Study On Microstructure And Mechanical Properties Of Al_0.3CoFeNiC_xCu_y High Entropy Alloy

High-entropy alloys?HEAs?are defined as alloys containing five or more elements.Due to the multielement alloying,HEAs have some characteristics,such as atomic sluggish diffusion,high lattice distortion and a single structure.Being a single-phase alloy with FCC crystal structure,as-cast Al_0.3Co Fe Ni HEA has good plasticity and low strength.The comprehensive mechanical properties of Al_0.3Co Fe Ni can be improved by means of alloying,heat treatment and rapid solidification for the changes of its structure,which may provide experimental basis for the potential engineering applications in the future.In this thesis,Al_0.3Co Fe Ni C_xCu_y HEAs with a small amount of C and Cu were prepared by a non-consumable vacuum arc melting furnace,then treated by heat treatment and rapid solidification,and finally the structure and mechanical properties were analyzed and tested by OM,XRD,SEM-EDS,TEM,Vickers hardness tester and electronic universal testing machine.The main conclusions were as follows:The addition of a small amount of C element does not change the crystal structure of Al_0.3Co Fe Ni alloy but promots the precipitation of the ordered phase of L12 nanoparticles.With increase of C content,the particle size first decreases and then increases,and the shapes of the particles change from sphere to bulk and finally into rod.With increase of C content,the microhardness and compressive yield strength of the alloys first rapidly increase and then slightly decrease,but the compression rate linearly decrease.The as-cast Al_0.3Co Fe Ni C_0.1 has the best comprehensive mechanical properties with the compression yield strength of 1264MPa,the compression strength of 1975 MPa and the compression ratio of 41%.Besides of not changing the crystal structure of Al_0.3Co Fe Ni C_0.1alloys,the addition of a small amount of element Cu promots the formation of FCC phase and the refinement of L12nanoparticles.With increase of Cu content,the nanoparticle size first decreases and then increases,and the microhardness and compressive yield strength of the alloys first increase and then decrease.The as-cast Al_0.3Co Fe Ni C_0.1Cu_0.1 has the best comprehensive mechanical properties with the compression yield strength of 1460 MPa,the compression strength of 1936MPa and the compression ratio of 38%.Solution treatment does not improve the comprehensive mechanical properties and with increase of the solution temperature,the size of the L12 nanoparticles precipitated in the alloys decreases and the shapes change from bulk to sphere.After solution-treated at 900?and1250?for 24h,the microhardness and compression yield strength of Al_0.3Co Fe Ni C_0.1 alloy greatly decrease for the precipitation of?-carbide phase along the grainboundary and in the grain and then increase after solution-treated at 1250?for 24h for the formation of BCC phase and the decrease of?-carbide phase content.Compared with those of as-cast Al_0.3Co Fe Ni C_0.1Cu_0.1 alloy,the rapid-solidified alloy by melt spinning technique has finer and more uniform grains distributed with L12 ordered phase nanoparticles and higher microhardness.