An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

Abstract

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900 °C and 1060 °C and strain rates between 10−4 and 10−2 s−1. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and (\(\bar 1\)12) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.