Anisotropic Oxidation Behaviors Of Some Single Crystalline Alloys And Compands

Anisotropic is one of the most basic characteristics of crystalline materials. Some physical properties of crystalline materials presented anisotropic characteristics due to long-term orderly arrangement of atoms. The single crystals'property of high-temperature oxidation also depends on crystal orientation. Several typical crystal materials, including high-temperature refractory metal of Nb single crystal, Ni-based single crys-tal high temperature alloy (DZ4), intermetallic compound Ni3Al, and MoSi2 single crystal etc, are chosen in this papers to study the dependent relationship between the oxidation behavior and crystal orientation.The results showed that a pronounced anisotropy in oxidation behavior was observed in selected materials .For the BCC structure of Nb single crystal,under the condition of constant temperature oxidation at 250℃for 200min ,orientation-dependent oxidation rates ordered from big to small were as follows: (100), (111), (110), the product of the three faces after oxidation are all Nb2O5. For the FCC structure of DZ4 nickel-based single crystal oxidation at 900℃for 95h, the oxidation rate of (111) is the highest, while (100) and (110) are closer. There are different elements segregation between core and inter dendrite in Nickel-based single-crystal, so the oxidation products also have the obvious differences. The dendrite morphology of different planes along crystal growth direction is also different. For L12 structure of the Ni3Al single-crystal at 1,100℃, are also showing oxidation anisotropy behavior. Oxidation reaction rate of different faces from high to low order are (111), (100), (110), respectively. The oxidation formation of Ni3Al single crystal is the outer oxidation layer of NiO, the intermediate layer of spinel NiAl2O4 and the inner layer of Al2O3. For C11b crystal structure MoSi2 single crystal, the oxidation reaction rate from high to low order are (101), (100), (001), (110) respectively. MoSi2 oxidation crystallographic anisotropy has no relationship with temperature.The anisotropic oxidation is an universal behavior for the single crystalline materials. For BCC structured, Nb single crystal, the highest atomic density has the fastest rate of oxidation reaction; however, the lowest atomic density has not the slowest oxidation rate. The oxygen diffusion is not the only key factor for oxidation. For the FCC structured DZ4 alloy with different orientation has different dendrite morphology along crystal growth direction, dendrite spacing, elements segregation and space arrangement of two phase boundary. These differents result in the oxidation anisotropic. For L12 structured Ni3Al crystal, the highest atomic density has the largest oxidation reaction rate, the lowest atomic density has the slowest oxidation reaction rate. The diffusion of oxygen cause the inner oxidation, the oxide film formation rate of different faces and the diffusion of oxygen are the key factors of anisotropic. For the MoSi2 crystal (C11b), the rate of oxygen spread through the oxide film for all the different faces affects the anisotropy oxidation.