Solute cluster-induced precipitation and resultant surface hardening during nitriding of Fe–Al–V alloys

The small amount (0.1 at%) of V addition to Fe–Al alloys promotes AlN precipitation during nitriding and results in substantial surface hardening, in contrast to the normal sluggish precipitation kinetics of AlN during nitriding of Fe–Al binary alloys. The addition of V to Fe–Al alloys can increase the surface hardness to a greater extent than the addition of Ti. First-principles calculations of the interaction energy between an Al atom and an N–s cluster (s = V, Ti, Cr, or Al) show that a V–N cluster has an attractive interaction with an Al atom, and this interaction is stronger than that between a Ti–N cluster and an Al atom, in good agreement with the greater hardening induced by V addition than by Ti addition. These experiments and calculations indicate that solute clusters not only contribute directly to the hardening but also induce alloy nitride precipitation.