Ordering and the deformation mechanisms of Ti-Al alloys Anna Radecka

205 Pages·2015·23.08 MB·English

Imperial College London
Department of Materials
Ordering and the deformation mechanisms of Ti-Al alloys
Anna Radecka
A dissertation submitted for the degree of
Doctor of Philosophy at Imperial College London, October 2015

Declaration of Originality
I, Anna Radecka, hereby declare that this thesis and associated research are my own work
and that all sources I have used or quoted have been indicated and acknowledged by means of
complete referencing.
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tribute or transmit the thesis on the condition that they attribute it, that they do not use it for
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Acknowledgements
First of all I would like to acknowledge Prof. David Dye, Prof. Trevor Lindley from Imperial
College London and Prof. David Rugg from Rolls-Royce plc. They have been my supervisors
through this project and provided guidance, knowledge and inspiration.
My genuine thanks go to Dr Michael Moody, Dr Paul Bagot and Dr Tomas Martin from Univer-
sity of Oxford for their help with atom probe tomography (ATP) experiments and data analysis.
I would also like to thank the atom probe group from Northwestern University Center for Atom
Probe Tomography (NUCAPT) for the allocated experimental days and their invaluable help
and advice.
Thanks to the scientific committees from the Oak Ridge National Laboratory (ORNL) for the
allocated experimental days on the VULCAN diffractometer, the help on experiments and data
analysis. Research conducted at ORNL’s Spallation Neutron Source was sponsored by the Sci-
entific User Facilities Division, Office Basic Energy Sciences, US Department of Energy.
I appreciate the time taken by all employees from Rolls-Royce plc. for useful discussions. A huge
debt of gratitude is owed to Timet Whitton for supplying the material.
I am glad to be in a group where all ’metal bashers’ work hard to create a great environment so
that we can learn from each other and achieve goals. I would like to thank Mr Ben Wood, Mr
Russell Stracey, Mr Garry Stakalles, Mr Richard Sweeney for technical support and other mem-
bers of staff at Imperial College London. All of them have been equally supportive by providing
help and advice. Experimental assistance from Dr Ananthi Sankaran, Dr Vassili Vorontsov, Dr
Mezan Rahman, Dr James Coakley is gratefully acknowledged.
I am very grateful to Mrs Ecaterinea Ware, Dr Catriona Mcgilvery and Dr Mahmoud Ardakani
who taught me microscopy. Helpful discussions on the TEM aspects are acknowledged with
Prof. Ian Jones, Prof. Mike Loretto, Mrs Theresa Morris and Bo Pang at the University of
Birmingham and Prof. John Bradley and Dr Hope Ishii at the University of Hawaii.
Finally I would like to specially thank my family and friends for encouragement, help, support,
motivation and vast reserves of patience as I was working on this project.
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Abstract
Thefatiguebehaviourofnear-aanda+btitaniumalloysdependsstronglyonthemicrostructure,
the environment and the load cycling conditions. It is known that Ti-Al alloys can suffer from
a chemical decomposition on ageing above 500 C or air cooling which reduces fatigue life. The
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factors promoting the ordering, as well as its effect on fatigue crack nucleation and propagation
are still not sufficiently understood. This project aims to provide an improved understanding
of how ordering affects the extent of planar slip occurring during fatigue in the a phase of
aluminium-alloyed titanium alloys. For this purpose, samples of Ti-7Al, Ti-6Al-4V and IMI 834
were aged to promote ordering and the formation of nanometre-scale a2 (Ti3Al), at a range of
temperatures between 450-700 C for times up to 84 days.
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Ti-7Al wt.% (12 at.%) was selected as a model of the a phase in Ti-6Al-4V which has also
been studied in the literature. A reliable method for producing the alloy with the desired
microstructure has been established. The effects of post-rolling heat treatment on ordering of Al
wereanalysedusinglightmicroscopy,transmissionelectronmicroscopy(TEM),scanningelectron
microscopy (SEM) and electron backscatter diffraction (EBSD). The evolution of intergranular
lattice strain was characterised using in situ time-of-flight (TOF) neutron diffraction during
room temperature tensile loading. On ageing, at 550 C and 625 C, dislocations were observed
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to travel in pairs, and in planar arrays, which has been attributed to the presence of ordering. A
slightchangeinc/awasobserved, from1.6949to1.6945, andaslightincreaseinthemacroscopic
modulus. However, no changes were observed in the residual lattice strains, which are the grain-
orientationaverageelasticstrainsproducedbylocalisedplasticity. Thereforeitisinferredthatthe
changes in deformation mechanisms caused by ordering that result in an enhanced vulnerability
to dwell fatigue affect primarily the extent of slip localisation.
At long ageing times, a chemical decomposition results in the formation of Ti3Al precipitates in
the a phase of a+b titanium alloys. At reduced times or elevated temperatures, diffuse electron
or neutron diffraction peaks can be observed, indicative of short range ordering (SRO). Here,
correlative TEM and atom probe tomography (APT) results are presented showing that the
reaction proceeds through the formation of ordered Al-rich clusters in Ti-6Al-4V. In addition,
the V-containing a phase of Ti-6Al-4V formed ordered clusters much faster than the binary alloy
Ti-7Al. This implies that the ternary addition of b stabilisers exacerbates the problem of a2
precipitate formation in commercial dual phase titanium alloys.
Precipitate evolution was also studied in the a phase of IMI 834 using TEM after ageing at 550,
625 and 920 C for times up to 28 days. The TEM investigation was complemented by APT to
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give compositional information. It is found that precipitation occurs much faster, and is more
prolific in samples that were heat treated at higher temperatures. Additionally, a change in
precipitate morphology was observed. Particles were spherical after ageing at 550 C while after
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ageing at 625 C become ellipsoids with the major axis lying close to the [0001] direction.
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Insummary,itwasfoundthatprecipitationoftheordereda2 phaseappearstooccurmuchfaster,
and in greater proportions, in multi-component Ti alloys than in the binary alloy. Additionally,
the appearance of superlattice spots was much clearer, and occurred after shorter ageing times,
in Ti-6Al-4V and IMI 834 compared to Ti-7Al, despite all the alloys having a similar Al content
in the a phase. This indicates that the addition of the ternary elements to binary Ti-Al system
accelerates the formation of the a2 phase.
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