Comparison of the deformation behaviour of commercially pure titanium and Ti–5Al–2.5Sn(wt.%) at 296 and 728 K

Abstract

The tension and tensile-creep deformation behaviours of a fully-α phase commercially pure (CP) Ti and a near-α Ti–5Al–2.5Sn(wt.%) alloy deformed in situ inside a scanning electron microscope were compared. Tensile tests were performed at 296 and 728 K, while tensile-creep tests were performed at 728 K. The yield stress of CP Ti decreased dramatically with increasing temperature. In contrast, temperature had much smaller effect on the yield stress of Ti–5Al–2.5Sn(wt.%). Electron backscattered diffraction was performed both before and after the deformation, and slip trace analysis was used to determine the active slip and twinning systems, as well as the associated global stress state Schmid factors. In tension tests of CP Ti, prismatic slip was the most likely slip system to be activated when the Schmid factor exceeded 0.4. Prismatic slip was observed over the largest Schmid factor range, indicating that the local stress tensor varies significantly from the global stress state of uniaxial tension. The basal slip activity in Ti–5Al–2.5Sn(wt.%) was observed in a larger faction of grains than in CP Ti. Pyramidal ⟨c + a⟩ slip was more prevalent in CP Ti. Although twinning was an active deformation mode in tension tests of the CP Ti, it was rare in Ti–5Al–2.5Sn(wt.%). During creep, dislocation slip was the primary apparent deformation mechanism in CP Ti, while evidence for dislocation slip was much less apparent in Ti–5Al–2.5Sn(wt.%), where grain boundary sliding was dominant. A robust statistical analysis was carried out to assess the significance of the comparative activity of the different slip systems under the variety of experimental conditions examined.

Keywords:

• titanium
• tension
• creep
• Schmid factor
• deformation behaviour

Acknowledgements

This research was supported by the US Department of Energy, Office of Basic Energy Science through grant No. DE-FG02-09ER46637. The authors are grateful to Mr. James Seal and Mr. Zhe Chen of Michigan State University for their assistance with the sample preparation and in-situ tests. The authors are grateful to Thomas Van Daam of Pratt & Whitney, Rocketdyne for providing the Ti–5Al–2.5Sn alloy used in this study and Dr. Christopher Cowen, previously at National Energy Technology Laboratory, Albany, Oregon and currently at United States Mint, West Point, NY, who provided the CP Ti used in this study.

Notes

1. Henceforth, all alloy compositions are given in weight percent.