Hardness of Cu–Mn–Al alloys as a function of the annealing temperature in the β phase

Samples of Cu–10.0at.%Mn–18.8at.%Al and Cu–11.0at.%Mn–18.1at.%Al alloys were prepared in the metastable austenitic β phase, by quenching from 1123 K into water at room temperature. The as quenched samples were found to be L21 ordered. Aging of these alloys at different temperatures induced an evolution of the microhardness. These microhardness changes are interpreted using the information from X-ray diffraction (XRD), martensitic transformation temperature measurements, transmission electron microscopy (TEM) and microanalysis using scanning electron microscopy (SEM). Changes in microhardness at low temperatures are correlated with the martensitic transformation temperature Ms provided that Ms is close to room temperature. Microhardness changes at higher temperatures are caused by the decomposition of the β phase. The kinetics of β phase decomposition was followed measuring its 200 XRD peak intensity. The experiments suggest that the last vestiges of β phase are L21 long range ordered and that an α fcc phase is formed during the first stages of decomposition.