CoCrFeNiAlxTiy high entropy alloys were produced by electric current assistive sintering (ECAS) method and their surfaces were then subjected to laser re-melting. Microstructure, hardness, and wear ...behaviors were investigated using different parameters. While the CoCrFeNi alloy had FCC and oxide phases after ECAS treatment, only FCC phase was observed after laser re-melting. In addition to the FCC phase, also a BCC phase has been determined with the Al addition. FCC phases disappeared with the laser re-melting effect and the addition of Ti to the alloy. Mixing enthalpy values were found to be effective in microstructure and phase formation. Thanks to the rapid solidification provided by laser re-melting, the phases were distributed homogeneously and in much smaller particle size. Al and Ti elements have contributed positively to the hardness of the CoCrFeNi alloy. With additions of Al and Ti elements, a significant increase in the hardness of CoCrFeNi alloy was obtained. Significant increases in hardness values were observed after laser re-melting due to the smaller dispersion of the phases and the formation of hard phases (BCC/intermetallic). In the hardness values of all alloys, 1.5 times increase was observed by the effect of re-melting. The highest average hardness was determined as 859 Hv in laser re-melted CoCrFeNiAl0.5Ti0.5 alloy. The best wear resistance and the lowest friction coefficient was observed in the laser re-melted CoCrFeNiAl0.5Ti0.5 alloy. Laser re-melted alloys exhibited lower volume losses compared to only ECAS-alloys.
•Electric current assistive sintered CoCrFeNiAlxTiy high entropy alloys were subjected to laser re-melting.•Laser re-melting process has changed the phases occurring in alloys.•After laser re-melting, the hardness of the alloys increased by an average of 1.5 times.•Al and Ti provided an increase in hardness while replacing the phases occurring in alloys.•The LR has increased the wear resistance and the highest wear resistance has been seen in the CoCrFeNiAl0.5Ti0.5-LR alloy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
CoCrFeNiAl
x
Ti
y
high-entropy alloys were produced by the induction melting method and their oxidation behavior investigated when exposed to 1000°C for different durations. One or more body-centered ...cubic phases were found in all alloys, except CoCrFeNiTi
0.5
. In the CoCrFeNiTi
0.5
alloy, two different face-centered cubic phases and one tetragonal sigma phase were detected. Scanning electron microscopy elemental analysis showed that all the alloys exhibited homogeneous microstructure. Energy-dispersive x-ray spectroscopy analysis revealed that Cr and Fe elements were enriched in one phase and Al-Ni-Ti elements in another. The presence of Ti negatively affected the oxidation behavior. According to the oxidation test results, dominant Al
2
O
3
formation was observed in the CoCrFeNiAl
0.5
and CoCrFeNiAlTi
0.5
alloys. As a result, these two alloys exhibited the best performance among the five high-entropy alloys in terms of mass gain and oxide thickness.
Hot corrosion and oxidation cause very destructive damage in thermal barrier coatings (TBCs) during service conditions. In hot corrosion, TBCs exposed to molten salts lose their integrity easily due ...to the phase transformations while oxygen easily penetrates from the TBCs to bond coats and forms thermally grown oxide (TGO) layer which causes higher stresses at the interface of bond and top coating. In the current study, CoNiCrAlY powders were sprayed by high-velocity oxygen fuel (HVOF) technique on Inconel 718, and then yttria-stabilized zirconia (YSZ) and YSZ/Gd2Zr2O7 ingots were deposited by electron beam physical vapor deposition (EB-PVD) technique on the bond coated substrates. Isothermal oxidation tests were carried out at 1000 °C for 8, 24, 50, and 100 h, while hot corrosion tests were carried out at 1000 °C in the presence of NaCl, Na2SO4, and V2O5 molten salts with 5, 10, 15, and 20 h cycles. The produced coatings, as well as the oxidation and hot corrosion test results, were examined using SEM, EDS, XRD, and image analysis techniques. After the tests, the Gd2Zr2O7 layer was found to exhibit superior oxidation and hot corrosion performance as compared to the conventional YSZ TBC system.
