FLOW BEHAVIOR OF PST AND FULLY LAMELLAR POLYCRYSTALS OF TI-48AL IN THE MICROSTRAIN REGIME

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 x 10(-2)) at room temperature to understand the evolution of the anisotropy of flow stress with strain. After reso lving the stresses on to the slip systems, the variation of flow stres s with orientation was observed to be only 10-15 MPa at near-zero (10( -5)) strain, but it increases rapidly to similar to 70 MPa at 0.2% pla stic strain. The stress-strain response had discrete steps, indicating the possible effects of the progressive deformation of lameliae of va rying thicknesses within the distribution. The flow behavior of a poly crystalline fully lamellar (FL) alloy of the same composition was also studied in the same strain regime for comparison. The saturation engi neering flow stress (0.2%) and the strain-rate sensitivity of the poly crystal were found to be close to those of the 0 degrees orientation o f 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 ratio nalized using a mechanistic model that takes into account the distribu tion of the lamellar sizes (gamma/gamma and gamma/alpha(2)). The work- hardening rate, the saturation Bow stress of the hard orientations and the Hall-Fetch slopes are all predicted to be sensitive to both the m ean and the standard deviation of the lamellar thickness distribution, with dislocation sources from only a fraction of the distribution con tributing to the deformation process even at 0.2% plastic strain. Narr ower distributions of lamellar spacings are predicted to be beneficial in increasing 0.2% yield strength. (C) 1998 Acta Metallurgica Inc.