Martensitic transformations during ordering in CuAu

Abstract
It was found that the ordering process in an equiatomic CuAu alloy is nonmonotonic. The initial stage of ordering includes a diffusion nucleation of microdomains of an ordered L1₀o phase with a high degree of order and a tetragonal lattice axes ratio . Large elastic anisotropic coherent stresses, which arise in the two-phase structure, suppress the diffusion completion of the ordering process and facilitate relaxation shear processes realized in two stages. The first shear transformation leads to a polytype formation of a metastable Z phase of the martensitic class. The reverse martensitic transformation takes place in plates of the Z martensite and restores the atomic order typical of the L1₀ phase, but changes the orientation of the c-axis by 90°. The proposed mechanism of ordering, which includes shear processes, explains the cause of formation of a lamellar structure in the CuAu alloy.