Formation of a submicrocrystalline structure in TiAl and Ti 3Al intermetallics by hot working

Abstract

A method of producing a submicrocrystalline structure (grain size d<1 μm) in large pieces of hard-to-deform materials like titanium aluminides has been developed. The method is based on initiation of dynamic recrystallization (DRX) during hot working, utilizing the decrease in grain size during DRX which occurs when the hot working temperature decreases or the strain rate increases, both leading to an increase in flow stress. In practice, the method involves continuous grain refinement due to DRX as the working temperature decreases. A grain size of 0.1–0.4 μm was produced with no porosity in TiAl- and Ti 3Al-based alloys. Partial disordering was detected in a Ti 3Al alloy with a submicrocrystalline (SMC) structure. The grain refinement hardens the intermetallic alloys at room temperature. In a fully ordered Ti 3Al alloy, room temperature ductility increases when the grain size decreases, while ductility of partially disordered Ti 3Al and TiAl alloys is close to zero at room temperature. Materials with SMC structures show superplastic properties at temperatures in the range of 600–900°C, i.e. 200–400°C below the temperature range for superplasticity in the materials with micron-sized grains.