Microstructure And Properties Of CoCrNiFeMnAl_0.5 High Entropy Alloy By Powder Metallurgy

As a single-phase FCC structure,CoCrNiFeMn high-entropy alloys(HEAs)has excellent plasticity.However,its low strength severely limits its engineering applications,and thus improving its strength is a key to solve the problem Based on this,9.1 at.%Al element was introduced in CoCrNiFeMn HEAs and the CoCrNiFeMnAl0.5 HEAs were prepared by different methods.The different microstructures of CoCrNiFeMnAl0.5 HEAs were investigated deeply.Their mechanical properties were compared,and the strengthening mechanisms of them were discussed systematically.It was found that with the increase of particle size,the microstructure of gas atomization powder changes from microcrystalline to dendritic,and the distribution of elements on the surface of the powder is uniform.With the increase of ball milling time,the mechanical alloying powder transforms from spherical to flake,and is completely alloyed after 50h.The sintered alloys prepared by powder metallurgy have a more uniform microstructure and finer grains.The as-cast alloys showed a dendritic structure and more serious segregation of the Al and Ni elements between the dendrites.The mechanical properties of the sintered alloys are significantly improved compared with the as-cast alloy.The yield strength and microhardness of the as-sintered alloys were 173%and 104%higher than that of the as-cast alloy,respectively.The excellent mechanical properties of the as-sintered alloys originate from the second phase strengthening and the grain boundary strengthening.In order to fabricate HEAs with both high strength and great plasticity,a new material design idea of network structure is combined with the characteristics of high-entropy alloys.And the CoCrNiFeMnAl0.5 HEAs powder is used as the matrix and BN ceramic particles are used as the source of N.By regulating the reinforcement content and changing the size of the HEAs matrix,we attempted to prepare the BN/CoCrNiFeMnAl0.5 HEAs composites with a network distribution of the reinforcement by low-energy ball milling combined with oscillatory pressing sintering.It was found that high-density networked BN/CoCrNiFeMnAl0.5 HEAs composites could be prepared.The AlN reinforcements were generated by in situ reaction at the network grain boundaries.A good metallurgical bonding interface between the matrix and the reinforcements was exhibited.For a certain size of the matrix particles,the microhardness of the network structured BN/CoCrNiFeMnAl0.5 HEAs composites showed a trend of decreasing and then increasing with a slight increase in BN content.For a certain added BN content,the compressive strength of this composite increased significantly with the decrease of the matrix particle size.Especially for the network BN/15-45CoCrNiFeMnAl0.5 HEAs composites,the compressive strength reached 1803 MPa,and the plastic strain still remained at about 30%.