Experimental Determination of Equilibrium (Ti,Nb)N Composition in Fe–18Cr–2Mo Alloy by Controlled Annealing at 1473 K

In our previous work, it was found that there was a disparity between thermodynamically-calculated and experimentally-measured (Ti,Nb)N composition during the unidirectional solidification of Fe–18Cr–2Mo alloy. To investigate this disparity, controlled annealing experiments were conducted for Fe–18Cr–2Mo alloy samples at 1473 K. The minimum time to reach equilibrium was firstly determined to be 24 hours. Samples with different Ti contents were then annealed for 24 hours at 1473 K. The nitride inclusions converged toward a narrow composition range of Ti 0.98–0.99 Nb 0.01–0.02 N after annealing. The experimentally determined equilibrium nitride compositions agreed well with thermodynamic predictions. No clear monotonic evolution trend was observed in inclusion number density or size distribution, suggesting that chemical equilibration proceeded predominantly through solute exchange between inclusions and the surrounding matrix rather than through morphological evolution.