Plastic deformation of Ti-6Al-4V at small scale: a microstructural and mechanistic study

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Mohammed Ameen, Rayan Basheer (2017). Plastic deformation of Ti-6Al-4V at small scale: a microstructural and mechanistic study. University of Birmingham. Ph.D.

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Abstract

Two phase alpha/beta titanium alloys are used in a wide variety of applications such as aerospace, biomedical, gas turbine engine, sport and energy. These alloys have high specific strength and specific modulus as compared to magnesium and aluminium alloys as well as excellent corrosion resistance. The Ti-6Al-4V (α+β) alloy is the most widely used and the best known of all the Ti alloys. Although there are a number of observations in the literature reporting the mechanical responses of these two phase alloys, there exists very little understanding of the mechanisms of the individual phases and the alpha/beta interface’s role in strengthening. Additionally, it has often been reported in the literature that ‘smaller is stronger’ for different metals due to the presence of a size effect. There is no real understanding of the mechanism of the size effect in the alpha-beta titanium and HCP and its dependence on orientation.
Single alpha, beta and alpha-beta colony micro-pillars have been manufactured from a polycrystalline commercial Ti-6Al-4V sample using Focused Ion Beam (FIB). Alpha/beta pillar contained two alpha lamellae separated by a thin fillet of beta phase. A nano-indenter was then used to conduct uniaxial micro-compression tests on Ti alloy single crystals, using a diamond flat tip as a compression platen.
By controlling the crystal orientation along the micro-pillar using Electron Back Scattering Diffraction (EBSD) different slip systems have been selectively activated. The advantage of the micro-compression method over conventional mechanical testing techniques is the ability to localize a single crystal volume which is characterisable after deformation.
This study makes a contribution to knowledge in several key areas, including an understanding of the mechanical response of different crystals at micro/submicro-scale, the effect of phase interfaces on deformation, and an understanding of the strengthening mechanism in two-phase Ti alloys, the orientation size effect in HCP metals and the CRSSs for each phase in Ti-6Al-4V. Therefore, in order to evaluate the behaviour of these alloys for future applications, it is imperative that the microstructural features and characteristics be quantified and examined on a small scale.
The results showed that the beta phase in between alpha lamellae caused strengthening deformation. When the surface normal is parallel to the [0001] of the single alpha crystal, the material deforms with difficulty in the [0001] direction of the single alpha phase.
The mechanical responses of the alpha, beta, and alpha/beta crystals not only depended on the size of the pillars, but also on the crystallographic orientation, the initial dislocation density and the relationship between the two phases in the case of α/β crystals.
Extensive electron microscopy investigation revealed that the anisotropy in basal and prismatic slip systems can be directly correlated with the transmission of dislocations across the single crystal, the beta laths, and the accumulation of residual dislocation content near the interfaces and in the single beta phase. This information is essential in order to better model mechanical deformation in these materials. In addition, dislocation analysis indicated that the deformation of individual grains conformed to the Schmid factor (SF) analysis where slip primarily occurs on those slip systems where the resolved shear stress (SF) values are highest.
The results presented in this thesis bring to light several concerns for designing with titanium alloys and identify a number of phenomena of strong scientific interest. They will allow for the development of realistic models for the mechanical behaviour and provide a comprehensive analysis that can contribute to the theoretical development of the design and enhancement of the titanium allows. Moreover, the role of crystallography in plastic deformation provides a novel insight into the nature of the orientation size effect in HCP. This work points towards the need for further investigations into the higher and lower temperature deformation behaviour of Ti-64 to fully understand the phenomena identified within this study.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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