Shape-memory characterization of NiTi microtubes fabricated through interdiffusion of Ti-Coated Ni wires

Near-equiatomic NiTi microtubes were fabricated using an additive alloying method 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. Both the mechanical and thermal response of the NiTi microtubes were characterized in this study using a combination of dynamic mechanical analysis and differential scanning calorimetry, respectively, in conjunction with conventional metallography and X-ray tomographic microscopy. Due to slight compositional variations, both shape-memory and superelastic behaviors were observed within the same microtube, which achieved a total tensile strain of ∼8% before failure: the largest contribution to the strain recovery was the thermal shape memory effect showing near complete strain recovery occurring during multiple cycles. A second microtube exhibited only superelastic behavior, achieving a maximum, recoverable strain of 2.5% at 110 MPa, likely limited by the presence of a remaining Ni₃Ti core as a result of under-titanization. Finite-element analysis of elastic stresses in a wire segment modeled from actual tomography data illustrates the extent of stress concentrations due to inner and outer tube surface roughness. The stress concentrations are responsible for a 65% increase in the top 1% average von Mises stress, which may further affect the shape-memory behavior of the tubes.