Study the Correlation of B19' Phase Transformation and Grain Orientation Evolution on Shape Recovery of Niti Alloys Fabricated by Powder Bed Fusion

Abstract

In this study, NiTi shape memory alloys (SMAs) were fabricated by powder bed fusion (PBF), and different energy densities were found to be effective for the microstructure, phase transformation behavior and shape memory effect (SME). The results indicated that the NiTi SMAs exhibits a microstructure with coarse and long grains surrounded by fine grains, while the increase in energy density from 44.5 J/mm3 to 133.3 J/mm3 makes the grain size larger, and different energy densities induce various grain orientations. Specifically, the NiTi alloy produces more martensite B19' at two energy densities of 44.5 J/mm3 and 111.1 J/mm3 after aging at 773 K for 2 h. The phase transition behavior of B19' is related to the high angle grain boundaries (HAGBs) fraction and the grain orientation spread (GOS) distribution. SME tests showed that the NiTi alloy prepared by PBF had an unstable recovery strain compared to the specimens after aging, and the SME of NiTi alloy after aging corresponded more to the evolution of the residual B19' content and grain orientation of the B2 phase. Specimens prepared at an energy density of 111.1 J/mm3 exhibited the best recovery strain of 4.5%, depending on the greater residual B19' content and grain orientations of the B2 phase with higher Schmid factors. These results reveal the relationship between the recovery strain and microstructural evolution of NiTi SMAs prepared using PBF and provide new insights into the fabrication of NiTi alloys with better SME by a proper PBF process.

Keywords: NiTi shape memory alloys, Powder bed fusion, Microstructures, Energy density, Shape memory effect