DOI: 10. 1051/jp4: 20031072 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 nuéleation of microdomains of an ordered Llo phase with a high degree of order and a tetragonal lattice axes ratio c/a = 0. 87. Large elastic anisotropic coherent stresses, which arise in the two-phase AI+L Io 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 Llo phase, but changes the orientation of the c-axis by 90o. The proposed mechanism of ordering, which includes shear processes, explains the cause of formation of a lamellar structure in the CuAu alloy