Effect Of Ball Milling Speed On The Mechanical And Corrosion Properties Of CoCrFeMnNi High-entropy Alloy

As a new kind of material with excellent mechanical properties,good corrosion resistance,high temperature thermal stability and other characteristics,high entropy alloys(HEAs)have attracted extensive attention.Recent studies show that the comprehensive mechanical property of HEAs sintered with gas atomized alloy powders is apparently higher than that prepared by conventional mechanical alloying with pure metals.However,the influence of powder morphology and deformation state before sintering on the properties of alloys has few reported.In this paper,CoCrFeMnNi was prepared by the pre-deformed CoCrFeMnNi powers with different ball milling speeds.The effects of ball milling speed on both mechanical properties and corrosion resistance are explored based on the powder morphology and microstructure observation.The results will provide experimental and theoretical support for the research and development of high performance HEAs and their composites.To reduce the powder pollution and excessive deformation frequently occurred in common ball milling process,wet ball milling with anhydrous ethanol as process control agent was adopted.The milling speed was chosen to be 0(un-milled),100,300 and 500 rpm,respectively.Four kinds of CoCrFeMnNi with different milling speed were obtained.The mechanical properties and corrosion resistance were studied systematically.XRD measurements of CoCrFeMnNi powder with different milling speeds were carried out at the same conditions.It is found that only a single FCC phase and no second phase diffraction peak can be observed for pre-deformed powders with different milling speed.The corresponding lattice constant a and lattice distortion εd were calculated.With increasing milling speed,a decreases slightly at first and then increases while εd increased continuously.The maximum εd is 0.27%at 500 rpm.Microstructure observation and EDS analyses show that the powder gradually changed from spherical to plate-like with increasing milling speed.In addition,Fe pollution in CoCrFeMnNi powder can be observed at milling speed of 500 rpm,which was caused by the Fe element escaped from ball milling ball and tank.The corresponding DSC results show apparently exothermic phenomenon at 500-900℃,which should be caused by the recovery and recrystallization of pre-deformed powder.The exothermic effect is more apparent at higher milling speed.The optimum sintering temperature of CoCrFeMnNi is determined to be 1120℃through density test.XRD results show that all sintered alloys with different pre-deformed powders have a single FCC structure;furthermore,the ball milling speed has small effect on the lattice constant and lattice distortion of bulk alloys.It indirectly confirms the hypothesis that the diffraction peak broadening observed in powders is mainly related to lattice distortion.With increasing ball milling speed,the alloy density increases firstly and then decreases while the highest density(99.2%)can be obtained for 100 rpm.The grain size decreases from 12.39±0.31 μm for 0 rpm alloy to 4.21±0.41 μm for 500 rpm alloy.The hardness and compression test results show that the hardness of alloys increases from 209±10 HV to 378±18 HV while the compression yield strength increases from 314±22 MPa to 404±29 MPa with increasing milling speed from 0 to 500 rpm.The tensile test results show that CoCrFeMnNi alloy sintered with 100 rpm powder has the highest tensile strength without apparent loss of elongation(26.8%)although the tensile yield strength increases with increasing ball milling speed.The tensile fractures consist of many dimples.At the bottom of dimples,precipitate particles with size of about 1 μm can usually be observed.EDS analyses show that the precipitates are all Cr rich,which may be associated with refractory Cr compounds formed during vacuum sintering process.The corrosion behavior of CoCrFeMnNi alloys with different pre-deformed powders in 3.5%NaCl solution was studied.The results of static corrosion rate tests show that the alloy with 100 rpm has the lowest static corrosion rate.The corrosion morphology observation shows that pitting corrosion is the main corrosion mode of all alloys.The pitting pit density of 100 rpm alloy is the lowest.In addition,the results of both potentiodynamic polarization curves and electrochemical impedance spectroscopy also confirm this conclusion.In conclusion,this paper mainly studied the influence of ball milling speed on the mechanical properties and corrosion resistance of CoCrFeMnNi sintered with pre-deformed gas atomized alloy powders.The results and corresponding mechanism are discussed by means of XRD,microstructure observation and EDS analysis.It shows that the mechanical properties and corrosion resistance of sintered alloy can be further improved by appropriate ball milling of powder before sintering.The results can provide an experimental and theoretical reference for further improving the comprehensive properties of HEA and its composites.