Hot Deformation of IN718 with Various Initial Microstrucures- Experiments and State-variable Modelling

Abstract

Significant research has been directed towards characterising the precipitation and dissolution kinetics of the δ phase in IN718, a nickel-base superalloy, but rather less is known about the specific influence of the precipitate on microstructural evolution during thermo-mechanical processing. To investigate the effect of δ precipitate morphology on the hot deformation behaviour of IN718 a series of hot isothermal axi-symmetric compression 'upset' tests have been carried out at fixed nominal strain rates and temperatures relevant to industrial hydraulic press-forging (0.001-0.3/s and 990-1040°C). These test conditions span the δ solvus temperature for the alloy, 1015°C. In particular, three different types of material, each with its own distinct microstructure, were examined: I- solution-treated, δ-free material; II- material containing finely dispersed, intragranular, particulate δ; III- material containing a dense network of intragranular and grain boundary δ platelets along crystallographic habit planes. The presence of δ phase strongly influences both the peak stress and the rate of flow softening. Below the δ solvus temperature the type II material, containing dispersed particulate ö, exhibited the highest peak stress. The type III material, containing acicular δ, exhibited the most significant post-peak softening, whereas the δ-free, type I material, exhibited the lowest peak stresses of all materials. For testing above the solvus temperature the flow behaviour was found to be practically identical for the three microstructures, regardless of the initial presence of δ in two of them. The presence of δ in the microstructure had a marked effect on the evolution of recrystallised fraction and grain size. It also influenced the evolution of texture that resulted into markedly different texture from the classic FCC texture exhibited by solution-treated material after hot deformation. Model predictions from semi-empirical state-variable model in the form of constitutive equations, in the form of flow behaviour and volume fraction recrystallised for solutiontreated IN718, showed good qualitative agreement with measured data. It was found that the model predictions naturally divided the flow behaviour into two regimes of dynamic recovery and dynamic recrystallisation, based on the strain rates employed for hot deformation. This agreed well with observations from microstructure investigations. The model was successfully extrapolated to predict flow behaviour of IN718 available elsewhere in the literature, under temperature and strain rate conditions different from those used in the present work. To further validate the state-variable model, hot compression of a non-uniform double-trucated cone specimen was carried out and the measured volume fraction recrystallised was successfully predicted