi-Nb eutectic liquid which, after solidification, densifies the NiTi powders into dense struts. This processing technique allows for separate control of the macroporous structure, and the densification and composition of the NiTi struts.
Porous, ferritic steel was produced by blending, pressing and sintering Fe, Cr, Mo and NaCl powders. During sintering NaCl evaporated to form 40–58% interconnected open porosities, while the metal powders densified and interdiffused to create a nearly dense Fe–26Cr–1Mo matrix (E-Brite, developed for solid oxide fuel cell interconnects). The foam compressive properties at ambient temperature were in good agreement with the Gibson–Ashby scaling laws for stiffness and strength and demonstrate high mechanical energy absorption. The foam compressive creep response at 850 °C under an argon atmosphere followed the same power law stress dependence as the bulk material, suggesting similar deformation mechanisms in each case. Creep data under argon were compared with a variational composite model and a simple unit cell model taking into account thicker nodes connecting slender struts.
The densification kinetics of Ti–6Al–4V powders with spherical or angular shapes are compared in uniaxial die pressing experiments between isothermal conditions (at 1020 °C, in the β-field, where deformation occurs by creep) and thermal cycling (between 860 and 1020 °C, within the range of the α–β phase transformation of the alloy, where transformation-mismatch plasticity is activated). Densification kinetics are only moderately affected by powder shape, but are markedly faster under thermal cycling than under isothermal conditions, as expected from the higher deformation rate achieved under transformation-mismatch plasticity conditions as compared to creep conditions. The densification curves for both creep and mismatch plasticity deformation mechanisms are successfully modeled for various applied stresses and for partial cycling, when transformation is incomplete. Tensile properties of specimens fully densified under thermal cycling conditions are similar to literature values from Ti–6Al–4V densified by isothermal hot isostatic pressing.
Ti–6Al–4V foams are produced by the expansion of pressurized argon pores trapped in billets created by powder metallurgy. Pore expansion during thermal cycling (840–1030 °C, which induces transformation superplasticity in Ti–6Al–4V) improves both the foaming rate (by reducing the flow stress) and the final porosity (by delaying fracture of the pores and subsequent escape of the gas), as compared to isothermal pore expansion at 1030 °C, where Ti–6Al–4V creep is the controlling mechanism. Raising the argon content in the billet increases the foaming rates for both creep and superplastic conditions, in general agreement with an analytical model taking into account the non-ideal behavior of high-pressure Ar and the pore size dependence of surface tension. Superplastically foamed Ti–6Al–4V with 52% open porosity exhibits a combination of high strength (170 MPa) and low stiffness (18 GPa), which is useful for bone implant applications.
Metallurgical and Materials Transactions A 2010 Volume 41( Issue 7) pp:1706-1713
Publication Date(Web):2010 July
DOI:10.1007/s11661-009-0039-3
Interpenetrating phase composites are created by warm equal channel angular extrusion (ECAE) of blended powders of amorphous Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 (Vit106a) and a crystalline ductile metal (Cu, Ni, or W). Subsequent dissolution of the continuous metallic phase results in amorphous Vit106a foams with ~40 pct aligned, elongated pores. The extent of Vit106a powder densification in the composites improves with the strength of the crystalline metallic powder, from low for Cu to high for W, with a concomitant improvement in foam compressive strength, ductility, and energy absorption.
The coarsening kinetics of nanoscale, coherent precipitates in α-Al during aging of a supersaturated Al–0.06 Sc–0.02 Er (at.%) alloy at 300 °C are studied using transmission electron microscopy and local-electrode atom-probe tomography. Erbium and Sc segregate at the precipitate core and shell, respectively. The matrix supersaturations of Er and Sc, as well as the mean precipitate radius and number density evolve in approximate agreement with coarsening models, allowing the determination of the matrix/precipitate interfacial free energy and solute diffusivities. At 300 °C, the α-Al/Al3(Sc1-xErx) interfacial free energy due to Sc is about twice as large as for α-Al/Al3Sc. The diffusivity of Er in the ternary alloy is about three orders of magnitude smaller than that of Er in binary Al–0.045 at.% Er and about two orders of magnitude smaller than the diffusivity of Sc in binary Al–Sc. The measured Sc diffusivity is consistent with the literature values.
