Hot workability characteristics of Rene88DT superalloy with directionally solidified microstructure

Abstract

The hot deformation characteristics of Rene88DT superalloy with directionally solidified microstructure produced by electroslag remelting continuous directionally solidification (ESR-CDS^®) were studied in the temperature range of 1,040–1,140 °C and strain rate range of 0.001–1.000 s⁻¹ by hot compression tests. Flow curves for Rene88DT alloy with initial directionally solidified (DS) microstructure exhibit pronounced peak stresses at the early stage of deformation followed by the occurrence of dynamic softening phenomenon. Rene88DT alloy with DS microstructure shows higher flow peak stresses compared with HIPed P/M superalloy FGH4096, but the disparities in peak stresses between ESR-CDSed Rene88DT and HIPed P/M superalloy FGH4096 reduce as temperature increases. The improvement of hot workability of DS alloy with columnar grains avoiding the maximum shear stress comes true. A hot deformation constitutive equation as a function of strain that describes the dependence of flow stress on strain rate and temperature is established. Hot deformation apparent activation energy (Q) varies not only with the strain rate and temperature but also with strain. The strain rate sensitivity exponent (m) map is established at the strain of 0.8, which reveals that global dynamic recrystallization (DRX) shows a relatively high m value in a large strain compression. Optimum parameters are predicted in two regions: T = 1,100–1,130 °C, ${\dot{\varepsilon}}$ = 0.100–1.000 s⁻¹ and T = 1,080–1,100 °C, ${\dot{\varepsilon}}$ = 0.010–0.100 s⁻¹, which is based on processing maps and deformation microstructure observations.