Microstructure and texture evolution of an ultrafine grained Ti-6Al4V alloy during post-deformation annealing
Q. Chao1, P.D. Hodgson1 and H. Beladi1
1Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia
qchao@deakin.edu.au, peter.hodgson@deakin.edu.au and
hossein.beladi@deakin.edu.au
Fabrication of an ultrafine grained (UFG) microstructure in titanium and its alloys
is a promising route to enhance the strength and ductility simultaneously and widen
their application. Recently several successful trials have been reported in the Ti-6Al4V alloy through different approaches [1-3]. Thereby, it becomes imperative to
understand the thermal stability of the UFG structure at elevated temperatures, where
superplastic deformation or interpass holding of the material for multipass hot working
are typically conducted.
In the current study, the post-deformation annealing behaviour of an ultrafine
grained Ti-6Al-4V alloy was studied throughout the thickness of as-rolled sheet. The
evolution of hardness, microstructure and texture over the annealing cycle were
investigated for the surface and interior layers which had different starting
microstructures (i.e. fully and partially UFG).
Upon annealing, the surface layer revealed more significant softening than the
interior layer and the material softening rate at 800°C was ~50 times quicker than that
at 700°C. The microstructure evolution in the surface layer comprised of concurrent
grain growth of alpha and beta precipitation, whereas the interior layer also revealed
the replacement of partially fragmented alpha lath by equiaxed grains most likely
through static recrystallization. Despite the presence of relatively high dislocation
density, the continuous precipitation of beta on alpha grain boundaries significantly
retards the alpha grain growth through a pinning effect.
The overall texture was mostly preserved for both surface and interior layers
throughout the post-deformation annealing treatment. However, slight changes
appeared in the surface layer texture with holding time, mainly showing a gradual
strengthening of the (0°, 0°, 30°) component at the expense of the (0°, 0°, 0°) and (0°,
90°, 0°) components. The changes were mainly attributed to preferred grain growth of
specific orientations.
References
[1] S.V. Zherebtsov, G.A. Salishchev, R.M. Galeyev, O.R. Valiakhmetov, S.Y.
Mironov and S.L. Semiatin, Scr. Mater. 51 1147 (2004).
[2] C.H. Park, J.H. Kim, J.-T. Yeom, C.-S. Oh, S.L. Semiatin and C.S. Lee, Scr.
Mater. 68 996 (2013).
[3] Q. Chao, P.D. Hodgson and H. Beladi, Metall. Mater. Trans. A 45 2659 (2014).