Microstructure of rapidly solidified Cu–Al–Ni shape memory alloy ribbons

Abstract

Cu–Al–Ni shape memory alloys (SMAs) are currently the only available high temperature SMAs, showing a good resistance against functional fatigue. In polycrystalline state, they are very brittle and exhibit, in general, only small reversible deformations. By melt spinning, thin Cu–Al–Ni ribbons can be manufactured directly from the melt. Appropriate casting parameters can ensure a single layer columnar structure with a fibre texture, which significantly increases the maximal reversible strain in longitudinal direction. Cu–Al–Ni ribbons, containing 13, 14 and 15 wt.% Al were cast by free jet melt spinning. Because of the alloys’ low thermal conductivity, the cooling rate was surprisingly low – considering the crystal grain size – significantly below 10 3 K/s. Therefore, wide ribbons having a single layer columnar and (except the ribbons containing 13 wt.% Al) completely martensitic structure could not be obtained. Regardless the chemical composition, the ribbons have a single layer columnar structure only if the thickness does not exceed approximately 50 μm, otherwise the structure consists of at least two layers of equiaxed grains. In as-cast condition, only ribbons containing 13 wt.% Al seem to be completely martensitic. Heat treatments at temperatures up to 900 °C improved the structure of 13 and 14 wt.% Al ribbons. All ribbons exhibit one-way shape memory effect in as-cast condition. Heat-treated ribbons containing 13 wt.% Al exhibited two-way shape memory effect already after one bending and heating cycle.