稀土高熵合金在压力、温度及剪切应力作用下相变及性能研究

High-entropy alloys (HEAs) beak the limitation of traditional alloy whose composition is based on one or two elements. HEAs are loosely defined as solid solution alloys that contain more than five principal elements in equal or near equal atomic percent (at.%). In regular (nonideal) solutions, the mixing enthalpy is nonzero ΔHmix ≠ 0, which leads to phase segregation (ΔHmix > 0) and the formation of intermetallic compounds (ΔHmix < 0). The most essential characteristics (random solid solution) are covered for its inhomogeneous structure. Rear earth high entropy alloy (Re-HEA) is developed in the past two years with hexagonal closed-packed (HCP) structure new HEA. The mutual mixing enthalpies for each pair are nearly zero. Meanwhile, their atomic radius and electronegativities are similar. All this suggests that the Re-HEA may be considered a prototype of an ideal HEA, offering the opportunity to address the question on the intrinsic properties and phase structure of HEA. In this project, new Re-HEA with HCP structure will be selected to investigate the phase transition and subsequent properties evolution under pressure, temperature, and shear stress. The major research techniques include high pressure in situ synchrotron radiation X-ray, high pressure torsion, and high pressure annealing, which will deepen the understanding of the HEA nature.

高熵合金突破了传统合金以一种或两种元素为主的限制，将五种或五种以上元素按照等摩尔比或接近等摩尔比制成具有固溶体结构的合金。然而由于实际研究的高熵合金中元素之间的混合焓不等于0（ΔHmix ≠ 0），使合金发生相分离（ΔHmix > 0）或者产生金属间化合物（ΔHmix < 0），结构变得不均匀，而掩盖了其最本质的无序固溶体特征。稀土高熵合金（Re-HEA）是近两年开发出的具有密排六方结构的新型高熵合金，各元素之间具有相近的原子半径和电负性，混合焓接近于0，具有形成均匀的无序固溶体的潜力。因此，Re-HEA是研究高熵合金相变与本征性能的理想的研究对象。本课题以Re-HEA作为研究对象，通过高压原位同步辐射X 射线衍射、高压退火、高压扭转等手段，研究Re-HEA在压力、温度和剪切应力，及其耦合作用下结构和性能的变化，加深对高熵合金结构与性能的本质认识。

本项目针对稀土高熵的高压相转变问题，主要以YGdDyHoTb和LaCePrNd等稀土高熵合金为研究对象，采用原位高压同步辐射等先进的检测手段和研究方法，研究了稀土高熵的相形成规律，以及高压下组织结构和性能关联。该研究包括稀土高熵合金结构随合金元素种类变化规律，研究表明轻稀土元素添加，使合金结构从hcp结构向dhcp转变。进行了稀土高熵合金等静压条件下的结构转变，以及电学性质（电阻）变化的研究。稀土高熵合金相变过程与其组成的稀土单质相变过程类似，YGdDyHoTb高熵合金高压下相变过程为：hcp→Sm-type→dhcp→dfcc，LaCePrNd高熵合金的相变过程为：dhcp→fcc→dfcc，相变压力也遵循“相加原理”，并且相变过程都是可逆的。压力和剪切应力，以及压力和温度共同作用下，稀土高熵合金性能得到提高。系统研究了稀土Gd元素对CoCrCuFeNi高熵合金结构和机械性能影响，稀土元素Gd有效调控面心立方CoCrCuFeNi高熵合金结构和性能。