Coalescence of Al_0.3CoCrFeNi polycrystalline high-entropy alloy in hot-pressed sintering: a molecular dynamics and phase-field study 

Qingwei Guo, Hua Hou, Kaile Wang, Muxi Li, Peter K. Liaw & Yuhong Zhao 

npj Computational Materials  9: 185 (2023). Published: 10 October 2023

编辑概述：重定义屈服点-实现Al_0.3CoCrFeNi高熵合金精准模拟

Al_0.3CoCrFeNi高熵合金虽拥有出色的拉伸强度和塑性，但与新型镍基合金及先进高强度钢相比优势不明显。研究人员正通过粉末冶金和火花等离子烧结技术来细化晶粒并均匀化微观结构，以增强合金的抗拉强度。然而，在模拟这一烧结过程中，如何精确考虑压力对合金粒子凝聚动力学的影响，仍是目前科研的一个前沿问题。中北大学材料科学与工程学院赵宇宏教授领导的团队，对屈服应力进行了新的定义：当应力-应变曲线与弹性区域平行并偏离0.2%的直线相交时，即为材料的屈服点。他们研究发现，热压烧结后的Al_0.3CoCrFeNi高熵合金的屈服强度（5.87 GPa）低于其理想状态下的屈服强度（6.15 GPa），对应的屈服点为0.03。此外，该合金热压烧结样本的最终拉伸强度和伸长率分别达到了10.79 GPa和0.073，而在理想状态下这两个指标分别为11.20 GPa和0.07。这种性能上的差异主要归因于热压烧结过程中位错数量的急剧增加，这些位错导致每个晶粒中的滑移带在靠近晶界处终止，从而提高了合金的强度。由于含有更多的六方最密堆积（HCP）结构，热压烧结样本相比于理想状态下展现了更佳的延展性。而在烧结后主要分布在晶界附近的体心立方（BCC）晶体结构在拉伸过程中阻碍了位错的移动，导致应力集中并引发了裂纹的形成。高密度的位错累积在晶界处，进一步促进了应力集中，使得理想状态下的样本容易从晶界处产生裂纹。研究团队采用晶体分析工具对热压烧结和理想状态下的Al_0.3CoCrFeNi高熵合金的HCP结构在拉伸前后进行了深入分析。这些HCP结构类型包括层错、孪晶、连续排列的三层和四层HCP原子。对于热压烧结样本，拉伸前后的层错原子数由30,112微增至30,218，相干孪晶的原子数从23,287增加到25,176，连续排列的三层HCP原子从9,274减少到6,607，而四层HCP原子从6,997增加到7,250。通过这些数据可以观察到，经过拉伸后，连续排列的三层HCP原子数量有所降低，而相干孪晶数量有所上升。因此，可以得出结论，热压烧结的Al_0.3CoCrFeNi高熵合金在拉伸前后HCP结构的含量基本保持不变。最优的烧结参数以及粉末颗粒的形态和尺寸对最终烧结样品的机械性能有着决定性的影响。因此，通过模拟技术寻求最佳烧结参数，对于有效构建和精确设计Al_0.3CoCrFeNi多晶高熵合金具有重要的指导意义。

Editorial Summary: Redefining Yield Stress——Precision Simulation Achieved for Al_0.3CoCrFeNi High-Entropy Alloy

Although the Al_0.3CoCrFeNi high-entropy alloy possesses notable tensile strength and ductility, its advantages are not significant when compared to new nickel-based alloys and advanced high-strength steels. Researchers are currently refining the grain size and homogenizing the microstructure through powder metallurgy and spark plasma sintering techniques to enhance the alloy's tensile strength. However, accurately considering the impact of pressure on the dynamic kinetics of alloy particle coalescence during the simulation of this sintering process remains a cutting-edge problem in current scientific research. A team lead by Prof. Yuhong Zhao from School of Materials Science and Engineering, North University of China, defined yield stress as the point of intersection between a straight line deviating 0.2% from parallel to the elastic region and the stress-strain curve. The as-sintered Al_0.3CoCrFeNi high-entropy alloy exhibits a lower yield strength (5.87 GPa) than its ideal state (6.15 GPa), with a corresponding yield point of 0.03. The ultimate tensile strength and elongation of the as-sintered sample (ideal state) are 10.79 GPa (11.20 GPa) and 0.073 (0.07), respectively. This discrepancy is attributed to the surge of dislocations during the hot-pressed sintering process, which prompts the slip band in each grain to terminate near the grain boundaries, thereby enhancing strength. The as-sintered samples exhibit improved elongation due to the higher content of HCP structures compared to the ideal state. The BCC crystal structure, which mainly exists near grain boundaries after sintering, acts as an obstacle hindering dislocation movement during stretching, leading to stress concentration and crack formation. The high density of dislocations at the grain boundary facilitates stress concentration, thereby causing crack initiation from the grain boundary in an ideal sample.

Using the Crystal Analysis Tool, the authors analyzed the types of HCP structures present in the as-sintered and ideal state Al_0.3CoCrFeNi high-entropy alloy before and after tension. These types include stacking faults, twins, three layers of HCP atoms in a continuous arrangement, and four layers of HCP atoms in a continuous arrangement. For the as-sintered Al_0.3CoCrFeNi high-entropy alloy, before tension, the number of atoms in stacking faults is 30,112, in coherent twins is 23,287, three layers of HCP atoms in a continuous arrangement is 9274, and four layers of HCP atoms in a continuous arrangement is 6997. After tension, the number of atoms in stacking faults is 30,218, in coherent twins is 25,176, three layers of HCP atoms in a continuous arrangement is 6607, and four layers of HCP atoms in a continuous arrangement is 7250. It can be observed that after tension, the number of three layers of HCP atoms in a continuous arrangement decreases while the number of coherent twins increases. Therefore, the content of the HCP structure in the as-sintered Al0.3CoCrFeNi high-entropy alloy remains unchanged before and after tension. Optimal sintering parameters and the morphology and size of powder particles significantly impact the mechanical properties of the final sintered samples. Therefore, obtaining optimal sintering parameters through simulation can provide new insights for efficiently and accurately designing high-performance Al_0.3CoCrFeNi polycrystalline high-entropy alloy.