Promoting the heterogeneous nucleation and the functional properties of directed energy deposited NiTi alloy by addition of La2O3

Abstract

Grain morphology control is a challenging issue for additive manufactured NiTi alloy, which directly affects the functional properties. In this work, La2O3 addition was applied to control microstructure and improve functional properties of directed energy deposited (DED) NiTi alloy. The results showed that the DEDed NiTi alloy mainly consisted of NiTi (B2) columnar grains and some coarse NiTi2 phases within and at the boundaries of NiTi grains. The addition of La2O3 led to the promotion of columnar-to-equiaxed transition and grain refinement of NiTi (B2) phase. La2O3 and LaNi secondary phases can be found in the DEDed NiTi alloy with La2O3 addition. The La2O3 precipitate could act as the effective heterogeneous nucleation site and the NiTi2 or LaNi precipitates could pin the grain boundaries contributing to the grain refinement and the formation of equiaxed grains of NiTi(B2) phase. The introduction of La2O3 could also refine the phase size and adjust morphology of NiTi2 phase, which was attributed to the increase of nucleation sites and more dispersed L(Ti-rich).The temperatures and latent heat of phase transformation evidently increase with La2O3 addition due to the decrease in the Ni content and La dissolved into NiTi(B2) phase. Improved superelasticity property was achieved after La2O3 addition owing to the promotion of grain order and yield strength of NiTi(B2) phase and the reduction of resistance from NiTi2 phase for the interface movement.