Measurement of transformation temperatures of Ni-Ti shape memory alloys with using methods continuous change of resistivity as function of temperature and DTA

Abstract

System Ni-Ti is based on equiatomic compound of nickel and titanium. Ni-Ti based alloys are highly stable and resistant to corrosion. Production of these alloys is very difficult due to high reactivity of titanium. For successful use in practice it is very important to know transformation characteristics of these alloys. Fabrication of alloys on the base of Ni-Ti is realized mostly by vacuum melting processes (electron beam melting, arc melting, induction melting) and plasma melting. During melting of Ni-Ti based alloys, oxygen and carbon have adverse effect. Carbide and oxide from Ti formation lead to changes in concentration of elements and to changes of transformation temperatures. Another problem is formation of the low-melting phase Ti2Ni, which leads to formation of hot cracks. Alloys on the base of Ni-Ti melt usually at the temperature of approx. 1500 K. It is possible to measure transformation temperatures by deformation methods, by DSC method (differential scanning calorimetry) or DTA method (differential thermal analysis), by resistometric methods. Resistometric method enables simple, precise and rapid evaluation of transformation temperatures. In dependence on constitution of alloys, or additional alloying or heat treatment or mechanical treatment It is possible to get different types of dependence of electric resistivity on temperature. This article deals with method of continuous change of resistivity as function of temperature of Ni-Ti alloys, and with influence of deformation and subsequent heat treatment at these temperatures.