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

The properties, deformation microstructures and fracture characteristi cs 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 b rittle-ductile transition with an elongation at 20 degrees C of 2.2% i ncreasing to 36% at 850 degrees C. A much less pronounced brittle-duct ile transition exists for TiAl + W, with an increase in elongation fro m 1.3% at 20 degrees C to only 4.5% at 850 degrees C. In both alloys f racture occurs predominantly by transgranular cleavage at low temperat ures, changing to intergranular and then prior particle boundary failu re with increasing temperature. High densities of deformation twins an d 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 oc curs, even during tensile deformation at 850 degrees C. The inability of TiAl + W to accommodate strain by deformation twinning eliminates t he brittle-ductile transition that occurs for most near gamma-TiAl com positions. TiAl + Cr attains higher fracture strength at 20 degrees C (563 MPa) than TiAI + W (505 MPa). This improved strength is related t o the substructure developed during HIP consolidation. The increased 2 0 degrees C ductility of TiAI + Cr is associated with a more homogeneo us as-HTPed microstructure.