Discontinuous phase separation, interstitial ordering and recrystallization during nitriding of FeNiCo medium entropy alloy

The sequence of phase transformations triggered as a consequence of inward diffusion of interstitial N during nitriding of FeNiCo medium entropy alloy was investigated. Recrystallized and cold rolled specimens of FeNiCo alloy were nitrided at 580 °C in a flowing ammonia gas atmosphere. Detailed microstructural characterization was carried out using scanning and transmission electron microscopies, X-ray and electron back scattered diffraction and atom probe tomography. Firstly, N supersaturation of about 2 at.% was observed to be established without any nanoscale chemical heterogeneities. Further increase in N-content triggered the occurrence of a discontinuous phase separation reaction, resulting in the formation of alternating Fe+ N enriched and Co+Ni enriched phases with a characteristic lamellar morphology and cube-on-cube orientation relationship. The Fe+ N enriched lamellae then undergo rapid interstitial N ordering, forming a γ ′ - Fe ₄ N type nitride phase. The compositional partitioning however is only transient and is followed by a recrystallization of the lamellar microstructure into a compositionally homogeneous γ ′ -(FeCoNi) ₄ N type matrix. Thermodynamic calculations show that this discontinuous phase separation reaction and subsequent composition homogenization via recrystallization results from isothermally forcing the alloy composition to first enter the metastable miscibility gap in the FeNiCo-N FCC solid solution (at N-supersaturation close to 2 at.%) and then moving out of the miscibility gap at high N contents, beyond 20 at.%. Role of volume misfits, the chemical partitioning and homogenization on the sequential occurrence of different transformations is discussed.