Heat Treatment on Transformation Behavior of TiNiHf Bulk and TiNiCu Ribbon Shape Memory Alloys

• Main Authors: Tsai-Jung Ho, 何彩蓉
• Other Authors: 吳錫侃
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/49843471355985221224

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === Thermal cycling effect on transformation behavior and microstructure of Ti40.5Ni49.5Hf10 shape memory alloy (SMAs) is investigated. Experimental results show that Ti40.5Ni49.5Hf10 SMA exhibits one-stage transformation of B2 ↔B19’ to two–stage one of B2←→B19’1 and B2←→B19’2 within 10 thermal cycles. Transformation temperature rapidly decreases during the early 20 cycles and then keeps almost constant for the further cycles. The evolution of transformation behavior and the decrease of transformation temperature can be ascribed to the dislocations induced by thermal cycling, which can suppress the transformation temperature and cause (001)M micro-twins to replace <011>M Type II twins in B19’ martensite. The formation mechanism of (001)M micro-twins associated with the thermal cycling is also proposed. Aging effect on transformation behavior of Ti50Ni40Cu10 ribbons is also investigated. Experimental results show that aging treatment can eliminate the defect and retained strain induced by RSP and increases quickly the transformation temperature. However, at the same time, aging treatment can cause Ti2Ni precipitation inside grains which can lower the Ti content in matrix and result in the decreases of transformation temperature. Low temperature aging or aging for short period would induce multi-stage B2←→B19 transformation owing to composition deviation generated by non-uniform Ti2Ni precipitation inside the grains. The multi-stage transformation would merge to one stage with high temperature aging or aging for long period due to the homogeneous composition caused by uniform precipitation. When aging temperature is above 500oC, Ti2Ni particles precipitate inside grains and cause the multi-stage transformation during short period aging, while, except Ti2Ni, TiNi2Cu particles also precipitate at 700oC aging with homogeneous matrix composition.