Studies On Bulk HITPERM Alloys Rapidly Solidification Microstructure Evolution And Refinement Mechanism

Adopting undercooling solidification and undercooling solidification combined with copper mould chilling,the bulk Fe₄₄Co₄₄Nb₇B₄Cu₁ alloy was obtained.The microstructure evolution and refinement mechanism were researched systematically by Optical Microscopy?OM?,Scanning Electron Microscope?SEM?,X-ray Diffraction?XRD?,Energy Dispersive Spectrometer?EDS?and Electron probe microanalysis?EPMA?.The grain refinement mechanism of the undercooled alloys was analyzed by dendrite remelting theory model.The influence of copper mold chilling and alloying elements on the grain refinement in as-solidified microstructure of Fe₄₄Co₄₄Nb₇B₄Cu₁ alloy were analyzed by classical nucleation theory and the kinetics of grain growth of nanocrystalline materials.The hysteresis loop and microhardness were respectively tested by vibrating sample magnetometer?VSM?and hardness tester,the properties were discussed based on solidification structure.The main conclusions are listed as below:1.Within 45K<?T<145K,?-Fe?Co?phase experienced the change of dendrite structures to granular grains in Fe₄₄Co₄₄Nb₇B₄Cu₁ alloy prepared by undercooling solidification with the critical undercooling of 145K.With the increase of undercooling.the solidification microstructure was continuly refined,among?-Fe?Co?dendrites the lamellar eutectic structure of??-Fe?Co?+NbCo₃?with the lamellar spacing of about 500nm was formed.The grain refinement of?-Fe?Co?phase in the undercooled Fe-Co based alloys was the result of remelting upon recalescence.2.Within 45K<?T<95K,the solidification microstructure of Fe₄₄Co₄₄Nb₇B₄Cu₁alloy prepared by undercooling solidification combined with copper mould chilling showed the microstructural transformation of dendrite structures--mixed dendrite and granular grains--granular grains.Compared with undercooling solidification,the smaller grains and more homogenous microstructures were formed in the samples prepared by undercooling solidification combined with copper mould chilling with the same undercooling.3.Microstructures of FeCo multicomponent bulk alloys prepared by undercooling solidification combined with copper mold chilling is constituted by three zones,i.e.the fine grains close to the shaped wall,mixed dendrite and granular grains in the middle and equiaxed crystals in the center.4.The decrease of the grain size is mainly attributed to the melt undercooling,dendrite remelting and copper mold chilling which increases nucleation rate and inhibits the grain growth.While alloying elements gathering at the grain boundary is not the main factor.Adding microelement to improve amorphous forming ability can make a better grain refinement in the undercooling solidification with copper mold chilling.5.FeCo based alloys prepared by undercooling solidification combined with copper mould chilling have better soft magnetic properties,although,the saturated magnetic induction intensity B_s decreases,the coercivity H_c greatly decreases campared with conventional FeCo alloys.The microhardness of the alloys increases with the decrease of the grain size.