Isothermal oxidation behavior of two-phase TiAl-Mn-Mo-C-Y alloys fabricated by different processes

The isothermal oxidation behavior of two-phase Ti-46.6Al-1.4Mn-2Mo-0.3C-0.3Y based alloys in synthetic air at 800 and 900 degrees C was investigated. The main emphasis was focused on the effect of microstructures obtained by different processes of extrusion, melting and hot rolling, i.e. fully lamellar, nearly lamellar and duplex. On the exposure for 350 h at 800 degrees C, the growth rate and the spallation behavior of oxide scales of the alloys depended strongly on the fabrication process. The extruded alloy with a fine fully lamellar microstructure showed an excellent oxidation resistance due to the lowest mass gain and strong adhesion of the scale to the substrate, and Y-addition was effective in improving oxidation resistance because of its oxygen-scavenging effect in EPM processing. However, the overall oxidation process was dominantly controlled by the beneficial effect of the Mo-addition in the melted alloy due to the doping effect and formation of Mo-rich Al-depletion layer close to the substrate. The rolled alloy with the duplex microstructure greatly accelerated the oxidation rate because of a high volume fraction of alpha(2)-Ti3Al. At 900 degrees C, the melted alloy showed an excellent oxidation resistance during 350 h exposure. The oxidation rate and the spallation resistance of oxide scales did not depend strongly on the microstructure for the extruded or the rolled alloy. The poor spallation resistance of the oxide scales in the extruded alloy was probably related to thicker Mn-containing scales. Neither Mo-rich layer nor Y-rich zone in the oxide scales in the rolled alloy was observed at two exposure temperatures as a result of fast oxidation rate of the scale. (c) 2005 Elsevier Ltd. All rights reserved.