We report on the formulation of a factor, f, that when applied together with the activation energy for viscous flow (Q), can be used to provide important insight into the densification mechanism that are active during powder sintering. We ascertain the validity of this formulation by comparing the densification behaviour of atomized and milled powders for Ti-6Al-4V alloy and pure Ti during spark plasma sintering, and identifying the underlying mechanisms.We described a formulation that can be used to provide insight into the densification mechanism that are active in atomized and milled powders, during spark plasma sintering. The values of f (defined in terms of powder physical properties) and Q (activation energy for viscous flow) can be used to assess the mechanism that controls each stage of densifications.Download high-res image (143KB)Download full-size image

•In-situ formation of NbC in nanocrystalline Cu was systematically studied.•The formation NbC results in high thermal stability of nanocrystalline Cu.•High strength was achieved with nanocrystalline Cu and NbC nanoparticles.We report on an investigation of the in-situ formation of NbC nanoparticles in a Cu nanocrystalline matrix, paying particular attention to the role played by NbC on stabilizing and strengthening the matrix. The material was prepared by milling a Cu-5wt%Nb powder mixture with 1 wt% stearic acid for 36 h, followed by annealing of the milled powder at temperatures ranging from 400 to 1000 °C. Our results show that the NbC nucleates at an approximate temperature of 700 °C and that the associated enthalpy of formation and activation energy of NbC are approximately −0.8 kJ/mol and +312 kJ/mol, respectively. Moreover, the initial size of NbC particles formed at 700 °C was 8 nm, and showed limited coarsening (to 21 nm) even when annealed at 1000 °C for 1 h. With the presence of NbC, dislocation annihilation was impaired and grain boundary mobility was hindered, resulting in a dislocation density as high as 1014 m−2 following annealing at 1000 °C, and a grain size of 96 nm after annealing at 900 °C. The high thermal stability of NbC nanoparticles and the Cu matrix lead to appreciable strengthening of the composite powders as evidenced by a microhardness increase from 2128 MPa for the as-milled powder to 2665 MPa for the powder annealed at 700 °C for 1 h.

We report on a novel approach to synthesize (Ti100-x-yFexCoy)82Nb12.2Al5.8 (at.%) bimodal alloys and provide fundamental insight into their underlying microstructural evolution and mechanical behavior. In our work, a bimodal microstructure is attained via selection of phases and composition in a eutectic reaction followed by semi-solid sintering. Specifically, if one selects an atomic ratio of Ti/Fe corresponding to the eutectic composition, the resultant (Ti63.5Fe26.5Co10)82Nb12.2Al5.8 alloy shows a bimodal microstructure of micron-sized fcc Ti2(Co, Fe) embedded in an ultrafine lamellar eutectic matrix containing ultrafine bcc β-Ti and bcc B2 superstructured Ti(Fe, Co) lamellae. This structure forms from the complete eutectic reaction between β-Ti and Ti(Fe, Co). The phase boundary of β-Ti and Ti(Fe, Co) lamellae consists of a coherent interface with the orientational relationships: (110)β-Ti//(110)Ti(Fe, Co), (200)β-Ti//(100)Ti(Fe, Co) and (11¯0)β-Ti//(11¯0)Ti(Fe,Co). Such bimodal alloy exhibits ultra-high compressive yield strength of 2050 MPa with a compressive plasticity of 19.7%, which exceed published values of equivalent materials. These unusual mechanical properties are attributed to a mechanism that involves blocking, branching and multiplication of β-Ti lamellae, dislocation interactions in Ti(Fe, Co) lamellae, and the stability of coherent interfaces. In addition, unusual phenomenon of introduced high-density dislocations in B2 superstructured Ti(Fe, Co) lamellae, other than β-Ti lamellae, can be rationalized based on the formation and decomposition of superlattice dislocations according to classic crystallographic strengthening theory.Bimodal titanium alloy with ultrafine lamellar eutectic matrix fabricated by semi-solid sintering exhibits ultra-high yield strength of 2050 MPa with plasticity of 19.7%, which exceed published values of equivalent materials. Also, it displays distinct yield phenomenon of a noticed tensile plastic strain and possesses an ultimate tensile stress of 920 MPa with a maximum elongation of 1.6%, approximately equivalent to those of cast bimodal titanium alloys.Download high-res image (114KB)Download full-size image

