THE BRITTLE-DUCTILE TRANSITION IN HIP CONSOLIDATED NEAR GAMMA-TIAL+W AND TIAL+CR POWDER ALLOYS

The properties, deformation microstructures and fracture characteristics resulting from tensile tests between 20 degrees C and 850 degrees C are compared for Ti-48Al-2W and Ti-47.5Al-3Cr. TiAl + Cr exhibits a brittle-ductile transition with an elongation at 20 degrees C of 2.2% increasing to 36% at 850 degrees C. A much less pronounced brittle-ductile transition exists for TiAl + W, with an increase in elongation from 1.3% at 20 degrees C to only 4.5% at 850 degrees C. In both alloys fracture occurs predominantly by transgranular cleavage at low temperatures, changing to intergranular and then prior particle boundary failure with increasing temperature. High densities of deformation twins and 1/2[110] dislocations form in TiAl + Cr deformed at greater than or equal to 700 degrees C. For TiAI + W much less deformation twinning occurs, even during tensile deformation at 850 degrees C. The inability of TiAl + W to accommodate strain by deformation twinning eliminates the brittle-ductile transition that occurs for most near gamma-TiAl compositions. TiAl + Cr attains higher fracture strength at 20 degrees C (563 MPa) than TiAI + W (505 MPa). This improved strength is related to the substructure developed during HIP consolidation. The increased 20 degrees C ductility of TiAI + Cr is associated with a more homogeneous as-HTPed microstructure.