FRACTURE AND TOUGHENING MECHANISMS IN AN ALPHA-2 TITANIUM ALUMINIDE ALLOY

The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α2 + β microstructure have been characterized as a function of temperature by performing uniaxial tension and JIC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied by post mortem fractographic and metallographic examinations of fractured specimens, as well as by in situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasi-static crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α2 matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α2 + β interface due to strain incompatibility resulting from impinging planar slip originated in α2. The sources of fracture toughness in the 25°C to 450°C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600°C, a change of toughening mechanisms leads to a lowering of the initiation toughness (the KIC value) but a drastic increase in the crack growth toughness and the tearing modulus.