Selective growth of Ni4Ti3 precipitate variants induced by complicated cyclic stress during laser additive manufacturing of NiTi-based composites

Abstract

NiTi-based composites were in-situ synthesized by selective laser melting (SLM) additive manufacturing technology with formation of a great many Ni4Ti3 precipitates. The differential scanning calorimetry (DSC) were carried out to exhibit the similar phase transformation temperature (As = 51.82 °C, Af = 94.02 °C, Ms = 55.87 °C and Mf = 18.93 °C) by comparing with that of SLM-fabricated equi-atomic NiTi alloy, which could be attributed to the compensation of Ni4Ti3 precipitates for the effect of an addition of TiC particles. Three different Ni4Ti3 precipitate variants ((111)R//11¯1¯B2, (111)R//1¯1¯1B2, and (111)R//1¯11¯B2) with various precipitate diameters were observed within SLM fabricated part, showing apparently selective growth behavior. By high-resolution transmission electron microscopy, severe lattice defects formed surrounding the smaller variant, while relatively intact lattice structure was found around the larger one, which indicated that high strain energy was produced in the interface between the matrix and the smaller variant. Furthermore, finite element simulation method was applied to disclose the evolution features of complicated cyclic stress during SLM processing of TiC/NiTi composites. By predicting, the main constraint mode within as-fabricated track was tensile stress effect. Based on the redistribution behavior of thermal stress and the coupling effect between the thermal stress and strain field induced by Ni4Ti3 precipitate, the selective growth mechanism of Ni4Ti3 variant was revealed.