Formation of a submicrocrystalline structure in TiAl and Ti3Al intermetallics by hot working

A method of producing a submicrocrystalline structure (grain size d < 1 mu m) in large pieces of hard-to-deform materials like titanium aluminides has been developed. The method is based on initiation of dynamic recrystalliza tion (DRX) during hot working, utilizing the decrease in grain size during DRX which occurs when the hot working temperature decreases or the strain r ate increases, both leading to an increase in flow stress. In practice, the method involves continuous grain refinement due to DRX as the working temp erature decreases. A grain size of 0.1-0.4 mu m was produced with no porosi ty in TiAl- and Ti3Al-based alloys. Partial disordering was detected in a T i3Al alloy with a submicrocrystalline (SMC) structure. The grain refinement hardens the intermetallic alloys at room temperature. In a fully ordered T i3Al alloy, room temperature ductility increases when the grain size decrea ses, while ductility of partially disordered Ti3Al and TiAl alloys is close to zero at room temperature. Materials with SMC structures show superplast ic properties at temperatures in the range of 600-900 degrees C, i.e. 200-4 00 degrees C below the temperature range for superplasticity in the materia ls with micron-sized grains. (C) 2000 Elsevier Science S.A. All rights rese rved.