Texture Evolution

The texture analysis in the current work only focuses on the dominant alpha phase. ȕ texture is not examined due to its relatively small volume fraction, especially in the as-deformed condition. The overall texture is qualitatively similar for all annealing conditions in the current study, especially for the interior layer. This is not surprising as it is well known that the deformed texture is quite stable upon post-deformation annealing of Į+ȕ titanium alloys such as Ti-Al-4V [150, 200]. For example, no significant texture change was observed in a warm rolled Ti-Al-4V alloy after annealing at 800°C for 96 hr [150] .

Despite the preserved overall texture characteristics throughout the thickness during post-deformation annealing, some changes are observed 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 texture evolution can be a result of phase transformation and/or preferred grain growth of specific orientation/s.

As discussed earlier, the volume fraction of beta phase continuously increases with the holding time, which reduces its stability due to lowering of the V concentration. This results in the beta to martensite phase transformation on quenching beyond 2 hr holding time in the current study. As discussed in Section 5.4.1, the

martensitic phase transformation follows the Burgers orientation relationship, enhancing two main (90°, 90°, 0°) and (90°, 30°, 0°) orientation components.

However, the strength of these orientations is not strong in the overall texture. This suggests that the volume fraction of martensite is not high enough to influence the overall texture. In addition, the main texture changes are observed at a holding time of less than 2 hr, where the beta phase is mostly stable (i.e. no martensitic phase transformation).

The main change in the surface texture is attributed to a 30° rotation around the c-axis for the initial dominating basal texture from the {0001}1120! to the {0001}

10 10

 ! orientations. To the best of our knowledge, this type of texture alteration has not been reported for Ti-6Al-4V alloy before. However, an opposite 30° rotation around the basal poles, turning the pole parallel to RD from 10 10! to 1120!, is well known as the recrystallization texture for cold rolled CP titanium [212, 277]. The orientation rotation has been attributed to the preferred nucleation and growth of grains with a particular orientation characterized by high stored energy governing the final texture [212, 214]. For the current study, the observed texture evolution at the surface layer is most likely a result of preferred growth/nucleation orientation/s upon annealing, rather than texture induced by the martensitic phase transformation.

Generally speaking, the annealing behaviour of primary alpha phase at 800°C in the current study is somewhat similar to the post-deformation annealing of a Ni-30Fe austenitic steel through the metadynamic recrystallization mechanism (i.e. growth of DRX grains) [278], where the initial texture was similarly preserved throughout the annealing process. However, there are some differences between MDRX mechanism and the present observations. Here, there is continuous precipitation of beta phase, mostly on grain boundaries and triple junctions, with holding time, which significantly retards the grain growth behaviour compared with pure titanium [212]. Despite the presence of a second phase, the maximum annealed-to-as-rolled ratio for the Ti-6Al-4V alloy (~14.8) is much higher than the metals subjected to MDRX (i.e. ~1.5-2) [278].

The difference could arise from the distinct characteristics of the as-deformed structures (i.e. initial grain size and grain boundary character). In the current study, the as-deformed titanium alloy undergoes continuous dynamic recrystallization resulting in a much smaller starting grain size (i.e. less than 300 nm) than the austenitic alloy

(~18 ȝm) subjected to DRX prior to the post-deformation annealing process. Recently, it was demonstrated that the initial state of the deformed structure significantly influences the post-deformation annealing processes [48]. The ultrafine equiaxed Į grains formed (i.e. 300 nm on average) by the CDRX mechanism are enclosed by non-equilibrium high angle boundaries, having high dislocation density in the grain interiors (Fig. 6-1). This microstructure enhances the short-range grain boundary migration during post-deformation annealing, as shown by others [2]. This promotes the homogenous distribution of grain size throughout the microstructure and convert the non-equilibrium high angle boundaries to equilibrium.

The current result suggests that the post-deformation behaviour of surface and interior layers are largely similar. The main difference is the replacement of partially fragmented laths by the fine equiaxed grains in the interior layer, which would be most likely through static recrystallization. Despite this microstructural change, the texture of the interior layer largely preserved its characteristics with holding time, similar to the surface layer.

6.5 Conclusions

In the current study, the post-deformation annealing of an ultrafine-grain Ti-6Al-4V alloy was studied throughout the thickness of the as-rolled martensitic microstructure.

The evolution of hardness, microstructure and texture during annealing were investigated for the surface and interior layers having a different starting microstructure (i.e. fully and partially UFG). The following conclusions can be drawn from this work:

i) Upon annealing, the surface layer revealed more significant softening than the interior layer and the material softening rate at 800°C was ~67 times quicker than at 700°C.

ii) The microstructure evolution in the surface layer comprised concurrent alpha grain growth and beta precipitation, whereas the interior layer also revealed the replacement of partially fragmented alpha laths to equiaxed grains, most likely through static recrystallization. Despite the presence of a relatively high dislocation density, the continuous precipitation of

beta on the alpha grain boundaries significantly retards the alpha grain growth through a pinning effect.

iii) The volume fraction of beta phase increased with holding time, resulting in a lower level of V content in the beta. Consequently, the stability of the beta phase gradually reduced with annealing time, leading to the martensitic phase transformation of the beta phase on cooling.

iv) The overall texture was mostly preserved for the 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. These changes were mainly attributed to preferred grain growth of specific orientation/s, rather than martensitic transformation of beta phase.

Conclusions and suggestions for further work

7.1 Conclusions

The motivation of current research was to develop a thermomechanical processing route to produce an ultrafine grained structure in titanium alloys. Grain refinement generally enhances the strength of metals without scarifying the ductility and toughness. This ultimately broadens the application of titanium alloys in a wide range of industries, such as aerospace. Among the various grain refinement approaches, the thermomechanical processing of a martensitic starting microstructure is one of the most prospective routes to produce bulk UFG titanium alloys in an industrial scale.

This is mainly due to its effectiveness of grain size reduction with relatively less deformation, the simplicity of the processing procedure without special equipment requirements and the wide applicability for various Į+ȕ titanium alloys. Different aspects of this novel TMP approach were investigated in the current study using a Ti-6Al-4V alloy.

Initially, the microstructural evolution of martensitic Ti-6Al-4V alloy during warm deformation and the impact of thermomechanical parameters on UFG formation

were examined using uniaxial compression (see Chapter 4). Based on this, a grain refinement mechanism was introduced. Secondly, a trial was made to produce a bulk UFG titanium alloy in Chapter 5 by means of both symmetric rolling and asymmetric rolling approaches. This also allowed an insight into the influence of deformation mode on microstructural evolution and subsequent mechanical properties. The post-deformation annealing behaviour of as-rolled UFG Ti-6Al-4V alloy was examined in Chapter 6, which led to an in-depth understanding of the thermal stability of the ultrafine grained structure. In addition, the texture evolution was examined at different thermomechanical conditions. The key findings and conclusions drawn from the current investigation were summarized as follows.