Phase equilibria in the Fe-rich portion of the Fe-Ni-Si system

Investigations relating to the phase equilibria involving the α, γ, σ and liquid phases and the ordering reactions in the bcc aluminides of the Fe-Ni-Si system in the temperature interval 800∼1200°C have been carried out by energy dispersion X-ray spectroscopic (EDS) analysis and transmission electron microscopy (TEM). The equilibrium compositions of the γ (A1: fcc-Fe), α (A2: bcc-Fe), σ (σ phase) and liquid phases and the critical boundaries of the continuous ordering transition in the α phase region have been determined. It is observed that the critical temperatures of the continuous ordering transitions α(A2)/a′(B2) and α′/α″(D0₃) are mainly functions only of Si content, and not of Ni. It is also found that the tie-line configurations and the partition behavior of the constituents at the α/γ phase equilibrium are strongly affected by the order-disorder transitions in the α phase. Results pertaining to these phase equilibria and the stability of the ordered bcc aluminides are presented and discussed.