Oxidation of commercially pure Ti and Ti alloys
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Date
2004
Authors
Dhlamini, Mokhotjwa Simon
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
The surface temperature and composition of commercially pure Ti, Ti6AI4V
and Ti3AI8V6Cr4Zr4Mo were monitored during oxidation with AES (Auger
electron spectroscopy). Theory suggested the release of large amount of heat
by titanium during oxidation process at high oxygen pressures. The AES
surface technique was used to investigate if the increase in the surface
temperature due to oxidation at lower oxygen pressures is measurable.
The respective samples were cut into specially designed shapes to enable the
surface temperature change measurements without affecting the temperature
of the sample due to factors other than an exothermic oxidation reaction. Two
thermocouples were used in this study, the one spot-welded to the base of the
sample and the second one on the surface. There was about 100 DC- 200 DC
temperature difference at equilibrium between the base of the sample and the
surface temperature. The time delay in temperature change between the
surface and base made it possible to measure the changes in surface
temperature. The specimens were exposed to oxygen at various
temperatures and pressures. The Auger peak-to-peak heights for the
specified elements in the specimens were measured as a function of time. The amount of heat generated during the oxidation was infinitesimally small
and no significant change in the surface temperature was measured.
However, the theoretical calculated amount of heat generated during the
reaction of Ti atoms with oxygen to form Ti02 layer is 939.7 KJ. The change in
the surface temperature for the single layer due to the reaction was calculated
to be 34450 DC. For the sample thickness used, 0.9 mm, the calculated
amount of heat generated was 0.011 DC. The effect of both the electron and
the ion beams on the surface temperature was also monitored and it is clear
that there was an increase in temperature due to heating by electron beam
and ion beam.The segregation of the impurities (C and S) at the very low oxygen pressures
(5 x 10-8 Torr) was also observed. The decrease in the oxidation rate at higher
temperatures and lower pressures due to the segregating species and the
mean surface lifetime of oxygen on the surface was apparent. No clear
difference in the oxidation behaviour amongst the different samples was
found. The initial reaction for the three samples followed the parabolic rate
law. The impurity segregation profiles at different constant temperatures (400°C -
800°C) and linear heating ramp (0.05 °C/s) were experimentally investigated.
It was found that mainly C and S segregated at 400°C and Cl and S at higher
temperatures for the pure Ti sample. Sulfur was however the main
segregating specie for all three samples. Aluminium segregation was
measured at 800°C for the Ti6AI4V sample. But due to strong interaction
between the S and AI segregating species the surface was immediately
covered by S. The linear least square fit method was used to determine the
contributions of pure titanium and titanium carbide from the measured
APPH's. The AES peak fitting was done and confirmed the formation of TiC
on the surface at temperatures 400°C to 500 "C.
Description
Keywords
Oxidation, AES (Auger electron spectroscopy), Titanium, Rutile (Ti02), Ilmenite (FeTi03), Dissertation (M.Sc. (Physics))--University of the Free State, 2004