Interstitial strengthening in f.c.c. metals and alloys

In this short review, we highlight instances where interstitials have been shown to substantially increase the yield strength and work-hardening rate (WHR) of f.c.c. alloys, particularly high entropy alloys, medium entropy alloys, TWIP steels and stainless steels. However, the common practice of describing interstitial strengthening in f.c.c. alloys using models that are used to explain substitutional strengthening appears to be neither appropriate nor accurate. Here we suggest, based on the literature, that the yield strength increase due to interstitials in f.c.c. alloys is more appropriately described by a linear dependence on the concentration: due to a paucity of experimental studies, the dependence of the yield strength and WHR on misfit parameters is currently unclear. Thus, the source of the strengthening remains unclear. A feature that has been observed in several f.c.c. alloys is that interstitial additions lead to a change from wavy to planar slip although the origin of this change, which may be related to changes in stacking fault energy as well as other factors, remains unclear. The paper concludes by outlining areas of future research, including the need to develop a new model for interstitial strengthening in f.c.c. alloys.