CHARACTERISATION OF SHAPE MEMORY ALLOYS BASED ON NICKEL-TITANIUM SYSTEM

Abstract

This dissertation report intends to present new experimental evidence of a comprehensive investigation of the effects of low temperature ageing on the transformation behaviour of a Ni-Ti and Ni-Ti-Cu Shape memory alloys (SMA).Up to date, experimental evidence reported in the literature are exclusive of the transformation behaviour of the ageing characteristics of studied from 473 K to 993 K. The ageing temperatures investigated in this study are in the range of 423 K (new experimental evidence) and 473 K to 773 K (conventional ageing temperatures).The occurrence of the complex transformation behaviour indicates that the Ni-rich Ni-Ti and Ni-Ti-Cu alloys are unstable at even these low temperatures. This has the direct evidence and application for the property stability of Ni-Ti and Ni-Ti-Cu alloys in many applications. This study investigates the effects on ageing (423 K to 773 K) on the transformation behaviour of a Ni54.9Ti45.os and Ni-49.TiCu6.6 Shape Memory Alloy (SMA). It was found that ageing in the temperature range of 423 K induced R-phase transformation in Ni-Ti and Ni-Ti-Cu SMA's. The transformation behaviours were analyzed by means of DSC, XRD, Tensile test and Resistivity measurements. High temperature XRD was carried out to find the Transformations Temperature Region (TTR) of the aged and as-rolled samples. The effect of cold working (% straining) on Martensite transformation was analyzed by means of XRD, DSC and resistivity methods. When the percentage straining increases means intensity of the (1 10) B2 phase was increased progressively. This increases in intensity reveals that Ms Temperature of the SMA was increased due to the formation of the stress induced Martensite (SIM). The effect of annealing on the Ms Temperature and As temperatures of 30% cold reduced wire samples were analyzed using DSC and tensile test. The effect of thermal cycle on the transformation hysteresis and pseudo-elasticity were analyzed by means of DSC and Tensile test.