The influence of hot-deformation parameters on the mechanical properties and precipitation process in nickel based superalloy

Abstract

Experimental results on hot deformation and dynamic structural processes of nickel based alloy Inconel 718 are reviewed. The focus is the analysis of dynamic precipitation processes which operate during hot deformation of these materials at elevated temperatures. Hot compression tests were performed on the solution treated precipitation hardenable nickel based superalloy Inconel 718 at 720-1150°C with a constant true strain rates of 10-4 and 4×10-4s-1. True stress - true strain curves and microstructure analysis of the deformed nickel based superalloy is presented. The properties and dynamic behaviour are explained through observation of the microstructure using standard optical, scanning and transmission electron microscopy. Structural observations of solution treated Inconel 718 deformed at high temperatures, reveal non uniform deformation effects. The distribution of molybdenum-rich and niobium-rich carbides were affected by localized flow within the strain range investigated at relatively low deformation temperatures 720 - 850°C Microstructural examination of the alloy also shows that shear banding, cavity growth and intergranular cracks penetrating through the whole grains were responsible for decreased flow stresses at temperature of 720, 800 and 850°C and might result in sample fracture at larger strains. On the basis of the measured flow stress activation energies of high-temperature deformation processes were estimated. The mathematical dependence of the effect of flow stress on temperature and strain rate (σpl -T and σpl - ε̇) as well as compression data were used to determine material's constants. These constants allowed the derivation of a formula that describes the relationship between strain rate (ε̇), deformation temperature (T) and flow stress σpl. Interaction of precipitates developed during deformation below the solvus temperature and heterogeneous deformation (flow localization) can become a significant aspect of high temperature performance of precipitation hardenable alloys. This interaction could also potentially allow production of specific microstructures in deformed materials. The contribution of flow localization to die strain hardening or flow softening and the flow stress-strain behavior during hot deformation of precipitation hardenable alloys is still a subject of extensive research.