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    Determination of fracture toughness of boride layers grown on Co1.21Cr1.82Fe1.44Mn1.32Ni1.12Al0.08B0.01 high entropy alloy by nanoindentation
    (Elsevier Sci Ltd, 2022) Gunen, Ali; Makuch, Natalia; Altinay, Yasemin; Carboga, Cemal; Dal, Serkan; Karaca, Yusuf
    Multiphase boride layers consisting of (CoFe)2B, (Fe0.4Mn0.6)B, Cr2Ni3B6 and (Cr0.4Mn0.6)B were formed on the surface of Co1.21Cr1.82Fe1.44Mn1.32Ni1.12Al0.08B0.01 high entropy alloy by powder-pack boronizing at 900 degrees C, 950 degrees C and 1000 degrees C for 4 h. The nanohardness (H), modulus of elasticity (E) and fracture toughness (KC) of the multiphase boride layers were determined based on the load-displacement (P-h) curves obtained in the nanoindentation tests. Three distinct regions were identified on the cross-sections of the produced layers: an outer layer consisting of MeB-type borides, an inner layer consisting of Me2B-type borides and the transition zone. The microstructural aspects of the layers were investigated using scanning electron microscopy, energy-dispersive Xray spectroscopy, and X-ray diffraction. Detailed analysis of the influence of the chemical composition on hardness, elastic modulus and fracture toughness in the three regions indicated that the most critical factor influencing the mechanical properties was the presence of chromium, iron and cobalt borides in the microstructure. Especially the formation of chromium borides reduced the fracture toughness of the transition zone.
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    Microstructure, some mechanical properties and tribocorrosion wear behavior of boronized Al0.07Co1.26Cr1.80Fe1.42Mn1.35Ni1.10 high entropy alloy
    (Elsevier Science Sa, 2021) Karakas, Mustafa Serdar; Gunen, Ali; Carboga, Cemal; Karaca, Yusuf; Demir, Mehmet; Altinay, Yasemin; Erdogan, Azmi
    High-entropy alloys (HEAs) with face-centered cubic (FCC) structures exhibit high toughness and corrosion resistance, but their average strengths and relatively low wear resistance can limit their engineering ap-plications. In this study, FCC Al0.07Co1.26Cr1.80Fe1.42Mn1.35Ni1.10 HEAs were boronized for 4 h at temperatures of 900, 950, and 1000 degrees C to form hard, protective metal borides on their surfaces. The microstructural characteristics of the borides formed were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The mechanical properties of the boride layers were studied by nanoindentation measurements, and the tribological performances of the layers were evaluated by ball-on -disk type wear tests in air, 3.5% NaCl and 5% H2SO4. Irrespective of the boronizing temperature, (Fe0.4Mn0.6) B, (Cr0.4Mn0.6)B, (CoFe)B2 and Cr2Ni3B6 phases were detected in the surfaces of the boronized samples. The surface hardnesses of the boronized samples reached nearly ten times the hardness of the as-cast HEA. The borides were effective in reducing friction as well as wear. Increasing the boronizing temperature increased the thicknesses of the coatings and further improved wear characteristics. Wear rates in 5% H2SO4 were generally higher than the wear rates in 3.5% NaCl, but the highest wear rates were observed in air. (c) 2021 Elsevier B.V. All rights reserved.