Fracture Toughness Behavior of Unaged Beta-III Titanium

Abstract

The dynamic fracture toughness behavior of unaged Beta-III (Ti-10.2 Mo-6Zr-4.5Mo) titanium has been investigated. Both solution treatment practice and subsequent test temperature have been found to affect the fracture toughness. Decreasing the solution treatment temperature from 1255 to 1005 K generally led to a decrease in the fracture toughness over the entire range of test temperatures investigated. In addition, decreasing the test temperature from 473 to 77 K also resulted in lower fracture toughness. These reductions in fracture toughness have been shown to be related to the introduction of increasing amounts of a localized shear fracture mode with both decreasing solution and test temperature. Based on these observations, a model has been proposed which describes the stress/strain conditions at the tip of a propagating crack that appear to enhance shear fracture. Finally, it has been suggested that the principal metallurgical variable controlling the choice of fracture mode transition from either dimple to cleavage or dimple to shear dimple in unaged metastable β titanium alloys is the primary deformation mechanism. Twinning tends to promote shear instability while slip may be associated with low temperature cleavage crack propagation.