This study employed a TEM-based automated crystal orientation mapping technique that enables orientation mapping with nanoscale spatial resolution. This nanoscale orientation mapping technique was employed to study thermally-assisted grain growth and to quantify the attendant formation of nanotwins and twin junctions in nanocrystalline Cu. The grain size increased from 29 ± 14 nm to 57 ± 22 nm and the fraction of twin-containing grains increased from 0.18 to 0.70. Close inspection of the twins and twin junctions captured within orientation maps documented a frequency of junctions that was remarkably consistent with previous molecular dynamics predictions.Download high-res image (185KB)Download full-size image

Boron carbide has a wide range of solubility, but the effects of stoichiometry on its microstructure and mechanical response are not well understood. In this study, detailed microstructural characterization was carried out on three hot-pressed B-rich boron carbide samples. Lattice parameter measurements from XRD identified the compositions to be B4.2C, B5.6C and B7.6C. Local substitutional disorder was observed by Raman spectroscopy, particularly for more B-rich samples. Electron energy loss spectroscopy observations suggest that excess boron preferentially substitutes for carbon atoms in the B11C icosahedra; after which additional boron modifies the CBC chains. Moreover, the boron content has salient effects on boron carbide densification and microstructure. Improved densification was observed in the more B-rich samples (B5.6C and B7.6C), and there is a transition from few or no intragranular planar defects (B4.2C), to numerous stacking faults (B5.6C), to copious twins (B7.6C). Nanoindentation experiments revealed that the highest value for B4.2C is statistically larger than that for B5.6C or B7.6C, suggesting that the hardness of boron carbide is reduced by boron substitution.Download high-res image (329KB)Download full-size image

A new experimental technique is presented that combines in situ straining with transmission electron microscope-based automated crystal orientation mapping to document microstructural evolution with nanoscale resolution at sequential stages of deformation. Orientation maps of freestanding annealed nanocrystalline Cu films have been collected, and the resultant datasets provide direct measures of grain size, shape and orientation as well as local grain boundary character and position at various stages of applied strain. Numerous examples of stress-driven grain boundary and twin boundary migration were recorded and studied. Detailed analysis of the misorientation of mobile and immobile grain boundaries provided clear experimental evidence that a broad range of grain boundaries are susceptible to stress-assisted migration; no general correlation between grain boundary misorientation and mobility was detected. Incoherent Σ3 boundaries were observed to be significantly more mobile than coherent Σ3 twin boundaries. Nevertheless, deformation twins were observed to nucleate and grow from grain boundaries and triple points.Download high-res image (315KB)Download full-size image

•Si addition leads to a dense final product at a low sintering temperature.•The added Si preferentially formed SiC and borosilicate glass instead of replacing the chain atoms in boron carbide.•The hardness of our Si-containing boron carbide is 30–35% greater than currently in-use commercial boron carbide plates.Fully dense boron carbide discs were achieved by spark plasma sintering boron carbide powders with 10 wt% silicon. The silicon did not diffuse into boron carbide grains to produce a solid solution of Si-doped boron carbide; instead the silicon reacted with impurities in the starting powder to form β-SiC and borosilicate glass. The resultant new phases facilitated densification of the multiphase ceramic through liquid phase-assisted sintering. The resultant material exhibits improved hardness (34.3 GPa Vikers hardness under 1 kg load) with toughness comparable to both Si-free and commercially available boron carbide.Download high-res image (127KB)Download full-size image

Findings of laser-assisted atom probe tomography experiments on boron carbide elucidate an approach for characterizing the atomic structure and interatomic bonding of molecules associated with extraordinary structural stability. The discovery of crystallographic planes in these boron carbide datasets substantiates that crystallinity is maintained to the point of field evaporation, and characterization of individual ionization events gives unexpected evidence of the destruction of individual icosahedra. Statistical analyses of the ions created during the field evaporation process have been used to deduce relative atomic bond strengths and show that the icosahedra in boron carbide are not as stable as anticipated. Combined with quantum mechanics simulations, this result provides insight into the structural instability and amorphization of boron carbide. The temporal, spatial, and compositional information provided by atom probe tomography makes it a unique platform for elucidating the relative stability and interactions of primary building blocks in hierarchically crystalline materials.

