Saturation of yield stress and embrittlement in fine lamellar TiAl alloy

It has been proposed by Sun that the yield stress of lamellar materials is increased by refinement of lamellar spacing but there is an upper limit to the increase. This paper aims at the examination of this prediction by experimental results of a fully lamellar Ti–39 mol% Al alloy tested at room temperature and at 950 K. The lamellar spacing is varied from 20 to 590 nm, while keeping the same grain size of 90 μm. The Hall–Petch relation holds between the lamellar spacing and the yield stress of the alloy. However, the yield stress saturates at a level of 1 GPa, which is achieved below a critical lamellar spacing of about 100 nm. These results support the prediction by Sun. The Hall–Petch slope, the critical lamellar spacing and the saturation stress level can be explained consistently by a pile-up model of dislocations at lamellar boundaries. Contrary to the beneficial effect on the yield stress, the lamellar refinement deteriorates ductility of the lamellar material.