Oxidation of commerciaally pure Ti and Ti alloys

[摘要] The surface temperature and composition of commercially pure Ti, Ti6AI4Vand Ti3AI8V6Cr4Zr4Mo were monitored during oxidation with AES (Augerelectron spectroscopy). Theory suggested the release of large amount of heatby titanium during oxidation process at high oxygen pressures. The AESsurface technique was used to investigate if the increase in the surfacetemperature due to oxidation at lower oxygen pressures is measurable.The respective samples were cut into specially designed shapes to enable thesurface temperature change measurements without affecting the temperatureof the sample due to factors other than an exothermic oxidation reaction. Twothermocouples were used in this study, the one spot-welded to the base of thesample and the second one on the surface. There was about 100 DC- 200 DCtemperature difference at equilibrium between the base of the sample and thesurface temperature. The time delay in temperature change between thesurface and base made it possible to measure the changes in surfacetemperature. The specimens were exposed to oxygen at varioustemperatures and pressures. The Auger peak-to-peak heights for thespecified elements in the specimens were measured as a function of time. The amount of heat generated during the oxidation was infinitesimally smalland no significant change in the surface temperature was measured.However, the theoretical calculated amount of heat generated during thereaction of Ti atoms with oxygen to form Ti02 layer is 939.7 KJ. The change inthe surface temperature for the single layer due to the reaction was calculatedto be 34450 DC. For the sample thickness used, 0.9 mm, the calculatedamount of heat generated was 0.011 DC. The effect of both the electron andthe ion beams on the surface temperature was also monitored and it is clearthat there was an increase in temperature due to heating by electron beamand 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 highertemperatures and lower pressures due to the segregating species and themean surface lifetime of oxygen on the surface was apparent. No cleardifference in the oxidation behaviour amongst the different samples wasfound. The initial reaction for the three samples followed the parabolic ratelaw. 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 highertemperatures for the pure Ti sample. Sulfur was however the mainsegregating specie for all three samples. Aluminium segregation wasmeasured at 800°C for the Ti6AI4V sample. But due to strong interactionbetween the S and AI segregating species the surface was immediatelycovered by S. The linear least square fit method was used to determine thecontributions of pure titanium and titanium carbide from the measuredAPPH's. The AES peak fitting was done and confirmed the formation of TiCon the surface at temperatures 400°C to 500 C.

[发布机构] University of the Free State