Microtensile experiments have been performed to elucidate the mechanical response of ultrafine-grained Mg thin films. Strengths of 160 MPa and elongations up to 8% were measured. Post-deformation electron microscopy indicates a lack of intragranular dislocation confinement. While strength does increase with decreasing grain size, the size effect for hexagonal Mg is not as strong as that reported for face-centered cubic metals. Strength appears to be governed by a lack of dislocation pile-up as well as texture and Peierls effects.

Multi-layer insulation (MLI) blankets from the Hubble Space Telescope have been recovered during the last servicing mission, after 19.1 years of on-orbit service. Based on testing and analysis of returned insulation material from earlier Hubble servicing missions, the space environment is known to have detrimental effects on the mechanical properties. The most recently retrieved MLI blankets were highly degraded with many cracks, limiting the material available for full-scale mechanical testing. As a result, micro-tensile experiments have been performed to characterize the effect of space exposure on the mechanical response of the outermost layer of the MLI. This outer layer, 127 μm thick fluorinated ethylene propylene with a 100 nm thick vapor deposited aluminum reflective coating, maintained significant tensile ductility but exhibited a degradation of strength that scales with severity of space exposure. This change in properties is attributed to damage from incident solar flux, atomic oxygen damage and thermal cycling.

A diffusion-multiple method was applied to determine the effect of nine ternary elements (Pt, Ta, Cr, Mo, Re, W, Zr, Si, and Co) on the martensitic transformation in β-NiAl. The results show that martensite formation is highly sensitive to composition, temperature, and kinetics. Electron microprobe analysis was employed to establish the composition of the ternary martensitic regions on the diffusion multiples. With the exception of Pt, Cr, and Co, the solubility limits of the ternary additions in β-NiAl were low (less than ∼2 at%) and the elements with the highest solubilities, Pt and Co, appeared to substitute predominantly with Ni. The implications of the aforementioned ternary additions are also discussed with regards to the overall performance of NiAl-based thermal barrier coating bond coat alloys.

Martensite in a ‘two-phase’ NiCoCrAlY bond coat was observed to transform to the parent β-phase < ∼150 °C during in situ transmission electron microscopy heating. At these low temperatures, the transformation does not promote bond coat plasticity or rumpling of the thermally grown oxide that would lead to significant stresses in the top coat. A lower limit of 800 °C was measured for the γ′ solvus temperature. Dissolution of γ′ does not produce a volume change, owing to the coherent nature of the precipitates.

Reactive elements such as yttrium are known to have a beneficial effect on oxide spallation resistance. Yttrium-rich precipitates containing sulfur were detected by scanning transmission electron microscopy in the peg regions of a thermally grown oxide on a commercial NiCoCrAlY bond coat that has seen service in a gas turbine environment. The coexistence of yttrium and sulfur in these pegs provides clear experimental evidence for the widely held hypothesis that sulfur is gettered by reactive elements in bond coats for thermal barrier coatings.

A combined experimental/simulation approach has been used to characterize the underlying deformation mechanisms associated with stress-assisted grain growth in nanocrystalline Al. Strain rate sensitivity experiments on freestanding submicron thin films undergoing stress-assisted grain boundary migration have uncovered rate sensitivities up to two orders of magnitude larger than previously reported for microcrystalline Al. Molecular dynamics simulations have been used to illustrate that these high strain rate sensitivities coincide with those associated with grain boundary processes such as migration, sliding, and dislocation nucleation.

Room and elevated temperature tensile, creep and high-cycle fatigue properties of electrodeposited LIGA Ni microsamples have been measured and are being used to predict the reliability of LIGA Ni MEMS structures. Tensile specimens with dimensions of hundreds of microns have been LIGA fabricated and characterized in terms of their underlying microstructure, elevated temperature tensile and creep strength and their high-cycle fatigue performance. The stiffness of these LIGA Ni structures was found to be reduced by the introduction of porosity during the plating process. The strength of these structures was observed to decrease dramatically at temperatures above 200 °C. At stresses significantly below the yield strength, substantial creep deformation was observed at moderately elevated temperatures. The fatigue life of the LIGA Ni microsamples increased with decreasing stress amplitude in a manner comparable to what has been reported for wrought Ni. An apparent fatigue limit was observed for the LIGA Ni microsamples, but the importance of underlying microstructure and component geometry on the fatigue life was also highlighted.