Year

2014

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

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.

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