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Significant progress has been made in magnesium-based composites during recent decades, especially for the appearance of magnesium matrix composite reinforced by nanoparticles. The nanoparticles added not only exhibit a good strengthening effect, but also maintain the initial toughness of the matrix, effectively balancing the contradiction between the strength and plasticity in the traditional magnesium matrix composites. The magnesium matrix nanocomposites with excellent mechanical properties have pushed the development of magnesium matrix composites to a new stage. However, it is very difficult to disperse the nanoparticles in metal melt especially in magnesium melt which is different from other metal melts and dangerous during the cast processing. This means that the preparation of magnesium matrix nanocomposite is extremely challenging. Further, the magnesium matrix nanocomposites possess a distinctive characteristic in deformation behavior, strengthening and toughening mechanism due to their special size effect of nanoparticles. Accordingly, this review will focus on the new preparation technologies, deformation behavior, mechanical properties and strengthening and toughening mechanisms. The potential applications, development trends and future research ideas of magnesium matrix nanocomposite are also prospected. [ABSTRACT FROM AUTHOR]

Copper matrix composites were fabricated through mixing fixed amount of reduced graphene oxide and the different amounts of titanium. The dried copper/titanium/reduced graphene oxide mixture powders were firstly obtained by the wet‐mixing process, and then the spark plasma sintering process realized their faster densification. In the as‐sintered bulk composites, the layered reduced graphene oxide network, uniform titanium particles and copper‐titanium solid solution are observed in copper matrix. Investigations on mechanical properties show that the as‐prepared bulk composites exhibit the hardness and compressive yield strength compared with single reduced graphene oxide added composites. Increased titanium addition resulted into higher hardness and strength. The relevant formation and failure mechanisms of the composites and their influence on mechanical properties were discussed. [ABSTRACT FROM AUTHOR]

The effect of quenching temperature and tempering time on the microstructure, content of retained austenite and carbon content in retained austenite of a copper and niobium alloyed medium carbon steel were investigated by using optical, electron microscopy and X-ray diffraction. Results showed the microstructure of the quenching–partitioning–tempering steel consisted of lath martensite, retained austenite and carbides. With the increase of quenching temperature, the martensite laths became coarsened and the content of retained austenite increased. With the increase of tempering time, the content of retained austenite initially increased, reached to the maximum at 600 s and then gradually decreased. [ABSTRACT FROM AUTHOR]

Many interesting studies on composites with nano or submicron SiCp have been performed since the size advantages of fine SiC particles. However, it is extremely hard to disperse fine particles in metal melts due to their poor wettability and large surface-to-volume ratio, especially the fabrication of composites with multisized SiCp (5 µm, 0.5 µm, and ∼60 nm) is very difficult. The use of multisized reinforcements is required to solve the dispersion problem. The stir casting technology and hot extrusion method could be used to disperse the multisized SiCp in the matrix. The effects of multisized SiCp on the dynamic recrystallization behavior of the composites are discussed. The large-scale dynamic recrystallization caused by adding multisized SiCp results in a fine matrix microstructure. Compared with the as-accepted AZ31B alloy (YS: 195 MPa, UTS: 277 MPa), the yield strength and ultimate tensile strength of the AZ31B/SiCp/n-1 + S-4 + 5-10 composite were enhanced to 73.8 and 44.8%, respectively. The better tensile properties were attributed to the uniform distribution of reinforcement, strengthening effect of multisized SiCp, and grain refinement of magnesium matrix. [ABSTRACT FROM AUTHOR]

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si + Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si + Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to straininduced martensitic transformation from the retained austenite. [ABSTRACT FROM AUTHOR]

The effects of short-term use of oral glucocorticoid (GC) on the skeleton are not well defined. To address this gap, the influences of 7 days, 21 days of GC administration on femurs of intact rats were investigated. Forty 4-month-old female Sprague–Dawley rats were randomly divided into control group (Cont) and prednisone-treated group (Pre) and administered either distilled water or prednisone acetate at doses of 3.5 mg/kg/day for 0, 7 and 21 days, respectively. All the femurs were harvested for dual-energy X-ray absorptiometry scan, biomechanical testing and micro computed tomography scan. The whole body weight, femur bone mineral density (BMD), all three-point bending test parameters, microstructural parameters increased or improved significantly in Cont at day 21 when compared to day 0. The whole body weight, distal femur BMD, Young’s modulus, bending stiffness, density of tissue volume and trabecular thickness (Tb.Th) decreased, while structure model index and trabecular separation (Tb.Sp) increased significantly in Pre at day 21 when compared to age-matched control but had no significant differences between day 7 and day 21. Our data demonstrate that 7-day use of prednisone does not influence on rats’ femur, and 21-day use of prednisone slows in rate of whole body weight gain, decreases femur metaphysis BMD and bone stiffness which mainly due to the deteriorated bone microstructure. [ABSTRACT FROM AUTHOR]

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials. [ABSTRACT FROM AUTHOR]

