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