Structural mechanisms of high-temperature shape changes in titanium-nickelalloys after low-temperature thermomechanical treatment

High-Temperature Shape Memory Effect (HTSME) in Ti-Ni alloys and correspond ing structural and internal stress changes were studied using dilatometry, in situ electron microscopy and X-ray diffractometry. The HTSME induced by the Low Temperature Thermomechanical Treatment (LTMT) consists of two stage s. The temperature range of the first stage is limited to 250 degrees C, wh ile the second stage extends to 400-500 degrees C. The first stage is cause d by the oriented reverse martensite transformation. The heterogeneous resi dual stress field causes a different thermal stability for the different ma rtensite orientations. During the reverse transformation an anisotropic shi ft of martensite and austenite X-ray lines is observed that can be due to a relaxation of the orientated stresses and to changes in the martensite lat tice. The second stage of HTSME is caused by internal stress relaxation dur ing recovery and polygonization of austenite that are not typical shape mem ory mechanisms. The possible reasons for the martensite stabilization induc ed by LTMT will be discussed.