Effects of Strain Rate and Prestraining on Tensile Behavior of Duplex Gamma Titanium Aluminides

Abstract

The effects of strain rate and prestraining on tensile behavior of two-phase (γ+α₂) titanium aluminides at 20 and 730°C have been investigated. At 20°C the elongation remains at about 1.4% as the strain rate increases from 5ȕ10⁻⁵ to 5ȕ10⁻² s⁻¹ and it drops to nearly zero at 5ȕ10⁻¹s⁻¹. At 730°C ( i.e.above DBTT) the plastic strain is about 13% when tested at 5ȕ10⁻⁵s⁻¹, while it reduces significantly to less than 3% at 5ȕ10⁻⁴ and 5ȕ10⁻² s⁻¹. Again, the elongation is about zero at the highest strain rate tested, 5ȕ10⁻¹ s⁻¹. Regardless of the strain rate, fracture by an intergranular mode of the primary equiaxed γ appears to increasingly dominate as temperature changes from 20 to 730°C. Introduction of prior plastic deformation by prestraining beyond yielding at 945°C obviously increases the 20°C yield stress, however, with little influence on ductility. Transmission electron microscopy reveals that a number of dislocation loops are produced during prestraining. These loops are generally immobile resulting in the observed increase of flow stress and unchanged ductility.