| A method for "Al+Y codeposition" was developed to improve the resistance of high temperatrue oxidation of TiAl using a single process of EB-PVD. Isothermal and cyclic oxidation tests at 950℃ showed that the Al+Y codeposition improved the oxidation resistance of TiAl significantly. During oxidation, the oxides formed in the Al+y codeposition layer are composed of two parts ; the outer (Y,Al)O layer and the inner Al₂O₃, layers containing small amount of Y-rich oxide. Both the outer (Y,Al)O and inner Al₂O₃ layers have a fine-grained structure which prevents effectively inward diffusion of oxygen and consequently prohibits formation of porous TiO₂. During cyclic oxidation, these (Y,Al)O and Al₂O₃, layers relieve easily both the growth and thermal stresses by permitting easy plastic deformation. However, when exposed to high temperature for an extended period of time, an Al₂O₃, layer having a columnar structure is developed underneath the existing fine-grained Al₂O₃, layer by reaction with Al from the substrate TiAl. This columnar Al₂O₃, is responsible for degradation of the oxide layer. |
|
