The Role of Supercompatibility on the Fatigue of Shape Memory Alloys

• Applicant: Professor Dr.-Ing. Eckhard Quandt
• Subject Area: Synthesis and Properties of Functional Materials
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413288478

Shape memory alloys (SMAs) show two distinct properties that are attractive for many applications. First, the shape memory effect is the basis for many solid-state actuators, which relies on a reversible, thermally induced, high energy density phase transformation between austenite and martensite. Second, they exhibit superelasticity used for example in self-expanding medical implants or in elastocaloric cooling, which is based on a reversible, stress-induced austenite-martensite phase transformation. These first order phase transformations with large eigenstrains in both cases result in large work output and large enthalpy changes. Their reversibility is aided by the compatibility of the two phases. Essential for the implementation of SMAs in new devices is their fatigue characteristics, especially for high-cycle applications. In general, fatigue concerns two aspects: functional fatigue, which describes the cycle-dependent changes of their functional properties and structural fatigue, which refers to the integrity of the material. Commonly, both types of fatigue are closely interconnected.In our previous work we have laid out what we believe are the most important factors governing functional and structural fatigue in stress-induced phase transformations (superelasticity) in shape memory alloys: Crystallographic compatibility, grain size (that was well known) and precipitation. We have shown that, if compositions are tuned so that all three of these factors are fulfilled, alloys with unprecedented resistance to functional fatigue are possible. However, our results have revealed that one or more of these factors can be compromised a little and still remarkable functional fatigue is achieved. This is a very important finding as the perfect crystallographic compatibility – the so-called supercompatibility -- is a very restrictive condition which was up to now only found to be almost ideally fulfilled in only two specific alloys. For example, alloys satisfying supercompatibility to high accuracy have exceptional resistance to functional fatigue, but so do alloys that satisfy these compatibility conditions only approximately, but that have small grain size and a favorable array of fine coherent precipitates. Establishing this as a general finding would significantly increase the possibility to identify ultra-low fatigue compositions thus offering a wider selection to meet other criteria as e.g. transformation temperatures and strain or biocompatibility.Thus, our main objectives are: to verify the influence of the different factors, to determine the required accuracy in satisfying this supercompatibility and thus to derive directions for a future search of ultra-low fatigue SMAs. These objectives will be applied here in the context of metallic SMAs, although it is expected that they should be applicable to broad classes of solid-solid phase transformations.

• DFG Programme: Research Grants