Date of Award

2001

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Hamouda Ghonem

Abstract

The recommended continuos operating temperature for Timetal 21S is below 540 °C. Application temperature for this alloy can, however exceed this level, with attendant embrittlement due to oxidation. The present study examines the effects of exposures in air on the tensile ductility of fully aged Timetal 21S sheet specimens in temperature range 482 - 693 °C. In this thesis, the kinetics of the loss of tensile ductility is investigated as a function of exposure temperature and sheet thickness. The experimental embrittlement activation energies are estimated by determining exposure times at various temperatures to reach close to zero ductility at room temperature. The embrittlement kinetics is investigated for three different sheet thickness; 0.12, 0.39, and 1.0 mm. The morphology of the surface layer and changes in microstructure are examined using scanning electron microscopy (SEM) as a function of exposure conditions. Phase changes as function of the exposure parameters are investigated using transmission electron microscopy (TEM). Kinetics of weight gain during exposure is also investigated. Results of this study show that two distinct embrittlement mechanisms exist within the temperature range mentioned above. At higher temperatures, >550 °C, the activation energy is 57 kcal/mole indicating that the embrittlement process is controlled by diffusion of oxygen. Below 550 °C, the embrittlement activation energy approaches zero, a characteristic of a diffusionless athermal transformation. Furthermore, the SEM examinations reveal that a protective oxide layer is present only at temperatures higher than 550 °C. There is also an indication of increased volume fraction of α phase in the vicinity of the surface. TEM examinations reveal an increase of the isothermal ω phase as function of the exposure time increase. The general trend of these results lead to the conclusion that embrittlement at higher temperature is caused by enhanced diffusion of oxygen, which increases both the volume fraction of a phase and dissolved oxygen. At lower temperature, below 550 °C, embrittlement is probably caused by an athermal transformation of β to ω phase resulting from oxidation of molybdenum, which is a β stabilizer.

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