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A bulk in-situ Al/(TiAl3+Al2O3) composite was fabricated by accumulative roll-bonding (ARB) and spark plasma sintering (SPS) from pure Al sheets and TiO2 nanoparticles. The microstructure and mechanical properties of the composite were investigated. A uniform dispersion of TiO2 nanoparticles in Al matrix was first achieved by ARB processing in the primary Al/TiO2 composites. Subsequent SPS resulted in the formation of hierarchical TiAl3 (∼60 nm) and Al2O3 (∼750 nm) particles that were distributed uniformly in the ultrafine-grained Al matrix. Microhardness and strength (at ambient and high temperatures) of the final composite were substantially enhanced compared with those of pure Al and the primary Al/TiO2 composite, reaching ∼198.6 HV, ∼628.8 MPa at room temperature and ∼384.2 MPa at 300 °C, respectively. The high strength of the Al/(TiAl3+Al2O3) composite was attributed to a superimposition of different strengthening mechanisms.

Ultrafine-grained Al matrix composites reinforced by TiAl3 particles with volume fractions (Vp) up to ∼49% were fabricated from nano-sized Ti powders and pure Al sheets via room-temperature accumulative roll-bonding (RT-ARB) combined with hot-ARB at high temperatures. The RT-ARB was first applied to disperse the Ti nanopowders uniformly in Al matrix. In situ formation of TiAl3 particles via reaction of Ti and Al was induced during subsequent hot-ARB process at 600 °C. It was found that further hot-ARB at 700 °C can enhance the volume fraction and the dispersion of TiAl3 remarkably. The microhardness and tensile strength of the final composites are significantly improved as compared to those of pure Al and increase with increasing volume fraction of TiAl3, reaching a maximum of ∼180 HV and ∼455 MPa for Vp = 49%, respectively. The high strength of the TiAl3/Al composites can be attributed to the superimposition of strengthening mechanisms which are dominant at high Vp.

We investigated the Hall-Petch relations in Al-ZnO composites with different matrix grain size (D) relative to interparticle spacing (L). For samples with D > L, it was found that grain refinement as described by Hall-Petch relation and particle strengthening by Orowan mechanism both significantly contribute to the strengthening of the composites. For samples with D ≤ L, variation of microhardness with grain size was found to follow a good Hall-Petch relation with an enhanced Hall-Petch slope. Our study indicates the different coupling of strengthening mechanisms in metal matrix composites with different microstructure length scales.

Copper matrix nanocomposites reinforced by Al2O3 nanoparticles were fabricated by accumulative roll-bonding (ARB) processed to ultrahigh rolling cycles. The effects of ARB cycles on the distribution of Al2O3 nanoparticles and properties of the nanocomposites were investigated. It was found that a higher ARB cycle was needed to obtain a uniform distribution of Al2O3 nanoparticles with a larger volume fraction. Significant strengthening, as well as a good combination of tensile strength and electrical conductivity of the Cu-Al2O3 nanocomposites can be obtained, through optimizing the uniform dispersion of Al2O3 nanoparticles by applying ultrahigh ARB cycles. The enhanced strength (maximum: 735 MPa) of the nanocomposites can be attributed to the maximized contribution of nanoparticles by various mechanisms through optimizing the uniform dispersion of them.

A recent experiment (Scripta Mater., 65:990, 2011) shows that the Σ7 {132}/{132} grain boundary in Al can migrate under external stress but produces no strain. Here, based on a bi-crystallographic analysis, an atomic shuffling path was identified as the feasible mechanism for this grain boundary migration. By a density functional theory calculation, it reveals that the enthalpy barrier of this atomic shuffling path increases by external shear stress applied with shear of the grain boundary along the tilt axis 〈111〉, which is in good agreement with experimentally measured shear-direction-dependence of activation enthalpy for this grain boundary migration.

