Heteroepitaxial Recrystallization, a New Recrystallization Mechanism in Sub-Solvus Forged γ-γ' Nickel-Based Superalloys With Low Lattice Mismatch

Abstract : A new dynamic recrystallization mechanism has recently been reported in γ-γ' Nickelbased superalloys after hot-forging in the sub-solvus domain. The mechanism leads to recrystallized grains encompassing a primary γ' precipitate having the same crystallographic orientation. The nucleation of those grains actually occurs before deformation by inverse precipitation of coherent γ phase at the rim of the primary γ' precipitates. This makes the specificity of that recrystallization process. Once the matrix starts being deformed, the coherent γ phase particles grow under the conventional stored-energy-consumption driving force. A detailed study of this new mechanism in the René65TM alloy has recently been published by the authors. Heteroepitaxially recristallized grains grow first, at low strain levels; then discontinuous dynamic recrystallization takes place, through classical necklace nucleation. Contrary to the usual trends for dynamic recrystallization kinetics, heteroepitaxial recrystallization goes faster with increasing strain rate and at lower temperatures. In the present paper, hot-forging experiments have been designed to demonstrate that the same mechanism can be triggered in other γ-γ' nickel base superalloys (Udimet 720 and RR1000).