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Abstract: In this paper, mechanical properties of a deformed Al–Si eutectic alloy processed by isothermal extrusion at temperature from 573K to 773K with reduction ratio from 25% to 85% were investigated at ambient temperature. The results showed that a banded structure composed of matrix region and accumulation region of second phase particles was formed and a few cracks were generated in particles and evolved to voids among particles. The tensile strength of test specimens ranged from 250MPa to 400MPa and was directly related with temperature from 623K to 773K. The elongation of test specimens ranged from 2.8% to 13.1%, and had a peak value at 673K under each section reduction ratio. A reduction in elongation occurred at section reduction ratio larger than 75% because of particle bands splitting aluminum matrix severely. The effect of temperature on mechanical properties was more significant than that of section reduction ratio. Excellent balance between strength and ductility can be obtained by extrusion at temperature 623–723K and section reduction ratio 40–70%. [Copyright &y& Elsevier]

The isothermal compression (1050 °C) of a 304-type austenite stainless has been employed to understand the roles of strain rates (0.001, 0.01, 0.1, 1, and 10 s−1) and twinning in flow behaviors and microstructure evolution. Using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analysis, it was determined that the alloy underwent discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) in low and high strain rate regimes, respectively. In the low strain rate regime, twinning contributed to the separation of the bulged region for dynamic recrystallization (DRX) nucleation, and Σ 3 twin boundaries mainly formed by 'growth accident' during the subsequent growth of nuclei. In the high strain rate regime, twinning did not require boundaries migration, and altered the orientation of boundary fronts, thus promoting the extension of the DRX area. Grain size exponents in low and high strain rate regimes were calculated to be −0.68 and −2.9, respectively. It was also confirmed that different DRX mechanisms result in distinct evolution of grain size, and the dividing point was determined to be the strain rate of 0.1 s−1. • Effects of strain rate on flow behaviors and microstructure evolution during hot deformation were analyzed using TEM and EBSD. • DDRX and CDRX mechanisms for different strain rate regimes were proposed. • Twinning mechanisms for two types of DRX were clarified to be different. • Grain size exponents were calculated to be −0.68 and −2.9 in low and high strain rate regimes, respectively. [ABSTRACT FROM AUTHOR]

In this study, the influence of low-frequency vibrations on the room-temperature compression behavior of T2 copper was experimentally investigated. Upon superposing a low-frequency vibration, the forming load noticeably reduced. Among the three vibration parameters investigated, viz. frequency, amplitude, and feeding rate, vibration amplitude plays the most important role in load reduction. The superposition of stress waves can explain the mechanism of forming load reduction at low-frequency vibrations. Due to an increase in the internal stress caused by the superposition of stress waves, the required forming load reduced. The influence of the vibrations on the microstructures of compressed specimens was also discussed. Under low-frequency vibration conditions, cyclic impact loading leads to an increase in the density of dislocations and the formation of dislocation cells, which gradually evolve into new grain boundaries, causing grain refinement. [ABSTRACT FROM AUTHOR]

Controlled activation of flow units and in-situ characterization of mechanical properties in metallic glasses are facing challenges thus far. Here, vibrational loading is introduced through nanoscale dynamic mechanical analysis technique to probe vibration-accelerated atomic level flow that plays a crucial role in the mechanical behavior of metallic glasses. The intriguing finding is that high vibrational frequency induces deep indentation depth, prominent pop-in events on load–depth curves and low storage modulus, exhibiting a vibration-facilitated activation of flow units in Pd 40 Cu 30 Ni 10 P 20 metallic glass. Theoretical analysis revealed that vibration-moderated activation time-scale accelerate the activation of flow units and responsible for the above scenario. [ABSTRACT FROM AUTHOR]

Despite the intense interest in coherent twin boundary (CTB) of nanotwinned materials, many details of interactions between the complex dislocation (prismatic dislocation loop (PDL)) and CTB in a single crystal remain mysteries. In this study, molecular dynamics (MD) simulation of the nanoindentation on single crystal copper with CTBs has been presented. At the initial stage of nanoindentation, indenter moves downward and PDLs form. Thompson's tetrahedron notation has been adopted to scrutinize the process of PDL's formation without cross-slip and release from the indented place. Dislocations at PDLs' side edge are stair-rod partial dislocations, including two Lomer-Cottrell locks and two Hirth Locks. Then the interactions between PDL and CTB drive the migration of CTB. As a result, a parallelogram step which is bounded by Frank partial dislocations has been left, implying the formation of twinning dislocations. When the indenter moves further, the interactions repeat and twinning dislocations multiply. Our simulation results shed light on the interactions between PDL and CTB and would provide a new insight into understanding the role CTB plays during mechanical deformation. [ABSTRACT FROM AUTHOR]

