Al_0.3CoCrFeNi高熵合金雖擁有出色的拉伸強度和塑性，但與新型鎳基合金及先進高強度鋼相比優勢不明顯。研究人員正通過粉末冶金和火花等離子燒結技術來細化晶粒并均勻化微觀結構，以增強合金的抗拉強度。

Fig. 1 The distribution of nanoparticles and the change of particle boundaries when the temperature increases from 300 K to 1116 K.

然而，在模擬這一燒結過程中，如何精確考慮壓力對合金粒子凝聚動力學的影響，仍是目前科研的一個前沿問題。中北大學材料科學與工程學院趙宇宏教授領導的團隊，對屈服應力進行了新的定義：當應力–應變曲線與彈性區域平行并偏離0.2%的直線相交時，即為材料的屈服點。

他們研究發現，熱壓燒結后的Al_0.3CoCrFeNi高熵合金的屈服強度（5.87 GPa）低于其理想狀態下的屈服強度（6.15 GPa），對應的屈服點為0.03。此外，該合金熱壓燒結樣本的最終拉伸強度和伸長率分別達到了10.79 GPa和0.073，而在理想狀態下這兩個指標分別為11.20 GPa和0.07。

這種性能上的差異主要歸因于熱壓燒結過程中位錯數量的急劇增加，這些位錯導致每個晶粒中的滑移帶在靠近晶界處終止，從而提高了合金的強度。由于含有更多的六方最密堆積（HCP）結構，熱壓燒結樣本相比于理想狀態下展現了更佳的延展性。

而在燒結后主要分布在晶界附近的體心立方（BCC）晶體結構在拉伸過程中阻礙了位錯的移動，導致應力集中并引發了裂紋的形成。高密度的位錯累積在晶界處，進一步促進了應力集中，使得理想狀態下的樣本容易從晶界處產生裂紋。

研究團隊采用晶體分析工具對熱壓燒結和理想狀態下的Al_0.3CoCrFeNi高熵合金的HCP結構在拉伸前后進行了深入分析。這些HCP結構類型包括層錯、孿晶、連續排列的三層和四層HCP原子。

Fig. 6 Microstructure evolution and dislocation evolution during the first sintering stage.

對于熱壓燒結樣本，拉伸前后的層錯原子數由30,112微增至30,218，相干孿晶的原子數從23,287增加到25,176，連續排列的三層HCP原子從9,274減少到6,607，而四層HCP原子從6,997增加到7,250。

通過這些數據可以觀察到，經過拉伸后，連續排列的三層HCP原子數量有所降低，而相干孿晶數量有所上升。因此，可以得出結論，熱壓燒結的Al_0.3CoCrFeNi高熵合金在拉伸前后HCP結構的含量基本保持不變。

最優的燒結參數以及粉末顆粒的形態和尺寸對最終燒結樣品的機械性能有著決定性的影響。

因此，通過模擬技術尋求最佳燒結參數，對于有效構建和精確設計Al_0.3CoCrFeNi多晶高熵合金具有重要的指導意義。該文近期發表于npj Computational Materials 9: 185 (2023).

Editorial Summary

Redefining yield stress: Precision simulation achieved for ?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.?This article was recently published in npj Computational Materials 9: 185 (2023).

原文Abstract及其翻譯

Coalescence of Al_0.3CoCrFeNi polycrystalline high-entropy alloy in hot-pressed sintering: a molecular dynamics and phase-field study (熱壓燒結過程中Al_0.3CoCrFeNi多晶高熵合金的凝聚：分子動力學與相場研究)

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

Abstract?Existing hot sintering models based on molecular dynamics focus on single-crystal alloys. This work proposes a new multiparticle model based on molecular dynamics to investigate coalescence kinetics during the hot-pressed sintering of a polycrystalline Al_0.3CoCrFeNi high-entropy alloy. The accuracy and effectiveness of the multiparticle model are verified by a phase-field model. Using this model, it is found that when the particle contact zones undergo pressure-induced evolution into exponential power creep zones, the occurrences of phenomena, such as necking, pore formation/filling, dislocation accumulation/decomposition, and particle rotation/rearrangement are accelerated. Based on tensile test results, Young’s modulus of the as-sintered Al_0.3CoCrFeNi high-entropy alloy is calculated to be 214.11?±?1.03?GPa, which deviates only 0.82% from the experimental value, thus further validating the feasibility and accuracy of the multiparticle model.

摘要 現有基于分子動力學的熱壓燒結模型主要針對的是單一的晶體合金。而本項研究創新性地提出了一個新型的多粒子模型，該模型同樣基于分子動力學，旨在深入探究多晶Al_0.3CoCrFeNi高熵合金在熱壓燒結過程中凝聚動力學的行為。該模型的準確性與效果已通過相場模型得到了驗證。利用該多粒子模型，我們觀察到在粒子接觸區域受到壓力作用并演變成指數型蠕變區的過程中，一系列現象——包括頸縮、孔隙的形成與填補、位錯的積累與消解以及粒子的旋轉和重排——都顯著加速了。此外，通過拉伸測試得出的數據表明，熱壓燒結處理后的Al_0.3CoCrFeNi高熵合金的楊氏模量為214.11 ± 1.03 GPa。這一計算結果與實驗值相比，偏差僅為0.82%，從而進一步證實了我們提出的多粒子模型在預測合金性能方面的可靠性與高精確度。