Phase transitions and modulated structures in ordered (Cu-Mn) 3Al alloys

Abstract

Quenched and aged alloys along the composition line Cu 3Al-Cu 2MnAl have been studied by electron diffraction and microscopy. Quenching produces an ordered solid solution based on the DO 3 structure of Cu 3Al and the L2 1 structure of Cu 2MnAl. Below 350°C the alloys decompose inside a miscibility gap into a Cu 2MnAl-rich phase and a Cu 3Al-rich phase. The resulting microstructures possess all the metallographic characteristics of spinodal decomposition. The early stage of the decomposition is characterized by composition modulations having a range of wavevectors close to 〈100〉. The wavelength developes into resolvable values of 40–100 Å and gradually increases with aging time. During coarsening, the symmetrical alloy develops a complex microstructure of 〈100〉 oriented rods having widely different wavelengths, whereas the Mn-rich asymmetrical alloys develop {100} oriented Cu 3Al-rich plates. On coarsening the coherency strain (~2%) is generally relieved by the generation of 1 2 a 〈100〉 interfacial dislocations. Prior to the loss of coherency in the symmetrical alloy, the Cu 3Al-rich phase possesses a tetragonal structure whereas the Cu 2MnAl-rich phase has a cubic structure. After the loss of coherency, both phases are cubic. In the Al-rich asymmetrical alloy, the Cu 2MnAl-rich phase heterogenously nucleates at 1 4 a 〈111〉 APB's. In the symmetrical alloy, the structure of the Cu 3Al-rich particles varies with the decomposition temperature. At temperatures close to the miscibility gap, the structure of the particles is DO 3. These DO 3 particles are further modulated by a dense array of small regions having the L1 0 superstructure and exhibit tweed strain contrast. At lower temperatures, the particles are composed of a regular arrangement of twelve possible variants of the L1 0 phase. At all temperatures, the structure of the Cu 2MnAl-rich phase is L2 1. During overaging of the symmetrical alloy at temperatures close to the miscibility gap, the Cu 3Al-rich phase transforms to the γ phase having a composition near Cu gAl 4. Thus it is suggested that for the symmetrical alloy the decomposition tie line is close to Cu 9Al 4-Cu 2.2MnAl 0.8.