Microstructural development in commercial Cu-Ni-Al-Mn alloy

Abstract

The development of microstructure in a Cu-Ni-Al-Mn alloy has been observed by optical and transmission electron microscopy. Solution treatment followed by water quenching does not produce a completely random solid solution and electron diffraction suggests that there may be short range order. Isothermal aging produces precipitation of θ phase (Ni₃Al, L1₂ structure) homogeneously in the matrix, discontinuously at grain boundaries, and heterogeneously at grain boundaries. The homogeneously nucleated θ phase coarsens from spheres to cubes to plates, because of the necessity to reduce the elastic strain energy associated with the coherent precipitates, which also encourages particle alignment at and below 600°C. A classical discontinuous precipitation reaction is observed at all temperatures studied, but the proportion of affected grain boundaries is reduced as the temperature rises. After termination of the reaction which occurs very rapidly, the θ rods coarsen at two different rates depending on the growth direction in the matrix.