The thermo-mechanical treatment of a physiological NiTi-alloy

A physiological shape memory alloy transforms to martensite below room temp erature (M-s < 15 degrees C). The beta austenite forms on warming to the te mperature of the human body (37 degrees C). One such suitable material is t he binary compound NiTi, but with a slight excess of Ni. This compound can be "ausformed" in the temperature range 900 degrees C to 380 degrees C and rolled to a degree of deformation of up to 70% in a single pass. This defor mation stretches the grains, produces twins and other defects in the beta-N iTi, and alters the martensitic transformation temperatures, but does not s uppress the transformation, as can be seen from the DSC-analysis. The metallographical investigation carried out included both optical light - and scanning electron microscopy of the etched austenite and of the marte nsitic transformation and re-transformation by shearing in the surface of t he material. Finally, all of the microstructural changes taking place durin g the course of a tensile test up to fracture were observed. The results obtained clarify the improvements which can be made in the stre ngth of this alloy by "ausforming", whilst still retaining it's shape memor y characteristics. The optimised properties of this material can for exampl e be used in medical applications where precise movements must be made, or large forces exerted.