Theoretically probing intrinsic properties of γ′ phase in FeCoNiTiAl high-entropy alloys

Abstract

The intrinsic properties of the γ′ phase are well known to be of critical importance for the targeted control of the mechanical performance of γ/γ′ high entropy alloys (HEAs). In the present work, a composition tuning strategy is employed to modulate the thermal stability, elastic properties, and deformation mechanisms of the γ′ phase in (FeCoNi)₈₆Ti₇Al₇ HEAs using ab initio methods. Prior to tailoring the alloying elements, the temperature-dependent stability of the γ′ phase is meticulously investigated by considering both enthalpic and entropic contributions. The findings reveal that the primary vibrational entropy can be effectively substituted by an empirical parameter (δ) to expedite the design of stable HEAs. Subsequently, based on the individual effects of elements on the order–disorder transformation temperatures (T_od) and practical considerations for high-temperature applications, eight substituting elements (Nb, Mo, Ta, W, V, Cr, Mn and Cu) are judiciously selected from the 3d, 4d and 5d transition metal series. The results indicate that Nb and Ta are the most ideal substituting elements for the γ′ phase, as they concurrently enhance the T_od, shear modulus, hardness, ductility, and antiphase boundary energy. These insights open a promising avenue for the innovative design of strong-yet-ductile γ/γ′ HEAs.

γ/γ′型高熵合金的力学性能严重依赖于γ′析出沉淀相的本征性能, 包括高温热稳定性, 弹性性能以及独特的变形机理。利用第一性原理计算方法, 本文对 (FeCoNi)₈₆Ti₇Al₇ 高熵合金中γ′相的本征性能及其合金化策略进行了深入的探索。研究表明, γ′相的热稳定性取决于熵-焓的竞争: 随着温度的升高, 其晶体结构由低温下焓稳定的L1₂结构逐渐转变为高温下熵稳定的面心立方结构。其中, 熵贡献中最重要的振动熵与描述晶格畸变的经验参数δ具有粗略的正相关关系。基于这种正相关和熵-焓竞争关系, γ′相的有序无序转变温度 (T_od) 可快速地被预测, 极大地节省了计算成本。通过同时考虑T_od, 密度和熔点等实际高温应用的需求, 最终从元素周期表3d, 4d, 5d的过渡族元素中选出了Nb, Mo, Ta, W, V, Cr, Mn和 Cu八种潜在的有利元素, 并对其在γ′相弹性性能和变形机理上的影响进行了对比。结果表明, Ta和Nb是最理想的两种合金化元素, 可同时提高γ′相的高温热稳定性, 模量, 硬度, 塑性和层错能。这些见解将为开发强度塑性兼备的新型γ/γ′型高熵合金提供有效的指导。