Recrystallisation in Single Crystal Nickel-Based Superalloys
Plastic strains during investment casting of single crystal Ni-based superalloys arise from differences in thermal contraction between the metal and ceramic mould-and-core. If deformation is above a critical limit, subsequent solution heat treatment of the alloy causes recrystallisation. Crack nucleation and propagation is preferred at the recrystallisation grain boundaries, and this significantly reduces the creep and fatigue properties of the alloy. The as-cast microstructure (Chapter 3) is characteristic of high temperature deformation (≥1050°C), where dislocations primarily form loops and networks at the y/y' interface. This validates the process modelling performed by our collaborators. The dislocation density was found to be higher in the interdendritic areas and the dendrite cores were virtually dislocation-free, indicating that deformation occurs at temperatures close to the y' solvus, which is 1250-1310°C for CMSX 4, where dislocations accumulate in the interdendritic areas that are the first to precipitate the y'. On examining the cast surface of CMSX 4, two sources of nucleation for recrystallisation were identified (Chapter 4). Firstly, micro-grains of y', 2-30 μm deep, forming high angle boundaries with the bulk single crystal were found within the surface eutectic; these grow larger during the heat treatment and maintain high angle misorientations with the matrix. Secondly, in regions where surface eutectic is absent, the metal adheres to the mould and forms intense local deformation, 5-20 μm deep, during subsequent detachment. During the heat treatment local surface recrystallisation occurs, where small grains develop in orientations similar to the deformed matrix and subsequently twin to form high angle boundaries. Experimental trials show that in the presence of deformation in the bulk the nuclei in the casting surface can cause recrystallisation. By removing the cast surface with etching, recrystallisation can be completely mitigated. Recrystallisation studies on alloys varying systematically in composition (Chapter 5) show that high Co alloys (with up to 8 wt%) are more prone to recrystallisation. Co lowers the y' solvus temperature and the stacking fault energy of y. Ru, Mo and W appear to have no direct effect on recrystallisation. The nucleating grains for recrystallisation form in orientations similar to the deformed matrix, and subsequently twin to form high angle boundaries and proliferate within the deformed microstructure. The y' phase and topologically close packed phases hinder grain boundary migration.
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