The as-quenched microstructure of the Fe-30%Mn-9%Al-5%Cr-0.7%C (in mass%) alloy was single-phase austenite. When the as-quenched alloy was aged at 550 C-750 C, fine (Fe,Mn) 3 AlC carbides were formed within the austenite matrix. In addition, as the aging temperature increased, a M 7 C 3 carbide + D0 3 ! M 7 C 3 carbide + B2 ! M 7 C 3 carbide + (ferrite) phase transition had occurred on the grain boundaries. This grain boundary precipitation behavior has never before been observed in FeMnAlC and FeMnAlCrC alloy systems.

When the present alloy was aged at 550 C, Hägg carbides (M 5 C 2 -type carbides) formed at a/2h100i anti-phase boundaries of the D0 3 domains. The Hägg carbide has never been observed by previous workers in FeMnAlC and FeMnAlCrC alloy systems. The orientation relationship between Hägg carbide and D0 3 matrix was determined to be ð 5 510Þ M5C2 == ð1 1 10Þ D03 and ð13 4 4Þ M5C2 == ð10 2 2Þ D03 . The orientation relationship between Hägg carbide and bcc-type phase has also never been reported before.

As-quenched microstructure of the Fe-24.6 at% Al-7.5 at% Ti alloy was a mixture of (A2+L2 1 ) phases. When the as-quenched alloy was aged at 1173 K for moderate times, the L2 1 domains grew considerably and B2 phase was formed at a/2h100i anti-phase boundaries (APBs) as well as phase separation from well-grown L2 1 to (B2+L2 1 Ã ) occurred basically contiguous to the APBs, where L2 1 Ã is also a L2 1 -type phase. With continued aging at 1173 K, the phase separation would proceed toward the whole well-grown L2 1 domains. This microstructural evolution has not been reported in the Fe-Al-Ti alloy systems before.