Microstructure-processing-property relationships in high-temperature shape memory alloys

Hans Jürgen Maier, Leibniz University, Hanover, Germany

Shape memory alloys (SMAs) that can be employed at elevated temperatures are attractive for advanced applications in many industries, particularly in the automotive and energy conversion fields. The earlier SMAs relied on significant amounts of noble metals. The more recently developed ones combine large recoverable strains with reasonable price for their constituents. The major challenge for realizing the potentially very widespread use of these alloys lies in overcoming their microstructural instability, which results in functional degradation upon cyclic loading at elevated temperatures.

The present contribution will provide an overview of the recent advances made in the development of high-temperature shape memory alloys. The alloy systems Ti-Ta and Co-Ni-Ga will be addressed in detail, as these are particularly attractive in terms of reversible strain, operating temperature range, applicable processing routes and overall costs.

Data obtained under thermo-mechanical loading conditions that shed light on the intricate details of the microstructure-processing-property relationships will be presented. It will be demonstrated how the microstructure can be tailored by both specific changes of the alloy composition and heat treatments to overcome some of the current restrictions with respect to actual applications.

In fact, appropriate heat treatments can result in both martensite and austenite stabilization, which in turn affect the respective phase transformation temperatures. Moreover, a complete rejuvenation of the functionally degraded microstructures is possible. Consequently, structural fatigue triggered by oxidation-induced crack initiation currently sets the final limit regarding maximum operating temperatures.

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