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Abstract In this work, cohesive and adhesive properties of Ti thin films on polyimide substrates were studied systematically as a function of grain size by in situ optical microscopy, resistance measurements and synchrotron X-ray diffraction under uniaxial tensile testing. The characteristic of cohesive and adhesive behavior can be well reflected in the stress-strain curves determined by synchrotron X-ray diffraction. Generally, the yield strength decreases while the cracking strain increases with increasing grain size, due to the constraint effect of grain size on dislocation activity. Different from the yield strength, the fracture strength is almost unchanged with grain size. The fracture toughness is found to increase as the grain size increases, which is attributed to the enhanced plasticity during fracture process. Additionally, the interfacial adhesion energy is determined to be 3.8 J m−2. The buckling strain increases with increasing grain size as a result of enhanced plastic dissipation. The formation of patches, instead of buckles, in Ti film with smaller grain size is ascribed to the low deformation capability of the film. [ABSTRACT FROM AUTHOR]

The fracture behavior and interfacial adhesion of Cu/Mo nanostructured multilayer films (NMFs) are systematically investigated as a function of modulation period λ. A fracture mode transition from opening to shear has been reported as λ decreases from 250 nm to 10 nm, which is related to the constraining effect of ductile Cu layers on microcrack-initiating Mo layers. Within the λ regime below a critical size (λcri) of ~50 nm, a λ-independent adhesion energy of about 2.8 J m-2 has been determined. Within the λ regime greater than λcri, however, the measured adhesion energy increases with increasing λ. The λ dependence of evaluated adhesion energy is discussed according to the size-dependent deformation mechanism. A micromechanics model has been utilized to quantify the λcri, where the deformation mechanism is transited from dislocation pileup to confined layer slip. [ABSTRACT FROM AUTHOR]

Abstract: The hot compressive deformation behavior of a new Ni–Cr–Co based P/M superalloy was studied in the temperature range of 950–1150°C and strain rate range of 0.0003–1s−1 using hot compression tests. The effect of processing parameters on flow stress, activation energy, power dissipation η maps and instability maps were investigated. The results show that the flow stress decreases with increasing temperature and decreasing strain rate. The apparent activation energy of deformation decreases while the values of η in the peak zones increase with increasing strain, due to the occurrence of dynamic recrystallization. When the strain is 0.5, a domain with its peak η of 40% occurs at 1050°C and 0.0003s−1, which corresponds to dynamic recrystallization with grain size of ASTM 10–11 and can be as the optimum processing parameters for good workability. [Copyright &y& Elsevier]

Abstract: Grp/AZ91 and SiCp/Grp/AZ91 magnesium matrix composites were fabricated by stir casting with graphite particle size of 50μm and SiC particle size of 10μm. The as-cast composites were extruded at 350°C with an extrusion ratio of 12:1. The experimental results reveal that the reinforcements play an important role on the tensile properties and damping capacities of the composites. The addition of graphite particles improves the yield strength, and the addition of SiC particles further improves the yield strength. The addition of graphite particles improves the strain amplitude independent damping, but the addition of reinforcements does not change the critical strain significantly. Temperature dependence of damping capacities in as-cast and as-extruded materials both can be divided into two regions, and each region has different dominant damping mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: A particulate reinforced magnesium matrix composite fabricated by stir casting was subjected to multidirectional forging (MDF) at a temperature of 420°C. The results showed that grains of matrix in the composite were significantly refined after 1 MDF pass, and gradually refined till 3 MDF passes. The evolution of particle distribution in the composite during MDF was investigated by a window technique. It was shown that MDF led to an increase of the uniformity of the particle distribution. With increasing the MDF passes, both the tensile strength and the elongation to failure of the composite are significantly enhanced till 3 MDF passes. [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. The as-cast composites were extruded at 300°C with an extrusion ratio of 12:1. The experimental results reveal that the graphite particles play an important role on the tensile properties and damping capacities of the composites. The strength increases with the addition of 5% graphite particles, but decreases with further addition of graphite particles. The strain amplitude independent damping increases significantly as the graphite particle volume fraction increases from 0 to 10%, but almost keeps constant when the volume fraction exceeds 10%. Two damping peaks are found at 150 and 350°C, respectively. The damping peak around 150°C is considered to be caused by movable boundary slip, and the damping peak around 350°C is inferred to be recrystallization peak. [Copyright &y& Elsevier]

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: 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]

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]