A set of analytical models based on engineering beam analysis is developed to predict creep behavior of cellular materials over a broad range of relative density. Model predictions, which take into account the presence of mass at strut nodes and consider different possible deformation mechanisms and foam architectures, are compared to experimental creep results for a replicated nickel-base foam and a reticulated aluminum foam. As porosity decreases, the controlling creep mechanism in the foams changes from strut bending, to strut shearing, and ultimately to strut compression.
The kinetics of MgB2 synthesis is studied in situ by synchrotron X-ray diffraction, using pressed compacts of 200–400 μm magnesium powders mixed with three types of submicrometer, amorphous, high-purity boron powders. Reaction times for commercially available and plasma-synthesized boron powders decreases from 100 to 2 min as temperature increases from 670 to 900 °C. They can be described by diffusion-controlled models of a reacting sphere with kinetics characterized by diffusion coefficients increasing with temperature from 2 × 10−17 to 3 × 10−16 m2 s−1, with activation energies of 123–143 kJ mol−1. Plasma-synthesized boron powders doped with 7.4 at.% carbon show no significant differences in reaction kinetics as compared to undoped powders.
The creep of reticulated metallic foams is studied through the finite element method using three-dimensional, periodic unit cells with four different architectures characterized by struts which deform primarily by: (i) simple bending, (ii) compression, (iii) a combination of simple bending and compression and (iv) double bending (for Kelvin space-filling tetrakaidecahedra). The creep behavior of each of these models is examined with respect to temperature, stress and foam relative density. Calculated creep rates for both bending and compression models are below those predicted from simplified analytical models and bracket those of the combination model. The simple and double bending models predict nearly identical strain rates despite very different geometries, because in both cases the deflection rates of the fastest deforming struts are similar. Both analytical and numerical predictions are compared to published creep data for metallic foams.
A composite, consisting of 68 vol.% superconducting continuous MgB2 fibers aligned within a ductile Mg matrix, was loaded in uniaxial compression and the volume-averaged lattice strains in the matrix and fiber were measured in situ by synchrotron X-ray diffraction as a function of applied stress. In the elastic range of the composite, both phases exhibit the same strain, indicating that the matrix is transferring load to the fibers according to a simple iso-strain model. In the plastic range of the composite, the matrix is carrying proportionally less load. Plastic load transfer from matrix to fibers is complex due to presence in the fibers of a stiff WB4 core and of cracks produced during the in situ synthesis of the MgB2 fibers from B fibers. Also, load transfer behavior was observed to be different in bulk and near-surface regions, indicating that surface measurements are prone to error.
Dispersion-strengthened-cast aluminum (DSC-Al), consisting of a coarse-grained aluminum matrix containing two populations of particles (30 vol.% of 300 nm Al2O3 incoherent dispersoids and 0.2–0.3 vol.% of 6–60 nm coherent Al3Sc precipitates), was studied. At ambient and elevated temperatures, both populations of particles contribute to strengthening. At 300 °C, creep threshold stresses are considerably higher than for control materials with a single population of either Al2O3 dispersoids or Al3Sc precipitates. This synergistic effect is modeled by considering dislocations pinned at the departure side of incoherent Al2O3 dispersoids (detachment model) and simultaneously subjected to elastic interactions from neighboring coherent Al3Sc precipitates.
