Synthesis of NiTi microtubes via the Kirkendall effect during interdiffusion of Ti-coated Ni wires

An additive alloying method is developed to fabricate NiTi microtubes, consisting of two steps: (i) depositing a Ti-rich coating onto ductile, pure Ni wires (50 μm in diameter) via pack cementation, resulting in a Ni core coated with concentric NiTi2, NiTi and Ni3Ti shells, and (ii) homogenizing the coated wires to near equiatomic NiTi composition via interdiffusion between core and shells, while concomitantly creating Kirkendall pores. Because of the spatial confinement and radial symmetry of the interdiffusing core/shell structure, the Kirkendall pores coalesce near the center of the wire and form a continuous longitudinal channel, thus creating a microtube. To study the evolution of Ni-Ti phases and Kirkendall pores during homogenization, coated wires were subjected to ex situ homogenization followed by (i) metallography and (ii) X-ray tomographic imaging. Near equiatomic NiTi was obtained upon homogenization at 925 °C for 4 h with compositional fluctuations between 49 and 53 at.% Ni consistent with slight variations in initial coating thickness. Kirkendall pores initially formed near the NiTi/Ni3Ti and Ni3Ti/Ni interfaces and eventually merged into a continuous channel with an aspect ratio of at least 75.