Alpha-Case Kinetics and Surface Crack Growth in the High-Temperature Alloy TIMETAL 834 Under Creep Conditions

Investigations of Timetal 834 crept specimens showed that a uniform alpha-case layer was developed at the surfaces of all specimens tested in air at 823 K to 923 K (550 A degrees C to 650 A degrees C). The formation of this alpha-case was a result of the inward diffusion of oxygen, which is an alpha stabilizer, and other elements promoting the transformation of the beta matrix into an alpha phase and, thus, modifying the microstructure at the surfaces of these test pieces. The thickness of this alpha-case was increasing in a parabolic manner with increasing exposure time and temperature. Cracks were clearly observed at the surface of the crept specimens whose depths were greater at lower stresses. Interestingly, an alpha-case was also developed at the surfaces of the penetrating cracks with the case thickness decreasing towards the crack tip. This information was used to calculate the times at which the cracks initiated.