Microstructure and tensile performance evolution during Titanium - 6 Aluminium - 4 Vanadium (Ti-6Al-4V) temporary hydrogen treatment

This thesis uses temporary hydrogen treatment to refine the cast Ti-6Al-4V microstructure in order to optimise its mechanical properties. It investigates the influence of hydrogen on microstructure evolution, grain refinement and tensile performance. It exposes the unarticulated relationship between hydrogen and titanium aluminide (Ti3Al) as a gap in literature that has been sustained by the pervasive use of the repeated nucleation framework. It hypothesises that grain refinement of Ti-6Al-4V by temporary hydrogen alloying can cause embrittling effects which remain even when hydrogen is fully expelled from the alloy. It also hypothesises that the hydrogenation-disproportionation-desorption-recombination (HDDR) framework can account for the evolution of grain refinement in hydrogen treated Ti-6Al-4V. This thesis demonstrates that HDDR accounts for microstructure evolution in hydrogen treated Ti-6Al-4V and unveils the intimate relationship between hydrogen and Ti3Al. Cast Ti-6Al-4V was refined using hydrogenation-dehydrogenation (HDH), thermohydrogen processing (THP) and thermohydrogen and deformation processing (THDP). Cast Ti-6Al-4V was hydrogenated to 20 at.%H and subsequently dehydrogenated at 675⁰C (HDH 675), 700⁰C (HDH 700), 725⁰C (HDH 725) and 750⁰C (HDH 750). Hydrogenated Ti-6Al-4V was solution treated (at 900⁰C), aged (at 580⁰C) and dehydrogenated (at 675⁰C or 750⁰C) during THP treatment. The THDP treatment had a similar processing protocol to the latter, but it incorporated hot deformation (to 1.6 true strain), recrystallisation (at 900⁰C) and quenching before aging. Microstructure evolution was characterised by X-ray diffraction, electron backscatter diffraction, transmission Kikuchi diffaction and transmission electron microscopy. Temporary hydrogen treatment causes submicron grain refinement. It improves the yield, tensile strength by up to 70 MPa, 100 MPa (in HDH) or 130 MPa, 142 MPa (in THP) treated Ti- 6Al-4V. Hydrogen precipitates Ti-H and Ti-H2 hydrides which partition Al from its Ti-Al couple. This forms Al rich clusters in neighbouring regions where the degree of Al enrichment stabilises Ti3Al precipitates. Dehydrogenating at 675⁰C decomposes hydrides but it retains the Ti3Al precipitates which deteriorate tensile ductility by up to 75% (in HDH treated microstructures). Dehydrogenating at 750⁰C significantly dissolves Ti3Al to extents that retain full ductility (in HDH) and restores 20% ductility (in THP) treated microstructures.

Grantor

Department of Mechanical Engineering

Year dc:date.issued

2019

Author dc:creator

• Vilane, Velile

Advisors dc:contributor.advisor

• Knutsen, Robert Douglas
• Westraadt, Johan

• Materials Engineering

Handle dc:identifier.uri

http://hdl.handle.net/11427/37448

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/37448