Two-way shape memory effect of a TiNiHf high temperature shape memory alloy

The two-way shape memory effect in a Ti(36)Ni(49)Hf(15) high temperature shape memory alloy (SMA) has been systematically studied by bending tests. In the TiNiHf alloy, the martensite deformation is an effective method to get two-way shape memory effect even with a small deformation strain. When the TiNiHf alloy is deformed at a full martensite state, the deformation mechanism is the martensite orientation accompanied by the dislocation slip. The experimental results indicated that the internal stress field formed by the bending deformation is in the directions of the preferentially oriented martensite variants formed during the bending deformation. Upon cooling the preferentially oriented martensite variants form under such an oriented stress field, which should be responsible for the generation of the two-way shape memory effect. Proper training process and aging benefit the formation of the oriented stress field, resulting in the improvement of the two-way shape memory effect. When the training strain is 7.1% and the training temperature is room temperature, a two-way shape memory strain of 0.88% is obtained in the Ti(36)Ni(49)Hf(15) alloy. This value is the maximum two-way shape memory strain in the TiNi-based high temperature SMAs so far. In comparison with the TiNi alloy, the TiNiHf high temperature SMA shows the two-way shape memory effect with the relative poor stability due to the lower strength of martensite. (C) 2003 Elsevier B.V. All rights reserved.