Creep-driven nitride scale growth in γ-TiAl

The interaction between creep deformation and the scale growth process has been studied by performing nitridation experiments on TiAl under four-point bending. It was observed that a modest applied load can influence the nitride scale growth rate significantly through the generation of large stresses induced by creep rate mismatch between the scale and the underlying metal. On the other hand, a thin nitride scale is shown to slow the creep deformation of the nitrided specimen by promoting stress transfer from the metal to the scale. Stress measurement by X-ray diffraction also reveals that the large stress accumulated in the nitride scale (particularly on the compressive side of the bend sample) can cause cracking in the scale, which relieves the stress. The experimental data provide excellent validation for a model of the creep-scale growth interaction published elsewhere.