Antiphase boundary-like stacking fault in α''-martensite of disordered crystal structure in β-titanium shape memory alloy

The ordering and antiphase boundary (APB)-like fault found in the alpha''-martensite of beta-Ti shape memory alloys are studied. Long-range chemical ordering was not found, but APB-like faults were observed in every martensite plate studied by transmission electron microscopy. These faults have morphology similar to the APBs observed in ordered phases. The superlattice reflections observed in some previous works were a consequence of multiple diffractions. APB-like faults were not observed in the parent phase, leading to the conclusion that the faults were introduced by the martensite transformation. The fault took the form of a wavy tube running perpendicular to the habit plane. The fault was a 'transformation-induced APB' with an additional small displacement due to the pre-existing athermal omega phase. The displacement vector was determined to be [-3/50, -23/50, 1/2]. Geometrical aspects of the formation of APB-like faults are also discussed.