In this study, bulk samples of a CrMoNbWV high-entropy alloy (HEA) were obtained for the first time by spark plasma sintering (SPS) of mechanically alloyed (MA) powders at 1200 °C, 1300 °C, and 1400 °C. Microstructure evolution, phase formation as well as wear and corrosion behavior were investigated. The MA powders’ phase composition was found to be represented by body-centered-cubic (BCC) solid solution. The solid solution partially decomposed to Laves phases under the sintering, such as Cr2Nb and (Fe, Cr)Nb, and NbVO4-VO oxides mixture. The temperature increase to 1400 °C led to a grain coarsening of the BCC phase and decreased the Laves phase content accompanied by precipitation at the grain boundaries. The sintered samples showed high hardness and compressive strength (2700–2800 MPa) at room temperature. The wear tests demonstrated excellent results in comparison to conventional wear-resistant composites. The obtained samples also exhibited high corrosion resistance under electrochemical tests in H2SO4 solution. The CrMoNbWV HEA has comparable mechanical and corrosive properties with the WNbMoTaV type HEA, but at the same time has a reduced density: CrMoNbWV—10.55 g/cm3, WNbMoTaV—12.42 g/cm3.
The influence of the semi-finished products manufacturing process (forging, solution annealing and ageing) on the structure and corrosion properties of the EP718 nickel-based alloy (KhN45MVTYuBR) used in the oil and gas industry is investigated. The corrosion-electrochemical properties of the alloy were determined using gravimetric and electrochemical techniques. Microstructure was studied by optical and transmission electron microscopy. It is shown that the EP718 alloy in the delivery state (without heat treatment) has the highest corrosion resistance, and corrosion properties degrade during subsequent solution annealing at 1080°C.
High-nitrogen, high-manganese austenitic steels because of their non-magnetic behavior and corrosion resistance in combination with high strength and toughness serve various applications. The Schaeffler diagram is commonly used for predicting the as-cast microstructures but it does not reflect the nature of the austenite associated with the sequence of phase formation during solidification. Thermodynamic simulations are an aid to understanding the metallurgical nature of austenite. In this study, the microstructure was revealed by tint etching with examination using polarized light with the light optical microscope. The fragments of dendritic arms that share a common crystallographic orientation were identified by this technique. Two kinds of austenitic microstructures, both with grains formed by sharply defined primary and secondary dendritic arms and with grains having a blurred dendritic pattern, were observed. Solidification through austenite freezes the chemical inhomogeneity, whereas solidification through δ-ferrite, with subsequent transformation of δ-ferrite to austenite, leads to a “blurring” of the dendritic structure. These two kinds of microstructures were interpreted by a Schaeffler–Spiedel diagram which is modified in this paper. Electrochemical investigations showed that the chemical homogeneity of the austenite obtained by primary δ-ferrite solidification exhibited improved corrosion properties.
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