Abstract: The hot deformation behavior of ZK60 magnesium alloy reinforced with 20 vol.% of Al18B4O33 whisker has been studied by employing both processing maps and microstructural observations. The material of investigation was deformed by compression in the temperature range of T =523–723K and strain rate range of . From the hot compression tests, the flow stress data, strain rate sensitivity index (m), power dissipation efficiency (η) and instability parameter () were evaluated and processing maps were developed for the strain of ɛ =0.6. Domains for optimum hot working conditions were obtained using the processing maps and the instability zones were identified. The material showed a regime of dynamic recrystallization in the temperature range of T =623–723K and strain rate range of corresponding to the higher values of efficiency (η). The optimum temperature and strain rate combination for squeeze cast Al18B4O33w/ZK60 composite are T =673K and corresponding to an efficiency of η =24%. In the temperature range of T =573–623K and strain rate range of Al18B4O33w/ZK60 composite exhibited dynamic recovery. At the strain rate higher than 0.3s-1 and temperature range of T =623–723K, the material exhibited flow localization and these conditions should be avoided in processing. [Copyright &y& Elsevier]

Abstract: A particulate reinforced magnesium matrix composite fabricated by stir casting was extruded at 250°C with an extrusion ratio of 12:1 and constant RAM speed of 15mm/s. Extrusion of the composite causes large scale dynamic recrystallization resulting in a fine matrix microstructure. The reinforcing particles stimulate dynamic recrystallization, and dynamic recrystallization in the composite is sensitive to the particle content on a local scale. The particle distribution of the composite before and after extrusion was studied using a window technique. It is found that the segregation of particles in the as-cast composite is largely eliminated by extrusion and the particle distribution is significantly improved. Extrusion-induced damage to the reinforcement is observed in the extruded composites, and the particle fracture induced by extrusion is also sensitive to the particle content on a local scale. [Copyright &y& Elsevier]

Abstract: The effect of Si content on low frequency damping capacities and mechanical properties of as-cast Mg–Si alloys were investigated. Both hypoeutectic alloys with 0.3wt.%Si, 0.8wt.%Si and hypereutectic alloy with 2.3wt.%Si exhibit high damping capacities since these Mg–Si alloys possess large size α-Mg phase with very small amount of soluble atoms in it. The amount of the in situ formed Mg2Si determines the dislocation density and then influences the damping capacity of these Mg–Si alloys. The Mg–0.3wt.%Si alloy exhibits the lowest damping value at small strain region but the highest damping value at large strain region among these Mg–Si alloys. Two damping peaks were found in Mg–Si alloys at 80 and 230°C, respectively. But the addition of Al in Mg–Si alloy impeded the appearance of these two damping peaks since the soluble Al atoms hindered the movement of dislocations and the slide of grain boundaries. The presence of large size, brittle primary Mg2Si particles decreases the tensile strength and ductility of the hypereutectic Mg–Si alloy. [Copyright &y& Elsevier]

Abstract: Mechanical properties of coated AZ91 Mg alloy, Al18B4O33w/AZ91 and SiCw/AZ91 composites by microarc oxidation (MAO) surface treatment technique were measured and effects of MAO treatment on the mechanical properties were investigated. Tensile test shows that MAO treatment just slightly decreases ultimate tensile strength (UTS) and elongation of the Mg-based materials, while enhances their elastic modulus to some extent. But the effects of MAO treatment on the mechanical properties will be influenced by the energy parameters applied during surface treatment. The three materials coated under the same process conditions have exhibited differential surface morphology and fracture after tensile test, which indirectly indicates that the qualities of MAO coatings are also related with the substrate materials. [Copyright &y& Elsevier]

The effect of high temperature exposure on the interface and tensile properties of Al18B4O33w/AZ91 magnesium matrix composite fabricated by squeeze casting was studied. The interfaces of the composites were examined using transmission electron microscopy, and the mechanical properties were evaluated by tensile tests. The interfacial region was composed of MgO which was formed by the reaction between the Al18B4O33 whisker and the Mg matrix. An thin and uniform interfacial reaction layer with a thickness of 10–20 nm formed during squeeze casting could act as a barrier to prevent the further reaction between Al18B4O33 whisker and Mg alloys. The interfacial reaction rate was very low during thermal exposure below the liquidus temperature of AZ91 matrix alloy. Only after exposure above the liquidus temperature of the AZ91 matrix alloy for a long period, the thickness of the interfacial layer increased obviously. However, the interfacial reaction rate was much slower than that in the Al18B4O33w/Al–Mg composites. The interfacial reaction kinetics was controlled by the seepage of Mg and release of Al and B along the narrow diffusion channels between the fine MgO particle. The composite did not show a degradation in the tensile properties after exposure below the solidus temperature of AZ91 for long periods, suggesting that the Al18B4O33w/AZ91 magnesium matrix composite could be subjected to long-term service at elevated temperature. [Copyright &y& Elsevier]

