Abstract
The microstructural evolution following compression and subsequent sub-solvus and super-solvus heat treatment was studied in the Ni-based superalloy, RR1000, typically used for rotor disc applications in aero-engines. For a low strain rate of 0.1 s−1 at close to solvus temperature, 1413 K (1140 °C), the flow stress is constant. For larger strain rates of 1 and 10 s−1 at sub-solvus temperature, 1373 K (1100 °C) dynamic re-crystallization (DRX) of γ grains occurs during forging with accompanying stress decay. Incoherent primary γ′ precipitates form mainly via meta-dynamic re-crystallization (MDRX) at 1 s−1 and are as intergranular. For 10 s−1, the coherently nucleated or existing precipitates present in the initial as-HIP condition become incoherent when the grain boundary sweeps past them during DRX and subsequent grain growth. The incoherent primary γ′ precipitates are mainly intragranular. During sub-solvus heat treatment at 1373 K (1100 °C), dissolution of the incoherent precipitates occurs through coarsening of the coherent intragranular population with only sporadic incoherent precipitates remaining. The prior induced deformation (strain and strain rate) influences the evolution of precipitate morphologies during cooling following heating to super-solvus temperature. Using numerical simulations, a quantitative calculation of the different precipitate morphologies was carried out during slow cooling from super-solvus temperature, 1443 K to 1373 K (1170 °C to 1100 °C).
Highlights
NI-BASED superalloys strengthened by the presence of a dispersion of coherent c¢ (L12) precipitates in an FCC-disordered solid solution matrix of c (A2) phase find wide applications in aero-engine and land-based power generation designed for high-temperature applications
The true stress–strain curves obtained during hot deformation at 1373 K (1100 °C) are presented in Figure 1 for the strain rates of 1 and 10 sÀ1 and correspond to sub-solvus conditions
The following observations can be made: (i) The flow curves are characterized by the initial rapid increase in stress from work hardening up to a true strain of ~ 0.04, when a peak in the true stress is reached
Summary
NI-BASED superalloys strengthened by the presence of a dispersion of coherent c¢ (L12) precipitates in an FCC-disordered solid solution matrix of c (A2) phase find wide applications in aero-engine and land-based power generation designed for high-temperature applications. For turbine disc applications, the powder metallurgy (PM) route is followed. Superalloy powders synthesized by gas atomization, hot isostatic pressing (HIP) followed by blind die compaction (upset), and hot extrusion are used to produce the billet. The billet is subsequently isothermally forged and heat treated. The evolution of the microstructure, i.e., the c¢ precipitate morphology during these thermomechanical processes controls the grain.
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