Cu/C composites were prepared from Cu and graphite by accumulative roll-bonding (ARB) up to 30 cycles (N) with a 50% thickness reduction per cycle at room temperature. The microstructure and properties of the Cu/C composites were investigated. Results showed that ARB can remarkably decrease the size of graphite and improve the dispersion of graphite in the Cu matrix. Moreover, significant thickness reduction (down to ~5 graphene layers) of the graphite was found in the Cu/C composites fabricated by ARB. The microhardness of the Cu/C composites increases with increasing N and is ~3.3 times that of pure Cu for N = 30. The electrical conductivity of the Cu/C composites decreases slightly with increasing N, with a minimum of ~90% IACS for N = 30. Our study indicated that ARB can be an effective method for fabrication of Cu/C composites from Cu and graphite with a combination of hardness and electrical conductivity better than or as good as that of carbon nanotube or graphene reinforced Cu matrix composites as reported. Keywords: Cu/C composites, Accumulative roll-bonding, Graphene, Hardness, Electrical conductivity

The use of selective laser melting (SLM) has become more common with regard to the fabrication of high-strength Ti-6Al-4V components. However, the as-SLMed Ti-6Al-4V alloy parts typically exhibit low ductility because of the formation of acicular and brittle α′ martensites. It is essential to use post-heat treatment to change their microstructures to achieve superior mechanical properties. In this study, a particular multi-step heat treatment (MSHT) was applied to the Ti-6Al-4V alloy samples fabricated via SLM. Conventional post-heat treatments were conducted for comparison. The microstructures and tensile properties of the as-SLMed and heat-treated samples were investigated. The as-SLMed Ti-6Al-4V sample is dominated by plate and acicular α′ martensites, which exhibit high strength (1280 MPa) and low ductility (9.0%). The conventional solution plus aging or stress-relieving treatment cannot result in a good combination of strength and ductility. A single step annealing process at 700 °C for 2 h changes the martensitic structure to a fine lamellar α+β structure, which provides a strong and ductile Ti-6Al-4V with an ultimate tensile strength of 1108 MPa and a total elongation of 17.6%. In contrast, the MSHT process facilitates the globularization of α and, thereby generates a nearly equiaxed structure, resulting in superior ductility (21.8% of total elongation), while maintaining a moderate ultimate tensile strength of 953 MPa.

The effect of surface nanostructuring on the liquid-phase aluminizing of Ti-6Al-4V alloy is investigated. A composite interaction layer of TiAl3 and Al is formed in samples with a nanostructured surface layer after liquid-phase aluminizing, of which the thickness is significantly larger than that of the coarse-grained samples, due to the enhanced formation of TiAl3 with more dispersed distribution. Fragmentation and migration of the nanostructured surface layer of the matrix during liquid-phase aluminizing is evidenced, which is responsible for the distinct aluminizing microstructure in surface nanostructured samples.

It has been expected that mechanical properties of a composite can be enhanced effectively by increasing the content of reinforcing particles to high levels, for which a simultaneous fulfillment of high content and uniform distribution of reinforcing particles is key. In this study, we fabricated Al-TiAl3-Al2O3 composites from starting materials of Al sheets and TiO2 nanopowders by combining accumulative roll-bonding and spark plasma sintering processes. The microstructure and mechanical properties of the samples were investigated. The as-prepared samples contain hybrid TiAl3 and Al2O3 particles with total volume fractions up to 66.1–81.5 % that were formed in situ by the reaction of Al and TiO2 with much smaller volume fractions (6.7–13.9 %). The composites exhibit a microhardness up to ∼532 HV and high strengths up to ∼1312 MPa at room temperature, ∼919 MPa at 300 °C and ∼565 MPa at 600 °C. The superior mechanical properties of the composites at ambient and elevated temperatures can be attributed to the high volume fraction of reinforcing particles with meanwhile uniform distribution, for which different strengthening mechanisms were discussed.

The substantial reduction in strength has limited the widespread usage of Co–Cr based alloys at elevated temperatures. Here, we designed and fabricated an ultrafine-grained Co–28Cr–3Ta alloy which could retain over 80% of its room temperature yield strength at 300 °C, two times that of its equivalent without addition of Ta. Such high thermal stability was achieved via a processing route which precipitated nanoscale Laves phases. The results demonstrate that Orowan strengthening operates as the dominant mechanism in the alloy. The dislocation loops formed from interactions between dislocations and precipitates could effectively prevent the dislocation gliding at modest temperature.

