Structural Evolution And Metastable Phase Formation Of Fe-B Eutectic Alloy Under High Undercooling/Hyperundercooling Conditions

In this paper, the metastable phase was formed as a consequence of melt hyper-undercooling and preserved in final solidified microstructure by employing molten glass slag purification incorporated with cyclical superheating techniques in the bulk Fe-B eutectic alloy melts. The fundamental technique and engineering controlling condition of obtaining hyperundercooling in Fe-B eutectic alloy was systematically investigated. Moreover, the formation and microstructure evolution of metastable phase under hyper-undercooling condition was also studied. Based on the classical nucleation theory and the time-dependent nucleation theory, the thermodynamics and the kinetics for the competitive nucleation between the metastable and stable phases have been calculated, and hence a critical condition of forming the metastable phase was proposed. Finally, the material with metastable phase was identified by SEM, TEM , X-ray powder diffraction and DTA technique. The main results are as follows:(1) By the method of molten glass slag purification and recyclesuperheating under Ar atmosphere, a maximum hyperundercooling of 460K was obtained in Fe83B17 alloy melt.(2) While the undercooling of Fe83B17 alloy melt is more than 386K, the metastable phase will form directly from the undercooled melt by the competitive nucleation between the metastable and stable phases.(3) In the obtainable undercooling range, when the undercooling of melt is less than 340K, the microstructure is hypoeutectic structure because of offset of eutectic intergrowth region under the non-equilibrium solidification. In the microstructure, the primary phase is α - Fe, eutectic structure is the mixture of α - Fe and Fe2B , which is in the interspace of the primary phase. With the increase of undercooling, the microstructure of the primary phase will undergo the transformation from fine grain to rough grain and to fine grain again. The two critical undercooling for the transformation are 123 K and 250K respectively. When the undercooling of melt is more than 340K, the microstructure of the alloy will transform into complete eutectics structure. With the increase of undercooling, the α - Fe in eutectic structure become more thick and a complete anomalous eutectic structure is formed eventually.(4) It indicates that the microstructure of hyperundercooled Fe83B17 alloy is typical anomalous eutectics. When metastable phase is formed , more latent heat of crystal makes size of α-Fe phase increased further. The formation mechanism of anomalous eutectics in the metastable phase alloy is preliminarily studied, the results indicate that α- Fe phase first form dendrite frames in the eutectic melt ,then metastable phases(Fe3B) nucleate and grow in the interval of the frames.(5) Based calculateon results of the classical nucleation theory and the time-dependent nucleation theory, it indicates that the nucleation work and the incubation periods of metastable phase(Fe3B) are less than that of stable phase(Fe2B), the nucleation rate of metastable phase(Fe3B) is higher than that of stable phase(Fe2B)when the undercooling of Fe83B17 alloy melt is more than one critical undercooling. So the metastable phases(Fe3B) defeat stable phases(Fe2B) in the competitive nucleation, and nucleate directly from the melt. The calculated critical undercooling of two nucleation theory are 53K and 343K respectively. It suggest the undercooling of Fe83B17 alloy melt is a chief factor in determining the competitive nucleation of metastable phase and stable phase.(6) All the results of SEM, TEM and X-ray powder diffraction indicate that the samples undercooled more than 386K contain the metastable phase...