Wires of nickel-titanium have been tensile tested to evaluate their elastic constants, super-elastic characteristics and strength. These data are compared with the response of the same material to (hot stage) indentation testing, using both nano-indentation and micro-indentation equipment, and both a Berkovich and a spherical indenter. Indentation characteristics indicative of super-elastic behaviour are identified. In particular, the observation of enhanced indentation strain recovery when tested above the Af temperature, compared with tests performed at lower temperature, is recorded here and appears to represent a reliable indicator of super-elastic behaviour. Wires have also been joined together by liquid phase sintering, after a copper electroplating treatment, and by solid state diffusion bonding. Microstructural studies of these joints revealed the expected phases. Preliminary mechanical studies have given an indication that it may be possible to produce strong, highly porous super-elastic material in this way.
Several nickel-titanium alloys have been mechanically tested over a range of temperature, such that superelastic deformation should or should not take place. Both conventional uniform loading (tension and compression) and nanoindentation (using a range of conditions) have been carried out. Indentation tests were conducted using either spherical or Berkovich tips, with either high or low load configurations. These experiments were conducted at various temperatures, corresponding to above Af, below Mf, or at an intermediate temperature. Elastic constants, superelasticity parameters and plastic deformation characteristics obtained using conventional mechanical tests are compared with indentation responses and conclusions drawn about how best to identify superelastic deformation via nanoindentation data. In particular, it is concluded that use of spherical indenters facilitates such identification. Scale effects are also discussed. These experimental investigations are supplemented by preliminary development of an ABAQUS finite element model suitable for study of the indentation response of superelastic material.