Ni-Ti계 합금 선재의 변태특성 및 의탄성 거동에 미치는 열기계적 싸이클의 영향

Abstract

The effects of annealing temperature and time on transformation characteristics and pseudoelasticity in Ni-Ti based wires have been investigated. The effect of thermal cycling on transformation temperature, heat flow and force-deformation characteristics has been also studied in Ni-Ti based wires annealed at different temperature and time. The obtained results are as follows; Transformation temperature is risen with increasing annealing temperature up to 400 ℃, while transformation temperature range becomes narrower due to the disappearance of internal defects by annealing. However, transformation temperature is increased with increasing annealing temperature above 450 ℃because of the presence of precipitates. With increasing annealing time at 500℃, at which both R phase and 19R' transformations occur concurrently, Rs temperature is stable but Ms temperature is risen. On the other hand, reverse transformation temperature is increased, while separated two peaks change into one peak. When the annealed wire is deformed above Af temperature, force-deformation curve represents pseudoelasic behavior showing typical stress induced martensitic transformation. In the case of deformation temperature of Rs+30℃, Fmax and Fela show higher value than that below deformation temperature of Rs+30℃. Deformation is also retained even after unloading by deformation below Rs+30℃ i.e. Rs+10℃, Rf～10℃ and Rf～Ms+10℃. Rs and Rf temperatures of as-received wire are 21.1℃ and ～.56℃ but both temperature are fallen and heat flow is decreased with increasing the number of thermal cycling. After as-received wire is cycled thermally, critical stress is almost constant but elastic stress is decreased. Deformation is retained by unloading after deformation at Rf ~ Ms + 10℃ but it is decreased with thermal cycling due to trainning effect.