Microstructure Evolution And Metastable Phase Formation In Highly Undercooled Fe-Ni Alloys

Fe-7.5 at.% Ni alloys were undercooled with molten glass fluxing and cycling superheating methods. The solidification microstructure evolution with undercooling and solidification behaviors of metastable phase were investigated, systematicly. The dendrite core of metastable phase and its distributing in the solidification microstructures were observed by TEM and SEM techniques. Competitive nucleation in undercooled melts was analyzed with classic nucleation theory (CNT) and time-dependent theory (TDT), respectively. The growth of metastable phase and stable phase were described based on BCT model. The competitive growth between the metastable phase and stable phase were discussed and the evolution of metastable phase was elucidated. The phase transformation principle has been studied with TEM and XRD methods. Main conclusions are as follows:(1) The grains refining mechanism in low undercooling range is the dendrite remelting by the serious superheating. The grains refining in the medium undercooling range is due to the recrystallization induced by the high contraction stress, and in high undercooling range, the metastable phase δ solidifies firstly, thenY phase solidifies at a low undercooling; which makes grain size of Y phase increase, subsequently, the high interface energy leads to the mergence of the grains after solidification, so that the coarse grains form.(2) The critical undercooling (ΔT = 120K ) of metastable phase formation as primary phase was defined by the observation of solidification microstructures. The amount of metastable dendrite cores increase with the increasing of undercooling. Most of such dendrite cores are distributed in the grains with an elliptical morphology; some of them lie in the grain boundaries with triangular or lathy morphologies. There are few dislocations and substructures in the dendrite cores, and there are clear phase boundaries between such dendrite cores and the matrix.(2) The results of calculations based on CNT and TDT are accordance with the experimental results qualitatively, namely, the metastable phase will solidify asprimary phase from the undercooled melts when the undercooling of melts exceeds a critical value. By comparing the results of calculation of those two nucleation theories, we found that the TDT can describe the competitive nucleation in the undercooled Fe-7.5at.%Ni alloy melts better.(4) The growth of metastable phase is mainly controlled by interfacial kinetic undercooling and thermal undercooling. There exists the competitive growth in the low and medium undercooled melts, and the stable phase is growing phase, whereas, in the high undercooled melts, the metastable phase grows as the primary phase, and the stable phase grows as the secondary phase.(5) The fragmentation mechanism of metastable dendrite is the dendrite remelting due to the serious superheating in the melts, and the course evolution of metastable phase is nucleation, growth, dendrite remelting, ripening, and then enwrapped by the growth of stable phase;(6) The solid state phase transformation of the undercooled Fe-7.5at.%Ni alloy are that stable phase Y transform to martensite, metastable phase δ transform to Y , firstly, and then transform to martensite.