Effect of Astroloy powder characteristics on mechanical properties of Powder-HIP components

Abstract

Astroloy, Ni-based superalloy, with a high-volume fraction of γ´, is beneficial for high temperature applications but, at the same time, has limited forgeability. For this reason, powder metallurgy (PM) is the only viable processing route for this alloy. In particular, Near Net Shape powder-HIPping (NNSHIP) is attracting much interest since it offers a possible solution to the high cost of PM production by reducing the amount of post-process forging and machining steps. However, this method still has some issues to overcome, such as the presence of prior particle boundaries (PPBs) decorated with carbides and/or oxides, which drastically reduce the material’s mechanical properties. Many studies have attempted to solve this issue by optimizing HIP cycles and heat treatments (HTs), although powder characteristics, more specifically interstitial content, affect PPB presence and mechanical properties of HIP+HTed material. Additionally, powder boron content can have a major effect on material microstructure and mechanical properties. In this work, four powders with different particle size distribution (PSD), oxygen, carbon and boron content were studied. In doing so, the four powder batches and their HIP+HTed samples were fully characterized in terms of chemical composition, grain size and γ´ system, and these properties were linked to the mechanical properties of the samples in order to define optimized powder characteristics. The optimized powder demonstrated more than 23 vol. % of tertiary γ´ due to a high boron content (259 ppm), a carbon content below 160 ppm, and an oxygen content below 150 ppm. With these characteristics, HIP+HTed Astroloy component obtained an improvement, with respect to the aeronautical AMS 5852B standard, of 7 % in ultimate tensile strength (UTS), 3 % in yield strength (YS), 84 % in elongation, 29 % in impact toughness and 22 % in hardness at room temperature. Additionally, at high temperatures, 760 °C, values are around the standard demands with an increase of 2 % in YS and a decrease of 7 % in UTS and 28 % in elongation, being the last one the best result obtained among the tested powders; also impact toughness at 760 °C presents a value close to room temperature requirements with 30 J.