The aging behavior of SiC whisker reinforced AZ91 magnesium matrix composites was investigated with Vickers hardness measurement, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation process observed in the monolithic alloy was not altered by the addition of SiC whiskers. The composite exhibited an accelerated hardening response compared with the unreinforced matrix alloy. However, the addition of SiC whiskers altered the distribution of the Mg17Al12 precipitates. Mg17Al12 precipitated preferentially at the SiCw–AZ91 interface, with a definite orientation relationship with SiC whisker: (1¯10)Mg17Al12//(111)SiCw, [111]Mg17Al12//[011¯]SiCw. The preferential interfacial precipitate decreased the Al content in the matrix, resulting in a decrease in the amount of continuous precipitates. As a consequence, the age-hardening efficiency in the composite was lower. [Copyright &y& Elsevier]

Due to its service background, the thermal behavior of Ta films is substantially concerning to researchers, especially the heat-induced β-Ta to α-Ta phase transformation. In this work, the nanocrystalline Ta films with varying β/α phase ratios (0.85–0) were deposited and vacuum annealed at 200–800 °C to study the phase transformation behavior and mechanical properties. Partial phase transformation of β-Ta to α-Ta occurred at low temperature (200 °C) and complete transformation when the annealing temperature was raised to 800 °C. Moreover, an orientation relationship of β-<110>//α-<111> and β-{330}//α-{110} was concluded, and a hypothesis theory about the low-temperature phase transformation was proposed. Annealing hardening took place at 200–600 °C and softening at 800 °C in all the samples. The hardening is mostly due to the ordering of intergranular amorphous phases with the temperature increased, simultaneously bringing the increase of modulus. The annealing softening is governed by both grain coarsening and phase transformation, where the latter is dominated in high β fraction samples. • High-temperature induced grain boundary structure ordering promotes the hardening of Ta films at 200 °C–600 °C. • β-Ta to α-Ta phase transformation was found at a wide temperature range, which partially governs the softening of Ta at 800 °C. • A preferred orientation relationship of β-<110>{330}//α-<111>{110} was concluded • The influence of phase transformation on mechanical properties was independently quantified using a simple comparative method. [ABSTRACT FROM AUTHOR]

Microcompressive tests were performed to investigate the intrinsic modulation effect and extrinsic diameter effect on strength and deformation behaviors of nanolaminated Cu/Cu-Zr crystalline/amorphous micropillars within a diameter D range from 300 to 1500 nm as well as a modulation ratio η range from 0.1 to 3.0. Experimental results revealed a transition of D -dependent deformation mode from homogeneous-like to shear localization at large η , while D -independent homogeneous extrusion of Cu at small η . Furthermore, the flow strength increased with raising D at small η , but decreased at large η , exhibiting a D -insensitive strength at a critical η* ~ 0.5. These unusual results were rationalized in terms of a coupling effect of dislocation actions and shear transformation zone mediated operations. [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: 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 nanocomposites reinforced by SiC nanoparticles were fabricated by ultrasonic vibration with SiC nanoparticle size of 60nm and volume fractions of 0.5%, 1% and 2%, respectively. With increasing the volume fraction of the SiC nanoparticles, grain size of matrix in the SiCp/AZ91 nanocomposites was gradually decreased and morphology of the phase Mg17Al12 was refined compared with as-cast AZ91 alloy. Most of the SiC nanoparticles exhibited a homogeneous distribution in the 0.5 and 1vol.% SiCp/AZ91 nanocomposites. The ultimate tensile strength, yield strength and elongation to fracture of the 0.5 and 1vol.% SiCp/AZ91 nanocomposites were simultaneously improved. However, agglomerates of SiC nanoparticles along the grain boundaries were evidently increased when the volume fraction of nanoparticles was increased to 2, which caused the decrease of ultimate tensile strength and elongation to fracture. [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: The uniaxial compressive deformation behavior of a 10vol.% SiC particulate reinforced AZ91 magnesium matrix composite (SiCp/AZ91) fabricated by stir casting is investigated at elevated temperature (250–400°C). Peak stresses and flow stresses decrease as temperatures increase and strain rates decrease. The extent of dynamic recrystallization (DRX) becomes less as temperatures decrease at 250–350°C or strain rates increase, and recrystallization occurs mainly within the intergranular regions rich of particles. Dynamic recrystallization accomplishes at 400°C even at the strain rate of 1s−1. An analysis of the effective stress dependence on strain rate and temperature gives a stress exponent of n =5 and a true activation energy of Q =99kJ/kJ. The value of Q is close to the value for grain boundary diffusion in Mg. It is concluded that the deformation mechanism of SiCp/AZ91 composite during hot compression is controlled by the dislocation climb. [Copyright &y& Elsevier]

