Pattern bifurcation in the pseudobinary Ni₃(Al, V) alloy: Evidence of spontaneous L1₂ single crystallization

The structural change between substitution-type ordered structures basically includes the competition of pairwise interactions, which can describe the stability of such structures. The purpose of this study is to investigate the frustration effect resulting from competition related to the L1₂ → D0₂₂ structural change in the Ni₃Al_0.52V_0.48 alloy and to discuss the pattern bifurcation in the pseudobinary Ni₃(Al, V) alloy. The present data obtained by transmission electron microscopy show that the change in microstructures during the structural change is characterized by the appearance and the subsequent annihilation of D0₂₂ regions in the L1₂ matrix. The final product of the structural change is the L1₂ single phase including topological defects, which is different from the equilibrium state known as the L1₂ + D0₂₂ checkerboard pattern. This indicates that the suppression of the L1₂ → D0₂₂ structural change occurs in this alloy. Such suppression Can be explained as being due to an inability to accommodate the strain field in the vicinity of the L1₂/D0₂₂ boundaries when D0₂₂ precipitation occurs. The trigger of this inability is thought to be strong frustration originating from a small concentration deviation in the initial L1₂ matrix from the stoichiometry of the L1₂ structure. It is concluded that the formation of the L1₂ single phase as the final state, i.e. spontaneous L1₂ single crystallization, results from an effort by the alloy to accommodate the strain field, the occurrence of which is related to the frustration effect appearing during the L1₂ → D0₂₂ structural change.