The high-temperature deformation behaviour and microstructure evolution of an extruded Al-Cu-Li alloy were investigated by compression tests conducted at various temperatures (613, 673 and 733 K) with various strain rates (0.001, 0.01, and 0.1 s -1 ). The results indicated that the deformation activation energy increased from 208.7 kJ/mol to 255.7 kJ/mol with an increase in strain from 0.1 to 0.7. The electron backscatter diffraction maps indicated that a dynamic recrystallisation occurred during the high-temperature deformation. Two types of recrystallisation mechanisms, grain boundary bulging and a grain boundary transformation from low misorientation to high misorientation, were considered as the mechanisms for controlling the formation of the recrystallised grains. A new dynamic recrystallisation model containing these two mechanisms was proposed to describe the microstructure evolution of the extruded Al-Cu-Li alloy. At the early stage of the deformation, the recrystallised grains were formed by grain boundary bulging along the original grain boundaries. With increasing strain, recrystallised grains were gradually generated in the deformed grains due to the transformation from low angle boundaries to high angle boundaries. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration (UV) assisted forming is a promising way to manufacture high-performance metallic components. However, the acoustoplasticity of α-Ti is affected by the coupling effect of grain size and plastic anisotropy, making the UV-assisted forming process complicated and hard to control. In this research, UV-assisted compression tests were conducted on rolling textured α-Ti specimens with different grain sizes and orientations. The deformed microstructure was observed, and the grain size effect on the anisotropic acoustoplasticity was analyzed. The results indicate that the ultrasonic softening effect on α-Ti was anisotropic and grain size related. The higher ultrasonic softening fraction in coarse-grained materials indicates that the ultrasonic effect is more effective. Coarse grains provide longer dislocation mean free path and more room for dislocation storage, and reduce the resistance for nucleation and growth of twins. Meanwhile, the Hall-Petch effect on ND specimens is more considerable than that on RD and TD. The ultrasonic weakening of Hall-Petch effect was observed in all three orientations, and the weakening effect is less effective on ND specimens induced by the directionality of ultrasonic softening. Furthermore, A grain size-related acoustoplasticity model considering the coupling effect of ultrasonic softening and the Hall-Petch was developed, which can well predict the acoustoplasticity of α-Ti. These findings provide a fundamental understanding of the grain size effect on the anisotropic acoustoplasticity of α-Ti in UV-assisted forming process. • Grain size effect on the anisotropic acoustoplasticity of rolled α-Ti was investigated. • More effective ultrasonic effect in coarse-grained α-Ti is related to the enhanced dislocation multiplication and twinning. • The ultrasonic weakening of Hall-Petch effect is less pronounced on ND specimens due to the directional acoustoplasticity. • The developed grain size-related acoustoplasticity model well predicted the flow behavior of α-Ti. [ABSTRACT FROM AUTHOR]

The room temperature creep behavior of newly developed Ti 20 Zr 20 Hf 20 Be 20 Cu 20 and Ti 20 Zr 20 Hf 20 Be 20 Cu 20 Ni 10 high entropy bulk metallic glasses (HE-BMGs) was investigated using nanoindentation technique under different loading rates. A generalized Kelvin model was applied to describe the experimental creep curves. The strain rate sensitivity parameter, creep compliance and retardation spectra were derived to study the creep mechanism. The experimental results show that the studied HE-BMGs possess good creep resistance. Moreover, it has been found that the addition of Ni effectively enhances the creep resistance of Ti 20 Zr 20 Hf 20 Be 20 Cu 20 HE-BMG and the related mechanism has also been discussed. [ABSTRACT FROM AUTHOR]

A novel P/M nickel-based superalloy is developed with an excellent combination of strength and ductility for use in turbine disks. This superalloy is prepared through hot isostatic pressing (HIP) and subsequent hot extrusion (HEX) with different extrusion ratios (λ). The effects of λ on the evolution and mechanism of γ′ precipitation behavior, grain refinement, microtexture, and mechanical properties are systematically studied. As λ increases, the primary γ′ phase fraction decreases from 17.22% to 8.47%, and the precipitation morphology changes from an irregular to a spherical shape. Increasing λ is beneficial to dynamic recrystallization. Fully recrystallized fine grains with a diameter of 4.47 μm are obtained at an optimal λ = 5.0, a decrease of 76% compared with the HIPed alloy. Abnormal grain coarsening was observed at a higher λ with 8.5 owing to the lack of γ′ pinning and a stronger driving force for grain boundary migration induced by deformation heat. The yield strength (YS) and ductility of the HEXed alloy are over 30% higher than those of the HIPed alloy, thanks to refined grains and the γ′ phase. The alloy extruded with λ of 5.0 exhibits a high YS of 1376 MPa with a ductility of 22.6% at room temperature and a YS of 1208 MPa with a ductility of 18.3% at 700 °C, which are better than those of other superalloys. A multi-mechanistic strengthening model is constructed by correlating the chemical composition, size and fraction of γ′, grain size, and YS. The calculations suggest that precipitation strengthening contributes more than 60% of the YS. Extrusion can dramatically enhance the YS of the superalloy owing to the increment of grain boundary strengthening and precipitation strengthening. This work provides a practical route and theoretical support for optimizing microstructure and enhancing the mechanical properties of a HIPed P/M nickel-based superalloy. • A novel P/M nickel-based superalloy with high strength (1376 MPa) and ductility (22.6%) was prepared. • Role of extrusion ratio on microstructure evolution mechanism of the superalloy was revealed for the first time. • Abnormal property deterioration was observed at the highest extrusion ratio (8.5). • Dissolution of primary γ′ results in the abnormal grain coarsening at the highest extrusion ratio. • Enhanced mechanical properties were quantitively elucidated by a multi-mechanistic strengthening model. [ABSTRACT FROM AUTHOR]

