Distribution of Grain Boundary Carbides in Inconel 617 Subjected to Creep at 900 °C and 950 °C

A post-creep deformation analysis is carried out on the nickel-based superalloy Inconel 617 in order to identify the grain boundary carbide (GBC) distributions for different creep stresses and temperatures and to determine the related microstructural changes in terms of grain size and associated changes in the material’s creep ductility as a function of GBC distribution. Creep tests were conducted at two temperatures 900 °C and 950 °C for stresses of 35, 50, and 62 MPa. Post-creep rupture, carbide size, density, and spacing were measured as a function of grain boundary orientation with respect to the loading direction (i.e., trace angle). It is observed that non-uniform carbide distributions were present in the five test conditions associated with an increase in the carbide size, density, and area fraction along grain boundaries perpendicular to loading conditions (tensile boundaries) when compared to those on parallel boundaries (compressive boundaries). The magnitude of preferential distribution of GBC towards tensile boundaries is observed to govern the ability of the compressive boundaries to migrate which facilitates grain elongation in the loading direction which leads to increased creep ductility. A critical magnitude of preferential GBC distribution is determined below which compressive boundaries remain relatively pinned with a low grain boundary spacing. This condition corresponds to creep deformation accommodated by grain boundary sliding only, leading to a relatively low creep rupture strain. Above that magnitude, compressive boundaries are permitted to slide and migrate and, as such, facilitate grain elongation giving rise to increasing magnitude of total creep strain. A criterion for significant preferential distribution resulting in changes to grain morphology and mechanical response, has been proposed in the form of a temperature-stress map which identifies the creep loading conditions associated with significant preferential distribution prior to creep rupture. The critical GBC distribution coupled with the concept of identifying temperature and stress combinations resulting in significant preferential distribution provides guidelines for creep testing for the purpose of extrapolating short-term test data to long-term response.