Search

A microstructural investigation of oxide scales was carried out by analytical electron microscope (AEM) after the as-cast Ti42Al48Cr8Ag2 alloy was exposed to air at 1,173 K for 5, 30 and 180 min, respectively. The results indicated that the formation of oxide scale experiences a transformation from ternary-layer system consisting of an outmost rutile (TiO2-rich) layer, mid amorphous alumina and inner nitrides to a double-layer of TiO2 and Al2O3-rich with increasing time from 5 to 30 min. In the subsurface zone, a change was also found a metastable Cr-rich new phase with bcc structure of a = 1.371 nm formed in addition to the oxidation for X-phase during 30 min. After 180 min, a continuous Laves-phase layer formed instead at the same position. In addition, the process revealed that amorphous alumina changed to a metastable structure before stable α-Al2O3 formed.

The oxidation of Ti-48Al-8Cr-2Ag (at.%) was studied for 50 h at 900 and 1000 degrees C. The microstructure of the oxide scales was characterized by means of scanning-electron microscopy (SEM) and high-resolution electron microscopy (HREM). The results showed that the mass gain of the alloy at 1000 degrees C was lower than that at 900 degrees C. At 1000 degrees C, a continuous and dense scale of quite pure Al2O3 formed, but at 900 degrees C, a complex scale with an outer layer of Al2O3 + TiO2 and an layer of inner Al2O3. A Laves phase and a Z-phase were found at the scale/substrate interface, which might be a factor beneficial to the formation of Al2O3.