Microstructure And Mechanical Properties Of Al_xCoCrFeNi（x=0.3,0.5,0.7,1） High-Entropy Alloys By Selective Laser Melting

The preparation of high-entropy alloys by selective laser melting（SLM）has broad application prospects in the forming of complex precision parts.However,the extreme non-equilibrium solidification process of SLM is prone to defects such as holes and cracks,which seriously affect the performance of printing HEAs.In this paper,the effects of Al content on the microstructure and mechanical properties of SLM-formed Al_xCoCrFeNi high-entropy alloys were systematically studied,and the internal mechanism of hot cracking of high-entropy alloys produced by laser additive manufacturing was revealed.On this basis,Al_0.3Co Cr Fe Ni with good crack resistance was selected for heat treatment.The deformation and strengthening mechanism were analyzed,and a high-strength high-entropy alloy suitable for SLM process was obtained.The main conclusions are as follows:1)The effect of Al content on the phase of Al_xCoCrFeNi under non-equilibrium solidification is revealed.It is found that Al_0.3Co Cr Fe Ni and Al_0.5Co Cr Fe Ni are FCC phases,and Al_0.7Co Cr Fe Ni and Al_1.0Co Cr Fe Ni are BCC/B2 phases.Al_0.3Co Cr Fe Ni is mainly columnar crystals,while Al_0.5Co Cr Fe Ni,Al_0.7Co Cr Fe Ni and Al_1.0Co Cr Fe Ni are equiaxed crystals and columnar crystals.Al_0.3Co Cr Fe Ni has a large number of stacked and entangled dislocations,and also found that there are twins and stacking faults.There is no obvious element segregation.Al_0.7Co Cr Fe Ni has the best creep resistance.2)The formation mechanism of hot cracking of SLM-formed Al_xCoCrFeNi high-entropy alloys and its intrinsic correlation with the crystal structure are elucidated.It was found that with increasing Al content,the number and length of cracks increased.The cracks mainly propagate along the high-angle grain boundaries,and there is no element segregation near the cracks,and a large number of dislocations are found at the crack tip,indicating that the cracks in the SLM-formed Al_xCoCrFeNi are mainly stress-induced thermal cracks.It was found that the coarse-grained as well as the hard and brittle BCC phase made cracks more likely to occur.With the increase of Al content,the residual stress in Al_xCoCrFeNi increases significantly,leading to a higher degree of cracking.3)The deformation mechanism and strengthening mechanism of Al0.3Co Cr Fe Ni after heat treatment were explored.After heat treatment at650℃for different time,it is found that the main precipitation phase is the ordered L1₂ phase,and a small amount of acicular B2 phase and theσphase precipitated along the grain boundary.The multi-slip and cross-slip of dislocations are observed,and the deformation mechanism is mainly dislocation-slip mechanism.After heat treatment for 100 h,the microhardness of Al_0.3Co Cr Fe Ni increased from 240.01 HV to 335.67 HV,the yield strength increased from 640 MPa to 740 MPa,and the tensile strength increased from 810 MPa to 1123 MPa.There are 40 figures,7 tables and 97 references included in this thesis.