Reduced functional degradation of a magnetic shape memory alloy by aging under stress

• Applicant: Professor Dr.-Ing. Hans Jürgen Maier
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259317613

The objective of the research project proposed in cooperation with the Siberian Physical Technical Institute at Tomsk State University is a comprehensive analysis of the functional degradation in a CoNiGa magnetic shape memory alloy. Previous studies on a conventional NiTi shape memory alloy showed that aging of NiTi single crystals under stress can be used to obtain a microstructure that features only certain precipitate variants. This in turn resulted in a substantial improvement in functional fatigue behaviour. Preliminary work already demonstrated that such a tailoring of the microstructure is also feasible in CoNiGa single crystals. This material, however, can also show magnetic field-induced strains, and thus, a drastically higher power density could be realized than possible in conventional NiTi. With respect to the envisaged applications, cyclic stability of the microstructure is the key issue, which has, however, not been studied yet. The hypothesis of the proposed project is that - similar to the conventional NiTi system - a microstructure that features only certain types of precipitate variants, should demonstrate significantly improved functional degradation resistance. In the proposed research, the effect of precipitate variants on both the conventional as well as the magnetic field-induced shape memory effect will be studied. Firstly, the Russian project-partner will analyse the evolution of the precipitate variants that form during stress-assisted aging by transmission electron microscopy. Subsequently, the influence the various microstructures have on the stability of the conventional shape memory effect will be analysed. The German team will use high currents with short pulses to generate high magnetic field strengths (up to 40 kOe). This will allow for a systematic study of the effects that the microstructures with different types of precipitate variants have on the magnetic field-induced shape memory effect. The motivation here is to understand the microstructural conditions that provide for maximum reversible field-induced strains over many cycles. Therefore, the microstructural evolution will be studied in samples that feature different degrees of functional degradation in order to uncover the relevant damage mechanisms. This will also provide for data that can be used later on to develop a validated model that allows for life prediction under conditions relevant for actual service.

• DFG Programme: Research Grants
• International Connection: Russia
• Participating Person: Dr. Irina Kireeva