Casting voids in nickel superalloy and the mechanical behaviour under room temperature tensile deformation

TL;DR

This study investigates the formation and growth of voids in a nickel superalloy under tensile stress, examining microstructural changes, dislocation activity, and the influence of stress triaxiality on void evolution.

Contribution

It provides new insights into void behavior in nickel superalloys, combining experimental observations with crystal plasticity simulations to analyze void growth mechanisms and stress distribution effects.

Findings

Void volume fraction increases after tensile deformation.

Weak correlation between stress triaxiality and void growth.

Smaller notch ratios lead to higher strain localization and early failure.

Abstract

The microstructure of a second-generation nickel base superalloy is studied using X-ray computed tomography (XCT) and scanning electron microscopy (SEM). The as-cast material contains 0.15 (+-0.001) vol% voids and these are distributed in the inter-dendritic region. The volume fraction of the voids increases to 0.21 (+-0.001) vol% after tensile deformation. Surface observations show evidence of dislocation emissions from the void surface, a mechanism possibly facilitates the expansion of the voids and contributes to the increased void volume fraction. Phenomenological parameters such as stress triaxiality, often believed to control void growth, are investigated through crystal plasticity simulation and compared with literature reported data. The results indicate weak correlation between stress triaxiality and void growth, but this may be possibly due to the lack of data at higher level of plastic deformation, which is limited by the ductility of the material. The distribution of the stress triaxiality field within the sample is heterogeneous and the peak of the triaxiality field is a function of the ratio between notch diameter and sample width. A smaller notch diameter to sample width ratio tend to distribute the triaxiality peaks towards the centre of the sample but also lead to higher strain localisation, an effect that results in early sample failure.