Ultrafine grained structure formation in nickel-based superalloy EP741NP during hot deformation in the (γ+γ')-phase field

A. Ganeev1, V. Valitov1
1Institute for Metals Superplasticity Problems of RAS, 39 Khalturin Str., Ufa, 450001, Russia
The effect of thermomechanical treatment within (γ+γ')-phase field on microstructure of the powder nickel-based superalloy EP741NP was investigated. It was shown, that preliminary heterogeneous annealing (HA) leads to decrease in a flow stress by 10-15% due to an increase size and distance between γ' particles at the constant size of γ-grain. Localization of deformation in bands oriented at 45° to the axis of compression was observed during compression to the strain 70% at low temperatures (ts -185÷285°C). This re-sulted to a deep cracks on the samples surface. The microstructure investigation of samples subjected compression showed that the deformation was developed homogeneously at higher tempera-tures. This effected on formation of partially recrystallized structure. The temperature rising results in to an increase of recrystallized grains volume fraction. Increasing the deformation temperature and carrying out deformation fractional, with intermediate annealing at the deformation temperature, leads to reduction of cracking. EBSD method had shown that unrecrystallized zones were a matrix type structure with developed substructure. The recrystallization processes occurred during thermomechanical treatment at ts-45÷25 °C leading to transformation of a coarse-grained matrix structure into microduplex one that was accompanied by transformation of coherent γ/γ' boundaries into incoherent ones. It was revealed, that the parameters of formed microstructure can be controlled by temperature of deformation and preliminary HA. The size of microduplex γ-grains increases and the volume fraction of γ'-phase decreases with temperature rise of thermomechanical treatment. The size of forming after HA γ'-phase particles, grows with an increase of HA start temperature.
Accepted: 09 June 2015
Views: 111   Downloads: 30
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