In spark plasma sintering (SPS) it is widely acknowledged that there are specific physicochemical mechanisms associated with atomic diffusion; however, a reaction diffusion rate coefficient K has yet to be determined. In this work, we derive the K of titanium-copper diffusion couples using isothermal heat treatment of pulsed electric current (PEC), generated in SPS system. Our results show that the derived K for CuTi and Cu4Ti is at least two times higher than the corresponding ones determined under conventional annealing. Accordingly, this work substantiates, for the first time, the argument that PEC accelerates atomic diffusion compared with conventional annealing.The reaction diffusion rate coefficient K for the CuTi and Cu4Ti phases, derived using an approach involving isothermal heat treatment of pulsed electric current (PEC) generated in spark plasma sintering system, is at least two times higher than the corresponding ones determined under conventional annealing (CA) treatment.Download high-res image (198KB)Download full-size image

In this investigation we address the question of whether it is possible to simultaneously enhance the ductility and thermal stability of nanostructured (NS) Ni-based alloy by using an approach that involves the introduction of nano-precipitates. To address this question, both NS and ultra-fine grained Ni-based alloys with nanoscale γ'-precipitates were obtained via heat treatment of single-phase NS Ni-based alloy (~ 50 nm). Our results show that the thermally-stable γ'-precipitates contribute to both ductility and a remarkably high thermal stability close to 700 °C (0.62 Tm). The underlying mechanisms that are believed to be responsible for this behavior are discussed.Download high-res image (144KB)Download full-size image

•Nanostructured/ultrafine grained bulk Al is produced via cryomilling and SPS.•Its yield strength and compressive strength are 380 MPa and 420 MPa, respectively.•A semi-quantitative insight into the strengthening mechanisms is provided.•The changes in chemistry of the Al powder that occurred during SPS are studied.We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS)/ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial strain hardened aluminum (124 MPa). Microstructural studies reveal that the nanometric grains are embedded inside a matrix of ultra-fine grains in the bulk material. The corresponding average grain size is approximately 125 nm and with a distribution of grains that are smaller than 500 nm. Moreover, chemical analysis of the powder particles and the consolidated samples indicates that carbon and oxygen levels remain unchanged, and that there is a slight decrease in the nitrogen level and a significant reduction in the hydrogen level after spark plasma sintering. Semi-quantitative analysis suggests that the mechanisms that contribute to the strength of the consolidated material include: grain boundary strengthening, second phase strengthening and dislocation strengthening.

A novel equiatomic Co20Ni20Fe20Al20Ti20 (at%) alloy was designed and synthesized to study the effect of high atomic concentrations of Al and Ti elements on the microstructure, phase composition and mechanical behavior of high-entropy alloys (HEAs) fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Following the MA process, the Co20Ni20Fe20Al20Ti20 alloy was composed of a primary body-centered cubic (BCC) supersaturated solid solution and a face-centered cubic (FCC) supersaturated solid solution. However, following SPS, a primary FCC solid-solution phase, a BCC solid-solution phase and a trace amount of Al3Ti intermetallics were observed. Transmission electron microscopy (TEM) results confirmed the presence of the FCC solid-solution phase, the BCC (B2-type) solid-solution phase and Al3Ti intermetallics in the bulk alloy. The FCC and B2-type phases are ultrafine-grained, and Al3Ti intermetallics is nano/ultrafine-grained. Our results suggest that consideration of a single existing empirical design criterion is inadequate to explain phase formation in the Co20Ni20Fe20Al20Ti20 alloy. Solid-solution strengthening, grain-boundary strengthening, twin-boundary strengthening, the presence of the strong B2-type BCC phase, and precipitate strengthening due to the presence of a trace amount of Al3Ti are responsible for the ultra-high compressive strength of ~2988 MPa and hardness of ~704 Hv. The strain-to-failure of ~5.8% with visible ductility is dominated by the FCC solid-solution phase.

