Microstructure,mechanical And Tribological Properties Of Multi-principal Elements CrCoNi High Entropy Alloys System

Recently,high entropy alloys（HEAs）have attracted extensive attention in materials science due to their simple phase composition and excellent mechanical properties.However,single-phased HEAs still have limitations in practical applications,such as FCC HEAs,which are ductile but weak in strength and wear resistance.In this thesis,a new HEA system,CrCoNiMo_x is designed based on FCC Cr Co Ni alloy by adding Mo element to improve the yield strength and wear resistance.The effects of Mo content on the microstructure,mechanical and tribological properties of CrCoNiMo_x（x=0,0.25,0.5,0.75 and 1.0）are systematically investigated.The tribological behaviors of CrCoNiMo_0.5.5 HEA at different temperatures are studied,and the wear mechanisms at elevated temperatures are discussed.The high temperature oxidation behavior of the Cr Co Ni HEA on its high temperature wear mechanism is initially demonstrated.The main results and conclusions are as follows:1.The spark plasma sintered CrCoNiMo_x HEAs have uniform microstructures and high relative densities.With the increase of Mo content,the hardness and compressive yield strength of the CrCoNiMo_x alloys increase obviously,whereas the plasticity decreases.Meanwhile,the fracture toughness of the alloys increases firstly and then decreases with increasing the Mo content.The solid solution strengthening of Mo and the formation of intermetallic compounds（ICMs）are the main reasons for these changes.When x≤0.5,the wear resistance of the alloys is in consistent with the increases of hardness.When x>0.5,the wear rate levels off although the hardness is still increasing.This is because that the wear mechanism of the alloys gradually changes from abrasive wear to adhesive wear and brittle fracture with the increase of Mo content.2.CrCoNiMo_0.5.5 HEA has good resistance to high temperature softening.Its hardness at 800 ^o C can achieve to 365 HV,which is 1.9 times of CrCoFeNi HEA and2.7 times of 316L stainless steel.This is mainly due to the serious lattice distortion of HEAs,the strong solution strengthening of Mo and the formation of ICMs.In the temperature range of 200⁴00 ^o C,both friction coefficient and wear rate of the CrCoNiMo_0.5.5 HEA are high.The abrasive wear and adhesion wear are the main wear mechanisms of the alloy.With the test temperature further increases,the friction coefficient and the wear rate decrease,and the oxidation wear gradually become the main wear mechanism of the alloy.At 800 ^o C,The friction coefficient of the friction pair is about 0.34,and the wear rate of the alloy is only 1.14×10^-66 mm³·N^-1·m^-1.3.The good high temperature oxidation resistance of the Cr Co Ni alloy is mainly derived from the high Cr content.The oxidation weight gain rate of the CrCoNi alloy is obviously lower than that of the conventional Ni20Cr alloy at 800 ^o C,but is equivalent to that of Ni Cr_33.43.4 alloy with the same Cr content.At high temperatures,the wear surface of the CrCoNi alloy is covered by an oxide film,which has good performance on reducing the friction and resisting the wear.After friction testing at800 ^o C,Cr₂O₃ is the only oxidation products outside the wear surface,while Ni Cr₂O₄and Co Cr₂O₄ exist on the worn surface except Cr₂O₃,because the high temperature friction process of the alloy is a dynamic coupling process of chemical oxidation and mechanical wear.