Microscopic phase-field simulation of the structure transformation from DO₂₂ to L1₀(M=1) phase in Ni₆₄Al₂₁V₁₅ alloy

The structure transformation from DO₂₂ to L1₀ (M=1) phase in Ni₆₄Al₂₁V₁₅ alloy at 1150 K was studied by the microscopic phase-field kinetic model. The results show that L1₀ (M=1) structure precipitates at interphase during the middle stage of the aging process with three types of the formation mechanism. Firstly, the L1₀ (M=1) structure which aligns along [001] direction and nucleates at the interphase of L1₂ and DO₂₂ grows towards DO₂₂ phase. Secondly, the L1₀ (M=1) structure which aligns along [001] direction nucleates at 90° ordered domain boundary of DO₂₂ phase and expands to the [001] direction of DO₂₂ phase. Thirdly, the L1₀ (M=1) structure which aligns along [100] direction nucleates at the interphase of two [100] aligned DO₂₂ structure, and expands towards DO₂₂ phase. The phase transformation from DO₂₂ structure to L1₀ (M=1) structure is accomplished by atomic diffusion. In a perfect DO₂₂ phase, the β site is mainly occupied by V atoms, while the α₂ and α₃ sites by Ni atoms and the α₁ site by Al atoms. As the aging process proceeds, V atoms which migrate to α₂ site, leading to the atomic ordering on (002) plane takes the place of both β and α₂ sites ultimately, and the Ni atoms migrate from α₂ to α₁ site and occupy both α₁ and α₃ sites lastly, while Al atoms diffuse towards outside of DO₂₂ structure facilitating the formation of highly ordered L1₀ (M=1) structure finally.