•β-NiAl phases slightly depleted after oxidation tests and, the TGO layer dominantly composed of alumina.•The phase transformation or spallation were not observed after the oxidation test.•YVO4 and GdVO4 phases led to the phase transformation in the top coatings after the hot corrosion tests.•Gd2Zr2O7 layer completely spalled from YSZ layer while YSZ TBC was still compact at the end of the hot corrosion tests.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Oxidation is an inevitable failure mechanism under the operating temperature in gas turbines. To avoid negative effects of oxidation, ceramic-based materials having low thermal conductivity and high ...stability should be used to hot section components. In accordance with this purpose, thermal barrier coatings (TBCs) are used in order to increase the lifetime of gas turbine engine components that have not reached to desired levels yet. Yttria stabilized zirconia (YSZ) has been used as a conventional top coat material in TBCs. Increased the turbine inlet temperatures (TIT) promote to researchers to try higher stable material such as rare earth zirconates. In this study, CoNiCrAlY metallic powders were sprayed using a new emerging technique as called cold gas dynamic spray (CGDS) on Inconel 718 substrates. Single layer YSZ, Gd2Zr2O7 and double-layered YSZ/Gd2Zr2O7 were deposited by electron beam physical vapor deposition (EB-PVD) technique as top coat materials. In high temperature furnace, both TBC samples were isothermally oxidized at 1000 °C under different time periods. TBCs were examined as microstructural before and after oxidation tests. Thermally grown oxide (TGO) layer forming at the interface during oxidation were investigated and compared for each TBC systems. Oxidation and TGO growth behaviors were discussed.
•TBC systems were produced by the combination of CGDS and EB-PVD techniques.•Gd2Zr2O7 TBC did not show a good performance due to the including micro cracks after 50 and 100 hours oxidation tests.•Chemical incompatibility was not detected at the end of the oxidation tests.•Double layered YSZ/Gd2Zr2O7 TBC exhibited slightly better performance compared to conventional single layered YSZ TBC.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Nickel-based superalloy Nimonic 80A was pack-borided in a solid medium at temperatures of 850 °C and 950 °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 °C under dry sliding conditions. Oxidation tests were carried out in air at 1000 °C for 5 h, 25 h and 75 h. Characterization studies revealed a smooth, 22 to 86 μm 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.
•The Nimonic 80A was subjected to Boriding at different temperatures and times.•Microstructures, mechanical, wear and oxidation properties of Nimonic 80A with and without coating were investigated.•Boride layers of 22–86 μm thickness, 15.57–18.95 GPa hardness, 142–217 GPa elastic modulus were obtained.•With the increasing surface hardness (by boriding), wear losses decreased.•The boriding process can provide a slight improvement in the oxidation resistance of Nimonic 80A
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This study is focused on a detailed investigation of high-temperature corrosion and oxidation behavior of borided CoCrFeNiAl0.5Nb0.5 HEAs, considering their use in advanced engineering applications. ...CoCrFeNiAl0.5Nb0.5 HEA was produced by arc melting. XRD and SEM-EDS analysis before boriding determined that the alloy consisted of four different phases with different chemical compositions. Powder-pack boriding of a CoCrFeNiAl0.5Nb0.5 HEA was performed at 1000 °C for 3 h in a boriding media containing 90 % boron carbide and 10 % sodium tetrafluoroborate. As a result of the boriding process, complex boride layers consisting of (CoFe)B2, CrFeB, CoNbB, FeB and NiB phases were obtained on the surface with a thickness of 40 μm and hardness of 3004 HV. it was determined that the microstructure with cauliflower appearance evolved towards a dense and non-porous appearance around 20 μm as the boron diffusing into the HEA microstructure filling the gaps of intermetallic compound in the single structure, HEA and borided HEAs were each subjected to a cyclic hot corrosion test at 900 °C in molten corrosion salts of Na2SO4 and V2O5. After hot corrosion tests, long rod-like structures were observed in both samples due to the excessive corrosion, while borided alloys were more resistant to hot corrosion damage. After oxidation tests, HEA consists of a compact protective alumina scale that provided better oxidation resistance, while non-protective mixed oxides with cracks were dominant in the borided HEA.
