2010 Volume 51 Issue 4 Pages 679-684
Thermal-cycled Ti40.5Ni49.5Hf10 alloy exhibits B2↔B19′ martensitic transformation and two DSC transformation peaks appear after 10 thermal cycles. Transformation temperatures rapidly decrease during the early cycles. XRD tests reveal that both transformation peaks are associated with B2↔B19′ transformation. TEM observations indicate that as-homogenized B19′ martensite mainly consists of internal 〈011〉M type II twins. However, they are gradually replaced by internal (001)M compound twins with increasing the thermal cycles. This feature accounts for these two transformation peaks are associated with B2↔B19′ transformation with two different internal twinning modes. The microstructural evolution of B19′ martensite is ascribed to the dislocations induced by thermal-cycling as they can promote the formation of thin internal (001)M compound twins.