Research on grain refinement in bulk undercooled Fe-Co base alloys

Fe-Co base alloys, owing to their excellent high tempreture soft magnetic property, have been given more and more attention. Especially, Fe-Co base bulk amorphous-nanocrystalline materials have became an important development direction, while the preparation techniques have limited the wide applications of these high performance magnetic materials. Undercooling rapid solidification technique, independent of the sample size, has unique advantages in preparing bulk microcrystalline and nanocrystalline materials. However, upon large volume of alloy melt, more heterogeneous nucleus and latent heat of crystallization will occurr, which is disadvantageous to obtain a high undercooling and to repress grain growth in the process of solidification. Obviously, regular rapid solidification technique has not met the requirements for the preparation of industrial products with a large volume. So it is important and necessary to combine other rapid solidification techniques with regular rapid solidification technique to achieve high undercooling. In present work, copper mould chilling was used for undercooled Fe-Co base alloy melts. On one hand, the latent heat of crystallization can be transmitted to outside more rapidly by copper mould, on the other hand, by increasing the cooling rate, copper mould can also make undercooled melt achieve further undercooling. In this work, applying fluxing purification and cycling superheating method, Fe₄₄Co₄₄ Nb₇B₄Cu₁ melts were undercooled, and microstructure evolutions of the two different kinds of Fe₄₄Co₄₄ Nb₇B₄Cu₁ alloy samples prepared by undercooling solidification and copper mould chilling were studied, respectively. Using SEM and EDS, the grain refinement mechanism was investigated systematically. The experimental results show that the dendrite structures chang into granular grains in both the two kinds of samples with the increase of undercooling. The critical undercooling of dendrite structures changing into granular grains is smaller in the samples prepared by copper mould chilling than that by undercooling solidification. And the smaller grains and more homogenous microstructures are found in the samples prepared by copper mould chilling under the same undercooling. In combination with the calculations and the analysis of experiment results, it indicates that the decrease of the grain size is mainly attributed to the melt supercooling, remelting and copper mold chilling which increases nucleation rate and inhibits the grain growth. While alloying element gathering at the grain boundary is not the main factor.