THE SHAPE MEMORY HEAT TREATMENT AND ENVIRONMENTAL TEMPERATURE FOR IMPROVEMENT OF FORMING LIMIT ON TI-NI BASED SHAPE MEMORY ALLOY

Abstract

ABSTRACT Ti-Ni based shape memory alloy is the most useful of all kinds of shape memory alloys. But some principal problems of the alloys are exist in process of production working and heat treatment. Especially, it is expensive and difficult for forming [1] , because of composition of alloy, heat treatment temperature, heat treatment time, and so on. To improve the formability, it is a good effect of doing heat treatment for the purpose of softening to anneal material before shape memory heat treatment. So we call this heat treatment as preheat treatment [2] . So in this study, we investigate both heat treatment memory property and superplastic properties by using stress-strain diagram. ( Fig. 1 ) Results of this study are as follows; as environmental temperature is higher, shape recovery stress became increasing and is obtained as deformation is smaller, the plateau in stress-strain curve became longer, so it is a benefit that though the deformation increase, shape recovery stress is almost constant. Shape recovery stress is controlled to some degree by heat pretreatment temperature. When we use shape memory alloy as superplastic material, residual strain is smallest under the condition of heat pretreatment temperature is about 450°C and environmental temperature is about 450°C. In addition, as for energy storage, energy efficiency is best under the condition of heat pretreatment temperature is 300°C and environmental temperature is about 80°C, but the formability became rather inferior.