Kirkendall pore evolution during interdiffusion and homogenization of titanium-coated nickel microwires

Abstract

Abstract In-situ and ex-situ X-ray 3D-tomography is used to characterize the microstructure of Ni microwires, with wire diameters spanning 25–100\xa0μm, (i) after vapor-phase deposition of Ti onto their surface and (ii) after subsequent homogenization to achieve the near-equiatomic NiTi composition desired for shape-memory or superelastic behavior. After Ti deposition at 925\xa0°C, wires are partially homogenized, exhibiting a pure Ni core surrounded by concentric shells of Ni3Ti, NiTi and NiTi2 intermetallic phases. Because of the imbalanced Ni and Ti diffusive fluxes, Kirkendall porosity is formed near the center of the wire, which often merges into a single pore in cross-sections, due to spatial confinement of the wire geometry. During subsequent homogenization at 925\xa0°C, these Kirkendall pores grow due to further Ni-Ti interdiffusion, and they coalesce into a single, hollow channel near the central axis of the wire, thus forming a NiTi microtube. In some cases, off-center pores form in addition to the central pore, but these off-center pores do not form continuous channels.