Effects of a high magnetic field on the coarsening of MnBi grains solidified from isothermal annealed semi-solid melt

With the imposition of a high magnetic field up to 11.5 T, the morphologic transition of the MnBi phase and the magnetic properties of the Bi–4.36 wt.% and 8.25 wt.% Mn alloy specimens solidified from a semi-solid state were investigated. Compared with the irregular shape and unequal dimensions of MnBi grains under 0 T, the coarsened MnBi grains were connected together and tended to be parallel to the direction of the external field of 11.5 T. The changes might be attributed to the Ostwald ripening of ferromagnetic grains in the isothermal holding stage and subsequent aggregation along the direction of external field driven by mutual magnetic force among the grains. The uniform orientation of the ferromagnetic MnBi grains obtained in the microstructures effectively improves the saturation magnetization of the Bi/MnBi composite materials, but the oversized grains and the defects in grains make the coercivity decrease.