PHASE STABILITY AND ISOTHERMAL OXIDATION BEHAVIOR OF AL0.75COCRFENI ALLOY AT TEMPERATURES OF 900, 1000, 1100 C

• Main Author: Ali Akbar, Mohamad
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/74088
• Institution: Institut Teknologi Bandung

The Indonesian Aerospace Industry Ecosystem Road Map 2022-2045 has been issued. In the future, high demand for high temperature structural materials might occur. Currently, nickel-based superalloys are highly reliable but have limitations due to their melting temperatures. High entropy alloys (HEA) show a good combination between strength and resistance to oxidation and corrosion at elevated temperatures. Reviews of HEA often based on secondary observations, so that validation is essentially needed and alloy designs must be carefully carried out. Among relatively large number alloy systems that have been developed, Al0.75CoCrFeNi is grouped in the eutectic high entropy alloy that has potential to answer this challenge because of the unique phase and microstructure that present. This research aims to predict the microstructure of Al0.75CoCrFeNi and oxidation behavior at high temperatures. The alloy was melted using a mini single arc furnace purged with high purity argon gas with 5 repetitions. The alloy button is then homogenized at 1100? for 10 hours. The as homogenized alloy was then sectioned using an electric discharge machining into several coupon specimens. The specimens were subjected to atmospheric isothermal oxidation in a tube furnace at temperatures of 900, 1000 and 1100 ? for 2, 16, 40 hours, and 168 hours. The weight gain of each sample was measured. Characterization was conducted to each sample using optical microscope, XRD, SEM-EDS to have the microstructures and microconstituents of the scale as well as the substrate. The hardness of the substrate of each sample was done using Vickers hardness tester. The results of the empirical approach have confirmed that the as homogenized alloy has two solid solution phases with FCC and BCC crystal structures. The characterization showed that Al0.75CoCrFeNi has Widmanstätten with two phases of FCC-A1 and BCC-B2/A2. Spheroidization of B2 occurred during high temperature oxidation. The volume fraction of this phase decreased with increasing temperatures followed by decreasing the hardness of the substrates. Various oxide compounds of CoO, NiO, Cr2O3 and CrO3, ?-Al2O3, ?- Al2O3, and Ni(Cr,Al)2O4 were found as external scale as well as internal oxides. Basically the scale consists of semicontinuous external of Al2O3 with Cr2O3 subscale and a mixture of oxides consisting spinels, NiO and CoO at the bottom. In clusters that do not form an alumina scale, a complex oxide is formed which is overgrown by the alumina ring structure. The oxidation kinetics of this alloy at all temperatures follow the logarithmic oxidation rate.