W_(1-x)Ir_(x)固溶合金几何结构、电子结构、力学和热力学性能的第一性原理计算

A First Principles Investigation of W_(1-x)Ir_(x) Alloys:Structural,Electronic,Mechanical,and Thermal Properties

利用基于密度泛函理论的第一性原理方法结合声子谱计算和价键作用分析,研究了Ir含量对W_(1-x)Ir_(x)(x=0~12.96,原子分数,%)合金几何结构、相稳定性、力学性能以及热力学稳定性的影响,构建了Ir的添加量与WIr合金的相稳定性、力学以及热力学性能的变化关系。发现W-Ir合金的相稳定性随着Ir含量的增加而逐渐降低,这与W—Ir价键中部分反键态占据Fermi能级以下区域有关;Ir在W中的添加量小于7.4%时,W-Ir合金满足基态相稳定性要求;随着温度的升高以及Ir含量的增加,W-Ir合金的热力学稳定性得到提升,表明Ir适合添加到高温条件下应用的W中;Ir的加入能降低剪切模量,改善钨合金韧性,与实验观察一致,但Ir也能提升平面抗剪切变形能力。本征脆性的Ir对W的强韧化作用与W—Ir原子键合时轨道电子的转移和重叠方式有关。

Tungsten(W)possess comprehensive physical and chemical properties that are suitable for aerospace and space nuclear power applications,including the highest melting temperature(3410o C)among metals,high elastic modulus,thermal shock resistance,and high temperature strength.However,its poor ductility at room temperatures significantly hinders its fabricability and potential use in the abovementioned fields.Accordingly,to improve the ductility of W,solid solution strengthening is the primary method considered besides grain refining and deformation strengthening.Experimental studies have shown that Ir is a brittle metal with an fcc structure,but it can greatly improve the ductility of W;however,the corresponding mechanism is still unclear.Thus,using the first principles method based on density functional theory together with phonon spectrum calculations,the effect of the addition of different contents of Ir on the structure,phase stability,mechanical properties,and thermodynamic properties of W were studied.The relation between the addition of different contents of Ir and above-mentioned proper ties of W-Ir alloys were theoretically investigated.It was found that Ir can induce instability in the W-Ir alloy in the ground state due to the occupation of its antibonding electrons below the Fermi level.When content of Ir added is less than 7.4%,the formation of the W-Ir alloy becomes stable in the ground state.With an increase in temperature and the content of Ir,the thermodynamic stability is improved,implying that Ir is suitable for incorporation with W for application at high temperature.The addition of Ir helps to improve the toughness of the W alloy,which is consistent with the experimental observation.Besides,Ir can simultaneously improve the planar shear resistance.Furthermore,the pCOHP analysis revealed that the inherent mechanism of the ductile effect of brittle Ir in W is attributed to their different modes of electron transition and overlapping.For Ir,electrons transfer from its higher energy orbital of d_(x2-y2)to the lower energy dxz and d_(yz)orbitals.In contrast,for W,the electrons transfer from its low energy orbital of dz 2 to the d_(xz)and d_(yz)orbitals.The d_(xz)and dyz orbitals of Ir and W form a metallic bond,which is further enhanced with an increase in the content of Ir added.Therefore,Ir acts as a toughness-enhancing element in W-Ir alloys.