The microstructure, mechanical behavior and deformation mechanisms of Al–TiAl3 composites containing a wide range of volume fraction of TiAl3 (Vp: 24.6–89.0%) with uniform distribution were investigated. Microhardness and strength of the composites increase with increasing Vp of TiAl3 particles, reaching ~11.0 times and ~8.0 times those of the matrix respectively. While the strength of the composites can be correlated well to the comprehensive contributions of various strengthening mechanisms of the matrix for low Vp, that for high Vp was found to fit to direct contributions from both the matrix and particle following the law of mixture. Besides the deformation of the matrix, evident plastic deformation of TiAl3 particles by dislocation activities as well as twinning was observed in the composites upon loading at room temperature. Two mechanisms for crack initiation inside the TiAl3 particle were identified, namely by intersection of twins and by interaction between dislocations and twins. Microcracks tend to propagate along the twin boundaries in TiAl3 particles and were found to be suspended at the particle/matrix interface. The unique deformation behavior of TiAl3 particles in the composites can be attributed to the uniform distribution of TiAl3 particles that ensures a continuous network of surrounding matrix even at very high Vp.

The present work investigates the microstructural evolution of a biomedical Co-20Cr-15W-10Ni alloy during multi-pass thermomechanical processing (TMP) and clarifies its effect on the tensile properties. The results indicate that multi-pass TMP consisting of repetitive cold groove-rolling followed by annealing is attractive for fabricating the alloy rod for fine wire and mini-tube making. The multi-pass TMP provided significant grain refinement from 100 to 4 μm via five repetition and, can create bimodal grain structures consisting of fine (≈3 μm) and coarse grains (4−16 μm). The bimodal grain structure provided a superior strength-ductility combination, a yield strength of 593−738 MPa and ultimate strength of 1197−1304 MPa with a uniform elongation of 54.7−61.1%. Changing the annealing condition in the final TMP step enables to further refine the grains to 2 μm and also modify the grain size distribution and, the dispersion and fraction of nano-carbides. Thus, the mechanical properties of the Co-20Cr-15W-10Ni alloy can be further optimized, leading to a yield strength of 795−890 MPa and ultimate strength of 1328−1363 MPa with a uniform elongation of 49.4−64.0%.

The properties of nanoporous gold (NPG) were known to be dependent on the microstructure of NPG. In this study, the effects of cold rolling and annealing of the original Ag0.7Au0.3 alloy on the microstructure of NPG produced by dealloying under free corrosion condition were investigated. Ag0.7Au0.3 alloy samples were cold-rolled to different strain levels/thickness reductions up to 98% and annealed at 900 &deg, C for 3 h before dealloying. It was found that cold rolling and annealing of the original alloy can lead to reduced ligament and pore sizes of NPG. Moreover, post-deformation annealing of the original alloy was found to facilitate the formation of a homogeneous and continuous NPG structure. The minima of pore and ligament sizes (both being ~8 nm) with uniform distribution were obtained in the annealed sample with a thickness reduction of 60% for a dealloying time of 7 h. The present study indicated the significant effect of a pre-dealloying treatment of the original alloy (by plastic deformation and annealing) on the formation and optimization of the NPG microstructure produced by dealloying.

Hydrogen atoms absorbed by metals in the hydrogen-containing environments can lead to the premature fracture of the metal components used in load-bearing conditions. Since metals used in practice are mostly polycrystalline, grain boundaries (GBs) can play an important role in hydrogen embrittlement of metals. Here we show that the reaction of GB with lattice dislocations is a key component in hydrogen embrittlement mechanism for polycrystalline metals. We use atomistic modeling methods to investigate the mechanical response of GBs in alpha-iron with various hydrogen concentrations. Analysis indicates that dislocations impingement and emission on the GB cause the GB to locally transform into an activated state with a more disordered atomistic structure, and introduce a local stress concentration. The activation of the GB segregated with hydrogen atoms can greatly facilitate decohesion of the GB. We show that the hydrogen embrittlement model proposed here can give better explanation of many experimental observations.

Al2O3–Ti2AlN composite with a high volume fraction (~40%) of Al2O3 was successfully fabricated by in situ solid reaction of nanoparticles produced by co-evaporation. The composite, characterized by small Al2O3 particles dispersed homogenously in a fine-grained Ti2AlN matrix, showed a novel combination of high temperature mechanical properties and oxidation resistance.

