Effect of texture and slip mode on the anisotropy of plastic flow and flowsoftening during hot working of Ti-6Al-4V

The effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was deter mined by conducting isothermal, constant-strain-rate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transver se direction (TD), 45 deg, and normal) from hot-rolled plate. Testing was p erformed using a fixed strain rate (0.1 s(-1)) and various temperatures bel ow the beta transus. The flow cur;es from all of the experiments exhibited a peak flow stress followed by a large and a small amount of flow softening for the colony and globular microstructures, respectively. Although the fl ow softening response did not depend noticeably on test direction for a giv en microstructure and test temperature, the peak flow stress and developmen t of sample ovality did. This orientation dependence was interpreted using both lower-bound (isostress-type) and upper-bound (isostrain, Taylor/Bishop -Hill) models to deduce the operative slip systems in the alpha phase. Thes e analyses suggested that prism [a] and basal [a] slip are considerably eas ier than pyramidal [c + a] or [a] slip at hot-working temperatures. A compa rison of the flow curves for the colony and globular alpha microstructures suggested that slip transfer across alpha/beta interfaces and loss of Hall- Fetch boundary strengthening can account for a substantial portion of the f low softening observed during hot working.