Phase equilibria in the Mn-rich portion of the binary system Mn-Al

The phase equilibria in the binary Mn-Al system have been investigated over the temperature range 800 to 1200 degrees C and the composition range 50 to 80 at.% Mn using a combination of diffusion couple techniques, optical metallography, X-ray diffraction, and differential scanning calorimetry. The results show that: (i) the epsilon/delta-Mn phase boundary is located at approximately 72 at.% Mn, (ii) the epsilon phase region is wider and extends to higher Mn content than that reported before, and (iii) the eutectoid reaction (delta-Mn --> epsilon + beta-Mn) occurs at approximately 1040 degrees C, i.e. at a temperature higher than that previously reported (970 degrees C). While the results from this study on the epsilon/gamma and epsilon/beta-Mn phase equilibria are in satisfactory agreement with the data of previous workers, the results on the: epsilon/delta-Mn phase equilibrium differ from those reported previously. It is explained that this discrepancy may be due to the presence of the previously undetected transformation from the epsilon to the beta-Mn phase on quenching. which occurs only in the epsilon phase alloys with Mn content over 58 at.%.