Studies On High-temperature Oxidation And Its Influence Mechanism Of Fe-Cr-Al Alloy

FeCrAl alloys are being used under high temperature up to1300°C, because theyare able to form protective alumina scales. This allows application of FeCrAl alloys ina large variety of industrial and domestic facilitie. The lifetime of an component is acritical issue for all materials. In spite of the excellent oxidation resistance, thelifetime of FeCrAl-alloys can be limited by oxidation. This is because during hightemperature exposure of the FeCrAl alloys, the aluminium is constantly depleted fromthe alloy matrix, thus the lifetime of the component is shortened significantly. In thestudy of the theoretical aspects of the behavior of high temperature oxidation ofFeCrAl alloy could reveal the high temperature oxidation mechanism of FeCrAl alloy.These studies help to enhance the operating temperature, improve high temperatureperformance of Fe-Cr-Al alloys and more conducive to develop new Fe-Cr-Al alloys.In this thesis, we used the Castep module of Materials Studio5.5software byusing a first principle plane-wave pseudopotential method based on density functiontheory to set up models of surfaces and O absorbed surfaces of Fe-Cr-Al alloy，thehigh-temperature oxidation and its influence mechanism of Fe-Cr-Al alloys wasstudied and discussed on the theoretical level.Surface segregation energies resultssuggest that Cr, Al, Y atoms energetically prefer to be on the Fe（110） surface, and thedriving force of Y segregation to surface is strongest. The surface segregation of Y,Al and Cr will suppress the outward diffusion of Fe. Cr, Y and Al atoms on theFe（110） surface form the tight coherent films of Al₂O₃, Cr₂O₃and RE oxides whichcan restrain oxygen inward diffusion, these lead to the decrease of the growth of oxidefilms. The results of oxygen adsorption energies show that the O adsorption process atFe surface is spontaneous. Al, Cr and Y segregation at Fe surfaces decreases theamount of released heat in the oxygen adsorption process, this may be beneficial fordecreasing the oxidation rate of FeCrAl alloy. By analyzing results of DOS, mullikencharges and average overlap populations, we found that the interaction between O andFe, Cr, Al, Y atoms exists both ionic and covalent binding characteristics. Al, Yalloying increases ionic and covalent binding between Al, Y and O, which speed selective oxidation of Al and Y, and hence improves the oxidation performance ofFeCrAl containing Y alloys.This paper also studied the mechanism of rare earth on the high temperatureoxidation of Fe-Cr-Al alloy by the same method. We find out the results show that theaddition of Rare-Earth element increases the concentration of vacancy of Al₂O₃oxidefilm, and then speeds the formation of Al₂O₃oxide film. Al₂O₃oxide film can preventthe oxygen inward diffusion, and has a protection for FeCrAl alloys. Impurity S willsegregate to surfaces of FeCrAl alloys. When impurity S exists on surfaces of FeCrAlalloys, impurity S repels absorbed O, this leads to the formation of cavity in oxidefilm, as a result, the adhesion of oxide film decreases, and the rupture of oxide film iseasy occur. The addition of Rare-Earth Y can suppress the segregation of impurity Sto surfaces of FeCrAl alloys, Y atoms segregated on surfaces have strong interactionwith O atoms, and react with O atoms to form compounds which enhence thecohesion of the oxide film and the matrix, and hence improves the oxidationresistance performance of FeCrAl alloys.