Discontinuous coarsening of primary [formula omitted] lamellae in surface layer of the Ti 58Al 42 intermetallic alloy

Abstract

Some experimental description of discontinuous coarsening (DC) reaction observed in various ({alpha}{sub 2} + {gamma}) two-phase TiAl-based alloys can be found in previous papers. This paper aims at examining the DC reaction in the surface layer of the alloy, and investigating the reaction by thermodynamic analysis. The cells in small size (about 30--50{micro}m) with coarse {alpha}{sub 2}/{gamma} lamellae due to the discontinuous coarsening nucleate randomly within the fine primary lamellae of the alloy surface and extend into a limited thickness of the surface layer for the Ti{sub 58}Al{sub 42} alloy ({alpha}{sub 2}{sup ss}) treated at 1,000 C. The elastic strain energy in the surface layer of the specimen has great effect on the discontinuous coarsening reaction. With the further prolonging of time, the extension of DC cellular microstructures with coarse {alpha}{sub 2}/{gamma} lamellae into the interior of the specimen becomes extremely difficult, and the discontinuous coarsening reaction nearly stops.