Sc_(x)Y_(1–x)Fe_(2)合金固溶和V_(2x)Fe_(2)(1–x)Zr有序-无序转变的理论预测

Theoretical prediction of solution in ScxY_(1-x)Fe_(2)and order-disorder transitions in V_(2x)Fe_(2)(1-x)Zr

合金化是增加材料结构和性能多样性的重要手段.本文先从考虑最近邻相互作用的Ising模型出发,通过铁磁耦合研究二元合金的低温相分离、高温固溶体系,通过反铁磁耦合研究低温有序固溶、高温无序体系.以储氢合金中的Laves相V_(2x)Fe_(2)(1–x)Zr和Sc_(x)Y_(1–x)Fe_(2)材料为例,采用基于结构识别的高通量第一原理计算,考虑结构简并度对配分函数的贡献,可以对合金材料进行有限温度下的理论预测.先通过第一原理计算得到基态(零温下)形成能,形成能大于零的体系Sc_(x)Y_(1–x)Fe_(2)在低温下相分离,根据自由能符号确定合金固溶的临界温度;形成能小于零的体系V_(2x)Fe_(2)(1–x)Zr在低温下倾向于形成有序相,根据比热的计算可以确定体系出现有序-无序转变的临界温度.其中,高通量第一原理计算和对应的结构简并度统计可以通过我们课题组发布的程序SAGAR(structures of alloy generation and recognition)实现.

Alloying is an important way to increase the diversity of material structure and properties.In this paper,we start from Ising model considering nearest neighbor interaction,in which a ferromagnetic system corresponds to a low temperature phase separation and high temperature solid solution of binary alloy,while antiferromagnetic system corresponds to a low temperature ordered solid solution and a high temperature disorder.The high-throughput first-principles calculation based on the structure recognition is realized by the program SAGAR(structures of alloy generation and recognition)developed by our research group.By considering the contribution of structural degeneracy to the partition function,theoretical prediction of alloy materials can be carried out at finite temperature.Taking hydrogen storage alloy(Sc_(x)Y_(1-x)Fe_(2)and V_(2x)Fe_(2)(1–x)Zr)for example,the formation energy of ground state(at zero temperature)can be obtained by the first-principles calculations.It is found that the formation energy of Sc_(x)Y_(1-x)Fe_(2)is greater than zero,thereby inducing the phase separation at low temperature.The free energy will decrease with the temperature and concentration increasing,where the critical temperature of solid solution of alloy is determined according to the zero point of free energy.The formation energies of V_(2x)Fe_(2)(1–x)Zr are all lower than zero,and the ordered phase occurs at low temperature.The order-disorder transition temperature of V0.5Fe1.5Zr and V1.5Fe0.5Zr are both about 100 K,while the transition temperature of VFeZr is nearly 50 K.The calculation process will effectively improve the high throughput screening efficiency of alloy,and also provide relevant theoretical reference for experimental research.