ACTA

The mechanical and twinning behavior of Mg alloys containing a monolithic texture component have been extensively studied; however, studies on samples with multi texture components are much insufficient. In this study, the quantitative relationship between the mechanical behavior under tension along the extrusion direction (ED) of Mg AZ31 rods and the percentages of bimodal texture components <0002>// ED (19–87%) and <0002>⊥ED were systematically studied. The component <0002>// ED favors {10-12} twinning and is soft orientation; however, the component <0002>⊥ED favors prismatic slip and is hard orientation. The results show that the yield strength does not decrease linearly with an increase in the percentage of the soft orientations and show a deviation from the calculations using the conventional rule of mixtures. A higher percentage of the soft grains increases this deviation when the percentage is below 50%, and decreases it over 50%. The ultimate strengths, however, do not show an obvious dependence on the percentage of the soft orientations. Subsequently, the corresponding mechanisms are discussed and a modified rule of mixtures was developed. Bimodal texture components stimulate the activation of {10-12} twin variants with a low Schmid factor and the presence of more variants within one grain. Conventional strain accommodation rule was used to analyze this twinning behavior, but it cannot explain the experimental observations. Crystal-plasticity based finite element modeling was employed to analyze this strange twinning behavior. The results indicate that the interaction among twin variants, rather than deformation in neighboring grains, plays a major role in the activation of twins with a low Schmid factor, due to a hard deformation propagation from the soft grains to the hard grains. This finding provides an important complementation to the strain accommodation theory.

ACTA

A novel method for measurement of atomic interdiffusivity is applied to amorphous Cu-Zr close to its glass-transition temperature Tg. Sputter-deposited multilayers are examined in cross-section by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Mapping the evolution of composition profiles gives the interdiffusivity, which is orders of magnitude higher than if coupled to the viscosity expected near Tg. Kirkendall drift of interlayer interfaces in both amorphous and supercooled liquid states (i.e. below and above Tg), and associated voiding in the liquid, show that the diffusivity of copper greatly exceeds that of zirconium. Amorphous Cu-Zr is known to show maxima in atomic packing density at sharply defined compositions. The comparison of the two compositions in the present work provides the first direct evidence that denser packing is associated with lower atomic interdiffusivity. The lower interdiffusivity is governed by a lower diffusivity of copper, and reflects a lessened degree of decoupling of the copper (fast) and zirconium (slow) diffusivities in an efficiently packed glass. The new insights help to understand issues ranging from glass-forming ability to the controlled generation of nanovoided structures.

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ACTA

At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.

ACTA

向上滑动阅览英文摘要

We studied the microstructural stability and high-temperature mechanical properties of γ/γ’  Co-Ti-Mo based alloys of varying compositions (Co-12Ti-4Mo, Co-12Ti-4Mo-2Al, Co-12Ti-4Mo-4Al, Co-10Ti-4Mo-2Al, and Co-8Ti-4Mo-4Al (at.%)) using electron microscopy and density functional theory. The Co-12Ti-4Mo alloy underwent discontinuous precipitation into FCC (γ) and D019 (χ) phases after aging at 800°C for 500 h while the Co-12Ti-4Mo-4Al alloy maintained the γ/γ’ microstructure. Al additions improved the stability of the γ/γ’ microstructure by forming L21 (β’) precipitates along the grain boundaries, reducing the supersaturation of the γ matrix and the FCC/L12 interface energy. The Co-12Ti-4Mo-4Al alloy showed lower yield strength compared to the Al-free alloy, however, the difference in yield strength between these two alloys decreased with increasing temperature. This decrease is counterbalanced by a substantially enhanced specific yield strength at elevated temperatures as compared to other well-known Co-based alloys such as MarM302 and Co-9Al-9W.

Vol. 214，1 Aug. 2021, 116960