Martensitic transformations during ordering in CuAu

It was found that the ordering process in an equiatomic CuAu alloy is nonmonotonic. The initial stage of ordering includes a diffusion nuċleation of microdomains of an ordered $L1_0$o phase with a high degree of order and a tetragonal lattice axes ratio $\rm c/a = 0.87$. Large elastic anisotropic coherent stresses, which arise in the two-phase ${\rm Al}+L1_0$ 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 $L1_0$ 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