Martensite phases of Zr50Cu50 and Zr50Cu44Al6 were deformed by high-pressure torsion (HPT) at different temperatures ranging from 253 to 473K. Microstructural observations revealed that the crystalline-to-amorphous transformation induced by HPT showed remarkable temperature dependence: an increase in deformation temperature up to 473K significantly increased the fraction of the amorphous structure. This result suggests that the lattice instability near martensitic transformation temperature promotes crystalline-to-amorphous transformation, which is coherent to the generalized Lindemann criterion of defect-induced amorphization. [doi:10.2320/matertrans.M2013049]

The strain-induced continuous microstructural evolution of pure Cu (99.97%) was recorded in an archeological fashion in a single sample by inducing a high-gradient strain. Quantitative measurements revealed a coupled behavior with obvious inflection points for the variations of critical structure parameters (boundary spacing and misorientation) with strain, separating the microstructural evolution into different stages. The results were discussed by considering the different roles of intragranular dislocation activities and grain/subgrain rotation in the microstructural refinement of metals deformed to high plastic strains.

Nanostructured NiTi with a graded surface nanostructure was produced by surface mechanical attrition treatment (SMAT). Nanoindentation measurements revealed that the elastic modulus of SMAT samples increased significantly with decreasing distance from the SMAT surface and grain size, reaching ∼85 GPa for a grain size of ∼6 nm at the surface. This grain size dependence of the elastic modulus was found to be due to suppression of the stress-induced martensitic transformation in nanostructured NiTi.

Melting of solids is a common phenomenon in nature. It is also one of the most important phase transformations in materials science and engineering. In recent years, extensive experimental and theoretical investigations in conjunction with computer simulations on melting of solids, with various geometries ranging from bulk forms to nanometer-sized clusters, have greatly enhanced our understanding of the nature of melting. This paper reviewed the up-to-date research results on this classical and cutting-edge topic. Emphasis was made on melting and superheating of nano-sized particles and thin films including thermodynamic and kinetic analyses of the size effect and the interfacial structure effect on melting processes. (C) 2007 Elsevier Ltd. All rights reserved.

Cr-containing diamond-like carbon (Cr-DLC) nanocomposite coatings were synthesized by ion beam-assisted arc ion plating with varying hollow cathode ion source (HCIS) currents. The morphologies, compositions and microstructures were characterized by scanning electronic microscopy (SEM), atomic force microscopy (AFM), X-Ray photoelectron spectrometer (XPS), Raman spectroscopy, grazing incidence X-ray diffraction (GIXRD) and high-resolution transmission electron microscopy (HRTEM). Hardness and friction coefficient were investigated by using nanoindentation and ball-on-disc tribometer, respectively. With no HCIS current, the coating exhibits the maximal growth rate and a rough surface, as well as lower hardness and elastic modulus. With the increasing HCIS current from 40[Formula: see text]A to 80[Formula: see text]A, the growth rate and the content of chromium carbide decrease obviously, the [Formula: see text]/[Formula: see text] ratio increases initially to the maximum at the HCIS current of 60[Formula: see text]A, the highest hardness and elastic modulus are obtained at the HCIS current of 50[Formula: see text]A. It is also revealed that moderate HCIS current can reduce surface roughness obviously and promote tribological properties. The correlation of the HCIS current with the microstructure and performance of Cr-DLC coating has been established.

By modifying the accumulative roll-bonding (ARB) procedures, accumulative roll-bonding (ARB) processing up to 30 cycles (N) with a 50% thickness reduction per cycle (equivalent strain = 24) at room temperature was conducted on pure copper. The bonding condition, microhardness and electrical conductivity of the ARBed Cu were studied. Results showed that good bonding condition of the samples was achieved. As N increases, the microhardness of ARBed Cu increases, reaching ~2.9 times that of annealed Cu for N = 30. The electrical conductivity of ARBed Cu decreases slightly but with periodic fluctuations for N > 10, with a minimum of 90.4% IACS for N = 30. Our study indicated that ARB can be an effective way to produce high-hardness and high-conductivity pure copper better than or comparable to Cu alloys and Cu based composites as reported.