Relation between lattice expansion and nitrogen content in expanded phase in austenitic stainless steel and CoCr alloys

Vegard's law provides a rule of thumb for equilibrium stress-free lattice constants in nitrided austenitic stainless steel and CoCr alloys. However, when taking the austenitic, metallic phase and stoichiometric FeN or CoN phases as endpoints, the resulting calculated expansions are too high compared with experiments. When extrapolating a linear equation taken from literature for very low nitrogen concentrations in steel, satisfactory agreement seems to be possible. However, nitrogen concentration gradients and, especially, biaxial stress, while limited by the yield strength of about 1 GPa and strongly influenced by processing characteristics can lead to discernible differences, limiting the use of this equation to a rule-of-thumb approximation. For CoCr, more experimental data are necessary to establish a similar guideline.