Microstructural development inside the stress induced martensite variant in a Ti-Ni-Nb shape memory alloy

Abstract

The microstructural development inside the stress induced martensite (SIM) variants in Ti-Ni-Nb alloy with various degrees of deformation have been revealed by electron microscopic observations. The orientation relationship between the SIM and the parent phase has been found: [11̄0]M//[111̄]B2, (001)M 5° away from (101)B2. The lattice invariant shear of the SIM variants at the slightly deformed stage is dominantly (111̄) Type I twin. Besides the ordinary slip, the adjustment and development of the internal secondary twinning from (111̄) Type I twin to 〈011〉 Type II/or (011) Type I twin, (001) compound twin and (111) Type I twin happen concurrently or in combination inside the SIM variants with the further deformation. The corresponding deformation mechanisms include stress induced reorientation of SIM substructural bands by the most favorably oriented twin system, stress induced migration of the SIM substructural boundary through internal twinning and stress induced injection of foreign SIM variant to the preexisting substructural bands.