ACTA

Plastic deformation in elemental BCC metals and dilute alloys is controlled by the slower of the kink pair nucleation and kink migration processes along screw dislocations. In alloys nucleation is facilitated and migration inhibited, leading to a concentration- and temperature-dependent transition from nucleation dominance to migration dominance. Here, an analytical statistical model for the stress-dependent kink migration barrier in dilute BCC alloys is developed and validated. The barrier depends only on a clearly-defined solute/screw dislocation interaction parameter, the kink width, and dislocation length between jogs. The analytic model is extensively validated via fully atomistic nudged-elastic band calculations and stochastic simulations in a model Fe-Si alloy. Combined with a recent validated double-kink nucleation theory, a fully-analytic model for the temperature- and concentration-dependent flow stress is obtained that includes the transition from nucleation to migration control. The overall model is applied to Fe-Si and W-Re using independently-determined material properties and good agreement is obtained with experiments over a range of concentrations and temperatures. Overall, the two theories represent a unified, fully-statistical, parameter-free understanding of screw dislocation strength in dilute BCC alloys.

ACTA

The emergence of multi-principal element alloys (MPEAs) holds great promise for the development of high performance metallic materials. However, it remains unclear whether MPEAs can provide previously unknown deformation mechanisms to drastically enhance their mechanical performance. Here we report a new deformation mechanism of mechanically-induced dual phase transformations from the face-centered cubic (FCC) to hexagonal close- packed (HCP) phase and then back to the FCC phase with nanotwins in a CrCoNi medium-entropy alloy (MEA). During the two sequential steps of phase transformation, continued shear occurs in the same 〈110〉FCC∥〈11-20〉HCPdirection along different {111}FCC∥(0001)HCP planes, producing a total shear transformation strain up to 70%. The dual phase transformations stem from a unique capability of facile slip in between the close-packed {111}FCC∥(0001)HCP atomic layers in both FCC and HCP phases, leading to flexible stacking sequences of those close-packed layers with low stacking fault energies. Our work demonstrates that MPEAs can offer unconventional deformation mechanisms such as dual phase transformations in the CrCoNi MEA, thereby opening opportunities for enhancing the mechanical properties of advanced alloys.

ACTA

ACTA

ACTA

Predicting the failure of crystalline materials at high strain rates requires knowledge of the underlying failure mechanisms and their dependence on microstructure. In this study, a 3D-EBSD characterization experiment is performed on high-purity tantalum prior to and after partial spallation by plate impact, which allows for the statistical assessment of the microstructural neighborhoods surrounding incipient voids. In analyzing the resulting dataset containing 5884 grains and 467 voids, it is observed that the voids were roughly spherical and consistent in size throughout the spalled material. The voids are most likely to reside at quadruple points, at triple junctions, at grain boundaries, and within grains, in decreasing order of prevalence. Moreover, voids tend to form at grain boundaries with high degrees of plastic incompatibility, growing into the plastically soft grain but orienting primarily with or perpendicular to the loading direction. The statistics from these analyses of 3D microstructural data support dynamic cavitation models for ductile spallation.

ACTA

向上滑动阅览英文摘要

Although the deformation behavior of high-entropy alloys (HEAs) has been extensively studied at the macroscale, many important properties have yet to be explored for these alloys at the microscale, thus hampering accurate prediction of damage and failure processes. Synchrotron high-energy diffraction microscopy (HEDM) and fast-Fourier transform-based crystal plasticity modeling was conducted to investigate the three-dimensional (3D) grain-resolved micromechanical response for approximately 1,900 constituent grains within a single-phase FCC HEA up to 1% applied strain. The evolution of grain-resolved elastic strains, lattice reorientations, and maximum resolved shear stresses (mRSS) were evaluated to quantify elastic, yield, and fully plastic behavior. Overall, the initial critical resolved shear stress (CRSS), determined via in situ HEDM and companion modeling, was found to be > 20% higher than estimated using the classical polycrystalline Taylor factor (M = 3.06). However, a descriptive parameter based on the average grain-resolved Taylor factor (M¯) was found to show excellent agreement with plastic yielding behavior observed within HEDM datasets. Noticeable deviations in HEDM lattice reorientations compared to both EVP-FFT simulations and classical predictions for FCC polycrystals were discovered, highlighting the complexity in correlating local lattice reorientations, Taylor, and Schmid factors with plastic response for this material at the grain-scale. Therefore, it is anticipated that the overall trends and parameter identification of 3D grain-resolved properties in this study can serve as an important foundation for continued mesoscale investigation on both well-established and newly developed Cantor-like HEAs.

Vol. 215，15 Aug. 2021, 117118

Vol. 215，15 Aug. 2021, 117131