Flow behavior of PST and fully lamellar polycrystals of Ti–48Al in the microstrain regime

Abstract

The flow behavior of polysynthetically twinned (PST) crystals of a Ti–48Al alloy was studied as a function of orientation in the microstrain regime (from 10 −5 to 2×10 −2) at room temperature to understand the evolution of the anisotropy of flow stress with strain. After resolving the stresses on to the slip systems, the variation of flow stress with orientation was observed to be only 10–15 MPa at near-zero (10 −5) strain, but it increases rapidly to ∼70 MPa at 0.2% plastic strain. The stress–strain response had discrete steps, indicating the possible effects of the progressive deformation of lamellae of varying thicknesses within the distribution. The flow behavior of a polycrystalline fully lamellar (FL) alloy of the same composition was also studied in the same strain regime for comparison. The saturation engineering flow stress (0.2%) and the strain-rate sensitivity of the polycrystal were found to be close to those of the 0° orientation of the PST crystals. Combining the data of the hard orientations of the PST material and those of the polycrystal, an apparent Taylor factor for fully lamellar polycrystalline Ti–48Al was determined to be in the range of 3.2–3.8. The results on PST crystals were analyzed and rationalized using a mechanistic model that takes into account the distribution of the lamellar sizes ( γ/ γ and γ/ α 2). The work-hardening rate, the saturation flow stress of the hard orientations and the Hall–Petch slopes are all predicted to be sensitive to both the mean and the standard deviation of the lamellar thickness distribution, with dislocation sources from only a fraction of the distribution contributing to the deformation process even at 0.2% plastic strain. Narrower distributions of lamellar spacings are predicted to be beneficial in increasing 0.2% yield strength.