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

doi:10.1016/j.jmatprotec.2005.02.196

Journal of Materials Processing Technology

Volumes 162–163, 15 May 2005, Pages 220-229

https://doi.org/10.1016/j.jmatprotec.2005.02.196 Get rights and content

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 10³ 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.

Section snippets

Shape memory alloys (SMAs)

The technical importance of most engineering materials is based on their mechanical, electrical or magnetic properties, which should, normally, be as independent as possible from environmental influences. Besides these conventional materials, there is another group, known as functional materials. Functional materials are not interesting so much for their properties under certain conditions, but much more for how they react on changes of these conditions. Among others, shape memory materials

Cu–Al–Ni shape memory alloys

Among the Cu-based SMAs, the most frequently applied are Cu–Zn–Al and Cu–Al–Ni alloys. The latter are indeed more expensive than the former; but among the Cu-based SMAs, they are the most resistant to degradation of functional properties due to undesired aging effects.

Many properties of Cu–Al–Ni alloys are inferior to that of Ni–Ti or Fe-based SMAs. The most important disadvantage of polycrystalline Cu–Al–Ni alloys is the small reversible deformation (one-way shape memory effect: up to 4%;

Rapid solidification

Rapid solidification means cooling rates 10³ K/s or higher. At high cooling rates, during solidification and further cooling, there is only very short time available for diffusion processes and approaching the thermodynamic equilibrium. Therefore, rapid solidification may lead to extremely fine microstructures, higher mutual solubility (and supersaturations) in the solid state, less segregations and better homogeneity, less or no secondary phases, extremely fine dispersion of eventual present

Experimental

The primary goal of our experimental work was to obtain ductile rapidly solidified ribbons, on which mechanical testing would be easy to perform. So, ribbons as wide as possible were desired. An additional goal was to test, if the ductility can be improved with boron, without obstructing the formation of the single layer microstructure.

Cu–Al–Ni alloys of four different chemical compositions were melt-spun. All four alloys were prepared from industrial pure metals in a vacuum induction furnace.

Results and discussion

After melt spinning, the obtained ribbons were clumped together, but they could be untwisted. Their width is not uniform, even more non-uniform is the thickness. The width and the thickness of obtained ribbons are given in Table 2.

The most likely reason for non-uniform width and thickness was instability and braking of the melt jet, caused by high surface tension and low viscosity of the melt, geometric imperfection of the nozzle, not absolutely constant melt flow, vibrations and gas flow on

Conclusions

The described experiments once more proved that melt spinning is an adequate method to produce Cu–Al–Ni ribbons directly from the melt. The ribbons of appropriate chemical composition are already in as-cast condition predominantly martensitic and exhibit shape memory effect, even if spun with low wheel speeds. Because of the low thermal conductivity of the Cu–Al–Ni alloys, with a free jet melt spinner it is very hard to produce wide ribbons with fully martensitic single layer columnar structure

Acknowledgement

This research was supported by “Österreichischer Fonds zur Förderung der wissenschaftlichen Forschung” (FWF) under Project no. P14221.

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

Abstract

Section snippets

Shape memory alloys (SMAs)

Cu–Al–Ni shape memory alloys

Rapid solidification

Experimental

Results and discussion

Conclusions

Acknowledgement

Acta Mater.

Acta Mater.

Mat. Sci. Eng.

Shape Memory Alloys

Shape memory materials

Ingenieur-Werkstoffe 1