The 15 vol.% micron SiC particle (SiCp)-reinforced AZ31B magnesium matrix composite (AZ31B-SiCp) prepared with semisolid stirring-assisted ultrasonic vibration was subjected to a multi-step process. The influence of the multi-step processing route on the microstructure and mechanical properties of the AZ31B-SiCp was investigated. For comparison, the monolithic AZ31B alloy was also processed under the same conditions. The results showed that the grain sizes of the AZ31B alloy and the AZ31B-SiCp were gradually decreased with increasing the processing step. Compared with the AZ31B-SiCp, the grain size of the AZ31B alloy was much larger, and the grain size distribution was inhomogeneous at the same processing condition. The particles of the AZ31B-SiCp were dispersed uniformly through the multi-step processing. Moreover, the tensile properties of the materials were gradually improved with increasing the processing step. In particular, the strength of AZ31B-SiCp and the ductility of AZ31B alloy improved significantly based on the room-temperature tensile test results. [ABSTRACT FROM AUTHOR]

The main work of this paper focuses on the simulation of binary alloy solidification using the phase field model and adaptive octree grids. Ni-Cu binary alloy is used as an example in this paper to do research on the numerical simulation of isothermal solidification of binary alloy. Firstly, the WBM model, numerical issues and adaptive octree grids have been explained. Secondary, the numerical simulation results of three dimensional morphology of the equiaxed grain and concentration variations are given, taking the efficiency advantage of the adaptive octree grids. The microsegregation of binary alloy has been analysed emphatically. Then, numerical simulation results of the influence of thermophysical parameters on the growth of the equiaxed grain are also given. At last, a simulation experiment of large scale and long-time has been carried out. It is found that increases of initial temperature and initial concentration will make grain grow along certain directions and adaptive octree grids can effectively be used in simulations of microstructure. [ABSTRACT FROM AUTHOR]

In the present study, magnesium-based composites with three different volume percentages of nano-sized SiC particulates (SiCp) reinforcement were fabricated using a simple and inexpensive technique followed by hot extrusion. Microstructural characterization of the materials revealed uniform distribution of nano-size SiCp and obvious grain refinement. The tensile test result indicates a remarkable improvement on the strength for the as-extruded SiCp/AZ31B nanocomposite, while the elongation to fracture was decreased by comparing with the AZ31B alloy. Although, compared with the as-extruded AZ31B alloy, the ductility of the SiCp-reinforced AZ31B nanocomposite is decreased, but the ductility of the present SiCp-reinforced AZ31B nanocomposite is far higher than that of the conventional micron or submicron SiCp-reinforced magnesium matrix composites. It is concluded that, compared with the larger sized (micron or submicron) particles, the addition of nano SiCp in the AZ31B alloy resulted in the best combination of the strength and ductility. An attempt is made in the present study to correlate the effect of presence of nano-SiCp as reinforcement and its increasing amount with the microstructural and mechanical properties of magnesium. [ABSTRACT FROM AUTHOR]

In the present study, AZ31B magnesium matrix composites reinforced with two volume fractions (3 and 5 vol.%) of micron-SiC particles(1 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. The as-cast ingots were extruded at 350 °C with the extrusion ratio of 15:1 at a constant ram speed of 15 mm/s. The microstructure of the composites was investigated by optical microscopy, scanning electron microscope and transmission electron microscope. Microstructure characterization of the composites showed relative uniform reinforcement distribution and significant grain refinement. The presence of 1 μm-SiC particles assisted in improving the elastic modulus and tensile strength. The ultimate tensile strength and yield strength of the 5 vol.% SiCp/AZ31B composites were simultaneously improved. [ABSTRACT FROM AUTHOR]

In this paper, three volume fractions (2, 5 and 10%) of 10 μm SiCp/AZ91 magnesium matrix composite were fabricated by stir casting. The as cast ingots were forged at 420°C with 50% reduction first and then extruded at 370°C with a ratio of 16∶1. The results show that the grain is refined significantly after hot deformation process, which results in the enhancement of tensile properties. The volume fraction of micrometre SiCp has significant effect on grain size by influencing dynamic recrystallisation of magnesium. By the increase in particle volume fraction, the grain size decreases, the dislocation density increases and the load transfer effect is strengthened, thus leading to the significant improvement of yield strength. The ultimate tensile strength of the composite increases with the increase in particle volume fraction due to the improved deformation uniformity of matrix and the enhanced hindering effect on crack propagation. [ABSTRACT FROM AUTHOR]

TiO and SiO filled polytetrafluoroethylene (PTFE) composite substrates were fabricated for microwave circuit applications. The total content of TiO and SiO fillers was a fixed value of 55 wt%, and the content of TiO in the PTFE matrix varied from 1 wt% to 9 wt%. The influences of TiO filler loading on the microstructure, density, moisture absorption, microwave dielectric properties and coefficient of thermal expansion of filled PTFE composites were investigated. The distribution of filler was uniform throughout the PTFE matrix. The dielectric constant ( ε) showed an increasing trend as the TiO content increased, while the dielectric loss (tan δ) decreased up to 5 wt% TiO filler loading and then slightly increased till 9 wt% TiO loading. Composites showed a similar trend of decreasing moisture absorption, and increasing linear coefficient of thermal expansion (CTE), density as the TiO filler content increased. Our results revealed that the TiO and SiO filled PTFE composites exhibited good dielectric properties ( ε ~ 2.87, tan δ ~ 0.00075), acceptable moisture absorption (0.2 %), CTE (17 ppm/°C) and tensile strength (7.32 MPa) at filler loading of 5 wt% TiO and 50 wt% SiO. [ABSTRACT FROM AUTHOR]