The microstructures of B4C/Al-7093 composites synthesized using the Boralyn technique were investigated. The present results show that the B4C particles were distributed uniformly in the aluminum matrix. SEM fractographic investigation revealed that particle cracking was a dominant damage mechanism, indicating a strong B4C/Al interfacial bond which promoted high mechanical properties in the composite. TEM and EDS analyses showed MgO particles at and near the B4C/Al interfaces. The amount of MgO significantly influenced the microstructure of the matrix. The formation of MgO in the composite resulted in the depletion of the Mg atoms in the matrix and furthermore, the suppression of precipitation during the aging process.

Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed using transmission electron microscopy in the nanostructured Cr3C2/NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles, the phase transformations in the original NiCr amorphous phase, which was always observed in the as-sprayed nanostructured Cr3C2/NiCr coatings, were also identified. Internal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. The results of microhardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposure temperature, while that of the nanostructured coating increased significantly from 1020 to 1240 HV300. Compared with the conventional Cr3C2/NiCr coatings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat treatment led to further increase in scratch-resistance and further decrease in coefficient of friction of the nanostructured coatings. The increases in microhardness and scratch-resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nanoparticles precipitating within the coating as the exposure temperature increased.

The microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying is described. Stainless steel powders with a particle size in the range of 45–11 μm were mechanically milled for 10 h in liquid nitrogen to produce powders with a nanocrystalline grain size of 21±8 nm and an aspect ratio of 1.68. The cryomilled powders were subsequently sprayed onto a stainless steel substrate by high-velocity oxygen fuel spraying. The resultant coating exhibited a superior microhardness, despite an increased porosity, over that of the conventional coating sprayed with the same parameters. Transmission electron microscopy performed on the cross-sections of the nanocrystalline coating revealed the splat formation with a thickness ranging from 40 to 400 nm. Various oxide phases (Cr2O3, FeO, Fe2O3 and γ-Fe2O3) in the stainless steel matrix were identified using selected area diffraction. This observation suggests that in-flight oxidation may have occurred during spraying and/or during splat formation.

•A bilayer composite was synthesized using plasma activated sintering.•Precipitation behavior of the bilayer composites was discussed.•Densification mechanisms of the bilayer composites were discussed.•Fracture mechanisms of the bilayer composites were discussed.In the present study a novel bilayer composite consisting of a tough layer (AA7075) and hard layer (AA7075/B4C composite), was successfully synthesized using plasma activated sintering. No obvious defects such as porosity, cracks, or delamination were observed in the as-processed bilayer composite. Transmission electron microscopy results suggest that dislocation density in the vicinity of B4C particles is higher relative to that of the unreinforced layer, as imaged with the [011] zone axis. The precipitation behavior is significantly different in the two layers, despite identical process conditions. Moreover, no obvious precipitates free zones were observed in the interfacial region between the two layers. To provide insight into the mechanical behavior of the bilayer composites, we measured the bending strength on two different load-bearing surfaces. Our results indicate that when the AA7075/B4C layer was loaded under tensile conditions, the AA7075/B4C composite was prone to crack formation and catastrophic failure. In contrast, when the AA7075 layer was loaded in tensile, the bilayer composite achieved a notable bending strength value of 1234 ± 72 MPa, which is higher relative to that of aforementioned case (938 ± 85 MPa). Interestingly, no interfacial cracks or delamination were observed under these testing conditions. In addition, the fracture mechanisms that are active in the bilayer composites for the load bearing conditions studied were discussed in detail.