Deep Subcooling Rapid Solidification And Tissue Evolution Of Co-Cr-Fe-Ni-Sn Eutectic High-entropy Alloys

High-entropy alloy refers to a new alloy material composed of five or more elements in near molar ratio,which is endowed with unique structure and excellent properties due to its four major effects.Eutectic high entropy alloy combines the advantages of high entropy alloy and eutectic alloy,and has good strong plastic matching and casting performance.At present,the research of high entropy alloys is mostly focused on eutectic composition design and performance regulation,but there is little research on its solidification behavior mechanism.In this thesis,Co-Cr-Fe-Ni-Sn high-entropy alloys with different compositions were designed and prepared based on the simple mixing method of binary phase diagram,and their eutectic composition points were confirmed by DSC,XRD,SEM,TEM and other analytical tests,and then the alloy specimens with eutectic,sub-eutectic and super-eutectic compositions were subjected to deep subcooling rapid solidification experiments using the molten glass plus cyclic superheating method,and combined with infrared temperature measurement and The re-glow process was recorded by the combination of IR temperature measurement and high-speed photography,and finally,the phase composition,solidification behavior and tissue transformation patterns of Co-Cr-Fe-Ni-Sn high-entropy alloy solidified at different subcooling degrees were studied by XRD,SEM,EBSD and other characterization means,and the following main conclusions were drawn:（1）CoCr_0.8Fe_0.8NiSn_x（x=1.0,1.2,1.4）three high entropy alloys consist of Cr-Fe-Co solid solution phase with FCC structure and M₃Sn₂（M=Ni,Co）intermetallic compound phase with HCP structure.The eutectic composition point is CoCr_0.8Fe_0.8NiSn_1.2.The maximum subcooling degree of 200 K is obtained,and the histomorphology undergoes three transformations in the subcooling degree range of0～200 K.（2）CoCr_0.8Fe_0.8NiSn_1.2 eutectic high-entropy alloy:ΔT<40 K,the solidification organization is regular lamellar eutectic;40 K<ΔT<110 K,the solidification organization of lamellar eutectic transforms into feathery eutectic;110 K<ΔT<190 K,the feathery eutectic gradually disappears,and the solidification organization consists of fine lamellar eutectic and granular anomalous eutectic;ΔT>190 K The solidification organization is composed of fine lamellar eutectic and granular anomalous eutectic.（3）CoCr_0.8Fe_0.8NiSn_1.0 sub-eutectic high-entropy alloy:ΔT<41 K,the solidification organization is incipientα-FCC dendrites+lamellar eutectic;41K<ΔT<103 K,the solidification organization is transformed from lamellar eutectic to feathery eutectic;103 K<ΔT<167 K,the alloy deviates from the eutectic eutectic region,and the solidification organization consists of FCC phase,M₃Sn₂（M=Ni,Co）phase and a small amount of rod+lamellar eutectic organization;ΔT>167 K,the feathery eutectic disappears and the solidification organization is fine standard eutectic+anomalous eutectic.（4）CoCr_0.8Fe_0.8NiSn_1.4 over-eutectic high-entropy alloy:ΔT<37 K,the solidification organization is primaryβ-M₃Sn₂（M=Ni,Co）phase+rod eutectic;37K<ΔT<105 K,the solidification organization changes from rod eutectic to feather eutectic;105 K<ΔT<151 K,the alloy deviates from the eutectic eutectic zone,and the feather eutectic disappears.The solidification organization consists of M₃Sn₂（M=Ni,Co）phase,FCC phase and a small amount of rod eutectic;ΔT>151 K,the solidification organization is composed of fine standard eutectic+anomalous eutectic.（5）Three typical eutectic histomorphologies exist in the supercooled CoCr_0.8Fe_0.8NiSnx（x=1.0,1.2,1.4）high-entropy alloy:lamellar/rod-li Ke standard eutectic,feather-li Ke eutectic at moderate supercooling,and anomalous eutectic at large supercooling.The formation mechanism of lamellar/rod eutectic:when the volume fraction of the two phases in the eutectic phase is different,it is favorable to form a rod,and vice versa for the formation of lamellar.Mechanism of formation of feathery eutectic:Under the condition of non-equilibrium solidification,insufficient atomic diffusion leads to solute enrichment at the interface front,resulting in component subcooling.At this time,the solute conditions for lamellae growth cannot meet the needs of the two-phase coupling growth of eutectic and gradually transform into feathery eutectic.Mechanism of formation of anomalous eutectic:The eutectic lamellae in the adjacent grains of the supercooled alloy are extruded under the action of solidification stress,resulting in their fragmentation and partial remelting under the action of latent heat of crystallization released in the re-glowing process,resulting in granular anomalous eutectic.