Surface modification of titanium-based alloys

Two routes of Oxygen Diffusion Hardening (ODH) have been investigated on two alloys of titanium, Ti-6AI-4V and Ti-6AI-7Nb (by weight). The first route involves a controlled atmosphere where argon saturated with water was used to transport water into the test pieces at elevated temperatures. The controlled atmosphere would encourage the generation of mono-atomic oxygen through the dissociation of water vapour, and therefore change the kinetics of physical absorption and diffusion of oxygen into titanium. The second route of ODH investigated was the Oxygen Boost Diffusion Hardening (OBDH). The oxygen boost diffusion hardening process was carried out in two steps. The first step was oxidation of the samples in air at elevated temperatures and the second step was to further diffusion treat the pre-oxidised test pieces III a vacuum or argon. Various temperature and time combinations were used on both steps of OBDH.The results revealed that the ODH heat-treatment in a controlled saturated argon environment was unsuccessful in developing a significant oxygen diffusion hardened layer. The OBDH process can be carried out to modify the surface properties of titanium and alloys. Both steps of this process play a vital role in achieving a thick modified layer for improved tribological properties of titanium and alloys. Performing the oxidation step of OBDH heat-treatment at higher temperatures results in higher surface hardness and deeper diffusion zone than carrying the oxidation step at lower temperatures for longer times provided there is no peeling of the oxide scale during the high temperature oxidation. The Ti-6AI-4V achieves higher surface hardness than the Ti-6AI-7Nb upon the same OBDH heat-treatment. The second step of the OBDH can also be carried out in an argon environment instead of vacuum. Carrying out the second step in an argon atmosphere allowed for higher surface hardness and thicker hardened zone than carrying the same step in vacuum. The effect of the OBDH on the underlying microstructures of two alloys under investigation is the depletion of the ɑ phase on the modified surface as a result of the diffused oxygen which stabilises the ɑ phase. Although higher surface hardness was achieved for the Ti-6AI-4V alloy than the Ti-6AI-7Nb alloy after the same heat treatment, the Ti-6AI-7Nb alloy achieved higher wear resistance due to more adherence of the oxide scale after the oxidation step. Despite achieving higher surface hardness and thicker hardened zone upon carrying out the second step of OBDH in an argon atmosphere than in vacuum, samples which underwent the second step of OBDH heat-treatment in vacuum exhibited higher wear resistance. Performing a twin cycle OBDH heat-treatment results in even higher surface hardness and higher wear resistance despite the severe scaling of the alloys upon the heat treatments.

Grantor dc:publisher.institution

Centre for Materials Engineering

Year dc:date.issued

2007

Author dc:creator

• Camagu, Sigqibo Templeton

Advisor dc:contributor.advisor

• Knutsen, Robert D

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/4967

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/4967