HREM studies of twin boundary structure in deformed martensite in the cold-rolled TiNi shape memory alloy

The microstructural aspects and interfacial characteristics of deformed martensite variants in TiNi alloy, cold-rolled to different extents, have been investigated by TEM and HREM techniques. With the increasing reduction of thickness, the following phenomena happen: the dominant deformation mechanism changes from coalescence of the martensite variant to coalescence of the substructural bands and the introduction of secondary twinning plates; the quantities of (100) compound twins and 〈011〉 Type II twins are reduced, with an increase of (111̄) Type I, (001) compound twins and (011) Type I twins and the introduction of (111) Type I twins; the (100) intervariant boundary gradually loses its mobility and becomes wavy, the 〈011〉 Type II twin boundary changes from gradually and randomly curved to the distorted shape, the (111̄) Type I, (001) compound and (011) Type I twin boundaries become stepped, and the (111) Type I twin boundaries exhibit a wavy morphology.