Improvement in fatigue lifetime of Ti-6Al-4V alloy by boron implantation

Ti-6Al-4V alloy with and without boron ion implantation has been tested in stress-controlled tension-zero fatigue conditions. The alloy was implanted at energies of 120 keV and with doses of 2x1017 ions cm-2. A 12% increase in endurance limit was found with implantation. The implanted surface and fracture surface structure, the dislocation arrangement and the concentration profile of boron in implanted layers were examined using scanning electron microscopy, transmission electron microscopy and secondary-ion mass spectroscopy. It was found that boron implantation produced irradiation defects and fine titanium boride precipitates. The implanted layers are obstacles preventing glide dislocations from moving out to the surface; at the same time implanted atoms diffuse into the base metal during fatigue cycling. (orig.)