Al_0.3CoCrFeNi高熵合金雖擁有出色的拉伸強(qiáng)度和塑性，但與新型鎳基合金及先進(jìn)高強(qiáng)度鋼相比優(yōu)勢(shì)不明顯。研究人員正通過粉末冶金和火花等離子燒結(jié)技術(shù)來細(xì)化晶粒并均勻化微觀結(jié)構(gòu)，以增強(qiáng)合金的抗拉強(qiáng)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(jié)過程中，如何精確考慮壓力對(duì)合金粒子凝聚動(dòng)力學(xué)的影響，仍是目前科研的一個(gè)前沿問題。中北大學(xué)材料科學(xué)與工程學(xué)院趙宇宏教授領(lǐng)導(dǎo)的團(tuán)隊(duì)，對(duì)屈服應(yīng)力進(jìn)行了新的定義：當(dāng)應(yīng)力–應(yīng)變曲線與彈性區(qū)域平行并偏離0.2%的直線相交時(shí)，即為材料的屈服點(diǎn)。

他們研究發(fā)現(xiàn)，熱壓燒結(jié)后的Al_0.3CoCrFeNi高熵合金的屈服強(qiáng)度（5.87 GPa）低于其理想狀態(tài)下的屈服強(qiáng)度（6.15 GPa），對(duì)應(yīng)的屈服點(diǎn)為0.03。此外，該合金熱壓燒結(jié)樣本的最終拉伸強(qiáng)度和伸長(zhǎng)率分別達(dá)到了10.79 GPa和0.073，而在理想狀態(tài)下這兩個(gè)指標(biāo)分別為11.20 GPa和0.07。

這種性能上的差異主要?dú)w因于熱壓燒結(jié)過程中位錯(cuò)數(shù)量的急劇增加，這些位錯(cuò)導(dǎo)致每個(gè)晶粒中的滑移帶在靠近晶界處終止，從而提高了合金的強(qiáng)度。由于含有更多的六方最密堆積（HCP）結(jié)構(gòu)，熱壓燒結(jié)樣本相比于理想狀態(tài)下展現(xiàn)了更佳的延展性。

而在燒結(jié)后主要分布在晶界附近的體心立方（BCC）晶體結(jié)構(gòu)在拉伸過程中阻礙了位錯(cuò)的移動(dòng)，導(dǎo)致應(yīng)力集中并引發(fā)了裂紋的形成。高密度的位錯(cuò)累積在晶界處，進(jìn)一步促進(jìn)了應(yīng)力集中，使得理想狀態(tài)下的樣本容易從晶界處產(chǎn)生裂紋。

研究團(tuán)隊(duì)采用晶體分析工具對(duì)熱壓燒結(jié)和理想狀態(tài)下的Al_0.3CoCrFeNi高熵合金的HCP結(jié)構(gòu)在拉伸前后進(jìn)行了深入分析。這些HCP結(jié)構(gòu)類型包括層錯(cuò)、孿晶、連續(xù)排列的三層和四層HCP原子。

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

對(duì)于熱壓燒結(jié)樣本，拉伸前后的層錯(cuò)原子數(shù)由30,112微增至30,218，相干孿晶的原子數(shù)從23,287增加到25,176，連續(xù)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(shù)據(jù)可以觀察到，經(jīng)過拉伸后，連續(xù)排列的三層HCP原子數(shù)量有所降低，而相干孿晶數(shù)量有所上升。因此，可以得出結(jié)論，熱壓燒結(jié)的Al_0.3CoCrFeNi高熵合金在拉伸前后HCP結(jié)構(gòu)的含量基本保持不變。

最優(yōu)的燒結(jié)參數(shù)以及粉末顆粒的形態(tài)和尺寸對(duì)最終燒結(jié)樣品的機(jī)械性能有著決定性的影響。

因此，通過模擬技術(shù)尋求最佳燒結(jié)參數(shù)，對(duì)于有效構(gòu)建和精確設(shè)計(jì)Al_0.3CoCrFeNi多晶高熵合金具有重要的指導(dǎo)意義。該文近期發(fā)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(jié)過程中Al_0.3CoCrFeNi多晶高熵合金的凝聚：分子動(dòng)力學(xué)與相場(chǎng)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(xiàn)有基于分子動(dòng)力學(xué)的熱壓燒結(jié)模型主要針對(duì)的是單一的晶體合金。而本項(xiàng)研究創(chuàng)新性地提出了一個(gè)新型的多粒子模型，該模型同樣基于分子動(dòng)力學(xué)，旨在深入探究多晶Al_0.3CoCrFeNi高熵合金在熱壓燒結(jié)過程中凝聚動(dòng)力學(xué)的行為。該模型的準(zhǔn)確性與效果已通過相場(chǎng)模型得到了驗(yàn)證。利用該多粒子模型，我們觀察到在粒子接觸區(qū)域受到壓力作用并演變成指數(shù)型蠕變區(qū)的過程中，一系列現(xiàn)象——包括頸縮、孔隙的形成與填補(bǔ)、位錯(cuò)的積累與消解以及粒子的旋轉(zhuǎn)和重排——都顯著加速了。此外，通過拉伸測(cè)試得出的數(shù)據(jù)表明，熱壓燒結(jié)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(jì)算結(jié)果與實(shí)驗(yàn)值相比，偏差僅為0.82%，從而進(jìn)一步證實(shí)了我們提出的多粒子模型在預(yù)測(cè)合金性能方面的可靠性與高精確度。