Alloys used for high temperatures applications require combinations of mechanical strength, microstructural stability and corrosion/oxidation resistance. Nickel base superalloys have been traditionally the prime materials utilized for hot section components of aircraft turbine engines. Nevertheless, due to their limited melting temperatures, alloys based on intermetallic compounds, such as TiAl base alloys, have emerged as high temperature materials and intensively developed with the main aim to replace nickel based superalloys. For applications in steam power plants operated at lower temperatures, ferritic high temperature alloys still attract high attention, and therefore, development of these alloys is in progress. This paper highlights the important metallurgical parameters of high temperature alloys and describes few efforts in the development of Fe-Ni-Al based alloys containing B2-(Fe,Ni)Al precipitates, oxide dispersion strengthening (ODS) ferritic steels and titanium aluminide based alloys include important protection system of aluminide coatings.

The development of high-entropy alloys (HEAs) for high-temperature applications has been driven by the limitation of nickel-based superalloys in achieving optimal efficiency at higher temperatures for higher efficiency in power generation engines. The alloys must have high oxidation resistance and microstructural stability at high temperatures. Relatively equimolar multi elements involved in HEAs produce microstructure containing a single solid solution or multiphase that improves the mechanical properties and oxidation resistance resulting from sluggish diffusion and core effects. In this study, the oxidation behavior and microstructural changes of Al_0.75CoCrFeNi HEA at 900, 1000, and 1100 °C in air atmosphere were investigated. Based on the XRD and SEM-EDS analysis, the mechanism of oxide scale formation and microstructural changes of the substrate are proposed. The results show that the oxidation behavior of the alloy follows a logarithmic rate law. Different oxide compounds of CoO, NiO, Cr₂O₃, and CrO₃, θ-Al₂O₃, α-Al₂O₃, and Ni(Cr,Al)₂O₄ with semicontinuous oxides of Al₂O₃ with Cr₂O₃ subscale and an oxide mixture consisting of spinel of Ni(Cr,Al)₂O₄ and Co(Cr,Al)₂O₄ were found. During oxidation, Widmanstätten of FCC-A1 and BCC-B2/A2 phases in the substrate have changed. Spheroidization of B2 and a reduction in volume fraction decrease the hardness of the substrates.

A series of experiments have been conducted to study the resistance of isothermal oxidation in 69.5Fe-14Ni-9Al-7.5Cr (% wt) alloys with some temperature variations of 800, 900, and 1000°C and each temperature is tested for 1, 10, 50, and 150 hours. Based on the analysis conducted on the test results, it is known that the higher the oxidation temperature, the faster the oxidation rate. As for the duration of oxidation, the longer the testing time is, the smaller the weight change per unit sample area is, indicated by a sloping graph. The oxide products found in this study were Fe₂O₃, Fe₃O₄, α-Al₂O₃, NiO, Cr₂O₃, and NiCr₂O₄. The rate of oxidation of this alloy at each temperature follows a logarithmic equation indicating that the oxide layer formed is very thin and can protect the metal surface well. The oxidation rate constants at temperatures of 800, 900, and 1000°C are 5.15E-05, 5.57E-05, and 6.74E-05 gr.cm⁻², respectively.