The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys （HEAs） were heated at 500, 600, 700, 800 and 1000 ℃, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi2V0.5Nb0.75 HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe2Nb-type Laves phase. The NbNi4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 ℃. With increasing quenching temperature, the volume fraction of the NbNi4-type intermetallic phase increased while that of the eutectic regions decreased. The sample quenched at 800 ℃ showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. Moreover, the eutectic CoFeNi2V0.5Nb0.75 HEAs exhibited apparent age hardening, especially quenched at 700 ℃, the hardness reached up to the maximum value of HV 727.52.
In order to study the wear behavior of different kinds of contact wires,the dry sliding wear behaviors of Cu-Sn,Cu-Ag and Cu-Mg alloys prepared by up-drawn continuous casting and followed continuous extrusion were studied.The research was tested on a block-on-ring wear tester.The results indicate that the friction coefficient is remarkably influenced by the formation of a continuous tribofilm,which consists of oxidation film.The abrasion,adhesion,oxidation and plastic deformation are observed.Oxidation and abrasion wear mechanisms dominate at the lower sliding velocity and load.The combination of oxidation and adhesion play leading roles with the increasing load and velocity.Plastic deformation is detected under higher applied load and sliding velocities.
Hot corrosion rates of Super 304H austenitic stainless steel pre-coated in Na2SO4-25%NaCl mixture salt film in air at 650 ℃ and 750 ℃ have been investigated by corrosion kinetics curves.The corrosion products are examined by means of X-ray diffraction and scanning electron microscopy coupled with energy-dispersive spectrometer,and electron probe micro-analyzer to analyze the scale structure and element distribution of the cross section.The results indicate that the kinetics curves are similar and display parabolic growth,and the mass gain obviously increases with the increasing temperature.A two-layer oxide scale composed of Fe oxides containing CuFe2O4 and Cr2O3 forms on the surface of the alloy and easily exfoliates with extending corrosion time or with improving corrosion temperature.Moreover,a corrosion-affected zone with micropores and micro-cracks appears beneath the oxide scale.It is concluded that the selective oxidation occurs and a protective oxide scale forms at the early corrosion stage.The melt salts destroy the integrity of the oxide scale and accelerate hot corrosion of the alloy by the cyclical oxidation-chlorination during the further corrosion processing.In addition,internal sulfidation also contributes to the corrosion of the alloy.
