Effect of diffusion distance on evolution of Kirkendall pores in titanium-coated nickel wires

Microtubes of near-equiatomic nickel-titanium (NiTi) alloys can be created via the Kirkendall effect during Ni–Ti interdiffusion, when nickel wires are surface-coated with titanium via pack cementation and subsequently homogenized. This study explores the effect of diffusion distance upon Kirkendall microtube formation in NiTi by considering a range of Ni wire diameters. For Ni wire diameters of 25, 50 and 100 μm, titanized at 925 °C for 0.5, 2, and 8 h to achieve average NiTi composition, partial interdiffusion occurs concurrently with Ti surface deposition, resulting in concentric shells of NiTi₂, NiTi and Ni₃Ti around a Ni core, with some Kirkendall porosity created within the wires. Upon subsequent homogenization at 925 °C, near-single-phase NiTi wires are created and the Kirkendall porosity increases, leading to a variety of pore/channel structures: (i) for 25 μm Ni wires where diffusion distances and times are short, a high volume fraction of micropores is created near the final NiTi wire surface, with 1–2 larger pores near its core; (ii) for 50 μm Ni wires, a single, ∼20 μm diameter pore is created near the NiTi wire center, transforming the wires into microtubes, and; (iii) for 100 μm Ni wires, a ∼50 μm diameter irregular pore is formed near the NiTi wire center, along with an eccentric crescent-shaped pore of similar cross-section, resulting from interruption of a single diffusion path, due to the longer diffusion distances and times.