Effects of heat treatment on the ballistic impact properties of Inconel 718 for jet engine fan containment applications

The effects of heat treating Inconel 718 on the ballistic impact response a nd failure mechanisms were studied. Two different annealing conditions and an aged condition were considered. Large differences in the static properti es were found between the annealed and the aged material, with the annealed condition having lower strength and hardness and greater elongation than t he aged. High strain rate tests show similar results. Correspondingly large differences were found in the velocity required to penetrate material in t he two conditions in impact tests involving 12.5 mm diameter, 25.4 mm long cylindrical Ti-6-4 projectiles impacting flat plates at velocities in the r ange of 150-300m/s. The annealed material was able to absorb over 25 percen t more energy than the aged. This is contrary to results observed for balli stic impact response for higher velocity impacts typically encountered in m ilitary applications where it has been shown that there exists a correlatio n between target hardness and ballistic impact strength. Metallographic exa mination of impacted plates showed strong indication of failure due to adia batic shear. In both materials localized bands of large shear deformation w ere apparent, and microhardness measurements indicated an increase in hardn ess in these bands compared to the surrounding material. These bands were m ore localized in the aged material than in the annealed material. In additi on, the annealed material underwent significantly greater overall deformati on before failure. The results indicate that lower elongation and reduced s train hardening behavior lead to a transition from shear to adiabatic shear failure, while high elongation and better strain hardening capabilities re duce the tendency for shear to localize and result in an unstable adiabatic shear failure. This supports empirical containment design methods that rel ate containment thickness to the static toughness. Published by Elsevier Sc ience Ltd.