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 h 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.
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