Nanostructured crystalline Al/amorphous AlN multilayer films, with a wide layer thickness (h) range from 10 to 200 nm, were prepared and exposed to high-temperature annealing from 200 to 600 °C. Microstructure of all the multilayers was highly stable under the temperature up to 400 °C. Apparent grain coarsening happened in multilayers with h > ~50 nm under 500 °C. However, the Al grains in h ≤ ~50 nm multilayers were extremely stabilized under even 500 °C annealing, showing a high thermal resistance up to about 0.8 T m (T m : melting point of bulk pure Al). The stabilization mechanisms mainly include the constraint effect by heterophase interface and the nanotwinning in Al layers, as demonstrated by statistical measurement results and ex-situ / in-situ transmission electron microscope observations. When the annealing temperature was raised up to 600 °C, the heterophase interfaces disappeared in h ≤ ~20 nm samples but were well preserved when h > ~20 nm, revealing that the thermal failure modes were quite sensitive to h. Based on the microstructural evolution, a thermal failure map was finally developed in variation with h and annealing temperature. Furthermore, hardness of the multilayers was measured and hardening mechanism was also discussed according to the microstructural evolution after annealing. [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]

Coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) Al-2.5 wt% Cu (Al-Cu) alloys were respectively prepared, with and without 0.3 wt% Sc addition, for comparison. The influences of minor Sc addition on the microstructural evolution, tensile mechanical properties and pitting corrosion resistance were systematically studied at different grain scales. A significant Sc microalloying effect on the precipitation was observed that the minor Sc addition promoted the dispersion of finer θ ′-Al 2 Cu precipitates in the CG alloy and favored the intragranular θ ′-Al 2 Cu precipitation in the FG and UFG alloys, with the smaller grain size leading to a stronger Sc microalloying effect. The Sc addition induced convincing increases in the yield strength at all the three grain scales, and improved (in the CG and UFG Al-Cu alloys) or retained (in the FG Al-Cu alloy) the pitting corrosion resistance at the same time. This indicates that the inverse strength-pitting corrosion correlation as usually observed can be broken by minor Sc addition. The strengthening mechanisms were discussed and the grain size-dependent pitting corrosion resistance mediated by the Sc addition was rationalized in terms of a competition between the positive influence derived from the interfacial Sc segregation and the negative influence come from the deformation-induced dislocations. The present findings provide a possible approach to break the inverse strength-pitting resistance correlation in heat-treatable Al alloys by modifying the precipitate/matrix interfaces through the suitable microalloying atom segregation. [ABSTRACT FROM AUTHOR]

Al-XCu (X = 1.0, 1.5, and 2.5 wt%) alloys with and without 0.3 wt% Sc addition were prepared respectively. The effects of composition and heat treatment processes on the microstructural evolution were systematically investigated by using transmission electron microscope and atom probe tomography (APT). Both Al 3 Sc dispersoids and θ ′-Al 2 Cu precipitates coexisted after artificial aging, and a strong Sc segregation at θ ′-Al 2 Cu/matrix interfaces was detected and quantified through APT examinations. A Sc solute partitioning was demonstrated between in the Al 3 Sc dispersoids and at the θ ′-Al 2 Cu/matrix interfaces, mediated by the Cu content. Increasing the Cu content, both the size and volume fraction of the Al 3 Sc decreased after solid solution treatment. As a result, the interfacial Sc segregation was accordingly intensified during subsequent aging treatment. A parameter of reduction in interfacial energy (Δγ), derived from APT analyses, was proposed to characterize the degree of interfacial Sc segregation. The coupling effect of Al 3 Sc dispersoids and θ ′-Al 2 Cu precipitates on ductile fracture was discussed, where a micromechanical model was developed to describe a quantitative relationship between the fracture strain with Δγ as well as parameters of the Al 3 Sc dispersoids. [ABSTRACT FROM AUTHOR]

