Grain Boundary Engineering for Intergranular Fracture and Creep Resistance

Previous studies (see review) have shown that grain boundaries crystallographically described by low Σ Coincidence Site Lattice (CSL) relationships can often display a high resistance to intergranular sliding, cavitation, and fracture; these boundaries satisfying Brandon’s criterion (Δθ≤15°Σ−1/2), for Σ≤29. Figure 1 summarizes the grain boundary structure-dependence of intergranular cracking on stressed C> rings (0.5% strain) of Alloy 600 (75Ni-15Cr-10Fe) following autoclave exposure to 10% NaOH at 350°C for 3000 hours. Grain boundaries connected to intergranular crack paths were characterized using electron backscattered diffraction (EBSD) in a SEM. As shown in Fig. 1, general grain boundaries (Σ≤29) are the preferred paths for intergranular cracking; ‘special’ grain boundaries (Σ>29) show a high resistance to intergranular attack. In quantifying the effect of ‘special’ grain boundary frequency on bulk intergranular cracking susceptibility, a geometric model has been formulated which considers that a propagating crack can arrest at a triple junction when both of the available intergranular paths for crack continuation are inaccessible owing to either (1) intrinsic resistance to cracking (e.g., low Σ CSL grain boundary) or (2) unfavourable orientation to the applied stress axis. Figure 2 summarizes the effect of ‘special’ grain boundary frequency on the probability of intergranular crack propagation as predicted by this model. Increasing the fraction of ‘special’ grain boundaries by only moderate amounts can considerably reduce the maximum attainable intergranular crack length in a material. For example, the probability of a crack propagating beyond 5 grain diameters is approximately 35% in a material having 15% special grain boundaries; the probability is reduced to approximately 1% in a material containing 45% ‘special’ grain boundaries, and to less than 0.001% in a material containing 75% special grain boundaries. These concepts have been recently applied in developing a proprietary thermomechanical processing methodology which serves to increase the frequency of special grain boundaries in austenitic stainless alloys to values in excess of 60%; such a treatment having been demonstrated to render Alloy 600 (see Fig. 1) immune to intergranular cracking.