ULTRAMICROHARDNESS AND MICROSTRUCTURE OF TI-6 WT-PERCENT-AL-4 WT-PERCENT-V ALLOY NITRIDED BY ION-IMPLANTATION

A study of various aspects of nitrogen implantation into Ti-6 wt.%Al-4 wt.%V followed by annealing at 705-degrees-C is presented. N+ implantation with fluences of 10(17) and 1.5 x 10(17) ions cm-2 at 80 keV induced a significant improvement in ultramicrohardness, and annealing was found to cause a reversal of this effect to the values of the unimplanted material and lower. The decrease in hardness after annealing was higher for the higher implantation dose, indicating a dose-dependent effect. Rutherford backscattering results on the sample implanted with 1.5 x 10(17) ions cm-2 show a buried nitrogen layer which decreased significantly after the annealing treatment. Transmission electron microscopy data show that isolated TiN islands form directly during implantation and grow during annealing. The defect density in the implanted material is markedly reduced after annealing. The results indicate that the increase in hardness in the as-implanted material is only due to the extended lattice damage and not to TiN formation.