Key microstructures controlling the mechanical properties of two-phase TiAl alloys with lamellar structures

The objective of this study is to identify key microstructural parameters which control the mechanical properties of two-phase γ-TiAl alloys with lamellar structures. TiAl alloys with the base composition of Ti-47Al-2Cr-2Nb (at.%) were prepared by arc melting and drop casting, followed by hot extrusion at temperatures above the α-transus temperature, T_α. The hot extruded materials were then heat treated at various temperatures above and below T_α in order to control microstructural features in these lamellar structures. The mechanical properties of these alloys were determined by tensile testing at temperatures to 1000°C. The tensile elongation at room temperature is strongly dependent on grain size, showing an increase in ductility with decreasing grain size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing an increase in strength with decreasing lamellar spacing. Hall-Petch relationships hold well for the yield strength at room and elevated temperatures and for the tensile elongation at room temperature. Tensile elongations of about 5% and yield strengths around 900 MPa are achieved by controlling both colony size and interlamellar spacing. The mechanical properties of the TiAl alloys with controlled lamellar structures produced directly by hot extrusion are much superior to those produced by conventional thermomechanical treatments.