The influence of strain rate on the visibility of dislocations in transmission electron microscopy images of deformed Ti–6 wt% Al–4 wt% V and in Timet 550

Abstract

Samples of Ti–6 wt% Al–4 wt% V and Timet 550 (Ti–4 wt% Al–4 wt% Mo–2 wt% Sn–0.5 wt% Si) have been subjected to strain rates between 10⁻¹ and 10³ s⁻¹and detailed examination of the dislocation structure in the α grains has been carried out using transmission electron microscopy (TEM). For samples deformed to a strain of 0.1 at 10⁻¹ s⁻¹, detailed analysis of the defects can be carried out using all diffracting vectors and the presence of (c + a) dislocations and a dislocations thus confirmed. In contrast, for samples strained to the same strain of 0.1 but at 5 s⁻¹, it is not possible to obtain images of dislocations when using any diffracting vectors other than 0002. Thus the presence of dislocations which have a Burgers vector containing a c component can be confirmed in the samples strained at 5 s⁻¹ but the presence of a-component dislocations can only be inferred from TEM of these samples because of the difficulty of obtaining images with diffracting vectors other than 0002. Limited observations on samples strained at 10³ s⁻¹ show that similar difficulties are found in imaging dislocations as are found in samples deformed at 5 s⁻¹ but at this strain rate, the highest used, the difficulties are reduced since images can be obtained in some grains using diffracting vectors other than 0002. These results are discussed in terms of the nature of damage as a function of strain rate and the factors that influence contrast from dislocations in crystals.

Acknowledgements

This work is partly supported through Rolls–Royce through the University Technology Partnership and by the Engineering and Physical Sciences Research Council. The help from Dr. S Fox of Timet and from I Wallis of QinetiQ for supply of the strained samples is gratefully acknowledged. Support through the Overseas Research Studentship (ORS) scheme is also acknowledged.