Pure molybdenum (PM) and La 2 O 3 dispersion strengthened Mo alloy (ODS-Mo) were prepared by powder metallurgy through two different mixing method (solid-solid or SS mixing and solid-liquid or SL mixing). After annealed at 1050 °C for 1 h, the PM, SS-Mo and SL-Mo were compared in microstructure and mechanical properties. The microstructural examinations showed that the PM was mostly recrystallized with rather coarse grains, but the ODS-Mo alloys remained fine elongated grains. This discrepancy is due to a higher recrystallization temperature held in the ODS-Mo alloys. The uniaxial testing results showed that the ODS-Mo had higher tensile mechanical properties compared to the PM, which are attributed to the remarkable strengthening and ductilizing effect induced by the La 2 O 3 particles. The low cycle fatigue (LCF) testing results revealed that the PM experienced cyclic hardening behaviors, while the ODS-Mo alloys exhibited cyclic softening behaviors. The Basquin-Manson-Coffin analyses results demonstrated that the ODS-Mo possessed a higher fatigue ductility and longer fatigue life than the PM. Two kinds of cracks, i.e., cleavage cracks and intergranular cracks, were experimentally observed to coexist on the fracture surface. In particular in the SL-Mo alloy, the two crack interacted and propagated forward until final fracture, leading to a step-like crack growth path and concomitantly enhanced fatigue ductility. In addition, different cycling dislocation structures were revealed between the PM and ODS-Mo alloys. Cycle fatigue mechanisms responsible for the fatigue behaviors and microstructure evolution during LCF testing were discussed. The best LCF resistance combined with superior uniaxial tensile mechanical properties found in the SL-Mo alloy were rationalized. [ABSTRACT FROM AUTHOR]

High quality WE43 magnesium alloy ingots with diameter of 500 mm are successfully fabricated by direct-chill (DC) casting. The alloying elements distributed almost homogeneously across the large ingot. With the average grain size decreasing from the center to the edge of the ingot, the mechanical properties are improved gradually. After solution treatment at 525 °C for 8 h, the Mg 14 Nd 2 Y phases distributing along the grain boundaries are well dissolved into matrix. The direct-chill (DC) casting ingot exhibits remarkable age-hardening response from 85 Hv to 117 Hv after T6 treatment at 250 °C. The T6 peak-aged sample exhibits ultimate tensile strength of 274 MPa, yield strength of 215 MPa and elongation to failure of 3.4%, which are higher than those of sand-cast and permanent mold cast WE43 alloys. The large amount of nano prismatic β ′ and β 1 phases precipitated during T6 treatment contribute to improved strength of the WE43 alloy. [ABSTRACT FROM AUTHOR]

In this work, hcp-to-bcc Ti phase transformation was uncovered in Ti/Ta nanostructured multilayers. The Ti nanolayers were fully transformed to bcc phase when the layer thickness ( h ) smaller than ~7.5 nm, accompanied with the formation of fully coherent interfaces. While in the greater layer thickness, Ti layers were partially phase-transformed, resulting in the simultaneous existence of bcc/bcc Ti/Ta coherent interfaces and bcc/hcp Ti/Ti phase boundaries. A thermodynamic model was proposed to account for the layer thickness h -dependent Ti phase transformation. The phase transformation caused an unusual size-dependent hardness, i.e., the smaller the layer thickness and the lower the hardness, which was quantitatively addressed by an ad hoc strengthening model. [ABSTRACT FROM AUTHOR]

The effect of Ca/Al ratio on microstructure and mechanical properties of the as-cast and as-extruded Mg-Al-Ca-Mn alloys were investigated. The as-cast Mg-Al-Ca-Mn alloys with Ca/Al mass ratios less than 0.50 contain divorced and lamellar eutectic (Mg,Al) 2 Ca. When Ca/Al ratio is greater than 0.90, fine lamellar Mg 2 Ca phase is formed in the as-cast Mg-Al-Ca-Mn alloys. After extrusion, the second phases are broken and dispersed along the extrusion direction. The area fraction of DRXed regions decrease with increasing Ca/Al ratio, the microstructure changes from an almost fully recrystallized structure to a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains and coarse deformed regions. With increasing Ca/Al ratio, the DRXed grains and dispersed second phases are refined. The as-extruded alloys exhibit a strong basal fiber texture, the intensity of which is increased with increasing Ca/Al ratio. With increasing Ca/Al ratio, the {0002}<11-20> texture component in the DRXed regions is suppressed, and shifted to a {0002}<10-10> texture component. The strength of the as-extruded Mg-Al-Ca-Mn alloys increases significantly with increasing Ca/Al ratio. The as-extruded Mg-2.7Al-3.5Ca-0.4Mn (wt%) alloy exhibits a tensile proof strength of 438 MPa and an ultimate tensile strength of 457 MPa. The extraordinary high strengths are attributed to the ultrafine DRXed grains pinned by fragmented secondary phases, strong basal texture and dense nano-scale precipitates. The present research indicates that the Mg-Al-Ca-Mn alloys have a great potential as next generation low-cost ultrahigh-strength wrought Mg alloys. [ABSTRACT FROM AUTHOR]

