Design And Rapid Solidification Of Mn-Mn-Fe-Co-Ni-B Eutectic High Entropy Alloy Under Deep Undercooling

Eutectic high entropy alloys（EHEAs）have the advantages of both eutectic and high entropy alloys（HEAs）,having excellent comprehensive mechanical and casting properties.However,current research on EHEAs mainly focuses on composition design and performance optimization.The solidification behavior of EHEAs,such as nucleation,growth characteristics,and phase transformation,remains to be discussed.The classical eutectic solidification theories are inappropriate to explain the complex solidification behavior under high entropy conditions.Therefore,the nucleation and growth phenomena of EHEAs and extending them to conditions such as deep undercooling and rapid solidification is one of the important directions for the research and development of eutectic solidification theory at present.In this thesis,a new Mn-Fe-Co-Ni-B EHEAs were designed and prepared using the binary eutectic mixing method.Hypoeutectic Fe₇（Co Ni Mn）₈₄B₉,eutectic Fe₇（Co Ni Mn）₈₀B₁₃,and hypereutectic Fe₇（Co Ni Mn）₇₈B₁₅ were selected as the research objects.Using the method of molten glass purification and cyclic superheating,samples were obtained for deep undercooling and rapid solidification at different undercooling.Microstructures and partial properties of as-cast specimens were tested and analyzed.Cooling curves,microstructures,and phase compositions of the deeply undercooled solidified specimens were characterized and discussed.The interface morphology during deep undercooling solidification of the alloy was observed in-situ using a high-speed camera.And the growth rate of EHEAs was calculated.The main conclusions are as follows:（1）The as-cast microstructures of Mn-Fe-Co-Ni-B EHEAs consists of（Mn,Fe,Co,Ni）solid solution and（Mn,Fe,Co,Ni）₂₃B₆ complex intermetallic compound.Under the combined action of various factors such as phase composition,crystal structure,interface energy,and interface type,a complex and regular trilobal rod-shaped eutectic structure is formed.（2）Three types of microstructures are observed during Fe₇（Co Ni Mn）₈₄B₉undercooling solidification.Under medium and small undercooling conditions,the alloy exhibits two recalescence phenomena,corresponding to the formation of（Mn,Fe,Co,Ni）primary phases and eutectic structure,respectively.At small undercooling,the trilobal eutectic disappears,and the eutectic structure changes from rod-shaped to short layered.The volume fraction of eutectic structure decreases as the undercooling increases.At high undercooling,only one recalescence occurs,corresponding to the rapid growth of primary（Mn,Fe,Co,Ni）dendrites.And the（Mn,Fe,Co,Ni）₂₃B₆phase nucleates and grows between dendrites,completely forming divorced eutectic.（3）The eutectic Fe₇（Co Ni Mn）₈₀B₁₃ alloy exhibits single point nucleation within a small undercooling range,forming（Mn,Fe,Co,Ni）primary phases and short rod-shaped eutectic structures.Within the medium undercooling range,the alloy undergoes non simultaneous and multi-zone nucleation,as well as competitive nucleation of primary phases.As the undercooling increases,the primary phase transforms from（Mn,Fe,Co,Ni）to（Mn,Fe,Co,Ni）₂₃B₆.The alloy exhibits single point nucleation under deep undercooling,generating two types of anomalous eutectic structures.When the undercooling is lower,some primary phase dendrites and eutectic structures are melted to be broken,then recrystallize and grow to form anomalous eutectic during the slow solidification stage.Under higher undercooling,the two phase non-coupled growth of the alloy forms anomalous eutectic.（4）The solidification structure of Fe₇（Co Ni Mn）₇₈B₁₅ in a small undercooling range is（Mn,Fe,Co,Ni）₂₃B₆ primary phase and lamellar eutectic,and the eutectic structure is refined with increasing undercooling.At medium undercooling,the recalescence phenomenon is mild,and partial eutectic remelting occurs to form anomalous eutectic.The microstructures of the alloy are composed of primary（Mn,Fe,Co,Ni）₂₃B₆ phase,eutectic structure,and anomalous eutectic.The primary phase and eutectic remelted severely with obvious recalescence under deep undercooling.When the undercooling increases to 277 K,the solid-liquid interface advances in a nearly circular arc shape.The primary phase of the alloy changes from（Mn,Fe,Co,Ni）₂₃B₆ to HCP-（Mn,Fe,Co,Ni）solid solution phase.The alloy is composed of HCP-（Mn,Fe,Co,Ni）solid solution and（Mn,Fe,Co,Ni）₂₃B₆ complex intermetallic compound.（5）The nucleation of the alloys are observed using high-speed photography and infrared temperature measurement.Within the range of undercooling obtained in the experiment,Mn-Fe-Co-Ni-B HEAs exhibit single point nucleation,non simultaneous and multi-zone nucleation,and simultaneous multi-zone nucleation.As the undercooling increases,the dendritic growth rate changes from a power exponential growth dominated by thermal diffusion to a linear growth dominated by kinetics.