Atomic level observations of mechanical damage in shot peened TiAl

High-resolution transmission microscopy has been used to reveal the defect processes occurring during shot peening of a high-strength titanium aluminide alloy with a nearly lamellar microstructure. Deformation is characterised by intensive dislocation glide and mechanical twinning, involving all potential slip systems available in the 2(Ti3Al) and (TiAl) phases. The outermost surface layer consists of extremely fine crystals that are probably a contaminant titanium nitride phase, embedded into an amorphous phase. The mechanisms involved in this mechanically driven solid state transformation will be elucidated. Particular emphasis is paid on the thermodynamic and kinetic factors involved in the amorphisation reaction.