Thin metal sheets are widely used in manufacturing miniaturized components through the micro forming process. However, the size effect on the plastic deformation behavior of thin metal sheets is significant, and the plastic anisotropy of rolled α-Ti sheets makes the micro forming process more complicated, which greatly limits the application of micro α-Ti components. A series of in-situ tensile tests were conducted using specimens with different sizes and orientations to investigate the size effect on the anisotropic deformation behavior of the α-Ti sheet. The microstructure and fracture morphology were observed, and the grain-scale deformation behavior of α-Ti was explored through full-field crystal plasticity simulation. The results indicate that the size effect on the tensile deformation behavior of α-Ti is anisotropic. The TD specimen has a greater size effect sensitivity and shows a more considerable fracture stain reduction with the specimen size decreasing. The twinning behavior in terms of types, morphology, and fractions is anisotropic and evolves with the specimen size. TD specimens show a more significant inhomogeneity of strain distribution than that of RD, and the difference increases with specimen size decreasing. More hard-oriented grain and massive local needle-like twins generated in small-sized TD specimens lead to severer strain localization. These findings provide a fundamental understanding of the anisotropic size effect in the micro forming process of α-Ti. • The anisotropic size effect on α-Ti was investigated through in situ experiment and crystal plasticity simulation. • The plastic anisotropy of α-Ti is enhanced as the specimen size decreases. • The flow behavior and fracture behavior of TD specimen are more sensitive to size effect than that of RD. • More hard-oriented grains and massive local needle-like twins in small TD specimens lead to severer strain localization. [ABSTRACT FROM AUTHOR]

Tungsten-particle (Wp)-reinforced bulk-metallic-glass composites (Wp/BMGCs) with a Wp volume fraction of 0%–50% were prepared by spark plasma sintering (SPS). The sintering process parameters were optimized through an orthogonal experiment. With increasing volume fraction of Wp, the compressive yield strength of the Wp/BMGCs decreased, while their ultimate strength and plastic strain increased. Nanoindentation experiments revealed that the elastic modulus and hardness of the composites increased with increasing volume fraction of Wp. The elastic modulus mismatch between the tungsten particles and metallic glass matrix resulted in a stress concentration at the interface of the two phases, promoting the initiation and propagation of shear bands in the metallic glass matrix. Fracture morphology analysis of the composites showed that the failure mode of the composites was shear fracture mode, which is beneficial to the self-sharpening of the composites. With increasing volume fraction of Wp, the plastic deformation of Wp became more obvious, and the deformation was most obvious near the shear fracture surface. The degree of plastic deformation of Wp in the region far away from the shear fracture surface gradually decreased. In addition, the reliability of the mechanical properties of Wp/BMGCs sintered in the same preparation parameters was verified by three-parameter Weibull statistical analysis. These results help deepen our understanding of the preparation and mechanical properties of Wp/BMGCs and further promote their application. [ABSTRACT FROM AUTHOR]

The effects of deep cryogenic cycling treatment (DCT) on the rejuvenation and room-temperature compressive properties of a TiZrHfBeCu high-entropy bulk metallic glass (HE-BMG) were studied. After DCT, the fully amorphous structure of TiZrHfBeCu HE-BMG remained, but the relaxation enthalpy showed obvious rejuvenation behavior. Upon increasing the number of cycles, the room-temperature plasticity of TiZrHfBeCu HE-BMG gradually increased from 0.62% to 5.38%, with no obvious change in the yield strength. The hardness and density of the high-entropy amorphous alloy decreased with the number of DCT cycles. The DCT-induced rejuvenation and plasticization mechanisms were explained based on the shear transformation zone (STZ) model. The effects of high mixing entropy and fragility on the rejuvenation behavior were also considered. This work confirmed that DCT is an effective method for the rejuvenation and plasticization of TiZrHfBeCu HE-BMG. • TiZrHfBeCu high entropy metallic glass can be rejuvenated by deep cryogenic cycling treatment. • The room temperature plasticity can be improved from 0.62% to 5.38%. • The plasticization results from the induced free volume content. • The small fragility of TiZrHfBeCu alloy favors the rejuvenation and plasticization. [ABSTRACT FROM AUTHOR]