Analysis of the stress field by finite element model in boride layers formed in the Inconel 718 superalloy

Abstract

This research numerically investigates the stress fields in boride layers on Inconel 718 superalloy, generated by Vickers indentations at various distances from the system interface in cross-sectional views. Three powder-pack boriding conditions at 850, 900, and 950 °C for 2, 4, and 6 h, respectively, were applied to Inconel 718. Employing X-ray diffraction, mainly the Ni₄B₃, Ni₂B, and Ni₃B phases were identified. In addition, a hardness (H) range between 23.8 and 26 GPa was determined by Berkovich instrumented indentation, while 280 to 380 GPa for Young's modulus (E). Vickers indentations were made in a load range between 200 and 1500 N at different distances from the diffusion zone towards the substrate. The cracks were identified in the boride layer without advancing to the substrate or at the interface with the diffusion zone and only the thinnest thickness presented delamination at 1500 N. The stress fields were analyzed using the finite element method with explicit dynamic analysis. The numerical model consists of a Vickers indenter modeled as a discrete, rigid body and a 3D deformable solid defined through cross sections. As the layer thickness increases, the system becomes less sensitive to applied loads, and the magnitude of the stress fields decreases. Simulation results indicate that maximum principal stresses lead to cracking within the layer, while the shear stresses are insufficient to cause delamination. Forming layers with low porosity is crucial to avoid stress concentrators that could eventually propagate into cracks. The thickest layer/substrate system, produced at 950 °C for 6 h, demonstrates greater resistance to cracking.