基于第一性原理计算Rh含量对Ir-Rh合金力学性能的影响∗

摘要

Platinum metal Ir-Rh alloy presents a promising candidate as future ultra-high-temperature gas turbine material due to its excellent high-temperature properties. In this paper, the mechanical properties of Ir-xRh (x=0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100) alloys with different rhodium content are investigated. Self-consistent, periodic, density functional theory calculations, Perdew-Burke-Ernzerhof functional, virtual crystal approximation are employed to calculate the elastic constants C11, C12, C44, Cauchy pressure (C12-C44), Young modulus E, shear modulus G, bulk modulus B and the ratio G/B, anisotropic factor A, and strain energy of dislocation per unit length. These parameters are adopted to characterize and assess the effect of Rh content on the mechanical property of Ir-Rh alloy. The results indicate that it is reasonable to use the virtual crystal approximation to calculate the mechanical properties of Ir-Rh alloys. The Young modulus E, shear modulus G and bulk modulus B increase rapidly with the increase of rhodium content, and the maximum value is reached at rhodium content 10%. Then it fast dereases down to a minimum value at 40% after the slowly rises and then slowly drops down. It is found to be in remarkable agreement with the strain energy of dislocation per unit length. This indirectly explains its changing trend. The Cauchy pressure (C12-C44), G/B value and the Poisson’s ratio reflect the change of the brittleness of the alloy. Therefore, we can come to a conclusion: the addition of Rh can cause the brittleness of the Ir-Rh alloys. The value of the brittleness first increases and then decreases with the increase of Rh content, and its maximum value is reached at 50%. The charge densities and the densities of states of pure Ir, Ir-10Rh, Ir-50Rh and pure Rh are calculated and compared. At the same time, we also establish a 2 × 2 × 1 solid solution supercell structure of Ir-Rh alloy and calculate its differential charge density. The results show that in the Ir-Rh alloys exists a“pseudo covalent bond”, which leads to the abnormal mechanical properties. The“pseudo covalent bond”is not a metal bond nor a covalent bond but a kind of transition bond or a mixed type. Finally, the experimental results show that the calculation method is reasonable and it can play an important role in understanding the microscopic mechanism of the abnormal mechanical properties of Ir-Rh alloys.%采用密度泛函理论的第一性原理平面波赝势方法,基于虚拟晶体势函数近似,通过计算Ir-xRh (x=0,5,10,20,30,40,50,60,70,80,90,100)合金的弹性常数、体积模量、剪切模量、杨氏模量、泊松比等力学常数、电荷密度和能态密度,研究Rh含量对Ir-Rh合金力学性能的影响。通过实验对计算结果加以验证,证明将虚晶近似法运用在Ir-Rh合金力学性能的计算中是合理的。研究结果表明, Ir-Rh合金的强度和硬度随Rh含量的升高迅速增大,在Ir-10 Rh 处达到最大值后快速下降到Ir-40 Rh处后先缓慢上升再缓慢下降。 Rh的添加会引起材料脆化,其脆性大小随着Rh含量的升高先增大后减小,在Ir-50 Rh处达到最大值。另外,计算了纯Ir, Ir-10 Rh, Ir-50 Rh、纯Rh的电荷密度、Ir和Rh的差分电荷密度以及能态密度,结果表明在铱铑合金中存在一种“伪共价键”,从而导致其力学性质的异常。