Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy

• Autor(a) principal: Trigo, Hugo Guilherme
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/38532
• topic: Shape memory alloys; Precipitates; Magnetic characterization; High temperature shape memory alloys; Martensitic transformation; Heat treatments; Electron microscopy; Microstructure

Co– Ni –Ga alloy is a Heusler type ferromagnetic material and High Temperature Shape Memory Alloy that demonstrates good shape memory and superelasticity effects, even at high temperatures. The fine tuning of thermal hysteresis, temperature range and shape memory effects can be done by adjusting composition, changing the microstructure, and/or controlling the growth of precipitates with adequate heat treatments. The precipitates that are formed are known to heavily vary on shape, size and number, based on the composition of the initial alloy and the heat treatment that is done. They can precipitate as γ or γ′ phase. The γ phase has a A1 structure, higher Co content, provides ductility to the alloy, it can precipitate in the grain boundary and inside the grains, and can be as big as few microns. The γ ′ phase has a L12 structure, also a higher Co content, helps strengthen the matrix, it precipitates inside the grains, and has a smaller, nanometric, size than the γ phase. Due to their high Co content, they are ferromagnetic. As these precipitates have a significant impact on the shape memory properties of the alloy, their characterization usually requires High-Resolution Scanning Electron Microscopy (HR-SEM) or Transmission Electron Microscopy (TEM), which is resource and labour intensive. Therefore, an easier method to characterize is sought after by the scientific community. In this work, such alternative is presented. First, SEM images are taken and treated to have an indication of the amount of precipitates that are forming for diferent heat treatments. Then, magnetic characterization is used to have an idea of the impact of the precipitates on the phase transition temperatures, the saturation magnetizations and the hysteresis. It is confirmed that an increase in precipitates lowers the phase transitions temperatures, it doesn’t negligibly affect the saturation magnetization values, and it presents a complex behaviour in terms of hysteresis areas. Although this information is useful for knowing the presence of precipitates and the overall magnetic characteristic of the material, it does not present a clear correlation to the amount of precipitates that are being formed. As a result, the Law of Approach to Saturation, a phenomenological model, is applied and tested, showing promising results for the correlation to the amount of precipitates, which means that it could be a viable option for an easier quantification to the current methods.