| High-entropy alloys have attracted much attention due to their excellent mechanical,physical and chemical properties,and unique microstructures.In recent years,different solidification conditions and heat treatments have been adopted to tailor the structures and mechanical properties of high-entropy alloys.In current work,Fe26.7Co26.7Ni26.7Si8.9B11.0 high-entropy alloy was fabricated into ribbons,bulk alloys and droplets,respectively,by melt-spinning,suction casting,and the system of space environment microgravity.The microstructures and properties of alloys were changed by controlling solidification methods and heat treatments.Firstly,the as-quenched fully amorphous ribbons and amorphous-crystalline ribbons were prepared by controlling rotation speeds of the copper wheel during rapid solidification,respectively.The phase formation can be further changed by different heat treatments.(1)For the fully amorphous ribbons(AR),BCC solid solution and Ni31Si12 intermetallic compounds precipitated in amorphous matrix when the annealing temperature was determined to be 786 K.With further increasing annealing temperature to 852 K and 1010 K,the BCC crystals gradually precipitated and then transformed into two kinds of FCC solid solutions accompanied by precipitation of(Fe,Co)2B and Ni31Si12 crystals.(2)For the amorphous-crystalline ribbons(ACR),the primary FCC solid solutions appear in the amorphous matrix.After annealing at 786 K,the metastable FCC solid solution gradually transformed into BCC structure with the precipitation of Ni31iSi12 crystals.When annealing at 852 K and 1010 K,the phase formation was similar to that of AR samples,which transformed from metastable BCC structure to two types of FCC solid solutions.This polymorphic phase transition also resulted in the changes of their magnetic properties.Both as-cast AR and ACR samples showed good soft magnetic properties.After heat treatments,the saturation magnetization(Bs)and coercivity(Hc)of the investigated samples changed obviously due to different phase transformations.Bs increased firstly and then decreased with increasing annealing temperature,while the Hc of other samples increased with rising annealing temperature except the ACR samples annealed at 852 KSecondly,the as-cast rods were prepared by suction casting and then different heat treatments were adopted.The as-cast rods was hypoeutectic structure,in which primary FCC dendrites distributed in the lamellar eutectic matrix.The eutectic matrix was composed of FCC crystals,(Fe,Co)2B,and a small amount of Ni31Si12 intermetallic compounds.After annealing at 786 K and 852 K,the eutectic structure coarsened,and the volume fraction of FCC phase increased with the gradual dissolution of the eutectic matrix.Meanwhile,(Fe,Co)2B and Ni31Si12 precipitated in the matrix.After heat treatments,the solute redistribution also changed the compositions of different phases.After annealing to 1010 K,the eutectic structures were completely dissolved,and the interconnected FCC crystal and(Fe,Co)2B formed as the framework of microstructures.When the annealing temperature further rose to 1118 K,the FCC solid solution became the dominant framework,while the isolated(Fe,Co)2B intermetallic compounds worked as the main reinforcement particles distributing in the interdendritic region.This gradual microstructural transition from hypoeutectic to quasi-duplex structures led to the change of the dominant deformation mechanism from crack-controlled to dislocation-dominated deformation.This transition further affected the mechanical properties:(1)for samples annealed by furnace cooling,the yield strength decreased from 2111±40 MPa at 786 K to 1159±150 MPa at 1118 K,while the plastic strain and ultimate strength increased from 0.9±0.1%and 2438±61 MPa to 25.7±4.4%and 2577±222 MPa respectively;(2)for samples annealed by quenching,the yield strength decreased from 1595± 164 MPa at 786 K to 918 ±78 MPa at 1118 K,while the plastic strain and ultimate strength rose from 2.5±0.6%and 2377± 54 MPa to 39.8± 3.4%and 3227± 138 MPa respectively.Finally,the supercooled high-entropy alloy droplets were fabricated by the microgravity suspension.The different undercooling degrees of alloys with different droplet sizes were strongly affected by different cooling rates,which further affected their microstructures.The microstructures of all droplets consisted of dendritic FCC solid solution,granular(Fe,Co)2B crystals,and a few unknown crystals.For larger droplets,the lower undercooling induced dendrites coarser,which interconnected with each other and the dominant framework could be induced.Besides,there were also coarser(Fe,Co)2B particles appearing in the interdendritic regions.With decreasing of droplet size,the degree of supercooling and cooling rate obtained during solidification gradually increased,respectively.Meanwhile,the critical nucleation size of the droplets decreased with increasing degree of supercooling,so that embryos with small sizes could gradually grow up.Therefore,the grain sizes of the alloy were refined to some extent.However,the FCC dendrites in the droplets were still the dominant phase.When the droplet size decreased to around 1000 microns,the undercooling degree further increased,which resulted in the refinement of FCC dendrites.At the same time,(Fe,Co)2B alloys distributed uniformly in the interdendritic regions.The mechanical properties were also affected by the changes of the microstructures.All droplets with different sizes exhibited high nanohardness and Young’s modulus.The nanohardness and Young’s modulus of samples with a droplet size of 2300±150μm were the largest.With decreasing of droplet size to about 1700±150μm,the fluctuation of naonohardness decreased obviously,while the decrease of intragranular segregation resulted in the decrease of nanohardness as well.As a reuslt,the present droplets displayed excellent mechanical properties at room temperature. |
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