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    Characteristics, high temperature wear and oxidation behavior of boride layer grown on nimonic 80A Ni-based superalloy
    (Elsevier Science Sa, 2021) Gunen, Ali; Doleker, Kadir Mert; Korkmaz, Mehmet Erdi; Gok, Mustafa Sabri; Erdogan, Azmi
    Nickel-based superalloy Nimonic 80A was pack-borided in a solid medium at temperatures of 850 degrees C and 950 degrees C for 2 h and 4 h using silicon-free boriding powders. To investigate the effects of the boriding treatments on mechanical properties (hardness, modulus of elasticity, fracture toughness) and high temperature oxidation resistance, the layers grown on the surfaces were characterized using optical and scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry, and evaluated using microhardness, nanoindentation, wear and oxidation tests. Wear tests were performed on untreated and borided Nimonic 80A alloys using a ball-on-disc tribometer at room temperature and at 500 degrees C under dry sliding conditions. Oxidation tests were carried out in air at 1000 degrees C for 5 h, 25 h and 75 h. Characterization studies revealed a smooth, 22 to 86 mu m thick crack-free boride layer consisting mainly of Ni2B and minor quantities of CrB, Cr2B and Cr5B3 in the borided samples. The hardness and elastic modulus of the boride layer was measured as 15.57-18.95 GPa and 142-217 GPa, respectively. Increasing the boriding temperature and time increased the concentrations of chromium in the boride layer. The hardness and elastic modulus of the boride layer increased with chromium content while its fracture toughness decreased. The boriding treatments improved the dry sliding wear resistance. Increasing boriding time and temperature generally led to a higher wear resistance values. However, the treatments had no significant effect on oxidation resistance. The results of this study show that boriding can significantly improve the wear resistance of Nimonic 80A without compromising its oxidation resistance.
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    Effect of severe vibratory peening on microstructural and tribological properties of hot rolled AISI 1020 mild steel
    (Elsevier Science Sa, 2020) Das, Turan; Erdogan, Azmi; Kursuncu, Bilal; Maleki, Erfan; Unal, Okan
    In this study, microstructural and mechanical performance of AISI 1020 were investigated after severe vibratory peening (SVP) for emerging the potential and performance of this novel treatment among surface severe plastic deformation (SSPD) methods. The specimens were subjected to SVP treatment of V1, V2, and V3 conditions at 20, 40 and 60 min. durations, respectively. Optical microscope (OM) and SEM images demonstrated two layered gradient structure. XRD analysis showed the oxide layer was completely vanished besides surface nanocrystallization by severe plastic deformation (SPD). The microhardness test revealed an average improvement of 48% compared to the untreated specimen. SVP caused raising of hardness from surface to a depth of approximately 900 mu m. In wear tests, the volume loss after SVP were less. The hardness improvement due to deformation overcame the negative effect caused by roughness increase. However, the friction coefficient of the unpeened specimen was the lowest at all loads.
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    Low-temperature aluminizing influence on degradation of nimonic 80A surface: Microstructure, wear and high temperature oxidation behaviors
    (Elsevier, 2021) Erdogan, Azmi; Yener, Tuba; Doleker, Kadir Mert; Korkmaz, Mehmet Erdi; Gok, Mustafa Sabri
    Nimonic 80A alloy has been subjected to aluminizing process to improve its high temperature oxidation and wear properties. Pack aluminizing process was carried out for 3 and 5 h at 650 and 700 degrees C temperatures without using a protective atmosphere. Aluminising treatment was carried out using metallic aluminium, ammonium chloride (NH4Cl) and alumina as a filler. The morphology and structure of aluminide layers was analysed by SEM-EDS. It was observed that there was a super bonding between matrix and coating layers which are smooth, dense and porosity free. Dominant phases of Ni2Al3 and Al80Cr20 were detected by XRD analysis. Layer thickness was measured from the surface to the matrix and changed from 20 to 70 mu m which was increased with increasing process duration and temperature. The matrix hardness is 400 HVN while the hardness of coating layer raised to 1025 HVN with the process time and temperature. Wear losses decreased with the increase in coating temperature and time in both high temperature and room temperature tests. After isothermal oxidation tests at 1000 degrees C up to 75 h, all aluminized samples consist of alumina scale while Nimonic 80A consist of Cr2O3 and TiO2 mixed oxides with high thickness. The increased aluminizing temperature and time provide better protection against high temperature oxidation.
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    Microstructural characterization, boriding kinetics and tribo-wear behavior of borided Fe-based A286 superalloy
    (Elsevier Science Inc, 2022) Gunen, Ali; Keddam, Mourad; Alkan, Sabri; Erdogan, Azmi; Cetin, Melik
    Iron-based superalloys are alloys produced for use in corrosive environments as an alternative to high-cost nickel-based superalloys. However, their average strength and hardness, attributed to their austenitic structures, limit their use in tribological applications. In an attempt to counter these drawbacks, boriding was applied to an iron-based A286 superalloy having an initial surface hardness of 320 HV. Boriding kinetics, some mechanical properties, and tribo-wear (ambient air and 3.5 NaCl environment) behaviors of the formed boride layers were investigated. Multicomponent boride layers (consist of FeB, Fe2B, CrB, NiB, Ni4B3) were formed on the surface of the alloy, with hardness and thickness values of 1498-1961 HV and 20-130 mu m, respectively, depending on the boriding temperature and the treatment time. The integral diffusion model was adopted to deal with the kinetics of monoboride and hemiboride layers formed on the surface. The boron activation energies of FeB, Fe2B, and DZ layer were estimated as equal to 175.86, 198.7, and 205.73 kJ mol- 1, respectively. As a result of increased surface hardness, all of the borided samples displayed reduced friction coefficients and higher wear resistance compared to the untreated alloy, in both ambient air and 3.5% NaCl. However, the increase in wear resistance was not proportional to the increase in hardness; while the best wear resistance was obtained in samples borided at 850-950 degrees C for 6 h, the lowest wear resistance was obtained in samples borided for 4-6 h at 1050 degrees C. This situation was caused by the Kirkendall effect and residual stresses in the structure of alloying elements with different diffusion rates due to the high-temperature effect of the boriding process.