Facilitation of detwinning through controlling crystal structure in Ti–Zr–Ni–Pd high temperature shape memory alloys

Novel Ti–20Zr–49.7Pd High Temperature Shape Memory Alloy with Facilitated Detwinning and Precipitation Strengthening

“…The formation of two types of precipitates and a related transformation temperature drop are also observed in a specimen aged at a lower temperature of 450°C for 3 h. The decrease in transformation temperatures after aging at low temperatures <550°C is due to fine precipitation which reduces the distance between precipitates, i.e., the size of the austenite matrix, and mechanically inhibits the martensitic transformation. 24) On the other hand, as seen in Fig. 2(b), H-phase is mainly precipitated in the 650°C3-h aged specimen, and Ti 2 Pd-type precipitates are barely observed even at austenite grain boundaries, unlike the 550°C aged specimen.…”

“…We have reported that a 1050°C24-h homogenized material without Ti 2 Pd-type and H-phase precipitates has A s = 144°C, M s = 134°C, and hysteresis (A s ¹ M s ) = 10°C. 24) In the first cycle after aging at 650°C, the specimen shows A s = 140°C and M s = 134°C, and the hysteresis (A s ¹ M s ) is small as 6°C, indicating that there is no significant change in the transformation temperatures after the 650°C3 h aging. On the other hand, the heating-cooling cycles caused transformation temperature decrease and peak broadening with each cycle.…”

“…2628) Therefore, in addition to the conventional design concept of HTSMAs, i.e., transformation temperature increase and precipitation strengthening, our research group has proposed an additional concept of facilitation of detwinning and a candidate alloy composition range in TiZrNiPd quaternary alloy systems. 24) It was revealed that Ti(1520)Zr49.7Pd (at%) and the surrounding Ni-containing alloys shown in the purple region in Fig. 1 satisfy the following three conditions: transformation temperature of 100°C or higher, martensite phase with orthorhombic B19 structure in which detwinning readily occurs, and strengthening ability due to H-phase precipitation during aging.…”

“…However, the addition of Zr or Hf promotes the introduction of thin and dense (001) B19A compound twins during the martensitic transformation in Ti(Zr, Hf )Ni alloys and the high-density twins in the self-accommodated martensite structure are difficult to be detwinned under stress. 24,25) The difficulty of detwinning causes a gradual increase in the reorientation stress without showing a stress plateau during deformation, resulting in a small recoverable strain generation of about 23% even though a high external stress >1 GPa is applied. 2628) Therefore, in addition to the conventional design concept of HTSMAs, i.e., transformation temperature increase and precipitation strengthening, our research group has proposed an additional concept of facilitation of detwinning and a candidate alloy composition range in TiZrNiPd quaternary alloy systems.…”

“…It has been confirmed that the H-phase precipitate forms even in Ti-rich compositions and Ti 2 Pd-type precipitate also forms in addition to the Hphase. 24) The effect of aging temperature and duration on the formation behavior of each precipitate is considered to be complicated, and the detailed precipitation behavior is still unknown. The deformation behavior and shape recovery strain in the shape memory effect have also yet to be investigated.…”