Semi-quantitative HRTEM for partially ordered materials: Application to Al-rich TiAl alloys

Microstructures of long-period ordered TiAl alloys that are rich in Al were investigated by the semi-quantitative high-resolution transmission electron microscopy (HRTEM). Local ordered arrangements of Al atoms in (002) Ti-rich layers of the L 1₀ matrix were identified from arrangements of very bright dots in the HRTEM images after appropriate image processing. For a short-range order (SRO) state, the Al atoms form three types of ordered-clusters: square (Ti₄Al), fat rhombus (Ti₃Al) and lean rhombus (Ti₂Al). These clusters are arranged periodically in a short-range to form microdomains of various long-period superstructures: Al₅Ti₃, h-Al₂Ti and so on. Below or above the temperature range showing the SRO state, the Al₅Ti₃ or h-Al₂Ti type cluster-arrangements selectively develop into their long-range order (LRO) domains. Some of the other cluster-arrangements are left between the LRO domains as anti-phase boundary structures.