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.
Ti42Al48Cr8Ag2 alloy AEM Oxidation Nitrides New phase
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