Investigation On Microstructure Evolution And Mechanical Behaviors Of The CrFeNi-based Multi-component Alloys

High-entropy alloys,known as multi-component alloys,are a new type of alloys,which have been rapidly developed in recent years.High-entropy alloys possess four core effects,including:high-entropy alloy effect,sluggish diffusion effect,lattice distortion effect,and cocktail effect.The unique microstructure and excellent performance of high-entropy alloys make them have huge potential in many important fields such as automotive,nuclear energy,and aerospace.At present,high-entropy alloys have been extensively studied,especially the face-centered cubic high-entropy alloys.Such alloys usually demonstrate good plasticity and excellent fracture toughness at low temperature,but the low strength at room temperature greatly limits their application and development as structural materials.In addition,there are relatively few reports on the medium-entropy alloys,which largely restrict the development of multi-component alloys.Therefore,there are two main works in this paper:on the one hand,strengthening the CoCrFeNi and Al₇Co₂₄Cr₂₁Fe₂₄Ni₂₄high-entropy alloys by elemental alloying method;on the other hand,strengthening the CrFeNi medium-entropy alloy by elemental alloying and cold rolling and annealing treatment,respectively,and the microstructure evolution and strengthening mechanism were systematically analyzed.Firstly,a series of(Al₇Co₂₄Cr₂₁Fe₂₄Ni₂₄)_100-xCr_x high-entropy alloys with different Cr contents were prepared,and the alloying effect of Cr on the microstructure and mechanical behaviors of the(Al₇Co₂₄Cr₂₁Fe₂₄Ni₂₄)_100-xCr_x high-entropy alloy were systematically investigated.It was found that the Cr addition resulted in the crystalline structure evolution,changing from single FCC structure to FCC+BCC structure.Transmission electron microscope?TEM?analysis indicated the presence of the ordered B2 phase.The alloying effect of Cr element resulted in a significant improvement in hardness and room-temperature compressive yield strength,but the plasticity decreased.Furthermore,by controlling the Al and Cr content in the CoCrFeNi matrix,the?CoCrFeNi?₄Cr₁Al₁ high-entropy alloy with unique solar-flower structure was successfully prepared,and the phase evolution,nano-and macro-mechanical behavior were investigated.The resluts showed that the?CoCrFeNi?₄Cr₁Al₁ high-entropy alloy demonstrated a dual phase structure,composing of FCC and ordered B2 phase.TEM analysis showed that there were two kinds of ordered B2 phases,one of which was enriched in Co,Fe and Cr,and the other was enriched in Al and Ni.The unique microstructure resulted in high hardness,good wear resistance,and excellent comprehensive room-temperature compressive properties of that alloy.The room-temperature compressive yield strength,fracture strength,and fracture strain of the?CoCrFeNi?₄Cr₁Al₁ alloy is 1653 MPa,2950 MPa,and 29.5%,respectively.In addition,a series of CrFeNiNb_0.1Ti_x medium-entropy alloys with different Ti addition were parepared,which all demonstrate FCC+BCC+HCP structure.The effects of Ti content on the hardness,strength and plasticity of the current medium-entropy alloy system were investigated.It was found that the hardness and strength of the alloy were significantly improved with the Ti addition.However,the plasticity decreased.The Vickers hardness of the current alloys increased from 512.35 HV to 866.92 HV,the room-temperature compressive yield strength increased from 1250 MPa to 2223 MPa,and the fracture strain decreased from 16.65%to 2.9%.At last,the effect of elemental alloying and cold-rolling on CrFeNi medium-entropy alloy was investigated,respectively.A series of CrFeNiAl_xTi_y alloys with different Al and Ti contents were prepared by introducing Al and Ti elements,including the bimodal structured CrFeNiAl_0.3Ti_0.3 and CrFeNiAl_0.4Ti_0.2 alloys.The bimodal structure was not only beneficial for the increment in strength,but also for maintaining the good plasticity.The room-temperature compressive yield strength of the CrFeNiAl_0.3Ti_0.3 alloy reached up to 1712 MPa,the fracture strength was 3700 MPa,and the fracture strain was 39.8%.The room-temperature compressive yield strength,fracture strength,and fracture strain of CrFeNiAl_0.4Ti_0.2 alloy were 1600 MPa,3819 MPa,and 45%,respectively.The density tests showed that the Al and Ti addition reduced the density of the CrFeNiAl_xTi_y alloys.The CrFeNiAl_0.3Ti_0.3 alloy possessed the smallest density?6.9294 g/cm³?and the highest specific strength?273.28 MP?cm³/g?.Furthermore,the effect of cold rolling and intermediate temperature annealing on the microstructure and mechanical properties of CrFeNi medium-entropy alloy were investigated.After 82%cold rolling,the CrFeNi alloy maintained the single FCC structure.However,the BCC phase precipitated on the FCC substrate after intermediate annealing at 600-800°C for 1 h.The cold rolling treatment significantly increased the hardness and strength of the alloy,but decreased the plasticity.After cold rolling,the Vickers hardness increased from 177.6 HV to 356.95 HV,and the room-temperature tensile yield strength increased from 136.55 MPa to 998.66MPa,while the uniform elongation decreased from 49.33%to 12.24%.The intermediate temperature annealing treatment decreased the hardness,yield strength,and tensile strength of the alloy,but increased the plasticity.