Additive Manufactured Ti-Ni-C Metal Matrix Composites for Ballistic Applications

Abstract

Functionally graded material (FGM) with the volumetric fraction of ceramic and metal changing gradually along the thickness is a prospective armour material with attractive properties. The gradually changing composition of FGM allows a relatively smooth transition from the ceramic rich zone to the metal to be achieved replacing the obvious interface in the bi-layered armour. This smooth transition theoretically leads to the improved ballistic performance of the armour. However, not many studies were reported on revealing the gradually varying microstructure and mechanical response under dynamic loading conditions of FGMs. This limits the practical application of FGM armour. Thus, this thesis aims to inspect the microstructures and mechanical responses of the single composition materials (SCMs) at different locations of an FGM under different loading conditions and understand the reinforcement and failure mechanisms of the SCMs. Ti-Ni-C metal matrix composites (MMCs) including FGMs and SCMs were fabricated with Ti-6Al-4V (Ti64) and Ni coated graphite (NCG) powders using LENS additive manufacturing system. The phase compositions and microstructures of SCMs and FGMs were examined by X-ray diffraction, optical microscopy and scanning electron microscopy. The results showed that Ti2Ni and TiCx phases were in situ synthesized with different quantity correlated to the chemical compositions. The variations of micro-hardness along the direction of composition variation were investigated using hardness tests. It was found that the hardness of the composite was enhanced by the higher content of TiCx and Ti2Ni. The quasi-static and dynamic compression tests were performed on SCMs with different NCG contents using universal testing machine and split Hopkinson pressure bar (SHPB) at various strain rates from 10-3 to 6×102 s-1. The larger amount of NCG in SCM resulted in the increase of strength and the reduction of ductility. The strain rate effect was negligible when the strain rate was lower than 10-1 s-1. However, SCM with higher NCG content exhibited decreased strain rate hardening effect and better resistance to shear fracture at a higher strain rate of approximately 6×102 s-1. The dynamic responses of SCMs with different NCG contents under shock loading were investigated using flyer plate impact experiments conducted with a gas gun. The SCMs containing more NCG exhibited lower spall strength and tended to break into larger number of fragments with smaller size.