An ultrahigh-carbon steel was heat-treated to form an in situ composite consisting of a fine-grained ferritic matrix with 34 vol.% submicron spheroidized cementite particles. Volume-averaged lattice elastic strains for various crystallographic planes of the α-Fe and Fe3C phases were measured by synchrotron X-ray diffraction for a range of uniaxial tensile stresses up to 1 GPa. In the elastic range of steel deformation, no load transfer occurs between matrix and particles because both phases have nearly equivalent elastic properties. In the steel plastic range after Lüders band propagation, marked load transfer takes place from the ductile α-Fe matrix to the elastic Fe3C particles. Reasonable agreement is achieved between phase lattice strains as experimentally measured and as computed using finite-element modeling.
Co-reporter:Scott M. Oppenheimer, Andrea R. Yung, David C. Dunand
Scripta Materialia 2007 Volume 57(Issue 5) pp:377-380
Publication Date(Web):September 2007
DOI:10.1016/j.scriptamat.2007.05.004
The compressive creep behavior of nickel-rich B2–NiTi (with 50–140 μm grain size) was studied over the stress range 3–11 MPa and the temperature range 950–1100 °C. The stress exponent (n = 2.7) and activation energy (Q = 155 kJ mol−1) are compared with a literature review of NiTi creep studies performed over lower temperature and/or higher stresses. Possible explanations for discrepancies between studies are discussed.
Co-reporter:Qizhen Li;Edward Y. Chen;Douglas R. Bice
Metallurgical and Materials Transactions A 2007 Volume 38( Issue 1) pp:44-53
Publication Date(Web):2007 January
DOI:10.1007/s11661-006-9020-6
Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity. Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average strain-rate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation. The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.
A martensitic NiTi foam was produced by hot isostatic pressing a blend of NiTi and NaF powders and subsequent dissolution of the NaF phase. The NiTi foam consists of 40 vol.% near-fully open pores, 240 μm in size, and with ragged surfaces due to incomplete NiTi powder densification. Near linear stress–strain curves are measured in compression with an average loading stiffness of 4 GPa, well below the unloading stiffness of 13 GPa because of detwinning on loading. Shape-memory recovery after unloading corresponds to 85–89% of the unloading plastic strain. After sintering at 1250 °C, the foam exhibits 20% porosity, smaller, smoother and partially-closed pores, and a shift in composition towards a martensite/austenite mixture at ambient temperature. This new composition allows for the activation of the superelastic effect in the austenite during loading and unloading resulting in average stiffnesses of 6–12 GPa, and the shape-memory effect in the martensite with 60–97% of the plastic strain recoverable.
Acoustic emission methods are used to investigate the nature and evolution of microfracture damage during uniaxial compression of ductile amorphous and brittle crystalline metal foams made from a commercial Zr-based bulk metallic glass, and to compare this behavior against that of aluminum-based foam of similar structure. For the amorphous foam, acoustic activity reveals evolution of the damage process from diffuse to localized damage through the foam stress plateau region, and reversion back towards diffuse damage in the foam densification region. Accommodation of microfracture by surrounding ductile struts, and significant point contact formation, permit high average compressive strains of ca. 80% in the amorphous foam without macroscopic failure.
Materials Science and Engineering: A 2006 Volume 421(1–2) pp:35-39
Publication Date(Web):15 April 2006
DOI:10.1016/j.msea.2005.10.014
Wires of 99.5% pure iron with 406 and 508 μm diameter were subjected to a uniaxial tensile stress while being thermally cycled about the α–γ allotropic transformation temperature. The strain increments per cycle are proportional to the applied stress in the range 1–22 MPa, indicating that transformation-mismatch plasticity is the dominant deformation mechanism. The strain increments for the wires have the same magnitude as those reported for bulk iron samples, thus, indicating that the internal mismatch strains responsible for this deformation mechanism are undiminished in the wires, despite their high surface-to-volume ratio. Very high average strain rates (up to 3 × 10−3 s−1) were achieved through resistive heating and convective/radiative cooling of the thinnest wires.
