Microstructure And Properties Of NiCoFeCuM System High Entropy Alloys And Their Composites Prepared By Mechanical Alloying

As a new class of alloys,high entropy alloys(HEAs)have broken up the design strategy of conventional alloys that select one or two elements as principal components and become one of the hot topics in the field of material science.HEAs with appropriate alloy compositions can exhibit a variety of superior properties.However,up to date,HEAs are mainly designed by varying the concentration of one component and the case of optimizing the microstructure and performances of HEAs by adjusting the ratios of multiple elements has rarely been seen.In this paper,NiCoFeCuM(M represents Cr or V or Al elements)system HEAs were designed based on NiCoFeCuCr alloy by varying the contents of multiple components or adopting the way of element substitution.Subsequently,these HEAs were prepared by mechanical alloying(MA)followed by spark plasma sintering(SPS)approach.In the case of NiCoFeCuM system HEAs,alloying behavior and phase formation mechanisms during MA as well as phase constitution,phase transformation,microstructure,mechanical behaviors and strengthening mechanisms after SPS were investigated systematically.Furthermore,the effect of in-situ synthesized VC particle reinforcements with different volume fractions on the microstructure and mechanical properties of Ni_1.5CoFeCu_0.8Al_0.2V_0.5alloy matrix composite was also explored.The main conclusions are summarized as follows:(1)In the process of MA,the alloying behavior of constituent elements mainly depended on two factors,namely crystal structure and atomic size.In other words,the elements with similar crystal structures and atomic sizes tended to dissolve each other.In terms of NiCoFeCuM alloy system,Cu and Co atoms might diffuse and dissolve into the crystal lattice of Ni,resulting in the formation of FCC structured solid solution phase;while,Fe might be miscible with Cr,V and Al elements,leading to the formation of BCC structured solid solution phase;NiCoFeCuM system HEA powders synthesized by MA were composed of FCC and BCC phases.With the increase of Ni/Cr molar ratio,the fraction of FCC phase increased while that of BCC phase decreased in the Ni_xCoFeCuCr_2-x alloy powder.The increase of Al content resulted in the increase of FCC phase while the reduction of BCC phase in the Ni_1.5CoFeCu_1-xAl_xV_0.5 alloy powder.(2)During SPS,phase transition in HEAs was influenced by three factors,namely enthalpy of mixing,crystal structure and atomic size.In terms of NiCoFeCuM alloy system,under the action of positive enthalpy of mixing,partial Cu atoms segregated out and FCC phase in the MA state was decomposed into FCC1 phase and Cu-rich FCC2 phase;under the action of negative enthalpy of mixing,Cr,V and Al elements with strong carbide forming abilities in BCC phase reacted with minor C element introduced into HEA powders,resulting in the formation of(Cr,V)₇C₃,V₂C and(V,Al)C_x carbides;Fe atoms migrated from BCC phase to FCC phase due to the crystalline transformation;in addition,BCC phase transformed into?phase in the Ni_xCoFeCu_2-x alloy,whileg-Al₂O₃ phase was precipitated from BCC phase in the Ni_1.5CoFeCu_1-xAl_xV_0.5 alloy;in view of the above,BCC phase disappeared after SPS.With the increase of Ni/Cr molar ratio,the fractions of FCC2 and?phases reduced in the bulk Ni_xCoFeCuCr_2-x alloy.With increase in Al content,FCC2 phase reduced whileg-Al₂O₃ phase increased in the bulk Ni_1.5CoFeCu_1-xAl_xV_0.5 alloy;moreover,V₂C evolved into(V,Al)C_x.(3)Microstructure analysis suggests that in the bulk Ni_xCoFeCuCr_2-x alloy the Cu-rich region gradually reduced with the increase of Ni/Cr molar ratio and it nearly disappeared when Ni/Cr?1.5/0.5;in the bulk Ni_1.5CoFeCu Cr_0.5-xV_x alloy,there were carbide particles whose average grain size was about 130 nm and volume fraction was approximately 4%dispersed in the FCC matrix phase;the bulk Ni_1.5CoFeCu_1-xAl_xV_0.5 alloy displayed bimodal grain microstructure and there wereg-Al₂O₃ particles whose average particle size was 22 nm and volume fraction was in the range of 1.0%to 1.3%in the fine-grained region of this alloy;with increase in Al content,the fraction of fine-grained region gradually increased and the grains in the coarse-grained and fine-grained regions got finer.(4)With the increase of Ni/Cr molar ratio,the compressive strengths of bulk Ni_xCoFeCuCr_2-x alloy at the quasi-static and dynamic conditions increased at first and then decreased and the alloy achieved the optimal comprehensive mechanical property when Ni/Cr=1.5/0.5.The bulk Ni_1.5CoFeCu Cr_0.5-xV_x alloy exhibited high compressive yield strength,which was attributed to the combination effect of grain boundary strengthening,precipitate strengthening,twin boundary strengthening and dislocation strengthening;additionally,the contribution of grain boundary strengthening was the largest among these of four mechanisms.In the case of bulk Ni_1.5CoFeCu_1-xAl_xV_0.5 alloy,its compressive yield strength was enhanced significantly and the number of strain-hardening stages reduced gradually with increase in Al content.(4)In terms of the bulk yVC/Ni_1.5CoFeCu_0.8Al_0.2V_0.5(y=5,10,15 Vol.%)composites,when the volume fraction of reinforcements was 5%or 10%,the addition of in-situ synthesized VC particles resulted in the grain refinement of HEA matrix and enhanced the contribution of grain boundary strengthening so that the hardnesses and strengths of composites were higher than those of HEA matrix;when the volume fraction of reinforcements was increased to 15%,these in-situ synthesized VC particles were in large sizes and they were agglomerated severely,which led to the significant grain coarsening of HEA matrix and reduction in the mechanical properties of composite.