高熵合金的磁性、相变机理及力学性能的理论研究

Due to unique microstructures and special performances, such as high strength, good wear, corrosion resistance, and thermophysical properties, high-entropy alloys (HEAs) have attracted increasing attention in experiments. The traditional alloy theory is used to predict the phase formation of HEAs, whereas theoretical studies on the fundamentals are particularly lacking. In calculations of the electronic structure of HEAs, the large supercell has to be constructed in the first-principles method based on Bloch theorem. Moreover the random chemical compositions do not be satisfied. Employing ab initio alloy method, coherent potential approximations (CPA) in combination with the Exact Muffin-tin Orbitals (EMTO), we can calculate the electronic structure of HEAs to investigate the structural and mechanical properties for the 3d-transition metal and refractory-metal HEAs, and compare our results with the available experiments. First the magnetic properties of HEAs composed of 3d transitional elements are investigated. Via the total energy of HEAs including magnetic entropy and vibrational entropy, we study the solid-solution phase and the fcc-bcc phase transition induced by the increased Al content. Second we use the elastic mechanics to calculate the elastic constants of cubic phase, further to get the polycrystalline elastic moduli, including bulk modulus, Young's modulus, shear modulus and ductility as well as the anisotropy and the ideal strength for two-type HEAs. Third we investigate the effect of the ordered element distribution on the magnetic property and mechanical performance. Forth using the Chen-Möbius inversion method, we extract the interlayer potentials along the special direction in HEAs and use the inversed potentials to calculate the common stacking fault energy, the interaction between different stacking faults, interface energy, and surface energy. The calculations derived from the quantum theorem are helpful to accelerate the predication of the low stacking fault energy induced high-ductility HEAs that naturally exhibit high mechanical strength.

高熵合金(HEAs)具有独特的微结构、高强度、耐磨性、耐腐蚀、热物理等性能，已得到广泛的实验研究。虽然人们利用传统合金理论对HEAs的形成特征进行归类总结，以揭示其形成规律，但HEAs的基础理论研究还相当缺乏，特别是电子结构。基于Bloch定理的第一性原理需要构建较大的超胞，且不能达到任意摩尔比。采用相干势近似(CPA)结合精确饼模轨道(EMTO)方法，从电子结构出发，研究3d过渡金属和难熔金属两类HEAs，观察3d-HEAs的磁性；研究固溶体结构，揭示铝掺杂引起3d-HEAs的fcc-bcc相变机理；分析力学性能，包括弹性常数、多晶弹性模量、塑性、各向异性及理想强度等；研究合金元素分布有序性对磁性和力学性能的影响。结合Chen-Möbius反演方法，发展层间相互作用势，计算HEAs的各种层错能、层错间相互作用、界面能及表面能等面缺陷能。探索低层错能所诱导的良好塑性和具有高强度的高熵合金。

突破传统合金设计理念，高熵合金的研究为合金的发展提供了更广阔的空间。自2004年高熵合金被提出以来，就备受材料科学和凝聚态物理领域的研究人员关注。10余年间，高熵合金的实验研究出现了井喷式发展，而在理论方面，特别是源于量子力学的计算模拟研究相当有限。其主要原因是基于Bloch定理的第一性原理方法并不能用于处理无序合金。常规的第一性原理方法在处理复杂合金时，需要构建占用大量计算资源的较大超胞，且在化学成分上也不能达到任意摩尔比，如等摩尔比五元高熵合金不能通过超胞实现。虽可以通过特殊结构方法，即special quasi-random structure (SQS) 来模拟固溶体，但目前此方法主要应用于二元或三元合金，对于多组元合金的关联函数难以匹配，且对于顺磁态的计算较为困难，因此，关于高熵合金的电子结构、磁性、微结构及力学性能的报道有限。.从第一性原理的精确饼模轨道结合相干势近似 (EMTO-CPA) 计算高熵合金的电子结构出发，结合弹性力学理论和Chen-Möbius 反演方法，针对高熵合金特别关注的磁性、微结构、fcc--bcc 相变、力学性能及与材料中特殊面簇相关的层错能等方面，开展对3d和refractory-两类HEAs的具体研究。同时，基于状态方程和弹性参数，采用准简谐近似的方法，计算并建立了二类典型高熵合金的热力学性质及热学参量，并与实验进行了对比，对计算结果的系统性偏差进行了修正并建立了实用的模型。.通过对大量高熵合金的实验结果收集及与实验课题组的深入讨论和分析，总结了目前实验上已经报道的具有单相高熵合金的基本特征并结合第一性原理计算预测了具有良好内禀塑性的高熵合金。针对3d和refractory-HEAs，使用第一性原理的EMTO-CPA计算方法，确定了典型高熵合金的计算参数，特别是3d-HEAs在不同磁态下的计算参数设置。与实验测量对比表明，计算和预测的弹性模量相当可靠。进而，进行了相稳定性、弹性力学和热力学参量等物性的计算。.从电子结构层次为3d-HEAs的宏观磁性提供理论指导；澄清了近程效应对弹性参数的影响；确定实验上不易准确测量单晶高熵合金的力学参数；结合设计具有高强度且良好内禀塑性的高熵合金。