Precipitation and its Hardening Effect in Ni-Rich NiTi

In order to clarify the precipitation and its hardening effect, and the relation between aging temperature and composition of precipitates in Ni-rich NiTi, the changes in hardness and microstructure with aging temperature and time have been examined. The composition of precipitates has also been determined by an electron probe microanalyser.In the solution treatment of Ni-rich NiTi, the higher the cooling rate the higher becomes the hardness. A significant hardening effect is obtained even by air cooling. The hardness of water-quenched specimens decreases with aging time in the temperature range from 400 to 700°C. No hardness peak is detected at these temperatures. Optical microstructure experiment shows that no precipitates exist in the water-quenched specimens, but the precipitates appear after aging for 1 to 10 hr at 600°C or 0.1 to 1 hr at 700°C. The compositions of the precipitates determined in the specimens aged at 600 and 700°C are close to Ni3Ti2 and Ni3Ti, respectively.The above results indicate that the quench-hardening is caused by quenched-in substitutional defects and that although age-hardening occurs the hardness peak does not appear because the hardening effect is cancelled by lowering of quench-hardening as the number of substitutional defects decreases.