We report the effects of thermal annealing on the characteristics of GeSn epilayers grown on Ge-buffered Si wafers with a high Sn content near a threshold value that affords a direct bandgap. On annealing at temperatures below 400 °C, the characteristics of the epilayer remain unchanged, compared to those of the as-grown samples. On annealing the samples at a temperature in the range of 440-540 °C, strain relaxation in the epilayer is observed, accompanied by generation of misfit dislocations at the GeSn/Ge interface. A further increase in annealing temperature beyond 580 °C causes not only a relaxation in strain but also a change in the microstructure of the epilayer. In addition, Sn forms clusters and segregates to the surface, resulting in a reduction in the Sn content of the epilayer. The present investigation shows changes in the characteristics of the film under thermal treatment, providing an insight into the physical properties of such devices. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium matrix composites reinforced by graphite particles were fabricated using stir casting with graphite particle size of 50μm and graphite particle volume fractions of 5%, 10%, 15% and 20%, respectively. A dynamic mechanical analyzer was used to measure the damping capacities of as-cast composites. The experimental results reveal that the graphite particles play an important role on the damping capacities of as-cast composites. The strain amplitude independent damping of as-cast composites increases significantly as the graphite particle volume fraction increases from 0% to 15%, but decreases when the volume fraction exceeds 15%. The damping values of as-cast composites rise slowly with increasing temperature from room temperature to 125°C, and have no obvious change with increasing temperature from 125°C to 250°C, but rise rapidly with increasing temperature from 250°C to 400°C. [Copyright &y& Elsevier]

Abstract: Magnesium matrix composite reinforced by graphite particles was fabricated using stir casting with graphite particle size of 50μm and graphite particle volume fraction of 10%. The as-cast composite was extruded at 250°C, 300°C and 350°C with an extrusion ratio of 12:1. The experimental results reveal that the extrusion temperature plays an important role on the microstructures and damping capacities. The aspect ratio of graphite particles increases as the extrusion temperature rises. Extrusion at 300°C leads to the highest damping values in low strain region at room temperature. It also demonstrates the highest damping values at elevated temperatures when the strain is 4×10−5. In addition, there are two definite damping peaks in the damping-temperature curves which occur at about 150°C and 350°C, respectively. [Copyright &y& Elsevier]

Abstract: In the present study, AZ91 magnesium matrix composites reinforced with six volume fractions (0.5, 1, 1.5, 2, 3 and 5vol.%) of submicron-SiC particulates (0.2μm) were fabricated by stir casting. The as-cast ingots were forged at 420°C with 50% reduction, and then extruded at 370°C with the ratio of 16 at a constant ram speed of 15mm/s. The microstructure of the composites was investigated by optical microscopy, scanning electron microscope and X-ray diffractometer. Microstructure characterization of the composites showed relative uniform reinforcement distribution, significant grain refinement and presence of minimal porosity. The X-ray diffractometer analysis showed that a strong basal plane texture was formed in both AZ91 alloy and SiCp/AZ91 composites during extrusion and the addition of submicron-SiC particulates could weaken the basal plane texture. The presence of submicron-SiC particulates assisted in improving the thermal stability, micro-hardness, elastic modulus and 0.2% yield strength. Both of the 0.2% yield strength and ultimate tensile strength increased with the increasing of submicron-SiC particulate content, however, decreased as the SiC particulate content overrun 2vol.%. [ABSTRACT FROM AUTHOR]

Abstract: The Mg/Al laminated composite was fabricated by the accumulative roll bonding (ARB) using the pure magnesium and Al5052 alloy at 400°C. Tensile properties along rolling direction and the transverse direction and the microhardness were evaluated at the ambient temperature. The tensile strength of the laminated Mg/Al composite along both directions increased gradually till two ARB cycles, but then decreased after the third ARB cycles. Optical microscopy and scanning electron microscopy (SEM) were utilized to reveal the microstructure evolution and the failure mechanism. Grain refinement of Mg layers was not obvious during the ARB process due to the high temperature and interval reheating. The obvious crack at the coarse intermetallic compounds and rupture of the Al layer after the third cycle led to the premature failure of the sample along the rolling direction during the tensile test. [Copyright &y& Elsevier]

Abstract: SiCp/AZ91 magnesium matrix composite was fabricated by stir casting. The as-cast ingots were cut into cylindrical billets, and then forged at different temperatures (320, 370, 420, 470 and 520°C) at a constant RAM speed of 15mm/s with 50% reduction. The microstructure evolution of the composites during forging was investigated by optical microscope, scanning electron microscope, and transmission electron microscope. The texture of the forged composites was measured by neutron diffraction. Mechanical properties of the composite at different forging temperatures were tested by tensile tests at room temperature. It was found that a strong basal plane texture formed during forging, and the intensity of basal plane texture weakened as forging temperatures increased. The particle distribution in the composite was significantly improved by hot forging. Typical microstructures were obtained after forging at different temperatures and the composite with different microstructures offered different mechanical properties during tensile test. [Copyright &y& Elsevier]

Abstract: Cyclic plasma oxynitriding and cyclic plasma nitriding catalyzed by rare earth La of AISI 420 martensitic stainless steel were performed and compared with conventional plasma nitriding. The nitrided layers were investigated by means of an optical microscope, microhardness tester, Auger electron spectroscopy (AES), X-ray diffraction (XRD), wear machine, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results show that the wear resistance of AISI 420 martensitic stainless steel is improved significantly by the two new rapid and deep plasma nitriding techniques. The new techniques increase the surface hardness of the nitrided layers and make the microhardness profiles gentler, which are consistent with the nitrogen concentration depth profiles. Meanwhile, the nitrided effect improves with increasing cycles. It was also found that the optimum phase compositions of nitrided layers with more γ′ phases and less ɛ phases for long-term service conditions can be obtained by the two new techniques, which is in agreement with the microstructure. In addition, traces of Fe3O4 were found in the cyclic plasma oxynitrided sample. Combining the SEM and EDS analysis indicated the existence of La in the nitrided layer of the sample under cyclic plasma nitriding catalyzed by rare earth La. [Copyright &y& Elsevier]

