CONSTITUTION OF MN-AL-(CU,FE,NI OR C) ALLOYS NEAR THE MAGNETIC TAU-PHASE

The effect of adding Cu, Fe, and Ni to the magnetically hard tau phase in Mn-Al was studied by investigating isothermal sections at 1000 and 700 degrees C. The Mn-Cu-Al alloys show no tau but only hexagonal eps ilon at 1000 degrees C. Likewise, in Mn-Fe-Al and Mn-Ni-Al only hexago nal epsilon exists at 1000 degrees C. The Cu alloys exhibit a hitherto unobserved splitting of epsilon (and therefore tau) into two phases h aving the same structure but different Cu contents. One of them can be understood as a continuation of the metastable binary a which can be undercooled even more effectively in the ternary. The other is the sta ble extension of epsilon into the ternary. This apparent decomposition of epsilon is described with the help of an isopleth at 55 Mn, 45 Al. Above similar to 10% Cu the tau phase is lost and is replaced by kapp a, resulting in a drop in Curie temperature and a loss of magnetic har dness. None of these three elements is a true stabilizer of the magnet ic tau phase; it always forms by transformation from the hexagonal hig h-temperature phase epsilon. This precludes processing in which fully dense magnets are obtained by sintering magnetically oriented powders at high temperatures. The Cu alloys offer an advantage for the type of processing applied to Mn-Al-C because they stabilize the tau precurso r epsilon to temperatures below 700 degrees C.