Selectivity of anti-phase boundary introduced in Al₅Ti₃ superstructure

Formation of anti-phase boundaries (APBs) in the Al₅Ti₃ long-period superstructure of Al-rich TiAl single crystals was examined by the high-resolution transmission electron microscopy (HRTEM) with image processing. The APBs were analyzed focusing on the unique units of ordered clusters such as Ti₄Al, Ti₃Al and Ti₂Al which form square, fat rhombus and lean rhombus shapes in (002) Ti-rich layers, respectively. The Al₅Ti₃-type anti-phase domains (APDs) with various sizes in diameter were obtained in Ti-62.5at.%Al crystals by heat treatment at 600°C or 750°C after the floating zone melting and the subsequent quenching from 1200°C. Almost all the Al₅Ti₃-type APBs lay on {110) and were geometrically classified into 19 types named Type A to Type S. Indeed, 6 types of APBs (Type A to Type F) composed of the units of Ti₄Al, Ti₃Al and Ti₂Al were found, but the other 13 types (Type G to Type S) were not clearly observed. Moreover, a specific type called Type A among the APBs was preferentially formed even in the initial stage of the APD growth. The APBs of Type A appeared with a quite higher frequency than those ideally formed by the random-nucleation and subsequent bounding of APDs. Thus, the APBs of Type A must be already introduced in the stage for nucleation of the APDs and grow stably on {110) due to the quite low APB energy in the long-period superstructure with a relatively large unit cell.