The self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Twelve pairs of minimum units consisting of two habit plane variants (HPVs) with V-shaped morphology connected to a _B19′ type I variant accommodation twin were observed. Three types of self-accommodation morphologies, based on the V-shaped minimum unit, developed around one of the {111}_B2 traces, which were triangular, rhombic and hexangular and consisted of three, four and six HPVs, respectively. In addition, the variant selection rule and the number of possible HPV combinations in each of these self-accommodation morphologies are discussed.

Four characteristic interface microstructures between habit plane variants (HPVs) in the self-accommodation morphologies of B19′ martensite in Ti–Ni alloys have been investigated by scanning transmission electron microscopy (STEM). The straight interface of a _B19′ type I twin is present at interface I. The relaxation of the transformation strain at interface II is achieved by a volume reduction of the minor correspondence variants (CVs) in the relevant habit plane variants (HPVs). The relaxation of the transformation strain at interface III is mainly due to the formation of a _B19′ type I twin between the two major CVs. Subsequently, local strain around the tips of the minor CVs perpendicular to the interface is released by the formation of micro-twins with the ⟨011⟩_B19′ type II and/or _B19′ type I relation. The major and minor CVs in each HPV are alternately connected through fine variants with the _B19′ type I twin relation parallel to interface IV. The results are compared with macroscopic observations and the predictions of PTMC analysis.