A novel B19′ martensite in nickel titanium shape memory alloys Original Research Article

A new type of the B19′ (monoclinic) Ni–Ti martensite—one which is internally twinned by the (0 0 1)m compound twinning mode, has been found in thermally cycled Ni–Ti shape memory alloys. This is an unexpected and remarkable martensite type on account of the fact that the (0 0 1)m compound twinning mode does not qualify to occur as the fine structure in the B2–B19′ martensite transformation. On the basis of the good concurrence of the observed crystallographic parameters with those predicted by the phenomenological theory of martensite transformations, it has been determined that this martensite is a product of the R phase-B19′ martensite transformation. However, the (0 0 1)m compound twinning mode can qualify to occur as the lattice invariant shear (LIS) only when the rhombohedral angle of the R phase is less than the critical value of 86.2°. The preference of this twinning mode as the LIS to the [0 1 1]m Type II twinning mode, which is the normally observed substructure of the Ni–Ti martensites, has been rationalized to be due to the closer proximity of the orientation relationships to the lattice correspondence and the lower magnitudes of twinning shear, shape strain shear, twin interface energy and nucleation strain energy.