Crystal structure and internal defects of equiatomic TiNi martensite

The crystal structure and internal defects of equiatomic TiNi martensites have been studied using the techniques of electron microscopy, electron diffraction, and X-ray diffraction. The crystal structure may be described as of distorted B19 type with a monoclinic unit cell, the lattice parameters of which are a = 2.889 Å, b = 4.120 Å, c = 4.622 Å, and β = 96.8°. The structure is derived from the matrix B2 in two steps. Namely, the matrix B2 transforms into a B19 first, and then a monoclinic martensite is derived by shearing the B19 in [100]B19 direction on (001)B19 plane. As an internal defect of the martensite, transformation twins with (111) twinning plane have been found without ambiguity, whose presence is essential for the understanding of the unique memory effect in this alloy. The orientation relationship between the matrix and martensite, and some characteristics of a pre-martensitic transition are also described.