Assessment of the reinforcing system and carbides evolution in hot isostatically pressed astroloy after prolonged exposure at 820°C

Hot Isostatic Pressing (HIP) is a powder metallurgy technique which densifies metallic powders via the contemporary application of high temperature and pressure. Such manufacturing route demonstrated highly efficient to produce components made of material with low forgeability or castability such as the Ni-based superalloys. Such materials are designed to withstand very high temperatures, which typically are met in aeronautical engines. Despite Astroloy, the Ni-based superalloy studied in the following work, is designed to work in the temperature range of 650–760 °C, only few research papers discussed its thermal stability in depth. In this work, the thermal stability of the alloy was studied in a more demanding condition, i.e., at 820 °C for 200 h. This experimental condition was chosen to assess the effects of repeated exposures of a component above the standard operating temperature. The over-ageing treatment applied to the alloy severely altered its microstructure, causing γ’ coarsening and second phases precipitation. More precisely, σ phase and M23C6 carbides precipitated inside and at the grain boundaries respectively. Nano-indentation trials and tensile tests at high temperature were used to assess how these microstructural alterations impacted on mechanical properties of the material. The main conclusions are: only a slight lowering of tensile properties and a strong increase of ductility was measured after tensile test up to 760 °C, as well as an higher reduction of both tensile properties and ductility when sample were tested at 820 °C.