Abstract: Equal channel angular pressing (ECAP) of the in situ Mg2Si reinforced Mg composite was performed with route Bc up to 8 passes using a 90° die. Refinement and re-distribution of the reinforced phase Mg2Si was investigated. Bulk texture development during ECAP was characterized by neutron radiation. Results showed that only the eutectic Mg2Si (Type-II) was proposed to be effective for preparing the in situ Mg2Si/Mg composite. Neutrons texture analysis briefly illustrated that a 〈00.2〉 basal fiber with the obtainment of less shear effect was formed. But the asymmetry of basal fiber around the ECAP direction was also observed. Both uniform texture and homogeneity distribution of Mg2Si particles played an important role in the tensile behavior of ECAPed composite. [Copyright &y& Elsevier]

Equal channel angular pressing (ECAP) was performed on the extruded pure magnesium at 250°C for 4 passes. After ECAP processing, the grain size was significantly refined. And the damping capacity was increased at low strain amplitudes, but decreased at high strain amplitudes. Three peaks were observed during the measurement of damping capacity at elevated temperatures. The mechanisms for damping capacity of pure Mg at ambient and elevated temperatures were discussed. [ABSTRACT FROM AUTHOR]

Abstract: Microstructures in γ+β/γ diffusion couples made of Ni–Cr–Al alloys were investigated numerically in two dimensions using a phase field model. With experimental thermodynamic and mobility data from the literature as inputs, the model was used to predict the evolution of the precipitate morphology, the dissolution of the two-phase γ+β regions and the diffusion paths as functions of alloy composition The dissolution rates and diffusion paths obtained from the simulations were in general agreement with experimental data, although quantitative differences were found. Differences in the dissolution rates and diffusion paths could be directly related to differences in the predicted phase diagram and the phase diagram reported in the experimental study. [Copyright &y& Elsevier]

Abstract: The hot deformation behavior of ZK60 magnesium alloy reinforced with 20vol.% of Al18B4O33 whisker was investigated by compression testing in temperature range of 523–723K and strain rate range of 0.001–10s−1. The deformation behavior was mainly dependent on strain rate and temperature on certain strain condition. Variations of flow behavior with deformation temperature as well as strain rate were analyzed. The microstructural evolution at different temperatures and strain rates are mainly characterized by twinning, dislocation slip and dynamic recrystallization (DRX) during the hot compression. They were observed by transmission electronic microscope (TEM). It is shown that twinning occurred at lower temperature and earlier deformation stage. Twinning is the main reason for the high strain hardening rate at relatively low temperature. The material showed DRX at higher temperature and moderate strain rate, which is the reason for flow softening at relatively high temperature. Al18B4O33 whiskers promote nucleation of DRX. A fine dislocation substructure forms upon hot deformation, leading to subgrain formation. [Copyright &y& Elsevier]

Most earlier simulation work on diffusion couples containing dispersed phases was limited to one-dimensional models. The models assumed that all diffusion occurs in the matrix phase and that precipitates are point sources or sinks of solute. Microstructural evolution in the interdiffusion zone and its effect on diffusion kinetics were ignored. In this study, spatial and temporal evolutions of two-phase microstructures are treated explicitly by applying the phase field method. Using a two-phase diffusion couple as an example, it is shown that both precipitates and the so-called Type 0 boundary migrate as a result of the Kirkendall effect. The Type 0 boundary becomes diffuse spreading out over several rows of precipitates in the simulation. Accordingly, “horns” in the diffusion path are less sharp than seen in earlier work but are similar in that they point in the same direction. Also the initial slope of the diffusion path differs significantly from the earlier work. In addition, the phase field method provides a detailed picture of Kirkendall marker movement in a two-phase microstructure. The marker plane bends around precipitates and individual markers move along curved paths. [Copyright &y& Elsevier]

We develop a method for phase field modeling of interdiffusion microstructures in Ni–Al–Cr by linking directly the free energy and mobility data to available databases by the CALPHAD method. Its applications are demonstrated for microstructural evolution in γ+β/γ and γ+β/γ+β diffusion couples. [Copyright &y& Elsevier]

In this study, we present an easy way to create layered-nanoplatelets with well-defined geometry by controlling the cracking process of nanostructured multilayers. The geometrical dimension of layered-nanoplatelets is determined by the multilayer intrinsic size, the total strain, and the elastic mismatch between the substrate and multilayers, which was analyzed by statistical approach. Fracture behaviors characterized by critical strain to nucleate microcrack, fracture toughness, and evolution of fragment width were also studied for nanostructured Cu/Cr multilayers with modulation period (λ) spanning from of 5 to 250 nm and were quantified based on linear elastic theory and shear-lag theory. An optimal modulation period seems to be likely favorable for maximizing the ductility, strength, and fracture toughness of the nanolayered materials. [ABSTRACT FROM AUTHOR]

