Composition Design And Mechanical Properties Of CoCrNiTa_x Eutectic Medium Entropy Alloys

The concept of high entropy alloy greatly enriched the composition design method of alloy,and the concept of eutectic high entropy alloys solved the main problems of single phase high entropy alloys.CoCrNiTax alloy is an eutectic medium entropy alloy with FCC/Laves phase structure,which is expected to show good comprehensive mechanical properties.However,there are few studies on the mechanical properties of CoCrNiTax eutectic medium entropy alloy at room temperature and high temperature.In this paper,a series of CoCrNiTax eutectic medium entropy alloys were designed based on CoCrNi alloy by simple mixing method and thermodynamic phase diagram calculation.Firstly,the influence of Ta addition on the mechanical properties of CoCrNiTax alloys at room temperature was investigated by a series of mechanical properties experiments such as microhardness,nanoindentation,room temperature compressionand wear.Secondly,the high temperature deformation behavior of CoCrNiTax alloys was investigated by high temperature compression experiments at different temperatures and strain rates.At the same time,the phase structure,as-cast and high temperature deformation micro structure of CoCrNiTax alloys were investigated by XRD,SEM and EBSD.The main results are as follows:(1)With increasing Ta content,the micro structure of CoCrNiTax alloys evolves from hypoeutectic to eutectic and hypereutectic.CoCrNiTa0.1 and CoCrNiTa0.3 alloys are hypoeutectic alloys,and their microstructures are Ta-poor primary γ phase and γ/Laves eutectic phases.CoCrNiTa0.35 alloy is eutectic alloy,and its micro structure is lamellar eutectic of γ phase+Laves phase.CoCrNiTa0.4 and CoCrNiTa0.5 alloys are hypereutectic alloys,and their microstructures are Ta-rich primary Laves phase and γ/Laves eutectic phases.x=0.35 is the eutectic point of CoCrNiTax alloys.(2)With increasing Ta content,the microhardness of CoCrNiTax alloys increases gradually,and the hardness and elastic modulus of the primary phase increase,while the hardness and elastic modulus of the eutectic phases firstly increased and then decrease.The compressive fracture strain of CoCrNiTax alloys significantly decreases with increasing Ta content.The 0.2%compressive yield strength increases gradually,and the compressive strength increases first and then decreases with increasing Ta content.The CoCrNiTa0.35 eutectic alloy has the highest compressive strength(2852 MPa)and good plasticity(23.3%).At the same time,the wear mechanism of the alloy changes from adhesive wear to abrasive wear,and the wear rate and the depth of the wear trajectory gradually decrease.(3)The true stress-strain curves of CoCrNiTa0.35 eutectic medium-entropy alloy at 950℃,1000℃,1050℃ and 1100℃ and strain rates of 0.001 s-1,0.01 s-1,0.1 s-1 and 1 s-1 are all dynamic recrystallization.At the same time,when the strain rate is constant,the flow stress of CoCrNiTa0.35 alloy decreases with increasing deformation temperature.When the deformation temperature is constant,the flow stress of the alloy increases with increasing strain rate.As compared with some superalloys and high entropy alloys,CoCrNiTa0.35 eutectic medium entropy alloy has higher peak flow stress.(4)Based on the Arrhenius model and the true stress-strain curve,the constitutive equation of CoCrNiTa0.35 eutectic medium entropy alloy was constructed and compared with the experimental values,which had a high fitting degree.The constitutive equation can be described by the following formula:ε=7.1844×1014[sinh(0.0031σ)]3.7777exp(-415.44/RT)(5)After high temperature deformation,the coarsening phenomenon of γ/Laves eutectic lamellar appeared in CoCrNiTa0.35 eutectic medium entropy alloy.When the strain rate was constant,the coarsening degree of γ/Laves eutectic lamellar increased with increasing deformation temperature.When the deformation temperature is constant,with increasing strain rate,the coarsening degree of γ/Laves eutectic phases lamellar decreases.Meanwhile with increasing strain rate,the recrystallization volume fraction decreases,the grain size increases,and the geometric dislocation density increases,indicating that the decrease of strain rate will increase the degree of recrystallization of experiemental alloy.