Microstructure And Mechanical Properties Of FeCoNi(CuAl)_x High Entropy Alloys Prepared By Powder Metallurgy

Because of their complex effects,high-entropy alloys have better mechanical properties than traditional alloys,such as high strength,high hardness,high wear resistance and oxidation resistance,which are one of promising research directions of new metal materials.Since the research period of HEAs fabricated by powder metallurgy is short,its theory is very limited.In this work,FeCoNi?CuAl?_x HEAs with good comprehensive mechanical properties were fabricated by powder metallurgy,the effect of sintering temperature and heat treatment on the microstructure and mechanical properties of the HEAs were studied,and the effect of alloy element content on phase transformation was discussed.The main research contents are as follows:The alloying sequence and phase formation mechanism of FeCoNi?CuAl?_0.4 HEA powders during mechanical alloying were studied.The results showed that alloy elements with lower melting point dissolved more quickly.After ball milled for 40 h,FeCoNi?CuAl?_0.4 HEA obtained single FCC structure with lattice parameter of 0.36009 nm.The effect of Al and Cu content in FeCoNi?CuAl?_x on the alloying speed was studied.It was found that the ball milling time to complete mechanical alloying processincreased after adding Al and Cu.DSC results showed that there was no phase transformation in FeCoNi alloy.The precipitation and solution of the second phase occurred in the alloyed powders during the heating process after adding Al and Cu.The precipitation temperature of the second phase decreased with the increase of Al and Cu content.The effect of sintering temperature on phase composition,microstructure and mechanical properties of FeCoNi?CuAl?_0.4 HEA were studied.The phase composition was different depanding on sintering temperatures.It was found that the samples sintered at 950-1050?consisted of FCC and BCC phases.However,only single FCC phase could be clearly detected at the sample sintered at 1100?.According to XRD analysis,the proportion of BCC phase decreased with the increase of sintering temperature.In the microstructure of dual-phase HEAs,the BCC phase was dispersed on the FCC matrix with granular shape,and the average grain size of FCC matrix phase was increased with the increase of sintering temperature.With the increase of sintering temperature,the density and compressive yield strength of FeCoNi?CuAl?_0.4 HEAs increased first and then decreased,and the the bulk HEA sintered at1050?exhibited highest yield strength of 1562.4 MPa.The effects of various strengthening mechanisms on sintered samples at 1050?were analyzed.The result showed that the calculated yield strength was 1688 MPa,which was basically consistent with the experimental value.Grain boundary strengthening was the main strengthening mechanism.The effect of heat treatment temperature on the the phase composition,microstructure and mechanical properties of FeCoNi?CuAl?_0.4 HEA samples with single FCC phase sintered at1100?was studied.BCC phase was precipitated from the FCC phase after heat treatment at500,600 and 700?,respectively.The fraction and size of BCC phase increased first and then decreased with the increase of heat treatment temperature.The fraction of BCC phase was the highest when heat treatment was conducted at 600?.With the increase of heat treatment temperature,the yield strength and hardness increased at first and then decreased.Bulk FeCoNi?CuAl?_x?x=0,0.4,0.6,0.8,1.0?high-entropy alloys were sintered at 1100?,the effect of the content of Al and Cu on phase composition,microstructure and mechanical properties was investigated.It was found that the microstructure basically consisted of single FCC phase when x=0 and 0.4,while FCC+BCC phases were detected when x=0.6,0.8 and 1.0.The fraction of BCC phase was continuously increased with the increase of Al and Cu content,the distribution of BCC phase changed from granular discrete dispersion to aggregated morphology.Adding minor Al and Cu elements to FeCoNi alloy can effectively inhibite grain growth,the average grain size of FCC phase decreased from 0.95 to 0.30?m when the content of Al and Cu increased from 0 to 0.4.The grain sizes of FCC phase and BCC phase increased from 0.30 and 0.27?m to 0.68 and 0.84?m,respectively,while x continued to increase from0.4 to 1.0.The yield strength and hardness of HEAs increased at first and then slightly decreased while the plasticity dropped down with the increase content of Al and Cu.The bulk HEA obtained excellent mechanical properties with compressive yield strength of 1467.7 MPa and plastic strain of 24.9%at x=0.6.