Microstructural model of twinning and detwinning processes of the martensitic phase in shape memory alloys

Using the Cu–Al–Ni shape memory alloy as an example, the article substantiates an approach to describing at the microstructural level the processes of twinning and detwinning of the martensitic phase. The coordinated twinned martensitic structure is described by the Hadamard compatibility equation for deformations, the solution of which made it possible to determine the surfaces along which the shift occurs, the directions and the magnitude of sliding in an orthorhombic crystal cell corresponding to the material under consideration in the martensitic state, leading to the appearance and disappearance of the twin structure. It is shown that two types of twins simultaneously and inseparably exist in an alloy with shape memory: deformation and structural. The first is related to the deformation of a simple shear, that occurs in accordance with the Hadamard compatibility condition in a martensitic plate, which leads to bending this straight plate and the appearance of two elements rotated at a certain angle relative to each other, which form this twin. The structural twin is formed from two parts, in each of which the orthorhombic crystal cells of martensite are identically oriented, but one of the axes of these cells changes its direction by $90^\circ$ when moving to another part of the twin. The formation of a deformation twin initiates the occurrence of a simple shear strain in the medium, and a structural twin initiates a structural strain. It is shown that a certain position of the structural twin in the deformation one leads to the equality of these strains.