A model for the effect of creep deformation and intrinsic growth stress on oxide/nitride scale growth rates with application to the nitridation of γ-TiAl

The growth rate of oxide and nitride scales can be considerably affected by the application of stress to the material when the underlying substrate is able to creep. This occurs even though the applied stress may be quite small. In this paper, we present a model to calculate the residual stress induced in growing scales during this process. It is shown that the applied stress causes the metallic substrate and the scale to creep at different rates leading to the accumulation of a large stress in the thin scale layer. This stress influences the diffusional flux of anions towards the metal/nitride interface, which controls the scale growth kinetics. The model is applied to the growth of TiN scale on γ-TiAl upon exposure at high temperature in a pure N2 atmosphere. As a consequence, the nitride scale growth rate is dependent on the sign and magnitude of the applied stress wherein a tensile stress accelerates the growth process and a compressive stress slows it down. The model predictions are in good agreement with experimental data for scale thickness on the tensile and compressive faces of TiAl bars following flexural creep tests in N2.