Phase transformations and microstructural evolution in aged Mn-Cu-based alloys

Abstract

The transformation behavior of aged Mn-Cu-based alloys was studied via transmission electron microscopy. The evolution of several unique forms of diffraction contrast, such as 'tweed' and 'flickering', was characterized and correlated with the underlying transformation behavior. Convergent beam microbeam compositional analysis and high resolution lattice imaging techniques were employed to assist in the interpretations. It was determined that upon aging these alloys, a FCC solid solution decomposes into a fine dispersion of Mn-rich and Mn-poor regions which coarsen with further aging. This decomposition was modeled using finite difference techniques. The Mn-rich regions were determined to undergo a crystallographic transformation which involves a slight tetragonal distortion upon cooling. For short aging times, when the elastic strain fields around each of the finely dispersed FCT regions overlap sufficiently, tweed contrast is observed.