A new shape memory alloy with up to now unprecedented functional stability was developed by researchers from the Institute for Materials at the Ruhr-Universität Bochum in cooperation with researchers from the USA and Japan. Based on a theoretical prediction, they used combinatorial materials science methods, i.e. so-called materials libraries, for a targeted search of optimized alloy compositions. The result consists of four components: titanium, nickel, copper and palladium. From the new material, the researchers expect a stable shape memory effect and improved lifetime, e.g. for applications in medical devices such as stents.
The scientists report their results in the noted journal Advanced Functional Materials, which selected their contribution as cover story.
Shape memory alloys
Shape memory alloys (SMAs) are materials that after being deformed mechanically can return to their original shape upon heating (shape memory effect) and/or allow for "elastic" strains up to 10 % (superelasticity). Those remarkable effects are based on a reversible martensitic phase transformation: a change in the crystal lattice as a function of temperature or stress. However, such changes do not leave the material untouched. Defects are formed during cyclic deformations, which accumulate and lead to decreasing shape memory properties. "The defects originate from the interface between the high-temperature phase (austenite) and the low-temperature phase (martensite) as a result of the crystallographic incompatibility," explains Robert Zarnetta from the Materials Research Department at the RUB.
Four matching partners
Theoretical calculations from the co-workers in the USA predicted that the incompatibility can vanish for alloys with special lattice parameters, such that the high-temperature and the low-temperature phase are compatible. As optimal partners for such an alloy, titanium, nickel, copper and palladium were identified by theory. The successful experimental "matchmaking" was realized by using thin film materials libraries, which enabled the screening of a large portion of the four component (quaternary) composition space using dedicated high-throughput characterization tools. "To find or optimize the special composition in the quaternary alloy system using conventional methods would have been extremely challenging," explains Prof. Dr. Alfred Ludwig (Chair Materials for Microtechnology) and thus highlights the advantage of the combinatorial materials science approach.
Compatible crystal lattices promote stability
Next to the discovery of the special alloy composition, the scientists also determined the underlying composition-structure-property relationship, which was subsequently used to successfully transfer the thin film results to bulk material. Thus, the fundamental relation between the crystal structure of a shape memory alloy and its functional stability could be proven for the first time. "An improved compatibility of the high- and low-temperature crystal lattice results in improved functional stability" summarized Robert Zarnetta , going on to explain "that this relation could only be discovered by bridging the fields of combinatorial SMA thin film and the conventional bulk materials development."
Collaborative Research Center and Research Department
The results were conducted, based on the work within the collaborative research center "SFB 459," at the Chairs "Materials for Microtechnology" (Prof. Dr.-Ing. Alfred Ludwig, Institute for Materials) and "Materials Science and Engineering" (Prof. Dr.-Ing. Gunther Eggeler, Institute for Materials) and in cooperation with the Materials Research Department at the RUB.
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