Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
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DOI：
10.1016/j.actamat.2016.07.038
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发表时间：
2016-10-01
期刊：
影响因子：
9.4
通讯作者：
George, E. P.
中科院分区：
文献类型：
--
作者：
Laplanche, G.;Kostka, A.;George, E. P.
在低温(低至低温)下，CrMnFeCoNi高熵合金具有良好的强度、加工硬化率(WHR)、塑性和断裂韧性。为了更好地了解影响其力学性能的变形机制，在液氮和室温(77K和293K)下进行了拉伸试验，并在不同应变下进行了拉伸试验，以利用透射电子显微镜量化组织的演变。测定了位错密度、孪晶宽度、孪晶间距和体积分数。在77K和293K的真应变分别为7.4%和25%后，首次观察到纳米孪晶；在较低的应变下，变形是通过位错塑性发生的。产生孪生的拉应力为720+/-30 Mpa，与温度基本无关，由此推导出孪生的临界分辨切应力为235+/-10 Mpa。在位错塑性变形区域，在77K和293K下，归一化的WHR随应变的增加而减小，超过7.4%的真应变后，由于孪生的激活，WHR在77K时保持在较高的G/30值不变，逐渐在组织中引入新的界面。相反，室温下的WHR随着应变的增加而继续下降，因为孪生直到很久以后才被激活(接近断裂)。因此，与293K相比，77K时增强的强度-塑性组合主要是由于孪晶在变形过程中较早开始并提供了额外的加工硬化。与此一致的是，当拉伸试件在77K预应变以引入纳米孪晶，并随后在293K进行测试时，流动应力和延性都比未预应变的试件增加。(C)2016 Acta Materialia Inc.，由Elsevier Ltd.出版。
At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of similar to 7.4% at 77 K and similar to 25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 +/- 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 +/- 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond similar to 7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd.