Mechanism of the TiNi Martensitic Transformation and the Crystal Structures of TiNi-II and TiNi-III Phases

Based on combined x‐ray and neutron‐diffraction data, an inference for the coexistence of the B2(CsCl) and $\text{P3̄m1}$ structures in the TiNi‐II phase is made. Other experimental evidence indicates the proportion of the B2 to $\text{P3̄m1}$ structures to depend critically on the thermal and mechanical history of the alloy. Through a mechanism of an inhomogeneous‐but‐cooperative atomic shear in the ${\text{〈111〉}}_{\text{B2}}$ direction and a factor sequence, ${\text{−(0−1−1̄)}}_n-$ in the (110)_B2 plane, the B2 and $\text{P3̄m1}$ structures transform into $\text{P1̄}$ abd P6/m structures, respectively, during the TiNi‐II to TiNi‐III martensitic transformation. Subsequent submicrotwinning at atomic level of the $\text{P1̄}$ structure leads to the P1 structure. Therefore, the TiNi‐III (``martensite'') phase may consist of the $\text{P1̄}$ , P1, and P6/m structures.