In this study, both AZ91 alloy and nano-SiCp/AZ91 composite were subjected to multi-pass forging under varying passes and temperatures. The microstructure and mechanical properties of the alloy were compared with its composite. After six passes of multi-pass forging at a constant temperature of 400 ℃, complete recrystallization occurred in both the AZ91 alloy and composite. The decrease of temperature and the increase of passes for the multi-pass forging led to further refinement of dynamic recrystallized grains and dynamic precipitation of second phases. The grain size of the nano-SiCp/AZ91 composite was smaller than that of the AZ91 alloy under the same multi-pass forging condition, which indicated that the addition of SiC nanoparticles were beneficial to grain refinement by pinning the grain boundaries. The texture intensity for the 12 passes of multi-pass forging with varying temperatures was increased compared with that after nine passes. The ultimate tensile strength is slightly decreased while the yield strength was increased unobviously for the AZ91 alloy with the decrease of temperature and the increase of the passes for the multi-pass forging. Under the same condition of multi-pass forging, the yield strength of the composite was higher than that of the AZ91 alloy due to the Orowan strengthening effect and grain refinement strengthening resulting from externally applied SiC nanoparticles and internally precipitated second phases. By comparing the microstructure and mechanical properties between the AZ91 alloy and nano-SiCp/AZ91 composite, the strength-toughness properties of the composites at room temperature were affected by the matrix grain size, texture evolution, SiC nanoparticles distribution and the precipitated second phases. [ABSTRACT FROM AUTHOR]

Nanostructured face-center-cubic/body-center-cubic Cu/(NbMoTaW) multilayers were prepared with equal layer thickness ( h ) spanning from 5 to 100 nm. The hardness was found to increase with reducing h to ~50 nm, following the Hall–Petch relation. Below this critical thickness, a hardness plateau emerged indicative of an interface barrier strengthening mechanism. The high density of misfit dislocations at interfaces facilitates the yielding of high entropy alloy NbMoTaW layers at stresses far less than the strength of Cu/(NbMoTaW) estimated by rule-of-mixture. Compared with reported multilayers, the present Cu/(NbMoTaW) samples show the size-independent hardness at a larger size-range h  < 50 nm. [ABSTRACT FROM AUTHOR]

Nanoindentation methodology was used to investigate the plastic deformation characteristics, including the hardness ( H ), strain rate sensitivity (SRS, m ) and activation volume ( V * ), of Cu/Mo nanostructured metallic multilayers (NMMs) with equal layer thickness ( h ) spanning from 10 to 200 nm before and after He-implantation at room temperature. Compared with the as-deposited Cu/Mo NMMs, the irradiated Cu/Mo samples exhibited the enhanced hardness particularly at great h , which is caused by the bubble-hardening effect. Unlike the as-deposited Cu/Mo NMMs displayed a monotonic increase in SRS (or a monotonic decrease in activation volume) with reducing h , the irradiated Cu/Mo samples manifested an unexpected non-monotonic variation in SRS as well as in activation volume. It was clearly unveiled that the SRS of irradiated Cu/Mo firstly decreased with reducing h down to a critical size of ~50 nm and subsequently increased with further reducing h , leaving a minimum value at the critical h . These phenomena are rationalized by considering a competition between dislocation-boundary and dislocation-bubble interactions. A thermally activated model based on the depinning process of bowed-out partial dislocations was employed to quantitatively account for the size-dependent SRS of Cu/Mo NMMs before and after irradiation. Our findings not only provide fundamental understanding of the effects of radiation-induced defects on plastic characteristics of NMMs, but also offer guidance for their microstructure sensitive design for performance optimization at extremes. [ABSTRACT FROM AUTHOR]

SiC particles reinforced AZ91D Mg matrix composites with different particle contents fabricated by a ultrasonic method were extruded at 350 °C. Hot extrusion eliminated the “island” particle distribution, and evidently improved SiC particles distribution of the composites. However, hot extrusion induced particle cracking besides the improvement of SiC particles distribution. There existed more cracked particles in the particle-rich zones. During extrusion, SiC particles stimulated the nucleation of dynamic recrystallization in the matrix. Dynamic recrystallization is also sensitive to the particle content on a local scale in the composites. As SiC contents increased, the yield strength of the composites increased. However, the ultimate tensile strength increases of the composites were small as the SiC contents increased from 10% to 20%, because the improvement of ultimate tensile strength caused by the strengthen effects of SiC particles was offset by extrusion-induced particle cracking. [ABSTRACT FROM AUTHOR]

Abstract: Ageing treatment at 200°C and 225°C for various periods of time was introduced to the Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt.%) alloy, the age-hardening behaviour and phase transformation were analysed systematically. The alloy aged at 225°C exhibits accelerated ageing progress compared with that aged at 200°C. The precipitation sequences of the alloy aged at both 200°C and 225°C are supersaturated solid solution (SSSS)→β′′ (D019)→β′ (bco). Stacking faults and 14H LPSO phase formed in the alloy over-aged at 200°C and 225°C, respectively. The equilibrium β phase precipitated at the grain boundaries with the formation of precipitate free zone (PFZ) at the peak-aged stage, after further ageing treatment, the β phase got coarsened, which was accompanied with the widening of the PFZ. [Copyright &y& Elsevier]

