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 Ni3Al0.52V0.48 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. (author)