•Borided HEA's surface degradation properties were investigated at high temperature.•Boriding process enhanced the hot corrosion resistance of alloy.•The presence of B2 phases provided alumina scale as oxidation product in the alloy.•The boriding process weakened the oxidation resistance as it caused the aluminum to migrate deeper.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Inconel 718 has a wide area of applications in gas turbines, aircraft, turbocharger rotor equipment, and so many other applications, including applications with temperatures up to ~700 °C. In this ...study, Ni-based aluminide coating on Inconel 718 alloy was applied by pack cementation technique. Phase structures, layer coating thickness, hardness, wear, and oxidation behaviors were studied for different aluminization temperatures and times (600 and 700 °C for 3 and 5 h). The change in the layer from the surface to the inside was varied from 8 to 80 μm and this was increased with higher aluminization temperature and more time. The hardness value of 960 HV was determined in the coating layer whereas the hardness of the matrix was 260 ± 10 HV. The wear resistance of the samples was tested under dry sliding conditions with different loads. The wear resistance of Inconel 718 has increased with aluminization. Increased aluminization duration and temperature reduced wear losses. Isothermal oxidation test results at 1000 °C for 5, 25, and 125 h show that Inconel 718 has a poor oxidation resistance under these conditions while aluminized Inconel 718 samples exhibit better results. The comparison of the aluminized Inconel 718 samples after oxidation tests showed that the aluminization at higher temperatures allowed the formation of a compact and continuous alumina layer while the formation of porosities under the oxide scale weakened the adherence and durability in the others.
•The Inconel 718 was subjected to low temperature aluminizing at different temperatures and times.•Microstructure, wear and oxidation properties of Inconel 718 and coatings were investigated.•Aluminization improved the wear and oxidation properties of the Inconel 718 alloy.•With the increasing surface hardness, wear losses and friction coefficient decreased.•The formed NiAl rich phases enable the formation of alumina phases during the oxidation tests.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Purpose
Thermal barrier coatings (TBCs), which are used in high temperature applications of gas turbines, are damaged due to fuels and airborne minerals under working conditions. Stable zirconia ...coatings, which are usually used as topcoat materials in TBCs, are damaged by interacting at high temperatures with elements such as vanadium and sulfur from low quality fuels. The purpose of this paper is to see the failure mechanism of TBC systems after hot corrosion damages.
Design/methodology/approach
CoNiCrAlY metallic bond coatings of TBC samples were produced by cold gas dynamic spray method which is a new trend production method and stabilized zirconia ceramic top coating was produced by atmospheric plasma spray method. In total, 50% by weight of V2O5 and 50% Na2SO4 salt mixtures were placed on TBC samples and subjected to hot corrosion test at 1000°C.
Findings
Hot corrosion behaviors of TBC samples were examined by scanning electron microscopy, elemental mapping analysis, energy dispersive X-ray spectrometry analysis and X-ray diffraction analysis. TBC samples were damaged at the end of 12-h cycles.
Originality/value
The paper provides to understand the mechanism of hot corrosion of TBCs with cold sprayed metallic bond coat.
The cold gas dynamic spray (CGDS) method has been considered a promising technology to produce a metallic bond coat for thermal barrier coating (TBC) systems. In this study, CoNiCrAlY bond coats ...produced by CGDS method were coated with yttria-stabilized zirconia (YSZ) by electron beam physical vapor deposition (EB-PVD). TBCs were subjected to 50 wt % V2O5 and 50 wt % Na2SO4 molten hot corrosion salt combinations at 1000 °C. In the case of YSZ top coat on TBCs, the reaction between Na2SO4, V2O5, and Y2O3 salts generates YVO4 crystals, and these structures cause the transformation of tetragonal ZrO2 to monoclinic ZrO2. This situation occurs under operating conditions that lead to TBC failure. Hot corrosion behavior and the related failure mechanisms of TBC systems were investigated and discussed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, and X-ray diffractometer (XRD).
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