SiC particles exhibited necklace-type distribution in SiCp/AZ91 Mg matrix composites fabricated by stir casting , and thus, the SiC particles could be divided into segregated particles region and particle free region. The effects of this particle distribution on the discontinuous precipitation of Mg 17 Al 12 phase during ageing were investigated. The high density dislocations were induced due to the sharp mismatch between the thermal expansion coefficients of SiC and AZ91 alloy. The overlapped mismatch among the segregated particles causes higher density dislocations. And as a result, the discontinuous precipitations as the ageing products started born mainly near particles, especially near segregated particles. However, few Mg 17 A1 12 precipitated in the matrix within the particle segregation. The high-density dislocations and the numerous SiC/Mg interfaces made Mg 17 A1 12 phases much easier and earlier to precipitate in the composite. Thus, the SiCp addition reduced the peak ageing time from 38 h (for AZ91 alloy) to 28 h (for the composite). The ultimate tensile strength of the composite was enhanced by 46% after peak-ageing. SiC particles could also improve the age hardening efficiency of the composite because the high-density dislocations, the fine grain sizes and the numerous SiC/Mg interfaces cause the finer and more uniform precipitation of Mg 17 A1 12 Above all, the addition of SiC particles is effectively to improve the mechanical properties of magnesium matrix composite materials. [ABSTRACT FROM AUTHOR]

Nanoindentation methodology was used to measure the hardness of Cu/Ta crystalline/amorphous nanostructured multilayers (CANMs) before and after He ion-implantation at room temperature. These CANMs have a constant modulation period ( λ =25 nm) but different modulation ratios ( η = h Ta / h Cu ) spanning from 0.11 to 1.0. It is found that in sharp contrast to previously reported Cu/Cu-Zr CANMs by Zhang et al. [9] , the He-irradiated Cu/Ta samples exhibit much greater microstructure stability without radiation-induced devitrification (RID) of glassy Ta nanolayers at smaller η (except the sample at η =1.0). Both the as-deposited and irradiated Cu/Ta CANMs manifest the monotonically increased hardness with decreasing η , however, there is an unexpected transition from size-dependent irradiation hardening at η <1.0 to softening at η ≥1.0 caused by a competition between dislocation-bubble interactions in crystalline nanolayers and RID in glassy nanolayers. These findings not only provide fundamental understanding of the radiation-induced defect effect on plastic characteristics of CANMs, but also offer guidance for their microstructure sensitive design for performance optimization at a critical modulation ratio under extreme conditions. [ABSTRACT FROM AUTHOR]

Multidirectional forging (MDF) under decreasing temperature conditions was adopted to process SiC nanoparticles reinforced magnesium matrix composite. The results showed that grains of the nanocomposite were effectively refined while the amount of precipitated phases was significantly increased after MDF under decreasing temperature conditions. The precipitated phases as well as dispersed SiC nanoparticles could hinder the growth of dynamic recrystallization. The yield strength and ultimate tensile strength of the nanocomposites were increased compared with that of as-cast nanocomposite. This improvement could be attributed to obvious grain refinement as well as Orowan strengthening effect introduced by the dispersed SiC nanoparticles and the precipitated phases. Microcrack could initiate within the SiC nanoparticle dense zones during tensile test at room temperature, resulting in decrease of the elongation. [ABSTRACT FROM AUTHOR]

Both twinning and its reversible process, i.e., detwinning, play important roles in crystalline plasticity, especially for nanostructured metals and alloys. Contrary to the popular belief that the nanotwinned (NT) microstructure is quite stable, in this work, we reported the twinning/detwinning-mediated grain growth (GG) and dislocation nucleation-controlled strength softening in the high stacking fault energy (SFE) NT-Ni foils with different grain sizes but nearly identical twin lamellar thickness during tension at room temperature. We further proposed two possible dislocation–boundary interaction mechanisms that cause GG via twinning/detwinning processes, and elucidated strain rate effect on the double-inverse grain size dependence of twinning/detwinning in nanostructured Ni samples. Our findings provide in-depth insights into the manipulation of internal features of metals and alloys with high SFE to achieve optimum robust performance for practical applications and fundamental understanding of the twinning/detwinning behavior of NT-materials to predict their stability and evolution of microstructures. [ABSTRACT FROM AUTHOR]

