Perspectives on Titanium Science and Technology

Effect of interstitial solutes on the strength and ductility of titanium

While it is widely recognized that alloy factors other than stacking fault energy play a role in promoting planarity of slip, no detailed model has been advanced to explain the mechanism of planar vs wavy slip mode Therefore, friction stress effects of alloying on disolcation behavior are reviewed, as well as the role of stacking fault structure in inhibiting the clustering of dislocations in planar slip metal A model of cross-slip inhibition (and thus planar slip behavior) is developed from the idea that the joining of partials is resisted by frictional effects Planarity of slip is promoted not only by low stacking fault energy but by increase in shear modulus, atomic size misfit and solute content A critical solute concentration is predicted by the model for the transition from wavy slip to planar slip and this is shown to be in good agreement with observations for copper base solid solutions and other alloy systems

Mechanisms of slip mode modification in F.C.C. solid solutions

Active glide and twinning systems have been studied by transmission electron microscopy (TEM) in samples of three Ti-alloys, T40 (Ti+1000 ppm O), T60 (Ti+2000 ppm O) and TiAl6V4 deformed up to 5% by uniaxial or biaxial tension. The aim of the work was to understand more clearly which glide systems are activated during deformation of polycrystalline material and how they are related to the formation of the cold rolling textures. In order to estimate the stresses necessary for the activation of the observed glide systems, the resolved shear stresses for the different deformation systems were calculated from measured crystal orientations. The main results are: in TiAl6V4 〈a〉, basal slip has a lower critical resolved shear stress, τc, than prismatic slip. 〈c+a〉 pyramidal glide shows a very low τc, which is up to two times larger than that for prismatic slip. Nevertheless, 〈c+a〉 glide systems were only rarely activated and twinning systems were never activated. Therefore, deformation with c-components may be accommodated by β-phase deformation or grain boundary sliding. The observed c-type texture is due to the strong basal glide. In T40, τc for 〈c+a〉 glide is up to 13 times higher than that for prismatic glide. However, 〈c+a〉 glide and twinning were strongly activated, leading to the observed t-type texture. In T60, the high oxygen content completely suppressed twinning and strongly reduced 〈c+a〉 glide. The less developed t-type texture is due to the combination of 〈c+a〉 and basal glide.

A study of active deformation systems in titanium alloys: dependence on alloy composition and correlation with deformation texture

This is an overview of micromechanical deformation mechanisms in hexagonal close-packed metals. We start with an in-depth discussion of single-crystal behaviour concerning crystallographic slip, plastic anisotropy and deformation twinning. We move on to discuss some complexities involved in polycrystalline deformation and modelling approaches, focusing on rate effects in titanium alloys that are thought to play a significant role in dwell fatigue. We finish our review with a brief commentary on current understanding and state-of-the-art techniques, and outline some key areas where further study is recommended.

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On the mechanistic basis of deformation at the microscale in hexagonal close-packed metals

The temperature dependence of the long range (internal) and thermally activated components of the flow stress have been measured by a stress relaxation technique over the temperature range 200 to 550 K in α titanium containing five different levels of oxygen. In addition, the dislocation arrangements have been studied using thin foil electron microscopy techniques. In the higher oxygen materials it has been found that a transition from wavy to planar slip occurs towards lower temperatures. The internal stress varies more strongly with temperature than would be predicted by the temperature dependence of the elastic modulus; simultaneously, the thermally activated component of the flow stress (t*) obtained as the difference between the flow and internal stresses, goes through a maximum at the temperature where the internal stress (Tint) becomes strongly temperature dependent. An increase in t* and rise of Tint accompany the onset of planar slip.

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J. C. Williams

Papers

The influence of oxygen concentration on the internal stress and dislocation arrangements in α titanium