PHASE-TRANSFORMATION IN AN FE-9.0AL-29.5MN-1.2SI ALLOY

The microstructure of an (alpha + gamma) duplex Fe-9.0Al-29.5Mn-1.2Si alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, extremely fine D0(3) particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at 550-degrees-C, the extremely fine D0(3) particles existing in the as-quenched specimen grew preferentially along [100] directions. With increasing the aging time at 550-degrees-C, a (Si, Mn)-rich phase (designated as "L phase") began to appear at the regions contiguous to the D0(3) particles. The L phase has never been observed in various Fe-Al-Mn, Fe-Al-Si, Fe-Mn-Si, and Mn-Al-Si alloy systems before. When the as-quenched specimen was aged at temperatures ranging from 550-degrees-C to 950-degrees-C, the phase transformation sequence occurring within the (alpha + D0(3)) region as the aging temperature increases was found to be (alpha + D0(3) + L phase) --> (alpha + D0(3) + A13 beta-Mn) --> (B2 + D0(3) + A13 beta-Mn) --> (B2 + A13 beta-Mn) --> (alpha + A13 beta-Mn) --> (alpha + gamma) --> alpha.