EFFECT OF GAMMA-PRECIPITATES ON THE MARTENSITIC-TRANSFORMATION IN CU-AL-MN ALLOYS

The effect of gamma precipitates on the martensitic transformation has been studied for two Cu-Al-Mn alloys containing 13.7 wt.% Al and 1.55 or 3 wt.% Mn. Precipitates were made to grow by aging at 570 K. Subsequent thermally induced martensitic transformations were monitored by differential scanning calorimetry. Optical microscopy and transmission electron microscopy, including in-situ cooling-heating experiments, were carried out for microstructural studies and to ascertain the martensite-precipitate interaction. Precipitates grow faster in the alloy with lower manganese content because of its closer composition to that of the equilibrium gamma phase. Precipitates cause substantial hardening, which increases with increasing aging time, being similar in martensite and in the parent phase. The evolution of characteristic transformation temperatures and heat of transformation is discussed in terms of matrix composition changes and stresses produced by the precipitates. Different precipitate-martensite interactions are related to the two gamma' martensite morphologies observed, one having large twinned plates in which the precipitates are embedded, and the other with fine heavily dislocated plates formed preferentially between the precipitates and predominating in samples with larger precipitates.