Tensile mechanical behavior of functionally graded NiTi alloy manufactured via laser powder bed fusion

Abstract

Laser powder bed fusion (LPBF) offers the potential to modify the characteristics of NiTi alloys, allowing for the creation of bespoke solutions tailored to specific applications. In this study, four functionally graded NiTi alloys with varying martensite/austenite distributions were designed and manufactured. A comprehensive investigation was conducted into the local microstructure, phase transformation behaviour, and tensile mechanical behaviour. The findings illustrate that, in comparison to homogeneous NiTi, gradient NiTi displays a propensity for preferential deformation within the martensitic region, attributable to the presence of localised performance variations. Gradient NiTi exhibits evident strain hardening and a wider range of plateau stress due to its intricate step-by-step phase transformation behaviour. The strain gradient strengthening near the interface between martensite and austenite endows gradient NiTi with enhanced fracture strength. Furthermore, while ensuring a discernible gradient in strain, the design of the interface with narrow spacing and parallel to the tensile direction can effectively enhance the mechanical properties of gradient NiTi. This study validates that gradient design based on LPBF is an effective approach for controlling the macroscopic mechanical behavior of NiTi alloy.