Eutectic Transformation Mechanism Of Fe-B Alloy During Rapid Solidification

In industrial production,off-eutectic alloys are widely used with excellent characteristics,but the solidification process is relatively more complicated than eutectic alloys,the classical eutectic transition theories cannot be applied.Therefore,studying the eutectic transition mechanism and phase transition path of off-eutectic alloys are of great significance to theory and practice.In this thesis,glass purification method and cyclic superheating method are used to make Fe-B alloy melt with a stable undercooling.The melt is solidified under different recalescence degree through triggering nucleation.With help of high-speed camera and high-sensitive pyrometer,the interface morphology and temperature changes during the recalescence process were investigated,as well as relationship between the interfacial movement rate and recalescence rate.The effect of recalescence degree on microstructure and mechanical properties was also studied.After that,three extreme cooling methods were used:highly undercooling,atomization and single-roller spinning are used to prepare extremely cooling samples,which were heated at high temperature to study the phase change path of the alloy.Finally,Fe-B alloy was isothermal treated at a certain undercooling,and the effects of different holding times on the microstructure,primary phase fraction and microhardness were studied.The main conclusions are as follows:The solidification characteristics of off-eutectic Fe-B alloy were studied by highly undercooling.The recalescence rate and interfacial movement rate increase with the increasing of recalescence degree.At the same recalescence degree,the recalescence rate and interfacial movement rate increase with the increase of boron content.It is also found that the ratio of interface movement time to recalescence time is a constant value with the increase of recalescence degree,recalescence degree and the ratio of interface movement rate to recalescence rate is an inverse proportional function.The microstructure and mechanical properties were studied.It was found that the primary phase of?-Fe was refined and microhardness increased with the increasing of recalescence degree?undercooling?.The morphology of eutectic structure changed from lamellar eutectic to granular abnormal eutectic,and the microhardness of Fe₂B decreased.The microhardness of the alloy increased with the increasing of boron content.The Fe-B and Ni-B alloys were solidified by extreme cold in different ways,and the samples were heated in situ and analyzed by XRD.The Ni-B alloy existed the metastable phase Ni₄B₃through single-roller stripping method.During the heating process,Ni-B alloy had two transitions,and the microstructure showed that dendrites arranged in a certain direction and gradually coarsened.At room temperature,Fe-B alloy does not retain the metastable phase Fe₃B with?T=250 K,the precipitation of the supersaturated phase occurs during heating process,and eutectic microstructure completely become abnormal granular eutectic;Under the atomization condition,the powder of small particles had Fe₃B and Fe B phases,and was transferred to?-Fe and Fe₂B at 800?.Fe₃B in large particles did not change.After heating,the fine powder became sticky,and the precipitated phase on the particle surface increased,some of which were seriously distorted.Samples of Ni-3.2wt.%B and Fe-3.4wt.%B alloys with undercooling and isothermal treatment of up to 508 s and 593 s were obtained respectively.It was found that with the extension of holding times,the microhardness of primary phase?-Fe and?-Ni were decreased,the microhardness Fe₂B and Ni₃B are increased.The morphology of primary phase changes from rod shape to ball shape,and the edges become smooth.The morphology of the eutectic phase Ni+Ni₃B changed from strip lamellar to nets and then to fine lamellar.The lamellar regular eutectic structure of Fe+Fe₂B becomes granular anomalous eutectic.The phase fraction of?-Ni increased significantly with the increase of recalescence degree.Under the same recalescence degree,the primary phase fraction increases with the extension of holding times,but the growth rate decreases gradually.This shows that the primary phase fraction of the undercooled melt is affected by the recalescence degree and the isothermal time.