Influence of Composition and Thermal Treatments on the Martensitic Transition of Cu-Al-Mn Alloys

We have studied the martensitic transformation and the order-disorder transitions in two families of composition related Cu-Al-Mn crystals. X-ray diffraction and calorimetric measurements have been performed to characterize order-disorder transitions. The entropy change at the martensitic transition has been computed from calorimetric measurements. Results show that its magnitude depends only on the martensite structure which, in turn, depends on the electronic concentration e/a. We have obtained ΔS=1.51 +/- 0.05 J.mol-1K-1 for a transition to a 2H phase and ΔS=1.26 +/- 0.06 J.mol-1K-1 for a transition to a 18R structure. We discuss the possibility of a magnetic contribution to ΔS. Finally it is found that quenching has a marked influence on the martensitic transition temperature, but it does not affect the entropy change