Study On Structure Evolution Of Multi-component Co-based Alloys Under Undercooling Conditions

It is rather important to study microstructure evolution of amorphous alloys, microstructure under different undercoolings and universal solidification regulation of BMG formation from a thermodynamic undercooling perspective. The method of glass fluxing and recycling overheating in air atmosphere is applied to study the undercooling behavior of Co-based alloys systematically. The main conclusions as follows:The phase constitution of (Co_0.68Fe_0.32)₈₀B₂₀ alloy under different undercooling is investigated in the thesis.The compositon of (Co_0.68Fe_0.32)₈₀B₂₀ alloy is near the quasi-peritectic point in Co-Fe-B phase diagram.The volume fraction of primary phase increased with increasing the undercooling because the quasi-peritectic reaction is lagged by solute traping. At the undercooling of 68K, the microstructure is composed of γ(FeCo)and (Fe,Co)₂B phase. There is little Co_B phase distributing in the main microstructure when the undercooling increases to 80K. The Co₃B phase becomes the main phase in the ultimate undercooled microstructure when the undercooling up to 110K.The microstructure evolution of (Co_0.68Fe_0.32)₈₀B₂₀ alloy is investigated in the thesis subsequently. With increasing the undercooling, the microstructure undergo a coarse dendrite, equiaxed dendrite, eutectic, anomalous eutectic and pseudo segregation free structure transformation. The crystal grain size and primary dendrite space decrease with increasing the undercooling, meanwhile, the dendrite remelting phenomenon occurres. The second phase distribute in the primary phase arranging as "Y" shape when the alloy becomes pseudo segregation free structure.The nucleation and growth mechanisms of undercooled (Co_0.68Fe_0.32)₈₀B₂₀ alloy are also studied in the dissertation.The classic nucleation theory is applied in the section. The calculations show that the nucleation rate of primary phase is always bigger than that of second phases, but they nearly come to the same value when the undercooling exceed 101K. When the undercooling below 82K, the growth velocity of Co₃B phase is smaller than that of the quasi-peritectic phases. It becomes the biggest one gradually when the undercooling exceeds 82K.A further analysis of solidification character is implemented on multi-component...