Tailoring the superelasticity of NiTi alloy fabricated by directed energy deposition through the variation of residual stress

https://doi.org/10.1016/j.matdes.2022.111311

Abstract

The dynamic thermal cycle of the directed energy deposition process inevitably leads to considerable residual stress within the as-deposited NiTi components. The high residual stresses in the as-deposited NiTi components were mostly removed by stress-relief annealing at 200°C for 3 hours without detectable change of its chemical composition and microstructure. The incremental hole drilling method was used to measure the residual stresses in samples of both the as-deposited and stress-relief annealed variants. Meanwhile, the changes in residual martensite and dislocation density during cycling were examined by quasi-in-situ EBSD observations. The results indicate that the stability of the cyclic process is jointly influenced by the residual stress and dislocation. When the residual stress was 1.6%, the superelasticity of the specimens increased from 69.81% to 95.72%; After releasing the residual stress at a strain of 2.5%, the superelasticity of the specimens increased from 63.09% to 73.70%. This study provides useful guidance for improving the superelasticity of NiTi alloys fabricated by directed energy deposition.