CRACK-GROWTH MECHANISM AND FRACTURE-TOUGHNESS IN EXTRA LOW INTERSTITIALS GRADE TI-6AL-4V ALLOY

Three-dimensional location analysis and source characterization of acoustic emissions (AE) were conducted in the fracture toughness test to understand the microfracture process of Extra Low Interstitials (ELI) Ti-6Al-4V alloy with equiaxed α microstructure. Scanning Electron Microscope observation was also carried out for both fracture surfaces and microstructures near the notch tips of the unloaded specimens to supplement the AE analysis. From about 50% of the maximum load in the fracture toughness test of ELI Ti-6Al-4V, many microvoids with diameters smaller than 10 μm began to nucleate near the crack tip and then microcracks of about 20 to 40 μm in diameter were generated by coalescence of these microvoids. These microcracks were located about 30 to 50 μm above or below the fatigue pre-crack plane. The stress relaxation near the crack tip due to the microcracks and zigzag type coalesence of microcracks was thought to enhance the fracture toughness of ELI Ti-6Al-4V alloys.