Microstructure,Properties,and Synergistic Effects Of Alloying Elements Of Co_xCr_yCuFeMnNi System High-entropy Alloys

Solid solution high-entropy alloys?HEAs?composed of four or more principle-elements usually exhibit simple structures,high hardness and strength,excellent high-temperature and low-temperature mechanical properties,good wear resistance and corrosion resistance,and good electromagnetic properties have been hailed as one of the three major breakthroughs in modern metal materials,and also one of the hotspots in contemporary metal materials research.Therefore,HEAs have great potential in novel structural and functional materials.However,the research of HEAs as a new metal material on design criteria,phase formation mechanism,strengthening mechanism,and physical and chemical properties is still insufficient.Five-element equimolar or near-molar HEAs designed from the six elements Co,Cr,Cu,Fe,Mn,and Ni have been extensively studied,and show stable structure,excellent low temperature,room temperature and high temperature mechanical properties.But relatively little research has been done on chemical and magnetic properties.Co element with HCP structure can promote the formation of FCC phase,and Cr element with BCC structure can promote the formation of BCC phase.However,the effects of Co,Cr and synergy effects of alloying elements on the mechanical,chemical and magnetic properties of HEAs are less studied.In this paper,Co_xCr_yCu Fe Mn Ni system HEA powders and bulks are prepared using mechanical alloying and hot-pressing sintering processes,respectively.The effects of Co and Cr elements and their synergistic effects on the mechanical alloying behavior,phase formation mechanism,microstructure,room temperature mechanical properties,chemical properties and magnetic properties of the HEA system were systematically studied.It provides theoretical and experimental basis for the application of HEAs in high temperature structural materials,corrosion resistant materials and magnetic materials.Through in-depth research and analysis,the main research results are as follows:?1?In the initial stage of mechanical alloying?5?15 h?,the solid solution structure of BCC-based and FCC-assisted solid solution structure is formed after the alloying of each element in Co_xCr_yCu Fe Mn Ni system HEA powders.After extending the milling time to 30 h,the metastable BCC phase transitions to the FCC phase.After 50 h of milling,a flaky nanocrystalline FCC+BCC or FCC solid solution structure with a size of about 50mm and a thickness of about 1mm is formed.As the increase of Co content,the phase structure of the alloys change from FCC+BCC to a single FCC structure.As the increase of Cr content,the phase structure of the alloys transform from a single FCC structure to an FCC+BCC structure.After hot-pressing sintering,the Co₀ alloy is composed of a Cr-rich FCC1 main phase,a Cu-rich FCC2secondary phase,and a small amount ofrphase.Cr_2.0alloy consists of Cu-rich FCC2main phase,Cr-rich FCC1 secondary phase and a small amount ofrphase.Other bulk alloys are composed of FCC2 main phase and FCC1 secondary phase.Co content increased,FCC1 phase content decreased and FCC2 phase content increased;Cr content increased,Cr-rich FCC1 phase content increased and FCC2 phase content decreased.The formation of FCC and BCC solid solutions in Co_xCr_yCu Fe Mn Ni system HEAs mainly depends on the competition mechanism of FCC formation ability and BCC formation ability.When is satisfied,a BCC+FCC or BCC phase structure is easily formed in the alloys;when formed in the alloys.?2?The hardness and yield strength of Co_xCr_yCu Fe Mn Ni system bulk HEAs decrease with increasing of Co content and increase with increasing of Cr content.Co₀ and Cr_2.0 alloys have higher hardness and yield strength,reaching 523 HV,1660MPa and 543 HV,2130 MPa,respectively.This is mainly due to the precipitation strengthening of therphase and the solid solution strengthening of the Cr-rich FCC1phase.The strengthening mechanism of other alloys is mainly solid solution strengthening.?3?The i_corr and V_corr of Co_xCr_yCu Fe Mn Ni system bulk HEAs in 3.5%Na Cl solution decrease with the increase of Co and Cr content.Among them,the i_corr and V_corr of the Co_2.0 and Cr_2.0 alloys are relatively low,and the relative resistance R_p and the relative thickness of the passivation film CPE₁^-1 are relatively high,thus showing good corrosion resistance.The corrosion resistance mechanism of Co_xCr_yCu Fe Mn Ni system bulk HEAs is mainly that Co promotes the formation of Co?OH?₂ adsorption layer on the alloy surface,which enhances the stability of the passivation film composed of Cr₂O₃ and Ni?OH?₂.Increasing the Cr content makes the Cr₂O₃passivation film formed on the alloy surface more dense,which inhibits or delays further corrosion of the corrosion medium.?4?The oxidation kinetics curve of Co_xCr_yCu Fe Mn Ni system bulk HEAs approximately follows the parabolic law.Increasing Co content leads to a decrease in oxidation resistance,while increasing Cr content increases the oxidation resistance of the alloy.In this alloy system,the Cr_2.0 alloy has the lowest K_p value in the rapid oxidation and slow oxidation stages,which are 2.29?10^-11 and 3.46?10^-12g²cm^-4s^-1,respectively,showing the best oxidation resistance.The oxidation products of the alloy system are mainly oxides of Mn₃O₄,Mn₂O₃,Cr₂O₃ and?M,Cr?₃O₄ type spinel structure.The oxidation mechanism is mainly selective oxidation.?5?Co element is beneficial to increase the Ms value of Co_xCr_yCu Fe Mn Ni system bulk HEAs powders and bulks,while Cr element is the opposite.The Cr₀alloy powder has the best soft magnetic properties.Its Ms and Hc are 84emu/g and 6Oe,respectively.After sintering,the Cr₀ high entropy alloy bulk material has 5emu/g and 11.5 Oe.Its soft magnetic properties are significantly reduced due to the increase of the grain size of the magnetic phase.The hysteresis loops of Co_xCr_yCu Fe Mn Ni system HEA powders and bulks both show typical asymmetry.Among them,the hysteresis loop of Co_1.0 alloy is shifted to the coordinate side as a whole and presents a large offset of approximately 2Hc.?6?The effects of Co and Cr elements on the microstructure and mechanical properties,chemical properties and physical properties of Co_xCr_yCu Fe Mn Ni system HEAs are essentially determined by the synergistic effect of alloying elements in HEAs.It is mainly reflected in the structural change of the phases and the components distribution caused by the competition mechanism between the FCC phase?FCC1 and FCC2 phases?and the BCC phase?orrphase?.Firstly,the increase of Cr_equiv promoted the formation ofrphase and the increase of FCC1 phase content,while the increase of Co_equiv suppressed the formation ofrphase and promoted the increase of plastic FCC2 phase content.Increasing of the content of the strengthening phase and the FCC1 phase significantly increased the hardness and yield strength of the HEAs,but reduced the plasticity of the alloys.Secondly,the increase of Cr_equivpromotes the increase of the FCC1 content and also increases the content of the corrosion resistance element Cr in the FCC1 phase,which makes the alloy easier to form a passivation film in the corrosive medium or high temperature environment.The density and thickness of the passivation film also increase with the increase of Cr_equiv,especially when Co_equiv is low.Therefore,alloys with low Co_equiv and Cr_equivhave better corrosion resistance and oxidation resistance.Finally,the increase in Cr_equiv has a small effect on the magnetic phase content and the relative content of ferromagnetic elements in the magnetic phase.Therefore,the effect on the Ms value of the alloys is more significant,especially when the Cr_equiv is lower,the effect of Co_equiv is more obvious,Therefore,by coordinating the increasing range of Co_equiv and the decreasing range of Cr_equiv,HEA powder or bulk materials with good soft magnetic properties can be obtained.