THE INFLUENCE OF THE STARTING GRAIN SIZE DURING HIGH-TEMPERATURE GRAIN BOUNDARY ENGINEERING OF Ni-BASE SUPERALLOY RR1000

摘要

Traditional grain boundary engineering (GBE) techniques employ iterations of cold work followed by short time annealing to obtain a high proportion of Σ3/twin boundaries. Although numerous investigations reported the improvement of the properties of various polycrystalline materials using GBE, this approach is not suitable for the fabrication of large and complex Ni-base superalloys components. In this investigation, samples of Ni-base superalloy RR1000 with a fine (～5.9 μm) and a medium (～10 μm) starting grain size were deformed at various temperatures, strains, and strain rates to study the effect of thermal-mechanical processing parameters on the formation of Σ3 boundaries upon annealing. As the magnitude of stored strain energy increased to promote strain-induced boundary migration (SIBM) and the formation of Σ3 boundaries during annealing, this led to an overall decrease in the density and fraction of pre-existing twin boundaries in the as-deformed microstructure. Consequently, subsequent annealing only resulted in minor changes in the Σ3 density and fraction as a function of the deformation parameters. The larger starting grain size in RR1000 was less conducive to the formation of Σ3 boundaries as strain incompatibilities and twin reorientations occurred when dislocation plasticity mechanisms were operative. This increased the tendency of the alloy to recrystallize and fewer Σ3 boundaries formed within the microstructure upon annealing.