Abstract: The extruded Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy were subjected to large-strain hot rolling with different final rolling temperatures. The microstructural evolution and mechanical properties of the sheets were investigated. The microstructure became homogeneous after hot rolling process and long period stacking ordered (LPSO) phase distributed at grain boundaries along rolling direction. The sheet rolled at 300°C was composed of deformed grains, substructures and excessive dislocations. With increasing final rolling temperatures from 300°C to 400°C, the volume fraction of recrystallized grains with relatively random orientations increased significantly. The strength of the rolled sheets was improved while the ductility was deteriorated with decreasing final rolling temperatures. The tensile yield strength and ultimate tensile strength of the sheet rolled at 300°C reached 320MPa and 416MPa, respectively with the elongation to failure of 5.3%. The mechanical anisotropy of the rolling sheet became unity when rolled at higher temperature of 400°C due to the formation of weaker basal texture. [Copyright &y& Elsevier]

Abstract: Different cooling processes, such as quenching in warm water and cooling in furnace, were introduced to homogenize Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy. Microstructure evolution and mechanical properties of the homogenized alloy were investigated. The as-quenched sample was comprised of α-Mg matrix, Mg5RE phase and 18R LPSO phase distributed at the grain boundaries and a few of RE-rich particles distributed randomly. During the process of cooling in furnace, Mg5RE and 18R LPSO phases were transformed into block-shaped 14H LPSO phase and lamellar-shaped 14H LPSO phase, respectively, due to the diffusion of solute atoms into the α-Mg matrix. Furthermore, the lamellar-shaped 14H LPSO phase grew and ran through the whole grains. The as-quenched sample exhibits tensile yield strength of 130MPa, ultimate tensile strength of 206MPa and elongation to failure of 5.5%, while the sample cooled in the furnace exhibits higher tensile yield strength but lower ultimate tensile strength and ductility due to the coarse grains and formation of block-shaped 14H LPSO phase. [Copyright &y& Elsevier]

Abstract: The Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt.%) alloy was hot rolled with different final rolling reductions at 400°C. The microstructure evolution and texture of the sheets were analyzed, and their effect on the mechanical properties was discussed. All of the rolled sheets consist of fine dynamically recrystallized grains with almost random orientation, large deformed grains with strong basal texture. In addition, bent lamellar-shaped long period stacking ordered (LPSO) phases due to kink deformation were observed in the deformed grains. The volume fraction of the dynamic recrystallization increased with increasing final rolling reduction and the basal texture weakened gradually. The strength of the alloy sheets changed a little with increasing final rolling reduction, while the yield anisotropy and the ductility were significantly improved. The sheet final-rolled with thickness reduction of 60% exhibits tensile yield strength of 306MPa, ultimate tensile strength of 393MPa and elongation to failure of 14.6% at ambient temperature, and tensile yield strength of 264MPa, ultimate tensile strength of 345MPa and elongation to failure of 19.4% at 250°C. [Copyright &y& Elsevier]

Abstract: Magnesium matrix composites reinforced with micro-SiC particles were fabricated by semisolid stirring assisted ultrasonic vibration with SiC particle size of 1μm and volume fractions of 3% and 5%, respectively. Compared with as-cast AZ91 alloy, with the addition of the micro-SiC particles grain size of matrix was decreased while most of the phase Mg17Al12 varied from coarse plates to lamellar precipitates in 3vol.% SiCp/AZ91 composite. Although some dispersed micro-SiC particles were found within the grains, most of the micro-SiC particles were located along grain boundaries in both 3vol.% and 5vol.% SiCp/AZ91 composites. The study of the interface between the micro-SiC particle and the alloy matrix suggested that micro-SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength and yield strength of the 3vol.% SiCp/AZ91 composite were improved while elongation to fracture was almost kept. [Copyright &y& Elsevier]

Abstract: Ultrasonic vibration with frequency of about 20kHz and power of 350W was introduced into molten AZ91 alloy. After ultrasonic vibration, morphology of the phase Mg17Al12 formed during the cooling and solidification varied from coarse plates to lamellar precipitates. With application of solution heat treatment, the phase Mg17Al12 was mostly dissolved in both the alloy with ultrasonic vibration and without ultrasonic vibration. The ultimate tensile strength and the elongation to fracture of the alloy were significantly increased after ultrasonic vibration. After solution heat treatment the tensile strength of the alloy without ultrasonic vibration increased remarkably while the alloy with ultrasonic vibration increased slightly. [Copyright &y& Elsevier]

Abstract: The pure Mg sheets were processed by ARB at 400°C and 300°C up to 8 cycles and 4 cycles, respectively. The microstructure was refined during the first cycle and only homogenized during the following ARB process due to the high deformation temperature, intermediate reheating and air-cooling. The ARB process did not show an obvious effect on the tensile properties of the as-ARBed pure Mg sheet due to the stable grain size and dominant basal plane texture during ARB. Further deformation should be introduced to improve the bonding quality of the as-ARBed sheet. [ABSTRACT FROM AUTHOR]

Abstract: Directional solidification (DS) of a binary Mg–4Al (wt.%) alloy was carried out to investigate the microstructures and microsegregation under controlled solidification conditions. In directional solidification the microstructure depends on the growth rate V because the cooling rate, which governs the solidification microstructure, is the product of the growth rate and the temperature gradient. The ability to produce simple and uniform microstructures in directional solidification enables us to correlate the formation of the microstructure and its characteristic length scales quantitatively with processing parameters. The morphology of the solid–liquid interface and the microstructure of both the mushy zone and the steady-state region were characterized at different levels of growth rates. With the help of an electron microprobe, microsegregation was determined in a specimen directionally solidified with cooling rates ranging from 0.06 to 0.8K/s. The calculated microsegregation results based on the Scheil model deviated significantly from the experimental data, which is anticipated since back diffusion was not included due to the lack of diffusivity data. [Copyright &y& Elsevier]