Co-reporter:A.H. Brothers, R. Scheunemann, J.D. DeFouw, D.C. Dunand
Scripta Materialia 2005 Volume 52(Issue 4) pp:335-339
Publication Date(Web):February 2005
DOI:10.1016/j.scriptamat.2004.10.002
Amorphous metallic foams with an open-cell structure are processed with the salt replication method by infiltration of a sintered salt pattern with liquid Vit106, a Zr-based bulk metallic glass. After pattern removal in nitric acid, the Vit106 foams exhibit highly uniform pores, about 250 μm in size, and relative densities in the range 15–22%. Processing parameters, including pattern selection, sintering, and removal, are investigated.
Co-reporter:Heeman Choe, Susan M. Abkowitz, Stanley Abkowitz, David C. Dunand
Materials Science and Engineering: A 2005 Volume 396(1–2) pp:99-106
Publication Date(Web):15 April 2005
DOI:10.1016/j.msea.2005.01.051
The alloy Ti-6Al-4V was modified by addition of 10 wt.% tungsten through powder metallurgy. Depending on the initial W powder size, different materials were formed after powder densification: (i) “alloys” for fine (0.7 and 2 μm) W powders which were almost completely dissolved in the Ti-6Al-4V matrix; (ii) “alloyed composites” for intermediate (12 and <45 μm) W powders which were partially dissolved; (iii) and “composites” for coarse (<250 μm) W powders which were nearly un-dissolved. In all cases, tungsten strengthens Ti-6Al-4V, but much more so when dissolved in the matrix than as a second phase. Ductility was not affected by W additions for the fully-dissolved alloys, but was reduced in the case of composites with W particles, which exhibited fracture or pull-out from the matrix. Flaw sensitivity was apparent from strain hardening being much lower in tension than in compression, and from a much reduced ductility exhibited by one specimen with residual porosity.
Co-reporter:Dorian K. Balch, John G. O’Dwyer, Graham R. Davis, Carl M. Cady, George T. Gray III, David C. Dunand
Materials Science and Engineering: A 2005 Volume 391(1–2) pp:408-417
Publication Date(Web):25 January 2005
DOI:10.1016/j.msea.2004.09.012
Syntactic foams were fabricated by liquid metal infiltration of commercially pure and 7075 aluminum into preforms of hollow ceramic microspheres. The foams exhibited peak strengths during quasi-static compression ranging from −100 to −230 MPa, while dynamic compression loading showed a 10–30% increase in peak strength magnitude, with strain rate sensitivities similar to those of aluminum–matrix composite materials. X-ray tomographic investigation of the post-compression loaded foam microstructures revealed sharp differences in deformation modes, with the unalloyed-Al foam failing initially by matrix deformation, while the alloy–matrix foams failed more abruptly through the formation of sharp crush bands oriented at about 45° to the compression axis. These foams displayed pronounced energy-absorbing capabilities, suggesting their potential use in packaging applications or for impact protection; proper tailoring of matrix and microsphere strengths would result in optimized syntactic foam properties.
Materials Science and Engineering: A 2004 Volume 384(1–2) pp:184-193
Publication Date(Web):25 October 2004
DOI:10.1016/j.msea.2004.06.045
Reticulated nickel foams were alloyed with chromium by pack-chromizing, resulting in Ni–Cr foams with 9–32 wt.% Cr and 2.6–3.5% relative density. The oxidation resistance at 1000 °C of the Ni–Cr foams and the corresponding bulk Ni–Cr alloys is the same, provided the foam’s higher surface area is taken into account. The foam compressive yield stress at ambient temperature is in agreement with model predictions. The foam creep behavior, measured between 725 and 825 °C in the stress range of 0.1–0.2 MPa, is compared to two models assuming strut compression or strut bending as creep deformation modes. These models, which originally consider dislocation creep as the sole deformation mechanism, are modified to incorporate diffusional creep, due to the relatively fine grain size of the Ni–Cr struts. Good agreement is then found between data and the strut creep compression model.
