Strain Path Effects on Titanium Alloy Timetal-834 under Hot Working Conditions

Abstract

The effects of strain path reversal under hot working conditions on the microstructure and crystallographic texture in the near-α titanium alloy Timetal-834 have been studied using high resolution electron backscatter diffraction (EBSD). The main objective of the work was to investigate the potential effect strain path may have on breaking up the well known clustering of similarly orientated primary alphas grains in the alloy, which significantly reduces its low cycle dwell fatigue lifetime. Deformation was carried out using forward torsion to an equivalent strain of 0.9 and forward/reverse torsion of two equal steps to produce a total strain of 0.9. The tests were performed at a typical industrial forging condition of 990°C (~50% alpha, ~50% beta) at an equivalent tensile strain rate of 2s-1. Investigation of the microstructure showed the primary alpha grains to align with the direction of torsion for the forward test and return to an equiaxed shape on strain reversal, though a significant numbers of deformation twins are formed and retained after the full strain reversal. Analysis of the texture of the starting material showed the typically clustering of primary alpha grains, which upon forward torsion and forward/reverse torsion did not break down. This indicates that during a typical forging operation the clustering of similarly orientated primary alpha grains inherited from the as-received billet will not be reduced. This suggests that improved in-service performance of this alloy can only be achieved by reducing the clustering upstream in the manufacture of the billet.