Grain-Boundary Parameters Controlled Allotriomorphic Phase Transformations in Beta-Processed Titanium Alloys

Allotriomorphic or grain-boundary alpha (GBA) is an unavoidable and important microstructural feature of the diffusional phase transformations in beta-processed titanium alloys. This phase has a negative influence on various mechanical properties, such as high cycle fatigue, fracture toughness, ductility etc. and has a profound influence on the overall microstructural evolution. In the present work, the evolution of GBA has been in explored relative to the grain-boundary parameters namely, misorientation angle and axis, and the GB plane. While the misorientation angle and axis relate to the crystallography of adjacent crystals and can easily be determined using diffraction based techniques, an accurate determination of the local crystallographic orientation of the GB plane is difficult because of its three-dimensional nature. To address this issue, two independent experimental approaches have been developed and validated by utilizing a combination of dual beam focused ion beam (FIB), SEM and electron back-scattered diffraction (EBSD) methods. Both these approaches considerably simplify the problem because of a relatively easier experimental set-up and a versatile methodology. The crystallographic variant selection and morphology of GBA have been subsequently evaluated relative to the quantified GB parameters. Results indicate that in alpha/beta- and beta-titanium alloys, the crystallographic variant selection and GB parameters control the evolution of the allotriomorphic alpha phase. In particular, misorientation angle/ axis parameters control the early precipitation of GBA. The grain-boundaries of those adjacent beta-grains that produce nearly parallel <111> and/or <101> poles are the preferred sites for the early nucleation of GBA. In addition, these closely related poles significantly influence the crystallographic variant selection criterion in a majority of cases and contribute to a considerable short-listing of the allowed variants of GBA. The morphology of GBA is primarily controlled by the inclination of the GB plane relative to the selected crystallographic variant of the Burgers-OR at grain-boundaries. In beta-titanium alloys, continuous GBA precipitates are produced when the GB plane orients nearly perpendicular to the invariant line of the contributing Burgers-OR. In case of alpha/beta-titanium alloys, the thickening of GBA is higher when the GB plane orients close to the broad-face or the habit plane of the Burgers-OR. It has been proposed for the first time that the crystallographic and morphological aspects of GBA and adjacent Widmanst¿¿tten alpha are interrelated and primarily dictated by the GB parameters. Specifically, once a particular crystallographic variant of GBA is chosen by a set of criterion, the crystallographic and morphological nature of adjacent Widmanst¿¿tten alpha is also predicted. The resulting Widmanst¿¿tten alpha morphology in combination with the nature of the GB parameters would control the growth (continuous or discrete) of GBA. Finally, transmission electron microscopy studies have revealed a variation in the interfacial structure between GBA and the adjacent beta grain, different from previously known well-defined interfacial structure between Widmanst¿¿tten alpha and the parent beta phase. The faceting of the interface as well as presence of new sets of misfit dislocations have been observed in this work.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Materials Science and Engineering

Grantor dc:publisher

The Ohio State University

Year dc:date

2013

Author dc:creator

• Dixit, Vikas

Contributors dc:contributor

• Fraser, Hamish L.

• Aerospace Materials
• Engineering
• Metallurgy
• titanium alloys
• grain boundary alpha
• allotriomorphic alpha
• variant selection, morphology
• precipitate thickening

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=osu1357240292

OAI identifier oai:identifier

oai:etd.ohiolink.edu:osu1357240292