AZ91 alloy was subjected to multidirectional forging (MDF) and the relationship between microstructures and mechanical properties generated by MDF were investigated. The results showed that after 1 MDF pass grain size of the alloy decreased compared with as-cast alloy, and the amount of recrystallization grains increased when the MDF temperature increased from 250 °C to 400 °C. With increasing the MDF passes at 350 °C, grains of the alloy were gradually refined. A typical basal texture was obtained after 1 MDF pass, and then the basal pole scattered with the increase of MDF passes. Mg 17 Al 12 phases were precipitated along grain boundaries during MDF, which could hinder the growth of recrystallization grains. Both tensile strength and elongation to failure of the alloy were significantly enhanced till 3 MDF passes with increasing the MDF passes. [ABSTRACT FROM AUTHOR]

Mg–5.3 wt%Zn alloy microalloyed with 0.2 wt%Ca and 0.5 wt%Ce was extruded, and the microstructure, texture and mechanical properties of the as-extruded alloy were investigated. The undissolved coarse T 1 ′during homogenization treatment stimulated nucleation of dynamic recrystallization (DRX) grains during extrusion. The double microalloying with Ca and Ce promoted the dynamic precipitation, resulting in high density precipitates after hot extrusion. Fine Ca 2 Mg 6 Zn 3 particles precipitated dynamically at the grain boundaries (GBs) could effectively hinder the grain growth during hot extrusion. A weaker texture in DRXed grains was attributed to the coarse-particle stimulated nucleation (PSN) and pinning of the grain boundaries by fine precipitates. The fine DRXed grains, high density of dislocation in the unDRXed region, and fine dense precipitates gave rise to the improved strength of the as-extruded Mg–Zn–Ca–Ce alloy. [ABSTRACT FROM AUTHOR]

Abstract: Bimodal sized (Micro-particles: m and Nano-particles: n) SiC particulates (SiCp) reinforced magnesium matrix composites with different volume fractions of micro-particles (1n+4p, 1n+9p and 1n+14p) were prepared by a semisolid stirring assisted ultrasonic vibration method. Both the as-cast SiCp/AZ31B composites and the AZ31B alloy were extruded at 350°C with the extrusion ratio of 12:1 at a constant ram speed of 15mm/s. The microstructure investigation was carried out by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results show that the distribution of bimodal size SiCp and the mechanical properties of the AZ31B alloy were significantly improved after hot extrusion. When the volume fraction of nano SiCp is 1vol%, both the yield strength and ultimate tensile strength of the AZ31B/SiC/1n+14p composite are stronger than that of the AZ31B/SiC/1n+4p and AZ31B/SiC/1n+9p composites, while the elongation to fracture was decreased comparing with the AZ31B/SiC/1n+4p and AZ31B/SiC/1n+9p composites. [Copyright &y& Elsevier]

Abstract: Carbon nanotube (CNTs) reinforced Mg matrix composites were successfully fabricated by a novel approach. After hot extrusion, the influence of CNTs on microstructure and mechanical properties of Mg–6Zn matrix was investigated. The results showed that most CNTs distributed uniformly and individually in the composites, and good interfacial bonding was achieved. CNTs significantly refined the grains of Mg–6Zn matrix, and the CNTs evidently improved the ultimate tensile strength (UTS), yield strength (YS) and Young's modulus. Grain refinement, load transfer mechanism and Orowan strengthening mechanism play important roles on the increase of the yield strength. [Copyright &y& Elsevier]

The Mg–4.50Zn–1.13Ca (wt%) alloy was subjected to double extrusion, and the microstructure and mechanical properties of the extruded alloys were investigated. Double extrusion refined the microstructure significantly, resulting in the enhancement of mechanical properties of the double-extruded alloys compared with the single-extruded alloy. The yield strength of the single-extruded alloy was 173MPa, while the value was increased to 320MPa and 370MPa after subsequent second extrusion at 300°C and 250°C, respectively. The second extrusion ratio had little effect on the microstructure and mechanical properties of the double-extruded alloys, while grain size was decreased and strength was increased with the decrease of double extrusion temperature. The double-extruded Mg–Zn–Ca alloy exhibited superior elevated temperature mechanical properties, which was attributed to the fine stable Ca2Mg6Zn3 intermetallic compound particles dispersed in the alloy. [ABSTRACT FROM AUTHOR]

