Phase selection and microstructural formation of rapidly directionally solidified peritectic Fe-Ni alloys by laser surface remelting

Rapid directional solidification of three peritectic Fe-Ni alloys with Ni-concentrations of 4.1 at.%, 4.33 at.%, and 4.6 at.% is achieved by laser surface remelting. It is found that phase selection of these alloys is controlled by the maximum growth temperature criterion. For the first two alloys, when the solidification velocity (Vs) is above a critical value (Vscrit=0.031m/s for Fe-4.1 at.%Ni alloy and Vscrit=0.012m/s for Fe-4.33 at.%Ni alloy), the growth temperature of γ phase (Tiγ) is higher than that of δ phase (Tiδ), leading to a transition from the solidification of δ phase to the solidification of γ phase. While for the third one, within the achieved range of Vs, Tiγ is always higher than Tiδ; only the solidification of γ phase is resultant. At Vscrit, an abrupt increase in cellular spacings (λ) of the first two alloys are observed, which can be attributed to the transition from the solidification of δ phase to the solidification of γ phase. For Fe-4.6 at.%Ni alloy, within the achieved range of Vs, only γ phase solidifies, which causes a monotonous decrease of λ with increasing Vs.