Scripta Materialia 2003 Volume 49(Issue 9) pp:879-883
Publication Date(Web):November 2003
DOI:10.1016/S1359-6462(03)00438-X
Titanium foam was produced by expansion of pressurized argon-filled pores within titanium at 860 °C. Alternating atmospheres of pure Ar and Ar/4% H2 resulted in cyclical hydrogen alloying/de-alloying. The associated internal-stress plasticity enhanced pore expansion, volume fraction and size, as compared to foams expanding by creep without composition change.
Scripta Materialia 2003 Volume 48(Issue 3) pp:219-222
Publication Date(Web):February 2003
DOI:10.1016/S1359-6462(02)00428-1
The thermal expansion coefficient of Al3Sc and Al3(Sc0.75X0.25), where X is Ti, Y, Zr or Hf, was measured by dilatometry between 25 and 1000 °C. The measured value, (16±1)×10−6 K−1, is constant between 25 and 900 °C and insensitive to alloying element. Good agreement is found with a literature value determined from first-principle calculations.
Co-reporter:Dorian K. Balch, Ersan Üstündag, David C. Dunand
Journal of Non-Crystalline Solids 2003 Volume 317(1–2) pp:176-180
Publication Date(Web):March 2003
DOI:10.1016/S0022-3093(02)02008-2
In situ diffraction experiments were performed with high-energy synchrotron X-rays to examine load partitioning and high-stress relaxation during uniaxial compression of a bulk metallic glass composite containing both ductile tantalum particles and crystallized matrix material. The tantalum particles yielded at an applied stress of −800 MPa, while the matrix precipitates remained elastic up to the maximum applied stress of −1250 MPa. The von Mises effective stress in the tantalum particles at yielding was 1500 MPa, well in excess of typical tantalum yield stresses, which is attributed to a combination of solid-solution strengthening and the inhibition of dislocation motion in the 1–2 μm particles. A series of constant crosshead-position measurements made at −1250 MPa suggested the possibility of room-temperature matrix relaxation under high applied loads.
Co-reporter:M Frary, S Abkowitz, S.M Abkowitz, D.C Dunand
Materials Science and Engineering: A 2003 Volume 344(1–2) pp:103-112
Publication Date(Web):15 March 2003
DOI:10.1016/S0921-5093(02)00426-4
Tungsten-reinforced Ti and Ti–6Al–4V composites were fabricated by powder metallurgical techniques from Ti, W and Al–V powders. The microstructure of the composites consists of partially dissolved tungsten particles within an α/β titanium matrix containing tungsten in solid-solution. Yield and ultimate tensile strengths increase linearly with tungsten content in the range 0–15 wt.% W and decrease near-linearly with temperature in the range 25–540 °C. Ductility follows the opposite trend and is within technologically acceptable values, except for Ti/15W at 315 and 425 °C and Ti/10W at 540 °C which fractured near the ultimate stress value. The Ti–6Al–4V/10W composite shows the best combination of high strength and ductility at all temperatures. At ambient temperatures, Ti/10W exhibits a stress–strain curve very similar to Ti–6Al–4V (with a slight decrease in stiffness), while eliminating aluminum and vanadium alloy elements. Further improvements in mechanical properties of these non-equilibrium composites are likely to be achieved through optimized heat-treatments, which affect the matrix microstructure and the degree of dissolution of tungsten and thus the relative importance of matrix solid-solution strengthening and composite strengthening.
Co-reporter:Emmanuelle A. Marquis, David C. Dunand
Scripta Materialia 2002 Volume 47(Issue 8) pp:503-508
Publication Date(Web):11 October 2002
DOI:10.1016/S1359-6462(02)00165-3
The general climb model for creep threshold stress for dislocations interacting with incoherent particles is modified for the case of coherent precipitates, by taking into account elastic interactions between matrix dislocations and particles due to particle/matrix stiffness and lattice mismatches. The model is in qualitative agreement with experimental data for the Al–Sc system.
