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Due to the vast compositional space of multi-principal element alloys (MPEAs), the rational design of MPEAs for optimized microstructures is difficult. Therefore, a high-throughput first-principles study of Mo-V-Nb-Ti-Zr, a refractory MPEA, was conducted to gain insights into the underlying microstructures. Using Monte-Carlo simulations powered by cluster expansion, we uncover the principles governing the MPEA’s microstructures across a large compositional space that includes non-equiatomic compositions and encompasses the constituent binaries, ternaries, and quaternaries. In the spirit of Hume-Rothery rules for complete solid solubility, we present a quantitative expression for predicting solid solution formation from the composition. Within a consistent framework, our results reproduce the microstructural observations (solid solution vs. segregation) from numerous experiments and provide microstructural predictions for unexplored regions in the compositional space. Our work illuminates the MPEA’s microstructures in terms of the separation and clustering tendencies of the elements, presenting a set of simple but powerful design principles for future experiments to rationally design MPEAs with the desired microstructures for superior mechanical properties.

ACTA

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laserprocessed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy.

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High Entropy Alloys (HEA) attract attention as possible radiation resistant materials, a feature observed in some experiments that has been attributed to several unique properties of HEA, in particular to the disorder-induced reduced thermal conductivity and to the peculiar defect properties originating from the chemical complexity. To explore the origin of such behavior we study the early stages (less than 0.1 ns), of radiation damage response of a HEA using molecular dynamics simulations of collision cascades induced by primary knock-on atoms (PKA) with 10, 20 and 40 keV, at room temperature, on an idealized model equiatomic quinary fcc FeNiCrCoCu alloy, the corresponding “Average Atom” (AA) material, and on pure Ni. We include accurate corrections to describe short-range atomic interactions during the cascade. In all cases the average number of defects in the HEA is lower than for pure Ni, which has been previously used to help claiming that HEA is radiation resistant. However, simulated defect evolution during primary damage, including the number of surviving Frenkel Pairs, and the defect cluster size distributions are nearly the same in all cases, within our statistical uncertainty. The number of surviving FP in the alloy is predicted fairly well by analytical models of defect production in pure materials. All of this indicates that the origin of radiation resistance in HEAs as observed in experiments may not be related to a reduction in primary damage due to chemical disorder, but is probably caused by longer-time defect evolution.

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向上滑动阅览英文摘要

Strengthening from single lattice defect such as dislocations or nanoprecipitates generally leads to the so called strength-ductility tradeoff, which becomes particularly pronounced at the strength level of above 2 GPa. Herein, we report a sustainable strain-hardening mechanism in ultrahigh-strength martensitic steels via manipulating interaction among different lattice defects. We show that fast precipitation of low-misfit B2-ordered Ni(Al, Fe) could efficiently prevent dense quench-in dislocations from recovery. During plastic deformation, the high cutting stress created by the ordered nanoprecipitates not only allows numerous retained dislocations to become mobile in planar mode, but also substantially expands the mean free path for dislocation movement in a heavily dislocated martensite. Simultaneously, the planar slips cause severe dislocation reactions with the pre-existing dislocations, which timely recover local cutting stress that has been weakened by cutting of the precipitates. This sort of timely established cutting stress minimizes simultaneously degree of slip concentration and magnitude of stored co-planar dislocations within planar slip bands while promoting pronounced band refinement as the main strain hardening mechanism, which gave rise to the simultaneous increment of the yield strength (2 GPa) and elongation to failure (9%). The current findings provide a possible means of simultaneously enhancing strength and ductility through tailoring the interplay among different types of lattice defects.

Vol. 213，1 Jul. 2021, 116969

Vol. 213，1 Jul. 2021, 116924