Uncovering the Intrinsic High Fracture Toughness of Titanium via Lowered Oxygen Impurity Content

Titanium (Ti) and its alloys are known to exhibit room-temperature fracture toughness below 130 MPa m^1/2, only about one half of the best austenitic stainless steels. It is purported that this is not the best possible fracture resistance of Ti, but a result of oxygen impurities that sensitively retard the activities of plasticity carriers in this hexagonal close-packed metal. By a reduction of oxygen content from the 0.14 wt% in commercial purity Ti to 0.02 wt%, the mode-Ι fracture toughness of the low-oxygen Ti is measured to be as high as K_{J Ic} ≈ 255 MPa m^1/2, corresponding to J-integral-based crack-initiation toughness of up to J_Ic ≈ 537 kJ m^-2. This extraordinary toughness, reported here for the first time for pure Ti, places Ti among the toughest known materials. The intrinsic high fracture resistance is attributed to the profuse plastic deformation in a significantly enlarged plastic zone, rendered by the pronounced deformation twinning ahead of the crack tip along with ample twin-stimulated 〈c+a〉 dislocation activities, in the absence of impeding oxygen. Controlling the content of a property-controlling impurity thus holds the promise to be a readily applicable strategy to reach for unprecedented damage tolerance in some other structural alloys.