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

Relying on high-temperature oxidation resistance, Ni-Cr-Al alloys become one of important high-temperature material. Ni-Cr-Al alloys with excellent properties play an important role in the fields of defense and industry, and are widely used as high temperature parts in aerospace, electric power and metallurgy. Ni-Cr-Al high temperature alloys of such good performance mainly rely on Cr and (or) Al to form a layer of Cr2O3 and (or) Al2O3 protective oxidation film. This oxidation film with good adhesion to the substrate, grows slowly, so it can have very good protection effect. Ni-Cr-Al alloy models are set up by using computer, and its behavior of high temperature oxidation was researched theoretically to reveal the high temperature oxidation mechanism of Ni-Cr-Al alloy. These researches play an active role in enhancing using temperature, improving high temperature performance and developing new Ni-Cr-Al alloys.In this thesis, the surface and 0 absorbed surface of Ni-Cr-Al alloy models were set up by using the Castep module of Materials Studio software, and the high-temperature selective oxidation and its influence mechanism of Ni-Cr-Al alloys was studied by applying a first principle plane-wave pseudo-potential method based on density function theory. The study shows that Al and Cr will not segregate on the surface (111), but Y will. The absorption of oxygen will promote inversion of Al and Cr segregation on the surface, which plays a decisive role on the selective oxidation of Ni-Cr-Al alloy. The order of surface segregation intensity of alloying elements is Y>Cr>Al. The alloying elements of Cr,Al,Y, especially Cr make it easier for the chemical absorption of O on the surface of Ni based alloy, which will explain the fact that why the outer oxidized scale of Ni based alloy is Cr2O3, while the inner oxidized scale is Al2O3. The mechanics why the doping of rare-earth elements promotes the selective oxidation is that the oxidants Y2O3 are helpful to nucleation of Cr2O3 or Al2O3. Alloying elements Al, Cr and Y will increase the surface activity of Ni based alloy, which will contribute to the formation of oxidants of Al, Cr and Y on the surface, as a result selective oxidation is brought up. During the process of absorption, the oxygen atom will form relatively strong covalent bonds with Al, Cr and Y, but the covalent effects with Ni are relatively weak, so there is prone to form thin oxidation film of Cr2O3 or Al2O3 on the surface of Ni-Cr-Y. The absorption of O will make it possible for electron d of Ni, Cr and Y transferred to the atom of oxygen, so that the chemical bonds are both covalently and electrovalently formed.In this thesis, the Ni-Cr-Al alloy and Al2O3 bulk models were set up by using the Castep module of-Materials Studio software, the Rere Earth elements-action mechanism on the high-temperature oxidation of Ni-Cr-Al alloys was also studied by the same method. Results show that the addition of Rare-Earth element prevents the formation of cavity and the rupture of oxide film, increase the adhesion of oxide film, then the oxide film has a better protection of the matrix. Rare-Earth element can form compounds with impurity S, which prevents S from destructing the adhesion of the oxide film. Rare-Earth element can not decrease the Al and O hole concentration in Al2O3 oxide film, and also cannot reduce their diffusion velocity. The influence of Rare Earth element on the growth rate of oxide film is from its rapidly forming of Al2O3 oxide film.