High Strain-Rate Deformation Evolution of Ni and Ti in Cold Sprayed Composite Ni-Ti Coatings

Cold Spray (CS) is a high deposition rate additive manufacturing technique for producing dense metal matrix and cermet composite coatings. In CS, particles are accelerated to high impact velocities (300-1200 m/s) and deposited on a substrate as a result of solid state deformation. In this study, composite Ni-Ti coatings are produced using blended Ni-Ti powders, to investigate their deformation and deposition behavior as well as their interaction during deposition. Single particle deformation and splat deformation analysis are carried out using scanning electron microscopy and electron backscatter diffraction.

Findings of this research revealed that Ni splats have a gradual deformation gradient from splat boundaries toward the interior of the splat with fine necklace-like recrystallized grains at splat boundaries, which is more prevalent at Ni/Ni interfaces. Ti splats presents fine recrystallized (subdivided) microstructure at all interfaces. High strain rate deformation in CS results in deformation mechanisms variation of FCC Ni from slip to twinning or combination of slip–microbands at splat boundaries. The high strain rate deformation results in severe grain refinement in Ti particles with no sign of twinning. Microstructural analysis of the composite NiTi coatings showed that the deformation evolution of both Ni and Ti components in composite coatings are different from that of single component coatings, attributed to material interaction in CS of composite coatings.

Keywords: Cold spray, Composite Coating, Deformation Evolution, Ni-Ti coating, Twinning- Microbands