INFLUENCE OF MICROSTRUCTURE ON CRACK-TIP MICROMECHANICS AND FRACTURE BEHAVIORS OF A 2-PHASE TIAL ALLOY

The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (gamma + alpha-2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr + V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25-degrees-C to 800-degrees-C range. In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600-degrees-C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25-degrees-C to 800-degrees-C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.