Grain boundary composition and effects on environmental degradation

Environmental degradation of metallic alloys is shown to depend on its local microstructure and microchemistry in many instances. Equilibrium and non-equilibrium processes can produce significant changes to the grain boundary composition and promote intergranular stress corrosion cracking (IGSCC). Austenitic stainless steels are used as an example to illustrate equilibrium effects of Gibbsian segregation and second-phase precipitation as well as non-equilibrium effects of quench-induced and radiation-induced segregation. Interfacial Cr concentration is shown to be the dominant material variable promoting IGSCC of austenitic stainless steels in oxidizing environments. Cracking susceptibility is a direct function of the boundary Cr content regardless of depletion width. However, grain boundary Cr depletion does not adequately explain IGSCC in stainless steels strengthened by cold work or neutron irradiation. Significant interfacial enrichment of Cr, Mo and B are often present in annealed stainless steels which may play a role in the IGSCC of cold-worked materials and delay IASCC to higher radiation doses. Impurity segregants (e.g., P) can promote IG hydrogen-induced cracking but do not have a strong detrimental effect on cracking in high-temperature water environments.