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Aluminum matrix composites (AMCs) reinforced with graphene nanoplatelets (GNPs) were fabricated by using an accumulative roll-compositing (ARC) process. Microstructure, mechanical and electrical properties of the nanostructured AMCs were characterized. The results showed that small addition (0.2 vol% and 0.5 vol%) of GNPs can lead to a simultaneous increase in the tensile strength and ductility of the GNPs/Al nanocomposites, as compared with the same processed pure Al. With increasing GNPs content, the tensile strength of the GNPs/Al nanocomposites can be enhanced to 387 MPa with retained elongation of 15%. Meanwhile, the GNPs/Al nanocomposites exhibited a good electrical conductivity of 77.8%–86.1% that of annealed pure Al. The excellent properties (high strength, high ductility and high conductivity) of the GNPs/Al are associated with the particular ARC process, which facilitates the uniform dispersion of GNPs in the matrix and formation of ultrafine-grained Al matrix. The strengthening and toughening of the GNPs/Al nanocomposites were discussed considering different mechanisms and the unique effect of GNPs.

Three-dimensional (3D) porous zinc (Zn) with a moderate degradation rate is a promising candidate for biodegradable bone scaffolds. However, fabrication of such scaffolds with adequate mechanical properties remains a challenge. Moreover, the composition, crystallography and microstructure of the in vivo degradation products formed at or near the implant-bone interface are still not precisely known. Here, we have fabricated porous Fe@Zn scaffolds with skeletons consisting of an inner core layer of Fe and an outer shell layer of Zn using template-assisted electrodeposition technique, and systematically evaluated their porous structure, mechanical properties, degradation mechanism, antibacterial ability and in vitro and in vivo biocompatibility. In situ site-specific focused ion beam micromilling and transmission electron microscopy were used to identify the in vivo degradation products at the nanometer scale. The 3D porous Fe@Zn scaffolds show similar structure and comparable mechanical properties to human cancellous bone. The degradation rates can be adjusted by varying the layer thickness of Zn and Fe. The antibacterial rates reach over 95% against S. aureus and almost 100% against E. coli. A threshold of released Zn ion concentration (~ 0.3 mM) was found to determine the in vitro biocompatibility. Intense new bone formation and ingrowth were observed despite with a slight inflammatory response. The in vivo degradation products were identified to be equiaxed nanocrystalline zinc oxide with dispersed zinc carbonate. This study not only demonstrates the feasibility of porous Fe@Zn for biodegradable bone implants, but also provides significant insight into the degradation mechanism of porous Zn in physiological environment.

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.

Co-Cr-based alloys have been widely used as mechanical components where high wear-resistance is required. However, conventional Co-Cr alloys usually have coarse-grained microstructures with phase coexistence of both hexagonal close packed (hcp) e-Co and face-centered cubic (fcc) γ-Co. The wear resistance of the Co-Cr alloys could be further enhanced by grain refinement and exclusion of γ-phase. Here, a Co-30Cr (at%) bulk alloy consisting of ultrafine-grained matrix of e-phase and uniformly dispersed nanoscale precipitates of σ-phase was fabricated by high energy ball milling and spark plasma sintering. Microstructure and dry sliding wear performance against alumina at room temperature and 600 °C were then investigated. The matrix and the precipitates have the average grain size of 227 nm and 16 nm, respectively. Hardness was determined to be 707 HV. The e-Co matrix, ultrafine-grained microstructure and the uniformly dispersed nanoprecipitates enable the alloy with excellent wear resistance. Grain growth of the matrix grain was found as the key factor to be responsible for the increase of wear rate at 600 °C. Subsurface microstructures of the alloy show no significant plastic deformation after wear tests, which is attributed to high hardness of the alloy and very limited slip in hcp system.

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.

The application and performance of graphene-reinforced composites can be benefited from the easy fabrication of graphene and good dispersion of them in the matrix. Here we reported the fabrication of in-situ graphene/copper (Gr/Cu) nanocomposites by an accumulative roll-compositing (ARC) process featured by ultrahigh rolling cycles/strains. By sandwiching in copper sheets, commercial graphite foils can be effectively delaminated into few-layer graphene sheets by ARC, without the need of any additives or chemical treatments. The ARC-induced delamination of graphite was mediated by the shear-induced slip and debonding of graphitic planes, for which the critical stress/strain conditions were analyzed. The plastic-deformation-based method simultaneously enabled the uniform incorporation of the delaminated graphene sheets into Cu matrix. The as-produced bulk Gr/Cu nanocomposites exhibit an excellent combination of strength, ductility and electrical conductivity, with the maximum tensile strength of ~686 MPa and electrical conductivity above 70% IACS. Our study showed the high effectiveness of ARC processing for exfoliating graphite into graphene nanosheets, and the great potential for enhancing the contributions of graphene by improving their dispersion and bonding with the matrix.