Abstract: The microstructures and mechanical properties of Mg–5.3Zn–0.2Ca–0.5Ce (wt%) alloy fabricated by semi-continuous casting were investigated. The as-cast alloy consists of α-Mg, Ca2Mg6Zn3 and T′1 phase with orthorhombic structure (a=0.57nm, b=0.75nm and c=1.00nm). T′1 phase mainly distributes at the triple conjunctions while the Ca2Mg6Zn3 at the grain boundaries. The as-cast alloy exhibits high strength retention due to the stable Ca2Mg6Zn3 and T′1 phase at the grain boundaries. [Copyright &y& Elsevier]

Abstract: A large Mg-4.50Zn-1.13Ca (wt%) alloy ingot 350mm in diameter and 1730mm long was successfully fabricated by semi-continuous casting. The microstructures and mechanical properties of the as-cast and as-extruded alloys were investigated. The as-cast alloy exhibits uniform chemical composition and microstructure from the centre to the surface of the ingot and mainly consists of α-Mg and Ca2Mg6Zn3. The coarse grains and network-like second phases that are distributed at the grain boundaries result in poor mechanical properties for the as-cast alloy. The mechanical properties of the as-extruded alloys are significantly enhanced. The high yield strengths of the as-extruded alloys mainly result from the fine DRXed grain and the stronger basal texture. [Copyright &y& Elsevier]

Abstract: Extruded Mg–8.2Gd–3.8Y–1Zn–0.4Zr (wt%) alloy was subjected to ageing treatment at 200°C. The microstructures, textures and mechanical properties of the as-extruded and peak-aged alloys were investigated. Recrystallization occurred during ageing treatment. A large number of β′ phases were observed in the peak-aged alloy, accompanied with the decrease of the volume fraction of LPSO phases. A basal fiber texture and an unusual texture component with basal planes perpendicular to the extrusion direction were found in the as-extruded alloy, and the basal fiber texture was strengthened after ageing treatment. The peak-aged alloy exhibited a tensile yield strength (TYS), an ultimate tensile strength (UTS) and an elongation to failure of 395MPa, 470MPa and 8%, respectively, at ambient temperature, and 271MPa, 311MPa and 19%, respectively, at 300°C. The superior strength of the peak-aged alloy was mainly attributed to the precipitation of fine β′ phases, the remaining LPSO phases and the texture modification. [Copyright &y& Elsevier]

Abstract: Equal channel angular pressing (ECAP) was performed on the as-extruded commercial pure magnesium at 250°C for 4 passes. The internal friction of the ECAPed pure Mg as a function of strain amplitude was investigated by dynamic mechanical analyzer (DMA), and the cyclic microplasticity of pure Mg was investigated by tensile loading and unloading test. After ECAP processing, the grain size is significantly refined, the texture component with basal planes parallel to extrusion direction is replaced by a new stronger one with basal planes having a tilting angle of about 40° to the extrusion direction. The stress in microplastic region is reduced with increasing ECAP passes, while the internal friction increases. The internal friction of Mg at high strain amplitude is closely related to microplastic deformation and can be interpreted by dislocation mechanism. The Granato and Lücke model only satisfies in anelastic regions, while the internal friction in microplastic deformation region should be explained in terms of the internal friction model postulated by Peguin. The internal friction related to microplasticity can be divided into two parts with different activation volumes of dislocation motion, which correspond to the two regions of microplasctic deformation process. The initial stage associated with dislocation motion on basal plane shows larger activation volume and lower friction stress of dislocations. The second stage related to the annihilation and tangle of dislocations is characterized by larger hardening exponent and friction stress. [Copyright &y& Elsevier]

Abstract: In this paper, 0.2μm SiCp/AZ91 composite was fabricated by stir casting technology. The influence of fine (<1μm) particles on the dynamic recrystallization (DRX) behavior of AZ91 matrix was investigated by researching the microstructure evolution of 0.2μm 1vol% SiCp/AZ91 composite during the hot extrusion process. Results showed that the DRXed ratio of AZ91 matrix increased as well as the average size of DRXed grains reduced due to the addition of fine particles. The SiCp could not only pin dislocation, but also was helpful to generate dislocations owing to the deformation mismatch between matrix and particles, thus causing the increase of dislocation density around SiCp, which was benefit to stimulate DRX nucleation. Meanwhile, the growth of DRXed grains was suppressed in the following deformation process due to the pinning effect of fine particles on grain boundaries. Thus, the fine particles had significant effect on stimulating DRXed nucleation and refining grain size. The yield strength and ultimate tensile strength of the as-deformed 0.2μm 1vol% SiCp/AZ91 composite are 275 and 335MPa, respectively, which are mainly attributed to the grain refinement, basal plane texture and uniform particle distribution induced by hot deformation. [Copyright &y& Elsevier]