Materials Science and Engineering: A 2002 Volume 335(1–2) pp:128-136
Publication Date(Web):25 September 2002
DOI:10.1016/S0921-5093(01)01901-3
A coupled thermo-mechanical finite-element model was developed to describe transformation-mismatch plasticity resulting from mismatch stresses produced by allotropic particles within a creeping matrix. A composite consisting of a NiAl matrix with 10 vol.% zirconia allotropic particles was modeled in two dimensions for a range of externally-applied stress values. The instantaneous composite strain developed during the zirconia transformation is found to increase linearly with the applied stress, in agreement with continuum-mechanical, closed-form models for transformation-mismatch plasticity. This instantaneous strain is smaller than the total strain accumulated over a half temperature cycle, indicating that mismatch stresses produced during the transformation relax by matrix creep long after the particles have transformed. Also, the total composite strain calculated after a full temperature cycle is in good agreement with strains determined experimentally on a NiAl–10% ZrO2 composite. Finally, the internal stress distribution within the transforming composite is determined numerically and compared to simple analytical averages.
Materials Science and Engineering: A 2001 Volume 298(1–2) pp:63-72
Publication Date(Web):31 January 2001
DOI:10.1016/S0921-5093(00)01343-5
NiAl composites containing 10 and 20 vol.% unstabilized zirconia particles were thermally cycled around the polymorphic phase transformation range of zirconia while being subjected to an external uniaxial tensile stress. The strain rate of the composite during thermal cycling is significantly higher than the isothermal creep rate of either composite or unreinforced matrix at the same average temperature. An established model for transformation-mismatch plasticity of an allotropic, creeping metal has been adapted to describe the results. To the best of our knowledge, this is the first demonstration of transformation-mismatch plasticity in a composite induced through transformation of the minority-phase reinforcement.
Co-reporter:Kristen L. Scotti, Emily E. Northard, Amelia Plunk, Bryce C. Tappan, David C. Dunand
Acta Materialia (1 February 2017) Volume 124() pp:
Publication Date(Web):1 February 2017
DOI:10.1016/j.actamat.2016.11.038
Porous materials exhibiting aligned, elongated pore structures can be created by directional solidification of aqueous suspensions—where particles are rejected from a propagating ice front and form interdendritic, particle-packed walls—followed by sublimation of the ice, and sintering of the particle walls. Theoretical models that predict dendritic lamellae spacing—and thus wall and pore width in the final materials—are currently limited due to an inability to account for gravity-driven convective effects during solidification. Here, aqueous suspensions of 10–30 nm TiO2 nanoparticles are solidified on parabolic flights under micro-, lunar (∼0.17 g; gl = 1.62 m/s2), and Martian (∼0.38 g; gm = 3.71 m/s2) gravity and compared to terrestrially-solidified samples. After ice sublimation and sintering, all resulting TiO2 materials exhibit elongated lamellar pores replicating the ice dendrites. Increasing the TiO2 fraction in the suspensions leads to decreased lamellar spacing in all samples, regardless of gravitational acceleration. Consistent with previous studies of microgravity solidification of binary metallic alloys, lamellar spacing decreases with increasing gravitational acceleration. Mean lamellar spacing for 20 wt% TiO2 nanoparticles suspensions under micro-, lunar, Martian, and terrestrial gravity are, respectively: 50 ± 8, 34 ± 11, 30 ± 6, and 23 ± 9 μm, indicating that gravity-driven convection strongly affects lamellae spacing under terrestrial gravity conditions. Gravitational effects on lamellar spacing are highest at low TiO2 fractions in the suspension; for 5 wt% TiO2 suspensions, the microgravity lamellar spacing is more than twice that under terrestrial gravity (182 ± 21 vs. 81 ± 23 μm). Results of this study are in good agreement with previous studies of binary metallic alloy solidification where primary dendrite spacing increases under microgravity. Literature data from ice-templating systems are used to discuss a dependence on lamellae spacing of the density ratio of particles and fluid.
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