Characterization of strain induced precipitation in Inconel 718 superalloy

Inconel 718 is a Ni-Cr-Fe superalloy which presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These characteristics can be optimized with an appropriate control of microstructural features such as grain size and precipitation. Precipitates of different nature can form in these alloys, i.e. γ’’ (a metastable metallic compound Ni3Nb), γ’ (Ni3(Ti, Al), carbides and/or δ phase (intermetallic Ni3Nb). Aging treatments are usually designed to obtain the precipitation required in order to optimize mechanical properties. However, precipitation can also appear induced by deformation and therefore interfere with hot forming operations, such as forging. In particular, the formation of δ phase is a concern regarding[j1] deformation processes, because it forms at the highest temperatures. In this work, stress relaxation tests were carried out in order to characterize the precipitation kinetics of Inconel 718 at temperatures ranging from 950°C to 800°C. Moreover, the effect of deformation on precipitation was also assessed by applying different degrees of deformation, i.e. 0.005, 0.1 and 0.2, prior to stress relaxation. The analysis of the relaxation curves, together with a proper characterization of precipitation by Scanning Electron Microscopy, was the basis for obtaining precipitation-time-temperature (PTT) diagrams, as well as the effect of deformation on precipitation kinetics.