The Laws of Generation and Relaxation of the Spherical Segments of Ti-Ni Shape Memory Alloys

It is experimentally shown that the spherical segments bent inversely to initial shape in martensitic state recover their initial shape by jump with a clap at heating to austenitic state. Such phenomenon is observed only when the spherical segment is unfixed on edges. The segments which are constrained or fixed on edges do not click. It is shown that the abrupt recovery of the pre-set shape is caused by the segment stability loss during the heating. The stability loss temperature (T_sl) at which one can observe the spherical segment shape recovery with a clap is close to the austenite finish temperature (A_f) and clearly depends on the dimensions of the spherical segments (D, R, h).

The papers are devoted to the research of changing laws of the force parameters which appear in the spherical segments’ material depending on the overheating temperature during one hour. It is shown that overheating of the spherical segments by 50, 100, 150, 200, 250……500°С comparatively to the austenite finish temperature (A_f) significantly affects on the impact force and reactive forces. In particular, the spherical segments subjected to the overheating (T_oh) in a free state click with an impact on the dynamometer at heating.  At that the reactive forces generate achieving the maximum values at the temperature of 200-250°С. After the heating up to >250°С the force parameters dramatically decrease due to relaxation of the internal stresses in spherical segments. The overheating of the segments by 200-250°С and above in constrained state drives out the shape memory effect. The obtained results can be used in designing of thermo-cut-off valves.