Degree Name

Doctor of Philosophy


School of Mechanical, Materials and Mechatronic Engineering


The high cost of titanium components restricts wider utilisation of Ti materials. Blended elemental powder metallurgy (BEPM) is a promising cost-effective approach to the manufacturing of Ti products for non-fatigue critical applications. In this series of investigations three near-b Ti alloys (Ti-5Al-5Mo-5V-1Cr-1Fe, Ti-5Al-5Mo-5V-2Cr-1Fe and Ti-10V-3Fe-3Al) produced by BEPM were subjected to different thermo-mechanical processing (TMP) schedules using a Gleeble 3500 simulator. The selected TMP schedules resulted in beneficial reductions in porosity and fragmentation of grain boundary α phase. Selection of specific cooling regimes to room temperature could be used to control the extent of b phase recrystallisation, volume fraction of intragranular a phase and the formation of athermal w.

To gain a deeper understanding of the resulting phase transformations and of the sequence of microstructural and nanostructural evolution a TMP sample cooled from α+b phase field (800 °C) at 10 Ks-1 was investigated using advanced methods of scanning transmission electron microscopy (STEM) coupled with atom probe tomography (APT). Athermal ω and α nuclei were found to be present both separately and interconnected. It was also confirmed by STEM that ω phase was formed via simple atomic displacements in theβ direction. Compositional fluctuations of Mo were discovered by APT. Possible nucleation sites for α formation were determined to be dislocations, β/ω interfaces and/or regions within ω and Modepleted areas.

The evolution of both nanostructure and microstructure during subsequent ageing at 650 °C for 120 s – 28.8 ks after slow heating (0.25 Ks-1) was also investigated by coupling STEM and APT. The early stage of ageing also shows partially transformed ω super cells both individually and in connection with α. It is suggested that α nucleates heterogeneously, preferentially at β/ω interfaces and dislocations. Only a slight compositional difference between b and newly formed α was detected, which supports a displacive-diffusional transformation mechanism. Whereas at shorter ageing times both nucleation and growth of a phase are responsible for the increase in its volume fraction, after 3.6 ks the growth becomes dominant.

Alloying element redistribution between α and β phases during processing (TMP and ageing) were monitored using energy dispersive spectroscopy (EDS) and APT. Consistent with the diffusion-controlled nature of growth of α phase, it was found that the later the stage of α phase formation (coarser α) the more pronounced was the redistribution of alloying elements (e.g. exit of β stabilisers from α phase). The composition of α was found to be affected by the composition of parent β phase, which varies with the stages of processing. Mo (and sometimes V) pile-up at the α/β interface was especially noticeable after TMP and early stage of ageing.

Mechanical properties of the samples (TMP and aged) were found to be dependent on the α volume, with increases in tensile strength and reductions in ductility with higher α fractions. A significant improvement in mechanical properties was found in the aged after TMP samples compared to the TMP, which can be attributed to the ω-assisted α formation in the aged samples.

The difference in Cr content of the studied alloys (Ti-55521 and Ti-55511) was found to affect the kinetics of α phase formation during isothermal holding in α+β phase field, resulting in a lower amount of a in the higher Cr content alloy.

Deformation behaviour of Ti-10V-3Fe-3Al alloy was determined by room temperature compression testing. It was found that deformation mechanisms in b phase change with varying β phase stability as a function of α phase fraction. With the least stable β phase, in addition to slip, {332}⟨113⟩β deformation twinning along with stress-induced α′′ martensite and stressinduced ω were found to be operating deformation mechanisms. With increasing β stability coexistence of {332}⟨113⟩β and {112}⟨111⟩β twinning was found. For samples with higher β stability only {112}⟨111⟩β twinning was noticed while the mechanisms of slip, stress-induced α" and stress-induced ω remain operational. Finally, deformation of samples with maximum β stability was found to occur only by slip.

The findings of the present study have enhanced the understanding of physical metallurgy of near β titanium alloys and will provide a basis for further development of both wrought and cost-effective powder metallurgy route produced alloys.