Ti6Al4V alloy was subjected to high pressure surface rolling (HPSR) deformation. Effects of initial microstructure (heat treatments) and HPSR parameter on the formation of surface nanostructure and mechanical property of Ti6Al4V were investigated. A gradient nano-microstructured surface layer was successfully produced on Ti6Al4V after HPSR, of which the thickness increases and the grain size decreases with the increase of HPSR load ( F ). Microhardness of HPSR samples increased with decreasing depth from the treated surface, and increased with the increase of F . Microstructure of the original sample including the morphology and size of the α and β phases was found to have evident effect on the thickness, grain size and microhardness of the surface deformed layer produced by HPSR. The HPSR sample showed good thermal stability after annealing at 700 °C for 1 h. The present results indicated that HPSR can be used as an effective method for controlled production of gradient surface nano-microstructure on Ti6Al4V alloy with enhanced property.

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%.

We investigated the combination effect of plastic deformation and phase transformation on the evolution of microstructure and property of NiTi alloy. Samples of Ni50.9Ti49.1 alloy were deformed by cold rolling to different strains/thickness reductions (4%–56%). X-ray diffraction, transmission electronic microscopy (TEM) and microhardness measurements were applied for characterization of the microstructure and property of the cold-rolled samples. Experimental results indicated the non-monotonic variations of microstructure parameters and mechanical property with strain, indicating the different processes in microstructure and property evolution of NiTi subjected to cold rolling. TEM observations further showed the dominating mechanisms of microstructure evolution at different strain levels, leading to the gradual reduction of grain size of NiTi to the nanoscale by cold rolling. The results were discussed and related to deformation of martensite, forward and reverse martensitic transformations and dynamic recrystallization. The present study provided experimental evidences for the enhanced formation of nanograins in NiTi by plastic deformation coupled with phase transformation.

TiC–DLC coatings were in situ synthesized on Ti target surface in acetylene ambient by cathodic arc evaporation. In this process, TiC–DLC composite phases were formed by chemical reaction between Ti target surface and ionized C ions. Different acetylene (C2H2) flow rate and synthetic time were designed as two independent variables to synthesize two series of coatings to study the effects of different synthetic parameters on structure of the coatings. Surface morphology, composition and structure of the coatings were investigated by SEM, EDS, XRD and Raman spectroscopy, respectively. The result showed that the structure and composition of the coatings on the titanium metal target surface could be controlled by changing the C2H2 flow rate. The uniform TiC-DLC composite phases were observed in the coatings, while both C2H2 flow rate and synthetic time could significantly affect the thickness and bonding state of the coatings.

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.

Nanocomposite CrC/hydrogenated amorphous carbon (nc-CrC/a-C:H) coatings were deposited by a hybrid beams system comprised of a hollow cathode ion source and a cathodic arc ion-plating unit with varying H2 flow rates. The influences of H2 flow rates on the morphologies, microstructures, and properties of the coatings were systematically studied. The morphologies and microstructures of the coatings were characterized by SEM, AFM, XPS, Raman spectroscopy, GIXRD, and HRTEM. The mechanical and tribological properties were measured by a nano-indenter, scratch tester, and ball-on-disk tribometer. The wear tracks were evaluated using 3D profilometer, optical microscope, and EDS analysis. It has been found that a moderate H2 flow rate can effectively smooth the surface, enlarge the fraction of a sp3 bond, and improve the properties. The coating exhibits the highest hardness and elastic modulus at the H2 flow rate of 40 sccm. A superior combination of adhesion strength, friction coefficient, and wear resistance can be achieved at the H2 flow rate of 80 sccm.

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.

Quantitative measurements were carried out on the pressure effect of superheating of Al nanoparticles encapsulated in Al2O3 shells without epitaxial interfaces. In situ XRD experiments revealed that encapsulated Al nanoparticles with different particle sizes can be superheated to 7–15 K beyond the bulk equilibrium melting point of Al, and that this is accompanied by a suppressed thermal expansion behavior. A value for the pressure build-up on the Al core due to the constraint of the rigid Al2O3 shell of up to 0.25 GPa was derived from the temperature dependence of the lattice spacing for the superheated samples. The correlation between the measured pressure and superheating verified that the observed superheating is a pressure-induced phenomenon which follows the Clausius–Clapeyron relation. It was demonstrated that substantial superheating can be achieved by a pressure build-up even without confinement of epitaxial interfaces for a metal/ceramic system. � 2004 Acta Materialia Inc. Published by 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.

We report a quantitative measurement of the pressure effect on melting point elevation of encapsulated nanoparticles without epitaxial interfaces. By means of in situ x-ray diffraction, a substantial melting point elevation of about 15 K in encapsulated Al nanoparticles in Al2O3 shells without epitaxial interfaces was observed. Meanwhile, a pressure buildup of about 0.25 GPa on the Al nanoparticles was determined due to constraint of the rigid Al2O3 shell. The correlation between the measured pressure and melting point elevation in the present system verified that the observed melting point elevation is a pressure-induced phenomenon and the pressure effect on melting point for encapsulated nanoparticles follows the rule for bulk solids.