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    Estudo do efeito do níquel na microestrutura, na dureza e na corrosão de ligas de alta entropia AlCoCrFeNiX
    (Universidade Federal de São Paulo (UNIFESP), 2020-02-17) Zemanate, Ana Maria [UNIFESP]; Cardoso, Kátia Regina [UNIFESP]; Universidade Federal de São Paulo
    High entropy alloys (HEAs), a new concept in metal alloys have been developed with the aim of producing new materials with unique properties for advanced applications. HEAs are defined as metallic alloys composed of multiple main elements in equimolar or quasi-equimolar atomic proportions, and which form simple solid solutions body-centered cubic (BCC), fase-centered cubic (FCC) or hexagonal close-pack (HCP), due to their high mixing entropies. In this work, three high entropy alloys were obtained: AlCoCrFeNi, AlCoCrFeNi1.5 and AlCoCrFeNi2, by electric arc melting under argon protection, to understand the effects of nickel (Ni) variation on the microstructure, and on the hardness and corrosion resistance. The composition of the alloys were defined based on the calculation of the empirical parameters that defined the HEAs and in computer simulations with the thermo-calc software, which it allowed predicting the equilibrium phase in the alloys depending on the temperature and the variation in the composition. The HEAs were characterized in three conditions: in the as-cast condition, after remelting and after heating treatment at 1125 °C during 5h. The HEAs microstructures were characterized by x-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). The three alloys have a much-differentiated microstructures; the equimolar alloy has a dendritic microstructure with fine precipitates, while the interdendritic region has a tissue-like structure with thicker precipitates. The AlCoCrFeNi1.5 alloy has a structure in the form of needles of the Widmanstätten type that grows from the grain boundaries to the center indicating the growth of the CFC phase, while the AlCoCrFeNi2 alloy has a microstructure of the eutectic type that alternates the CFC/B2 phases. With the heat treatment at 1125 °C, the microstructure of the alloys, despite of being essentially they were the same coarser and growth of grains and precipitates were observed. Differential scanning calorimetry (DSC) tests made it possible to evaluate the transition temperature of the phases especially those related to the transformation from the (BCC) phase to the (CFC) phase. The hardness of the alloys were determined, indicating that the increase in the nickel (Ni) content led to a reduction in the hardness from 461 HV in the equimolar alloy to 303 HV in the AlCoCrFeNi2 alloy due to the increased in the fraction of the CFC phase with the nickel content. The remelting of the alloys were followed by casting in a refrigerated mold, which resulted in a higher cooling rate with consequent microstructural refining and increased hardness; the equimolar alloy, for example, it reached a hardness of 520 HV in this condition. With the homogenization heat treatment, the hardness values decreased, which varied from 400HV for the equimolar alloy to 270 HV for the alloy with the highest Ni content. The corrosion resistance of the alloys were evaluated by using the potentiodynamic polarization technique in a 3.5% NaCl solution. In general, the three alloys presented corrosion potentials (Ecorr) in order to -0.31V and low corrosion current densities (Icorr) with the lowest values obtained for the alloy with the highest content of Ni, AlCoCrFeNi2, 41.5 mA/cm2 in the as-cast condition and 55.7 mA/cm2 after the homogenization heat treatment. Preferential corrosion of the B2 phase was observed due to the lower Cr content in this phase.