Distribution of Grain Boundary Carbides in Inconel 617 Subjected to Creep at 900 °C and 950 °C DANIEL SPADER, KIMBERLY MACIEJEWSKI, and HAMOUDA GHONEM 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, or 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 behavior. https://doi.org/10.1007/s11661-020-05798-x Ó The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION DEMANDS for increases in fuel efficiency and operational lifetimes in advanced engineering designs involved in applications such as aerospace and energy generation industries, some of which are operating at temperatures in the range of 900 °C, require the use of increasingly complex alloys to cope with these extreme loading environments. With the increase in time, tem- perature, and microstructural complexity, creep defor- mation and its associated diffusional processes make prediction of long-term creep response increasingly challenging. As the operational life of some of these materials approach 60 years, one-to-one creep testing cannot be conducted in laboratory environments and as a such, long-term predication requires a fundamental understanding of the rate at which the microstructure changes and how these changes impact the deformation rate of the material under these types of loading conditions. While the c¢-precipitate-strengthened nick- el-based alloys have proven beneficial in many high-tem- perature loading applications, at temperatures exceeding ~ 750 °C, the benefits of the strengthening phase are lost as the c¢ precipitate phase dissolves back into the matrix. [1] At temperatures exceeding 900 °C, one pre- cipitate phase which forms along grain boundaries in nickel-based superalloys containing C, as well as, Cr, Mo, Fe, and the M 23 C 6 -type carbides which can provide high temperature strength through the pinning of grain boundaries. [2] The scope of the following research is DANIEL SPADER, KIMBERLY MACIEJEWSKI, and HAMOUDA GHONEM are with the Mechanics of Materials Research Laboratory, Department of Mechanical, Industrial and Systems Engineering, University of Rhode Island, Kingston, RI, 02881. Contact e-mail: ghonem@uri.edu Manuscript submitted September 2, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS A