采用第一原理赝势平面波方法,基于虚拟晶体势函数近似(VCA),计算Fe合金化(浓度x<3.0%, 原子分数, 下同)时完整与缺陷B_2-NiAl晶体的弹性性质,并采用弹性常数C_(44)、Cauchy压力参数(C_(12)-C_(44))、杨氏模量E、剪切模量G 及其与体模量B_0的比值 G/B_0等,表征和评判Fe合金化浓度x对NiAl金属间化合物延性与硬度的影响.结果表明:无论是无缺陷的理想NiAl晶体,还是含Ni空位或Ni反位的NiAl缺陷晶体,x<0.6%的Fe合金化均可使其硬度大幅提高.Fe合金化浓度低于0.5%时,虽然完整NiAl晶体的延性变差,但含Ni空位的缺陷NiAl晶体的延性却可明显改善,并以x = 0.2%~0.4%时韧化效果最好.Ni空位或Ni反位降低B_2-NiAl晶体的本征延性.实验中0.20%~0.25%的Fe合金化对NiAl晶体延性的改善很可能源于Fe原子与NiAl晶体中Ni空位间的关联与协同作用.
Using the first-principles pseudopotential plane-wave methods based on the density functional theory, the elastic constants of B_2-(Ni_(1-x)Fe_x)Al (x=0-3.0 at%) supercells with or without Ni vacancy or Ni anti-site defect were calculated in the framework of Virtual Crystal Approximation. Several parameters, such as elastic constant C_(44), Cauchy pressure (C_(12)-C_(44)), Young modulus E, the shear modulus G and their ratio G/B_0, have been adopted to characterize and assess the effect of Fe alloying concentration on the ductility and hardness of NiAl intermetallic compounds. It is found that Fe addition with x < 0.6 at% is proved to be efficient to enforce the strength or hardness of NiAl intermetallic compounds either for perfect crystals or for defect crystals. No improvement of the ductility of perfect B_2-NiAl crystals can be demonstrated as Fe is added in the range from 0 to 3.0 at%. The Ni vacancy or Ni anti-site defects make the intrinsic ductility of perfect B_2-NiAl crystals without Fe addition to be weakened. However, an obvious decrease in the degree of the embrittlement of B_2-(Ni_(1-x)Fe_x)Al crystals with Ni vacancies can be seen as Fe alloying concentration x is lower than 0.5 at%, and the optimum additions locate in the range from 0.2 at% to 0.4 at%. From these calculations, a deduction therefore is conduced. That is for the distinct increase in the elongation rate of a B_2-NiAl single crystal with Fe addition of 0.20-0.25 at% in the experiments could originate from the correlative and cooperative effects between vacancies and Fe addition in the B_2-NiAl crystal.
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