Study On Solidification Process And Properties Of Multi-component Alloys

In this paper,（Co/Cr/Fe/Ni）₃Nb_x mutil-component alloys were prepared by vacuum arc furnace smelting as the research object.Modern analytic determination methods include SEM,EDS,XRD and DSC were used to research the microstructures,composition,phases and phase transition process of these alloys.The hardness and compressive properties were used to characterize the mechanical properties of the alloys.Besides,the parameters coveredδ,ΔH,ΔX,VEC,Md were also studied.A simple phase diagram was made by the Themo-Calc simulation results and experimental data.According to the previous experiments,CoFeNi,CrFeNi and CoCrNi alloys are constituted by single FCC phase,while the CoCrFe alloy is composed of FCC andσphases.When Nb element added,Laves phase appeares in all alloys,and the eutectic structure also appeares in all alloys apart from CoCrFeNb_0.5 alloy.Ternary alloy which constitute by single FCC phase shows good ductility.However,due to the formation ofσphase,CoCrFe alloy has bad plasticity which strain is only 17.7%.With the addition of Nb,the strength and handness of the alloys increase significantly,while the plasticity decreased compared to the ternary alloy.Overall,in this series of alloys,CoCrNi and CoCrNiNb_0.5 alloy have the best microstructure and properties.CoCrNiNb_x multi-component alloys are just composed by FCC and Laves phases,with the content of Nb increased.Besides,the volume fraction of eutectic structure in the alloy increased first and then decreased with increase of Nb content.According to the experimental results,it is proved that when x<0.46,the solidified microstructure is hypoeutectic;when x=0.46,the complete eutectic structure is formed;when x>0.46,the solidified structure is hypereutectic alloy.Hardness of dendritic region in hypoeutectic alloys increases with the increase of Nb element.Nevertheless,the hardness of dendritic district in hypoeutectic alloys is significantly higher than that of hypoeutectic alloys.In this series of alloys,the optimal mechanical properties happened with the ratio of volume fraction for FCC phase to eutectic structure is 2:1.At the same time,the phase formation parametersΔH,δ,ΔX and VEC are failed to predicted the formation of phase in the CoCrNiNb_x alloys,while the parameter Md can predict the phase formation law of the CoCrNiNb_x alloys accurately.According to the test,when Md≤0.882,only FCC single phase is produced in the alloy,while Md>0.882,forms compound phase.The thermal analysis shows that there are three phase transitions in the hypo eutectic alloy.During the solidification process,the dendrite FCC phase first precipitates,after this,eutectic reaction occurs.As the temperature decreases further,part of FCC phase occur solid-state transformation,and form Laves phase.During the solidification process of hypereutectic alloys,Laves phase in dendritic first nucleates and grows.With the decrease of temperature,eutectic reaction occurs.After different heat treatment process,the phases of CoCrNiNb_0.46 multi-component alloys do not change,but with the increase of heat treatment temperature and holding time,the stability of the alloy decreased.After holding at 900℃for 72 h,laminar structure in CoCrNiNb_0.46 alloy almost disappeared.When the eutectic structure disappeared,the hardness of the alloy is greatly reduced.