Phase Equilibrium Of The Fe-Cr-Ni-Al Quaternary System And The Effect Of Ti On Oxidation Behavior Of Fe-Cr-Ni-Al Alloy At High Temperature

Thanks to the high temperature strength, excellent oxidation resistance and highmelting point, Fe-Cr-Ni-Al alloys have found applications in a wide range ofhigh-temperature oxidation environments, such as heating furnace, nuclear reactor,automotive exhaust system and so on. By modifying the composition and controllingthe heating treatment, we may develop new type of superalloys with excellentmechanical properties. For a further study of superalloys used at high temperature, theknowledge of the Fe-Cr-Ni-Al phase diagram is necessary. The information of thephase equilibria of the quaternary system at high temperature is very important indeveloping new type of superalloys. The high temperature oxidation resistance playsan important role in the mechanical properties of superalloys, so the study on theoxidation behavior of Fe-Cr-Ni-Al alloys is also necessary.The900°C isothermal sections of the Fe-Cr-Ni-Al quaternary with Fe fixed at70at.%and that with Ni fixed at60at.%have been determined by scanning electronmicrocopy（SEM） coupled with energy dispersive spectroscopy（EDS） and X-raydiffraction（XRD）. Only one three-phase region, α+β+γ, is found in the900°Cisothermal section of70at.%Fe. One four-phase region, α+β+γ+γ′, and threethree-phase regions, namely α+γ+γ′, γ+γ′+β and α+β+γ′are found in the900°Cisothermal section of60at.%Ni. No quaternary compound is detected in bothisothermal sections.The effect of Ti on the oxidation behavior of nickel-based and iron-basedFe-Cr-Ni-Al alloys with microstructures of γ+γ′and α+β respectively wasinvestigated by means of the thermogravimetric method at900℃. SEM-EDS andXRD were used to reveal the morphologies and composition of the oxidationproducts and determine their phase structures respectively. The results indicated thatoxidation kinetics obeyed parabolic law. With regard to nickle-based alloys, certaincontent TiO₂with poorer protective effect would be formed on the surface of alloysby the addition of Ti. With the increase of Ti content, serious internal oxidationwould take place in the nickle-based alloys since the amount of TiO₂was increased.As a result the oxidation resistance was reduced. On the contrary, with the additionof Ti to iron-based alloys, Al2O3protection oxidation film was formed and Ti was segregated on the grain boundary. The formed TiO₂pinned together with thecontinuous outer oxide film and substrate which was beneficial to the adhesion ofthe oxidation film, so the oxidation resistance was improved. However, with theincrease of Ti content, Ti segregation on the grain boundary had become more intense.More TiO₂would be formed on the grain boundary which led to certain internaloxidation, making the oxidation resistance reduced.