Microstructure And Mechanical Properties Of Al_xSi_0.2CrFeCoNiCu_1-xHigh-Entropy Alloys

Asa new series of advanced materials,High-entropy alloys?HEAs?havebroken the traditional alloy design strategy that to select one or two elements as principal components.HEAs, which are composed of at least five principal elements with the concentration of each element between 5 at.% and 35 at.%, mainly consist of disordered/ordered FCC and/or BCC structures instead of complex intermetallics. HEAs exhibit excellent properties such as superior comprehensive mechanical properties, high hardness, excellent wear resistance,and good thermal stability. Thus, HEAs are expected to have high value and broad application prospects in science and engineering fields.In the present study,AlxSi_0.2CrFeCoNiCu_1-x?x=0.2, 0.4, 0.5, 0.6, 0.8, 0.9? HEAs were prepared by arc melting and injecting the melted alloy into the copper mold. The microstructure and mechanical properties of the HEAs were investigated, and the influences of elements content and cooling rate on HEAs were discussed.For the increase in Al content that was accompanied by a decrease in Cu content, the crystal structure of the AlxSi_0.2CrFeCoNiCu_1-x alloys gradually evolved from a FCC to a coexisting FCC and BCC structure. The final structures were BCC solid solutions, and the microstructure changed from columnar dendrites to equiaxed dendritic grains. The FCC structure consisted of a disordered A1 phase and an ordered L1₂ phase, and the BCC structure consisted of a disordered A2 phase and an ordered B2 phase. The nano-scaled spinodal decomposition?SD? phases were formed in both the FCC and BCC structures and increased with the increase in x values. The compositional segregation decreased with the increase of SDphases.The alloys with a FCC structure exhibited a good ductility but a low strength and hardness, and the yield stress increased and the ductility decreased as the Al content increased. In contrast, the alloys with the BCC structure possessed a high strength and hardness, and both the strength and ductility increased with the increase in the Al content.The stress of the alloys improved as the Al content was increased, which is attributed to the combined action of solid-solution strengthening, a decrease in compositional segregation,and fine-grain strengthening.The alloys with a FCC structureshowed shear fracturefeature,and the alloys with a BCC structureexhibited the mixedfeature of shear fracture and axial splitting. Almost all of the alloys exhibit a significant strain hardening effect.Comparedwith the alloys that had BCC structures, the alloys with FCC structures exhibited greater strain rate sensitivity.The increase of cooling rate promotes the formation of single BCC phase solid solution with simple structure, and is beneficial to the refinement of grain and microstructure, which is attributed to the increase of nucleation rate and the inhibition of atomic diffusion and grain growth under the high cooling rate.At the same time, the increase of cooling rate is beneficial to increase the hardness and strength of the alloy,which is due to the refinement of grain and the increase of BCC phase.