Abstract: Equal channel angular pressing (ECAP) has been conducted on as-cast Mg–4.3 wt.%Zn–0.7 wt.%Y Mg alloy containing quasicrystal phase at a temperature of 623 K. After 8 ECAP passes, the grain size of the as-cast alloy is decreased from ∼120 to ∼3.5 μm, and the coarse eutectic quasicrystal phases are broken and dispersed in the alloy. Tensile testing has been performed on the ECAPed Mg–Zn–Y alloy at temperatures of 523 K and 623 K with initial strain rates from 1.5×10−3 to 1.5×10−4 s−1. The ECAPed alloy exhibits a maximum elongation of about 600% when testing at 623 K using an initial strain rate of 1.5×10−4 s−1. Grain boundary sliding is considered to be the dominant deformation mechanism of the Mg–Zn–Y alloy in the temperature and strain-rate range investigated. [Copyright &y& Elsevier]

Abstract The microstructural features and hardness of He-irradiated miscible Cu/Zr crystalline/crystalline nanolaminates with a negative enthalpy of mixing were investigated at room temperature. The distribution of He bubbles within the Cu layers exhibits a high dependence on both layer thickness (h) and ion dose (Φ), while that at the Cu-Zr interfaces is only affected by ion dose. Compared with their as-deposited counterparts, the He-irradiated Cu/Zr nanolaminates tend to manifest radiation-induced solid-state amorphization (RSA), which was induced by ion beam mixing and initiated from crystalline interfaces. The size-dependent hardness of the as-deposited Cu/Zr nanolaminates is elucidated by a transition in the strengthening mechanism from the interfacial barrier strengthening mechanism to the confined layer slip of partials when h is increased. In the He-irradiated Cu/Zr samples, however, the size-dependent hardness is highly related to Φ. Under lower Φ , a nonmonotonically h -dependent irradiation hardening is evident and is associated with a transition from irradiation hardening at large h (≥10 nm) to softening at small h (<10 nm), and the maximum hardening occurs at a critical value of h ≈ 25 nm. In contrast, under higher Φ , the Cu/Zr nanolaminates merely display irradiation softening behavior, with the minimum softening at the critical value of h ≈ 25 nm. The underlying mechanisms are highlighted in terms of dislocation-bubble interactions and RSA. These findings indicate the effects of He ion dose and layer thickness on the He bubble distribution and the interface-mixing of Cu/Zr nanolaminates as well as provide thorough insight into how to tune the He ion dose to manipulate the mechanical response of nanolaminated FCC/HCP materials on a small scale. Graphical abstract Unlabelled Image Highlights • Cu/Zr nanolaminates with smaller layer thickness showed enhanced radiation tolerance. • Large ion dose enhances bubble diameter while reduces bubble spacing in Cu layers. • Cu/Zr nanolaminates suffered size- and dose-dependent amorphization at interfaces. • Low ion dose induces irradiation hardening, while high ion dose induces softening. [ABSTRACT FROM AUTHOR]

Alloying effect on tensile ductility of nanostructured Cu-X (X = Zr and W) thin films was studied in comparison. Both Zr and W atoms segregated at grain boundaries (GBs) and increased the GB cohesion energy, leading to similar increase of ductility in as-deposited Cu-X films. After annealing treatment, however, changes in ductility showed different alloying effect: the Cu-Zr one increased while the Cu-W one decreased when compared with their as-deposited counterparts. This discrepancy was rationalized by different microstructural evolution that intergranular CuZr amorphous layer was produced in the Cu-Zr film while intergranular W grains were formed in the Cu-W one. [ABSTRACT FROM AUTHOR]

Grain boundary (GB) engineering via alloying opens a pathway to design the interfaces in alloys for tuning their mechanical properties, thus it is quite critical to determine the optimum addition of alloying element. Here, we prepared immiscible Cu-Cr alloyed thin films by non-equilibrium magnetron sputtering deposition to study Cr alloying effects on the microstructure and mechanical properties of Cu thin films. It is found that Cr doping can notably refine the grains and promote the formation of fully nanotwinned columnar grains at low Cr volume concentrations (≤∼3.7 at.%) associated with the average GB Cr concentrations ≤ ∼15 at.%, beyond which the formation of nanotwins is significantly suppressed. The role of Cr atoms/clusters played in the twinning process is rationalized in terms of dislocation rebound-promotion mechanism. Importantly, a maximum hardness is discovered at the optimum Cr addition of ∼2.0 at.%. The Cr concentration-dependent hardness of Cu-Cr alloyed films was quantified by a combined strengthening model. The achievement of the maximum hardness was related to the greatest GB solute segregation-induced strength contribution. It is further uncovered that the Cu-Cr system exhibits strain rate sensitivity (SRS, m ) monotonically reduced with increasing the Cr concentration, ranging from a positive m of 0.0314 (for pure Cu) to a negative m of −0.0245, which is attributed to a competition between the dislocation-boundary interactions (increasing m ) and the dislocation-solute atoms interactions (decreasing m ). These findings provide valuable insights into tuning the composition of alloyed thin films to achieve optimized mechanical performance. [ABSTRACT FROM AUTHOR]

