Oxidation / Corrosion Resistance Improvements of TiAl and TiNi Intermetallics by Using Surface Modification

• Main Authors: Mu-Rong Yang, 楊木榮
• Other Authors: Shyi-Kaan Wu
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/83132697909710789652

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 88 === The oxidation resistance of Ti-50Al intermetallics has been improved by the duplex process of pre-oxidizing at 800oC’48hr in air and subsequently polishing the outer TiO2 layer of the oxide scale, pre-oxidation for 1, 4 or 16 hours in high pressure pure oxygen (~3.9 atm) at 900oC and anodic coating in phosphoric acid aqueous solution. Among these three methods to enhance the high temperature oxidation resistance, the anodic coating method exhibits the best performance and the pre-oxidation in pure oxygen gives slightly better result than the duplex process. The improvement of oxidation resistance of Ti-50Al by the duplex process is attributable to the compressive stress relief associated with the removal of the outer TiO2 layer. The stress relief will not only alleviate cracking or rupture of the scales, but also reduce the stress-assisted diffusion in the scale. Moreover, the catalytic effect of outer rutile titania, which favors the dissociation of molecular oxygen into oxygen ions, can be dodged by the duplex process. For the pre-oxidation in pure oxygen (~3.9 atm), specimen pre-oxidized for 1h exhibits better oxidation resistance than 4h-or 16hr-pre-oxidized specimen. Prolonged pre-oxidation time can deteriorate the oxidation resistance in 800oC air. The oxide mound occurrence is an important factor for evaluating the effectiveness of the pre-oxidation treatment in oxygen. The formation mechanism of Z-phase (Ti5Al3O2) in the Al-depleted layer beneath the flat oxide scale and the oxide mound is also proposed in this thesis. In the aspect of the anodic coating, cyclic oxidation test indicates that anodized Ti-50Al can remarkably reduce the oxidation rate at 800oC and the improvement increases with increasing the anodizing voltage up to 400V, at which the parabolic rate constant can be reduced to 1/600 of that for as-homogenized TiAl. The reduce of the oxygen vacancies in the titania, resulting from the doping effect of phosphorus ions, is attributable to the improvement against high temperature oxidation of TiAl from the analysis of Raman spectra and SEM observation analyses. The optimal conditions for anodic coating against oxidation resistance of titanium aluminides are also accessed in this thesis. The corrosion resistance of TiNi shape memory alloy can be substantially improved by the DC plasma-polymerized hexamethyldisilazane (PHMDSN) coatings. The higher DC voltage used in plasma polymerization will develop a film with a more inorganic nature from the IR spectrum analysis and water contact angle measurements. The PHMDSN coatings for DC voltage 3 1000V at fixed monomer pressure of 250 mTorr are protective against the corrosion in Ringer''s solution in potentiodynamic polarization test and can be deformed up to a 2% strain. The corrosion current density of the TiNi can be lowered by as many as 4 orders of magnitude. The pitting potential and re-passivation potential of TiNi alloy can also be enhanced to nobler potential after PHMDSN coatings. The durability of the PHMDSN coatings and the mechanism of the passivity of the PHMDSN coatings can be studied by the electrochemical impedance spectroscopy (EIS). It indicates that all specimens deteriorate with the time of immersion in 37oC Ringer''s solution. The deterioration can be attributed to the water uptake from the analysis of the EIS. The optimal parameters to form a protective PHMDSN coating are the higher DC voltage and high HMDSN pressure to get a thick and smooth coating.