Studies on dynamic behaviours of magnetic shape memory single crystal NiMnGa alloys for actuator applications

Omotosho, Emmanuel Olubambi (2023) Studies on dynamic behaviours of magnetic shape memory single crystal NiMnGa alloys for actuator applications. Doctoral thesis, Northumbria University.

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Abstract

Single crystal NiMnGa alloys present potential applications as sensors and actuators due to their high-frequency response and significant reversible strain. The aim of this dissertation is to study the thermo-magneto-mechanical behaviours of single crystal NiMnGa alloys. In this regard, both experimental and theoretical analysis are performed.

Firstly, the effects of thermo-magnetic loading conditions on the high-frequency, long-term (>100 s, achieving steady state), dynamic behaviour of single crystal NiMnGa alloys that are driven by cyclic magnetic fields are studied. A dynamic model that incorporates both magnetic-field-induced martensite reorientation and temperature-driven martensitic phase transformation simulates the material's dynamic behaviour at various degrees of ambient heat-transfer, ambient temperature, applied magnetic field frequency, and amplitude. An analytical expression of the material's long-term steady-state behaviour (i.e., stable strain amplitude and temperature) as a function of the thermal environment and the magnetic loading circumstances is further developed from the dynamic model. Weak ambient heat transfer, high ambient temperature, and high magnetic field amplitude are found to lead to large net heat generation from dissipative martensite reorientation and thus increase the material's stable temperature, which reduces twinning stress and increases stable strain amplitude.

Secondly, the experimental part presents the temperature-induced phase transformations of single crystal NiMnGa alloys. Two sets of experiments are carried out. At the start of both experiments, the sample is in the state of single martensite variant. For Experiment 1, the sample is heated and when the martensite-to-austenite transformation takes place by the propagation of an austenite-martensite interface (A-M interface), the heat supply is turned off, and the sample temperature drops below the austenite start temperature As. It is observed that the A-M interface continues propagating even below As. Experiment 2 involves cooling of the sample from above the austenite finish temperature Af when the austenite phase has been fully formed in the sample. During cooling when the austenite-to-martensite transformation starts to propagate below the martensite start temperature Ms, the sample is supplied with heat. It is observed that the sample temperature rises above the Ms temperature before the A-M interface stops. With these tests, two new critical temperatures are formulated: (1) the austenite stop temperature Astop from Experiment 1, at which the martensite-to-austenite transformation stops after the heat source is switched off, and (2) the martensite stop temperature Mstop from Experiment 2, at which the austenite-to-martensite transformation stops after the heat supply is turned on. It is found that Astop is below the As temperature and Mstop is above the Mstemperature. This new phenomenon happens to be in analogy with the stress-induced phase transformation of shape memory alloys (super-elasticity).

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