This study investigates the precipitation behavior in the graphene nanoplatelets (GNPs) reinforced bimodal structural Mg-6Zn (wt%) matrix composite. The GNPs with an increasing content progressively accelerate the age-hardening response in the local regions of the composite. The composite takes only half the time that the Mg-6Zn alloy needs to reach the peak strength when aging at 200 °C. The observation reveals that the planar and wrinkled GNPs in the composite act as the effective trigger of dislocations and collector of solute atoms to accelerate the precipitation. It is concluded that GNPs have a pronounced effect on the development of matrix microstructure. Moreover, the orientation relationship between the aligned GNPs towards the extrusion direction and the matrix grains with a fiber type texture makes the GNPs and [0001] Mg precipitate rods constitute a hybrid strengthening architecture in the composite. As a result, the synergistic strengthening effect of the GNPs and the precipitates is realized. [ABSTRACT FROM AUTHOR]

Tuning the microstructure to optimize the mechanical performance of nanocrystalline Cu thin films via the alloying strategy is quite important for their application in microdevices. In this work, we prepared nanocrystalline miscible Cu-Ti and immiscible Cu-Mo alloyed thin films to investigate alloying effects on the microstructure and mechanical properties of Cu thin films in terms of mixing enthalpies. It is found that the dopants of both Ti and Mo can notably refine the grains, and in particular promote the formation of nanotwins below a critical content of solute, beyond which the formation of nanotwins is notably suppressed. The nonmonotonic solute concentration-dependent twinning behavior observed in Cu-Ti and Cu-Mo alloyed thin films is explained by the coupling effects between grain size and grain boundary segregated dopants that affects the stimulated slip process of partials. The increased hardness of both Cu-Ti and Cu-Mo systems with increasing the solute contents are quantitatively explained by combining several strengthening mechanisms, including solid solution strengthening, grain/twin boundary (GB/TB) strengthening, solute segregation-induced strengthening. It unexpectedly appears that with increasing the solute contents, the Cu-Ti system exhibits monotonically reduced positive strain rate sensitivity (SRS, m ), whereas the Cu-Mo system manifests almost constant negative SRS. The fundamental difference in SRS m between Cu-Ti and Cu-Mo is rationalized in terms of the interactions between solute atomic clusters and dislocations based on the cross-core diffusion mechanism. [ABSTRACT FROM AUTHOR]

Mg-10Zn-6Y-0.5Zr (wt%) alloy and Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy with medium RE content have been fabricated by conventional casting and indirect extrusion. The as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy shows a bimodal microstructure containing dynamic recrystallized (DRXed) grains with average grain size of 2 μm and deformed regions. In addition to the fragmented W phase particle bands distributing along the extrusion direction, large amount of nano W phase and small amount of nano β 2 ′ phase are precipitated in the matrix. While the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy exhibits completely dynamic recrystallized microstructure with average grain size of 3 μm. The broken W phase particles are distributed homogeneously in the matrix. Icosahedral quasicrystal phase layer is observed at the surface of some W phase particles in the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr alloy. The as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy exhibits yield strength of 341 MPa, ultimate tensile strength of 368 MPa and elongation to failure of 6.8%. While the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy shows lower yield strength of 247 MPa, ultimate tensile strength of 330 MPa and higher elongation to failure of 19.8%. The higher strength in the as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy is mainly attributed to higher area fraction of unDRXed regions with strong texture and the precipitation of nano W and β 2 ′ phase. The fully recrystallized microstructure with weak texture and the formation of icosahedral quasicrystal phases with good coherent bond with α-Mg contribute to the good ductility of the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr alloy. [ABSTRACT FROM AUTHOR]

The magnetron sputtering technique was employed to prepare Zr-alloyed Cu thin films with Zr addition ranging from 0 to 7.0 at.%. Microstructure evolution with Zr doping was characterized by using the transmission electron microscope and atom probe tomography. Plastic deformation characteristics (hardness, strain rate sensitivity and activation volume) of the Cu-Zr alloyed films were examined by using nanoindentation testing. Significant Zr doping effects on the microstructure were clearly uncovered that (i) the Zr segregated at grain boundaries and tuned the grain boundary structure. With increasing the Zr addition, CuZr amorphous particles at grain boundaries (3.0 at.% Zr) and even three dimensional CuZr amorphous grain boundary network (7.0 at.% Zr) were formed; (ii) the grains (size and morphology) and nanotwins were notably influenced by the Zr doping. In particular, the nanotwin thickness was reduced from ∼25 nm in the pure Cu film down to ∼5 nm in the Cu-Zr alloyed films. Accompanied with the microstructure evolution, the Cu-Zr thin films displayed hardness and strain rate sensitivity highly sensitive to the Zr doping, i.e. , hardness increasing while strain rate sensitivity decreasing with raising the Zr addition. The strengthening and deformation mechanisms were discussed in terms of the microstructure-property relationship. Three regimes were divided within the studied Zr doping range: grain/nanotwin boundary-dominated strengthening mechanism in the regime I of pure Cu film, dislocation nucleation-controlled nanotwin softening mechanism in the regime II of Zr addition ≤3.0 at.%, and intergranular amorphous layer-mediated strengthening mechanism in the regime III of Zr addition up to 7.0 at.%. In addition, the reduction in strain rate sensitivity with Zr doping was quantitatively described by adopting a model that involves the thermally activated partials depinning mechanism based on the effective dislocation segment length. [ABSTRACT FROM AUTHOR]