Effect Of Mo And C On Microstructure And Properties Of CoCrFeMnNi High Entropy Alloy

Compared with traditional alloys,some multi-principal high-entropy alloys have the characteristics of low temperature,high plasticity and low strength.For example,The elongation of CoCrFeMnNi high-entropy alloy is close to 60% at room temperature and can reach 75% at liquid nitrogen temperature,but the yield strength is only 400 MPa at room temperature.Therefore,the microstructure of CoCrFeMnNi high-entropy alloy is regulated to improve its comprehensive mechanical properties by means of alloying and heat treatment in order to provide experimental basis for its engineering application.In this paper,CoCrFeMnNi-Mox and CoCrFeMnNi-Mo5%Cx high entropy alloys prepared by vacuum arc melting furnace were studied.Testing methods such as SEM,EDS,XRD were used to determine the microstructure of these materials,and the mechanical properties were tested by universal testing machine and Vickers hardness tester.The effects of Mo and C on the microstructure and mechanical properties of CoCrFeMnNi high-entropy alloy were analyzed.The effects of cooling mode,heating temperature and holding time on microstructure and mechanical properties of CoCrFeMnNi-Mo5% and CoCrFeMnNi-Mo5%C0.5% high-entropy were studied.The formation mechanism of the second phase was revealed in CoCrFeMnNi high-entropy alloys after adding Mo and C elements.The main conclusions were as follows:In CoCrFeMnNi-Mox(x=1,3,5,7,10,wt.%)high-entropy alloy,Mo is solid-dissolved in the matrix when Mo<7wt.%;When Mo?7wt.%,the ? phase(CrMo)is precipitated at the grain boundary,with the increase of Mo content,the yield strength and tensile strength of the alloy are improved,and the plasticity is still good,the elongation is more than 35%.When the CoCrFeMnNi-Mo5% high entropy alloy is cooled after holding for 2 hours at different temperatures(700?,800?,900?,1000?),the ? phase precipitated.and with the increase of temperature,the number of sigma phase in the alloy gradually increased and the phenomenon of crystal precipitation occurred;The highest hardness and tensile strength were 195 HV and 579 MPa at 1000?.In the subsequent cooling process of CoCrFeMnNi-Mo5% high-entropy alloy which was kept at 1000°C for 4h,the number of ? phase decreases,and the precipitation gradually changes from intragranular precipitation to grain boundary precipitation.The distribution pattern changed from intragranular strip and continuous grain boundary to grain boundary strip.The yield strength and tensile strength of the alloy were the highest after furnace cooling,308 MPa and 583 MPa respectively.In CoCrFeMnNi-Mo5%Cx(y=0.1,0.3,0.5,0.7,1,wt.%)high-entropy alloys,it is a single FCC phase when C<0.3wt.%.When C?0.3wt.%,the carbides phase begin to precipitates in the grain boundary and the crystal,the number of carbides increases with the increase of C content,which increases the hardness,tensile strength and yield strength of the alloys and decreases the plasticity.When C content is 0.5%,the alloy has the best properties,and its yield strength,tensile strength and elongation is 347 MPa,739MPa and 41.5%,respectively.The volume fraction of carbide phase in CoCrFeMnNi-Mo5%C0.5% high-entropy alloy is the highest,and the yield strength is up to 460 MPa after holding at 900? for 4 hours;the carbide phases in the alloy are gradually decomposed with the heat treatment temperature further increased;The volume fraction of carbides in the alloy decreased with the increased of cooling rate,after holding at 1150? for 4 hours.The tensile strength and yield strength of the alloy decreased,and the plasticity increased.The carbides in the alloy disappeared after holding at 1150? for 4 hours,but two new second phases,CrMn and MnCr,were formed,the strength of the alloy is further reduced.