Phase Transitions During Continuous Heating of Martensitic CuAlMn Alloys

Abstract
Phase transitions in CuAlMn alloys containing up to 12 at% Mn and 27 at% Al continuously heated up to 550°C 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 γ₂ precipitation. The additional endothermal peak above 500°C is caused by DO₃ disordering. Alloys of intermediate Mn and Al content show only a reversible martensitic and ordering transformations during repeated cycling in the range - 50 to + 550°C. Alloys of higher Mn (6 - 12 at%) and lower Al content exhibit a diffused exothermal peak in the range 320 - 450°C due to bainitic transformation and a broad endothermal effect above 500°C due to L2₁/DO₃ disordering. Bainitic plates possess 9R structure. They contain more Cu and less Mn and Al when compared to initial alloy compositions. Precipitates of γ₂ compositions lie close to its equilibrium phase limits.