Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite (γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2-λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.