Mechanical Properties And Wear Mechanism Of Ultrafine-grained FeCoMoNi High-entropy Alloys

High-entropy alloys(HEAs)have aroused extensive attention in recent years,due to their excellent mechanical performance.Single-phase face-centered-cubic(fcc)HEAs display outstanding plasticity but low strength,which severely hinders their structural applications.In this dissertation,we demonstrate a strategy that takes advantages of second phase strengthening and grain refinement to obtain an exceptional compressive property.Specifically,we firstly designed Fex(Co Mo Ni)100-x HEAs using the calculated phase diagram method,which indicates that the relative content of ductile fcc matrix and μ phase can be adjusted with varying Fe contents.The combination of mechanical alloying and spark plasma sintering was selected to fabricate a series of bulk HEAs.The as-fabricated Fe40Co20Mo20Ni20 HEA exhibits an ultrafine-grained microstructure,where the average grain sizes of fcc matrix and μ phase are 266 nm and 252 nm respectively.Compression tests at ambient temperature show that the HEA achieve an optimized performance with a yield strength of 1877 MPa and an appreciable fracture strain of ～40%,which outperforms most existing HEAs and shows promise in structural materials.X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)studies were carried out to unveil the microstructure evolution of Fe40Co20Mo20Ni20 HEA.The systematic characterization revealed that the fcc matrix provides the plastic accommodation via dislocation slip and the hard μ phase functions as strengthening particles to impede dislocation movements.Quantitative analysis of strengthening contributions in the HEA indicates that the high strength is mainly governed by dispersed μ phase and grain refinement,followed by dislocation strengthening,twinning strengthening and solid solution strengthening.On the other hand,we also focus on the sliding wear behavior of Fe40Co20Mo20Ni20 HEA at room temperature.Ball-on-disk wear tests showed that the HEA exhibits extremely low wear rate of the order of 10-6 ～ 10-5 mm3/(N·m),upon sliding against alumina ball under different loads and sliding speeds.Systematic studies on morphology,composition of the worn surface and generated debris were performed using SEM,energy dispersive X-ray spectroscopy(EDX)and 3D optical microscopy.Besides,subsurface microstructure evolution was characterized by TEM in detail,while the chemical state of the oxide layer was analyzed using X-ray photoelectron spectroscopy(XPS).The results show that the wear mechanisms remained almost unchanged as primarily abrasive,oxidation and fatigue wear from low to high load and sliding velocity.The massive μ particles are mainly responsible for the excellent wear resistance as they serve as obstacles to both dislocation movements as well as plastic deformation.In summary,this study can provide deep insights into the systematic design,fabrication,characterization and analysis of novel HEAs with ultra-high strength and outstanding wear performance.