PSI - Issue 14

320 M S Nandana et al. / Procedia Structural Integrity 14 (2019) 314–321 M S Nandana/ Structural Integrity Procedia 00 (2018) 000–000 The weaker PFZ, due to the depleted precipitates leads to localized plastic deformation promoting intergranular crack path as shown by Chen et al. (2013). Since the PFZ size observed in RRA treated sample is marginally higher than T6 treated sample, intergranular crack path is not observed on fractured surfaces of the RRA treated sample. Hence the influence of the PFZ size on reduction of FCGR can be negligible. The possible reason for reduction in crack growth rate is the microstructure of the alloy matrix. Study performed by Wang et al. (2014) showed that the coherent precipitates promote planar slip and hence dislocations glide back and forth during cyclic loading. The fine precipitates in the matrix of RRA with larger inter-particle spacing between them promote cyclic slip reversibility and retard the crack growth rate in near the threshold regime. The fractograhic observations in the final fracture regime confirm a similar characteristics of ductile dimple type of failure in both the alloys as shown in Fig. 7(c,f).

Fig. 7. SEM images of fractured surfaces in near-threshold, Paris regime and high ΔK regime: (a, b, c: RRA) and (d, e, f: T6)