Improving the Recovery Properties of Near-spherical Porous NiTi Alloys by Controlling Ni4Ti3 Precipitation

Abstract

Aging treatment to modulate Ni₄Ti₃ precipitates in NiTi alloys is an important strengthening method for improving their recoverability. In this study, NiTi alloys with 21–53% porosity were prepared using metal injection molding combined with the powder space holder method (MIM-PSH). The effects of the aging temperatures on the microstructure, transformation and mechanical properties were investigated. The coarse Ni₄Ti₃ was eliminated after solution treatment. The length of the precipitates increased gradually with the aging temperature while the martensitic transition (B2-B19′) was gradually suppressed. At low porosity (21%), the yield strength and modulus were substantially enhanced by increasing the aging temperature. The recovery rate improved fabulously from 82.0% to 95.9%. But at high porosity (53%), no significant changes in the yield strength and modulus of high porosity specimens were observed after aging treatment, as the recovery rate was only increased slightly from 79.7% to 83.7%. The change in mechanical properties was attributed to the control of the precipitation morphology. However, the strengthening effect gradually reduced with increasing porosity. Considering the greater susceptibility to oxidation, the aging treatment for porous NiTi with high porosity should be carried out with caution. This study provides an experimental basis for improving the mechanical properties of near-spherical porous NiTi alloys.