Apparent yield strength reduction behavior in solid-solution treated Inconel 718 subjected to surface severe plastic deformation

초록

The influence of residual stress on the tensile yielding behavior of solid-solution-treated Inconel 718 subjected to ultrasonic nanocrystalline surface modification (UNSM) was investigated. By using a solid-solution-treated condition in which precipitation strengthening was intentionally eliminated, the contribution of residual stress to apparent yielding could be evaluated independently of precipitation strengthening. Electron backscatter diffraction revealed a severely deformed surface layer extending to approximately 120 & micro;m, while Vickers hardness reached a maximum of 434 HV at a depth of approximately 30 & micro;m and decreased to a nearly constant average value of 305.7 HV beyond approximately 500 & micro;m. X-ray diffraction further identified a compressive residual stress field extending to approximately 300 & micro;m beneath the surface, with a maximum compressive stress of -537 MPa. Despite these surface strengthening effects, the apparent yield strength decreased from 585.6 MPa to 545.8 MPa after UNSM treatment, whereas the ultimate tensile strength remained nearly unchanged. This demonstrates that UNSM-induced apparent yield strength reduction also occurs in solid-solution-treated Inconel 718 and is not governed solely by precipitation related effects. As an additional analysis, a closed-form twodimensional plane stress solution was applied to incorporate the self-equilibrated residual stress field generated during SSPD, with the implementation procedure explicitly described. The analytical prediction showed good agreement with the experimental results, estimating an apparent yield strength of 530.3 MPa with a deviation of only 2.9%. These results indicate that the apparent yield strength reduction primarily originates from the residual stress distribution and demonstrate the applicability of the analytical approach to evaluating residualstress-induced apparent yielding in SSPD-processed metallic materials.

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