Microstructure and Thermomechanical Properties of Shape Memory Alloys TI₅₀-NI₅₀ Elaborated by Arc Melting and by Powder Metallurgy

Abstract
This study was focussed on the elaboration and transformation of Ti₅₀Ni₅₀ shape memory alloys in relation to structural and thermomechanical properties. An original method for producing TiNi alloys by powder metallurgy (PM), through combustion synthesis, was developed. After hot extrusion, intermetallic rods without porosity were obtained. Microstructural and thermomechanical properties of products obtained by this method were systematically compared to those of some alloys elaborated by the more conventionnal method of arc melting (AM). For comparison, cast products were hot rolled at different temperatures ; for all the transformation conditions used, the grain size remained coarser and more heterogeneous compared to PM products. Investigation of the thermomechanical behaviour was conducted by tensile and flexion tests. It was found that :
- The ultimate tensile strength and the recovery stress were in the same range for PM and AM products but the ductility of PM products was enhanced.
- The better one way shape memory recovery is obtained for a PM product with an oxygen content of about 1200ppm. Also, too low content of oxygen (<600ppm) seems to decrease the stress and strain recovery during thermomechanical cycling.