Abstract
The toughening mechanisms in the Ti-24A1-11Nb (Ti-24-11) alloy have been identified previously to include crack-tip blunting, bridging, and deflection by the ductileβ phase, while the fracture mechanisms involve the nucleation, growth, and linkage of microcracks with the main crack. By performing appropriate theoretical analyses and critical experiments, the relative contributions of intrinsic and extrinsic toughening mechanisms, including microcrack shielding, crack-tip blunting, bridging, and deflection by theβ phase, to the initiation and crack growth toughness values of the Ti-24-11 alloy have been studied for three microstructures. The results indicate that the microstructure affects not only the amount of toughness enhancement, but also the type of toughening mechanisms present in the Ti-24-11 alloy. The initiation toughness in Ti-24-11 arises from the matrix toughness, crack-tip blunting, and, occasionally, from crack deflection by the ductile phase. As a result, theKIC values increase with the volume fraction of the ductile phase. In contrast, the resistance curve behavior originates from (1) a change of crack-tip singularity, which occurs when the blunted crack extends into the plastic zone, (2) crack bridging by ductile phase and shear ligaments, and (3) microcrack shielding, which occurs mostly at elevated temperatures.
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