Microstructure And Properties Of CoCrFeMnNiGd_x High Entropy Alloys Prepared By Powder Metallurgy

Compared with traditional alloys,high-entropy alloys?HEAs?often show better properties such as high strength,high hardness,good plasticity and toughness,because of their special microstructure.At present,the research of high-entropy alloys is mostly focused on alloy systems with silimar atomic radius and close atomic ratio of elements,such as 3d transition metal high-entropy alloy system,refractory metal high-entropy alloy system,etc.Most of them tend to form single-phase solid solution.In this work,a small amount of rare earth element Gd was added to Co Cr Fe Mn Ni and CoCrFeMnNiGd_x series high-entropy alloys were prepared by mechanical alloying?MA?and spark plasma sintering?SPS?.The effects of Gd on the microstructure and mechanical properties were studied.Based on this,the friction and wear behavior of CoCrFeMnNiGd_x high-entropy alloys were also discussed.CoCrFeMnNiGd_0.15 high-entropy alloy powders with single-phase structure were fabricated by mechanical alloying.The microstructures of bulk alloy sintered by SPS consisted of FCC matrix phase,Gd-Ni-Mn-rich tetragonal-structure phase and a few BCC structured rare earth oxide particles.With the increase of sintering temperature,the volume fraction and size of precipitated phases increased,the hardness and compressive yield strength of sintered alloy declined,and the plasticity increased.The alloy sintered at 900?had balanced comprehensive mechanical properties,its compressive yield strength(?_0.2),compressive strength(?_max),plastic deformation??_p?and Vickers hardness reached 1662 MPa,2518 MPa,30.6%and 458 Hv,respectively.The effect of Gd content on mechanical alloying,microstructure and mechanical properties of CoCrFeMnNiGd_x?x=0?0.05?0.1?0.15?series HEAs was studied.The results showed that the increase of Gd content prolonged the time required for the elemental powder to complete alloying and increased the microstrain of milled powders.When x=0.05,the precipitated particles in the alloy could effectively suppress the growth of matrix grains during spark plasma sintering.With the increase of the Gd content,the size of precipitated phase grew up,which reduced the inhibition effect.Therefore,the grain size of HEA was the smallest when x=0.05.The compressive yield strength of high-entropy alloys containing Gd were generally higher than that of Co Cr Fe Mn Ni alloy.The grain boundary strengthening and second phase strengthening were the main strengthening mechanism.Co Cr Fe Mn Ni Gd_0.1 HEA exhibited excellent comprehensive mechanical properties,its compressive yield strength(?_0.2),compressive strength(?_max),plastic deformation??_p?and Vickers hardness reached 1621 MPa,2685 MPa,35.4%and 460 Hv,respectively.The friction and wear properties of CoCrFeMnNiGd_x high-entropy alloys were studied.With the increase of Gd content,the friction coefficient and wear rate of CoCrFeMnNiGd_xseries high-entropy alloys decreased first and then increased,with the minimum friction coefficient and wear rate reaching 0.46 and 5.08×10^-4 mm³N^-1m^-1 respectively at x=0.05.The main wear mechanisms of HEAs were adhesive wear and abrasive wear.As the load increases,the friction coefficient of Co Cr Fe Mn Ni Gd_0.1 high-entropy alloy was increasing,and the wear rate increased first and then decreased.When Co Cr Fe Mn Ni Gd_0.1 high-entropy alloy was rubbed at sliding speeds of 0.11 m/s and 0.22 m/s,the friction coefficient decreased slightly.When the sliding speed increased to 0.33 m/s,the friction coefficient was greatly improved.The wear rate continued to increase as the sliding speed increases.As the load and speed increased,the wear mechanism of this high-entropy alloy gradually changed from oxidative delamination wear to adhesive wear.