Novel Ti–20Zr–49.7Pd High Temperature Shape Memory Alloy with Facilitated Detwinning and Precipitation Strengthening
Hirobumi Tobe, Shunsuke Kojima, Eiichi Sato
It is desirable to develop high temperature shape memory alloys (HTSMAs) with a martensitic transformation start temperature (Ms) above 100°C and a recoverable strain of about 4–6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We previously demonstrated facilitation of detwinning for Ti–Zr–Ni–Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti–(15–20)Zr–49.7Pd (at%) and surrounding Ni-containing compositions are candidates of HTSMAs having low variant reorientation stress. On the other hand, the aging condition for precipitation strengthening was not optimized, since the candidate alloys show a complex precipitation behavior of two types of precipitates, Ti2Pd-type and H-phase. Therefore, in this study, the effects of aging on precipitation behavior, martensitic transformation temperatures, and shape memory properties were investigated for one of the candidate alloys, Ti–20Zr–49.7Pd, and an excellent shape memory effect with Ms above 130°C, a low reorientation stress around 200 MPa, a high plastic deformation stress around 1800 MPa, and a large recovery strain of 4.5% was achieved after an optimum aging treatment. On the other hand, short-range ordering of solute atoms occurs just above the reverse transformation temperature and decreases Ms, which would limit the number of shape recovery operations when the alloy is used as a device.