Exploration of Local Strain Accumulation in Nickel-based Superalloys

Deformation in polycrystalline nickel-based superalloys is a complex process dependent on the interaction of dislocations with both the intra-granular γ'' particles and the grain boundaries. An extensive body of work exists on understanding the interaction between dislocations and the γ'' particles, but understanding the interaction between dislocations and grain boundaries has been historically hindered by the experimental techniques. In this work a full field strain mapping technique was developed and utilized to explore surface strain accumulation at grain boundaries of René 104 samples with different microstructures. The full field strain mapping technique utilized Correlated Solutions VIC-2D software for digital image correlation to measure strain accumulation from secondary electron images taken during constant load tests at elevated temperature. This technique indicated that the two different microstructures of René 104, one with microscopically flat grain boundaries and the other with serrated grain boundaries, accumulate strain by different methods. Analysis of discrete offsets in grid lines placed prior to deformation indicate that grain boundary sliding (GBS) is an active deformation mechanism at these temperature and strain rate regimes, and that the development of serrated high angle grain boundaries can decrease the activity of this mechanism by 30%. Slip transmission parameters, which mathematically assess the ease of slip transmission across a grain boundary, were calculated based on grain boundary misorientation and grain boundary trace. These parameters proved unsuccessful at predicting strain localization sites in these materials, indicating that slip transmission is not the only factor dictating strain localization sites. AAA Full field strain maps were used to site-specifically extract grain boundaries of interest to study dislocation interaction and sub-surface grain boundary neighborhood. Representative from each of four types of boundaries was selected for scanning transmission electron microscopy (STEM) analysis: high angle grain boundaries that either did or did not experienced strain accumulation, high angle grain boundaries that experienced GBS, and special annealing twin boundary that experienced both strain accumulation and grain boundary sliding. The STEM analysis indicates that high angle grain boundaries that experienced strain accumulation showed increased dislocation content near the grain boundary, while a grain boundary that did not show accumulation and a grain boundary that experienced grain boundary sliding showed no indication of increased dislocation content near the grain boundaries. The STEM analysis also indicated that grain boundary surface roughness and the sub-surface grain boundary neighborhood was substantially more complex for the grain boundaries that experienced strain accumulation as compared to the boundary that experienced GBS. AAA Since STEM foils only provide a two dimensional representation of the grain boundary surface, serial sectioning data sets were reconstructed from stacks of 2D images acquired using the Focused Ion Beam (FIB). These reconstructions confirm that indeed the surface roughness of a boundary that experienced strain accumulation was an order of magnitude greater than a grain boundary that experienced GBS. The observations from the STEM and serial sectioning work indicate that grain boundary neighborhood and grain boundary topography also need to be considered if models are to predict strain localization and GBS sites.