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

Detalhes bibliográficos
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
Resumo: 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.
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spelling Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloyShape memory alloysPrecipitatesMagnetic characterizationHigh temperature shape memory alloysMartensitic transformationHeat treatmentsElectron microscopyMicrostructureCo– 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.A liga Co– Ni –Ga é um Heusler ferromagnético e com efeito de memória de forma a altas temperaturas, que demonstra bons efeitos de memória de forma e superelasticidade. O ajuste da histerese térmica, o intervalo de temperaturas e os efeitos de memória pode ser feito através da alteração da composição, da microestrutura, e/ou do controlo do crescimento dos precipitados pela escolha adequada dos tratamentos térmicos. É conhecido que os precipitados que se formam variam de forma, tamanho e número, com base na composição inicial da liga e dos tratamentos térmicos que são feitos. Eles podem precipitar como fase γ ou γ′. A fase γ tem uma estrutura A1, maior concentração de Co, providencia ductilidade à liga, pode precipitar na fronteira de grão ou dentro dos grãos, e chega a ter o tamanho de vários microns. A fase γ′ tem uma estrutura L12, também uma maior concentração de Co, ajuda a fortalecer a matriz, precipita dentro dos grãos, e tem um tamanho mais pequeno, da ordem dos nanometros. Por causa da maior concentração de Co estes precipitados são ferromagnéticos. Como estes precipitados tŸm um impacto significativo nas propriedades de memória de forma da liga, a sua caracterização costuma utilizar High-Resolution Scanning Electron Microscopy (HR-SEM) ou Transmission Electron Microscopy (TEM), que são técnicas intensivas em termos de recursos e trabalho de preparação. Como tal, um método mais fácil para caracterizar é desejado pela comunidade científica. Neste trabalho, essa alternativa é apresentada. Primeiro, são tiradas imagens SEM que são depois tratadas para se obter uma indicação da quantidade de precipitados que estão a ser formados. Depois, caracterização magnética é feita para se ter uma ideia do impacto dos precipitados nas temperaturas de transição de fase, na saturação magnética e na histerese. É confirmado de que um aumento de precipitados diminui as temperaturas de transição, não afeta de forma significativa os valores da saturação magnética, e apresenta um comportamento complexo nas áreas de histerese. Apesar desta informação ser relevante em saber que existem precipitados formados, não apresenta uma correlação clara da quantidade de precipitados que estão a ser formados. Por consequência, a Lei da Aproximação à Saturação, um modelo fenomenológico, é aplicado e testado, mostrando resultados promissores para uma correlação com a quantidade de precipitados, o que significa que pode ser uma alternativa viável para uma quantificação mais fácil dos que os métodos atuais.2023-07-11T14:50:57Z2022-12-13T00:00:00Z2022-12-13info:eu-repo/semantics/publishedVersioninfo:eu-repo/semantics/masterThesisapplication/pdfhttp://hdl.handle.net/10773/38532engTrigo, Hugo Guilhermeinfo:eu-repo/semantics/openAccessreponame:Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)instname:Agência para a Sociedade do Conhecimento (UMIC) - FCT - Sociedade da Informaçãoinstacron:RCAAP2024-02-22T12:14:39Zoai:ria.ua.pt:10773/38532Portal AgregadorONGhttps://www.rcaap.pt/oai/openaireopendoar:71602024-03-20T03:08:44.322317Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos) - Agência para a Sociedade do Conhecimento (UMIC) - FCT - Sociedade da Informaçãofalse
dc.title.none.fl_str_mv Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
title Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
spellingShingle Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
Trigo, Hugo Guilherme
Shape memory alloys
Precipitates
Magnetic characterization
High temperature shape memory alloys
Martensitic transformation
Heat treatments
Electron microscopy
Microstructure
title_short Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
title_full Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
title_fullStr Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
title_full_unstemmed Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
title_sort Study of cobalt-rich precipitates in Co–Ni–Ga shape memory alloy
author Trigo, Hugo Guilherme
author_facet Trigo, Hugo Guilherme
author_role author
dc.contributor.author.fl_str_mv Trigo, Hugo Guilherme
dc.subject.por.fl_str_mv Shape memory alloys
Precipitates
Magnetic characterization
High temperature shape memory alloys
Martensitic transformation
Heat treatments
Electron microscopy
Microstructure
topic Shape memory alloys
Precipitates
Magnetic characterization
High temperature shape memory alloys
Martensitic transformation
Heat treatments
Electron microscopy
Microstructure
description 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.
publishDate 2022
dc.date.none.fl_str_mv 2022-12-13T00:00:00Z
2022-12-13
2023-07-11T14:50:57Z
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