The microstructure determines the resistance of polycrystalline materials to intergranular stress corrosion cracking to a large extent. The random grain boundaries are prone to sensitisation and form paths of low resistance for intergranular cracks to follow. The non-sensitised special grain boundaries, such as twin boundaries, are observed to encourage crack bridging ligament formation. Computational models of intergranular cracking have been developed to investigate crack bridging and its effects on crack propagation in microstructures with different fractions of special boundaries. Grain refinement has been shown to be beneficial through experimental studies, but was not described by the model. This work introduces a two-dimensional model and presents results for microstructures with grain sizes that differ by a factor of two. A synergetic effect of grain size and special boundaries fraction is demonstrated. It is shown that the crack bridging zone size depends on the grain size, and the shielding contribution depends on the relative size of the bridging zone compared to the crack length. It is concluded that both grain refinement and increase in the fraction of special boundaries are important for improving microstructure resistance. These observations are consistent with the effects of grain boundary engineering on stress corrosion cracking resistance.

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