Electrodeposition of thin film shape memory alloys

Abstract

There is considerable potential for the use of thin film shape memory alloys in the field of microtechnology due to their high power to volume ratio. The main obstacles for fabrication arise mainly due to the narrow regime over which shape memory behaviour is observed and the paucity of process techniques. Shape memory transition in brass only occurs in the alloy composition range 38.5 - 41.6 wt %% zinc. This study used a pyrophosphate electrolyte containing Cu2P2O7, Zn2P2O7 salts and an excess of K4P207 and KNO3, for brass deposition as a replacement for cyanide electrolytes because it is non-toxic and noncorrosive. A rotating disc electrode was employed to systematically examine polarisation data and a rotating cylinder electrode was employed to produce thin brass films and deduce the current efficiencies of copper, zinc and brass deposition with respect to deposition potential. Thin films were plated between 5- 301im, they all displayed a smooth, uniform homogenous deposit with no precipitates or oxide inclusions. The current efficiencies were found to be < 45% for copper, < 15% for zinc and between 10 - 30% for brass. The microstructural characterisation of the Cu-Zn thin alloys was undertaken by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray fluorescence (EDAX), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). XRD showed all the electrodeposited Cu-Zn alloys to have same phase composition as those predicted by the equilibrium phase diagram for Cu-Zn. This confirmed the existence of the parent p-phase within the shape memory composition range, which undergoes the martensitic transformation. TEM showed these foils to be composed of a matrix of a, p and martensite nano sized grains (< 40nm) co-existing with a sparse distribution of larger grains (200-300nm). The larger grains were always martensite in nature, recognisable by their twinning planes. Differential scanning calorimetry analysis shows evidence of a martensitic transformation change for the thin brass films.