Phase Transitions During Continuous Heating of Martensitic CuAlMn Alloys

Phase transitions in CuAlMn alloys containing up to 12 at% Mn and 27 at% A1 continuously heated up to 5500C were studied using differential scanning calorimetry and transmission electron microscopy. Alloys containing less than 4.8 at% Mn exhibit an exothermal heat effect in the range 280 380°C due to y2 precipitation. The additional endothermal peak above 5000C is caused by DO3 disordering. Alloys o f intermediate Mn and AI content show only a reversible martensitic and ordering transformations during repeated cycling in the range 50 to + 5500C. Alloys o f higher Mn (6 12 at%) and lower A1 content exhibit a diffused exothermal peak in the range 320 450W due to bainitic transformation and a bmad endothermal effect above 5000C due to L211D03 disordering. Bainitic plates possess 9R structure. They contain more Cu and less M n and A1 when compared to initial alloy compositions. Precipitates o f yz compositions lie close to its equilibrium phase limits.