Changes in martensitic transformation temperatures during thermal cycling in Ti-Ni-Zr shape memory alloys

Changes in the transformation behavior and transformation temperatures of a 40Ti-50Ni-10Zr (at.%) alloy during thermal cycling have been investigated by means of differential scanning calorimetry, X-ray diffraction and thermal cycling tests under a constant load. A 40Ti-50Ni-10Zr alloy showed two stage transformation behavior, i.e., from the B2 to the B19 and then from the B19 to the B19', although they are not clearly separated. With the increasing of the number of thermal cycling, the decreasing rate of the transformation temperature corresponding to the B2-B19 was higher than that corresponding to the B19-B19', and consequently the former overlapped with the latter. Transformation temperatures of 40Ti-50Ni-10Zr greatly decreased (95K) during thermal cycling without applied stress, since only the thermal cycling effect which suppresses martensitic transformation had an influence on transformation temperatures. Decreases in transformation temperatures during thermal cycling with applied stress was smaller than that during thermal cycling without applied stress because both the structural refinement effect which assists martensitic transformation and the thermal cycling effect had an influence on transformation temperatures.