Ti-Ni계 형상기억합금 코일 스프링의 형상회복 거동에 미치는 열처리의 영향

Abstract

The effect of aging temperature on the microstructural change, transformation characteristics and phase change has been investigate in Ti-Ni based shape memory alloy wire. The recovery chracteristics of Ni-Ti based SMA coil spring, which has been aged at different temperature, was thermally cycled to 10,000th cycle, has been studied by using SMA coil spring with bias coil spring. (1) Precipitates alined longitudinal direction of cold drawn wire was dissolved in matrix by solutionizing at 900℃ for 1hr but not completely. The volume fraction of precipitates in solutionized wire was not changed by aging below 400℃ but it was slightly increased by aging above 450℃ and 500℃. (2) No martensitic transformation was observed even by cooling below -100℃ in cold drawn wire but mixture of parent phase and martensite could be observed by solutionizing at 900℃ for 1hr. Ms temperature was decreased with increasing aging temperature while B2→R transformation and R→B19' transformation coexited. (3) Recovery force was increased from 2,000gf to 3,600gf and displacement was also increased from 3.6㎜ to 9.8㎜ with increasing aging temperature. Although both coil springs are aged at same temperature. shape recovery finish temperature(Rf temperature) measured from recovery force vs. temperature or recovery displacement vs. temperature was lower than Af temperature calculated from DSC measurement (4) With increasing thermal cycling, Ms and As temperatures were fallen and transformation temperature range(Ms～Mf) was increased. On the other hand, Multi step martensitic transformation as well R phase transformation occured in the coil spring at aged 400℃. It imply that change in chemical composition is changed and the stress field is presented around precipitates formed during the thermal cycling. (5) Recovery force and displacement of coil spring were decreased with increasing the number of cycle but them of coil spring aged 500℃ were increased by 10th cycle because of training effect.