Interaction between creep deformation and oxy-nitride scale growth in gamma-titanium aluminide

During typical high-temperature service, a component is subjected to mechanical stress and/or thermal load in addition to the aggressive environment. Such a complex condition highlights the importance of understanding the interaction among various processes that lead to the degradation of the material. Two phenomena of interest are creep deformation and oxidation/nitridation as these processes limit the performance of high temperature structural materials such as gamma-TiAl.; In this contribution, three models have been developed. The first provides a first order estimate of stress induced during the growth of an oxide/nitride scale. The other two models simulate the scale growth process under uniform and bending loads. In these models, the mechanical and diffusional problems are coupled by calculating the stress induced by the creep rate mismatch between the thin scale and the underlying metallic substrate. The most important result of these models is the calculation of scale growth rate, creep strain evolution and residual stress in the scale.; Experimental work was performed to provide the required input for the model and to provide verification for the validity of the model. Creep experiments on TiAl under compression and bending load were performed in various environments to measure the creep deformation of the specimen with the occurrence of scale growth process.; Measurement of scale thickness was done by electron microscopy and residual strain was measured using x-ray diffraction technique.; Quantitative agreement was obtained between the modeling and experimental results in terms of the three parameters mentioned above. It is found that a thin scale can lower the creep rate of an oxidized/nitrided metal as the scale is more resistant to both elastic and creep deformation. Furthermore, a modest applied stress can alter the scale growth significantly through the generation of large stresses in the scale due to the creep rate mismatch. These results show a strong interaction between the scale growth and creep deformation at high temperature. Thus, these processes can simultaneously contribute to the degradation of materials in an aggressive operational condition.