Oxidation of commerciaally pure Ti and Ti alloys
Dhlamini, Mokhotjwa Simon
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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.