Lattice Transitions and Flickering Images in Aged Cu-Mn Alloys

Distinctive "flickering" movements are observed in TEM images of the microstructure of Cu-Mn alloys after aging within the miscibility gap. Analyses of two-beam image extinction and electron diffraction streaks indicate that the underlying tweed and "V-shaped images involve a static displacement field of the type {llO) <1iO>. Atomic force microscopy and other microanalyses show the presence of Mn-enriched colonies of 15-40 nm, formed throughout the microstructure, in which twinned fct crystallites are induced and confined. The flickering movements are interpreted as a direct manifestation of the fee-fct transformation event and in particular of a rotation of the fct c-axis, these effects being caused by an inelastic interaction between <110> <1iO> phonons and the accelerated electrons of the incident TEM beam.