Caracterització termomecànica d'actuadors amb memòria de forma NiTiCu

• Autores: Albert Fabregat Sanjuan
• Directores de la Tesis: Silvia de la Flor López (dir. tes.), Frederic Marimón Carvajal (dir. tes.), Francesc Ferrando Piera (dir. tes.)
• Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2015
• Idioma: catalán
• Tribunal Calificador de la Tesis: Francesc Roure Fernández (presid.), Cristina Victoria Urbina Pons (secret.), Xavier Fernández Francos (voc.)
• Programa de doctorado: Programa Oficial de Doctorado en Análisis Estructural
• Materias:
  • Física
    • Mecánica
      • Medida de propiedades mecánicas
    • Física del estado sólido
      • Aleaciones
      • Estructura cristalina
  • Ciencias tecnológicas
    • Tecnología de materiales
      • Ensayo de materiales
• Enlaces
  • Tesis en acceso abierto en: TDX
• Resumen

  • In this thesis the thermomechanical characterization of the shape memory alloy NiTiCu working as an actuator with different heat treatment (HT) temperatures has been carried out. The results from the tests carried on resistivity changes and differential scanning calorimetry (DSC) showed the dependence of the transformation temperatures with the heat treatment temperature. There have not been identified big differences between different HT temperatures in mechanical behaviour (tensile tests until break and microhardness measurements) at martensitic state on the working range (stress/strain) of most applications. However, there has been noticed a loss of mechanical properties for high stress/strain tests values from a heat treatment temperature above 550ºC. Dynamomechanical tests (DMA) allowed the study of the martensitic transformation evolution and its relation to microstructural changes. The results of the internal friction (IF or tan d) showed correlation with the results of DSC, showing the same trend in function of the HT temperature. The characterization of the microstructure has allowed the correlation of the results obtained in the previous sections with microstructural changes. The identification of particles that were not part of the matrix (carbides, oxides and precipitates) through the scanning electron microscopy (SEM) lead to a particle quantification and distribution analysis. A list of potential precipitates present was carried out through the energy-dispersive X-ray spectroscopy (EDS) technique. The results of synchrotron X-ray diffraction (SXRD) confirmed the hypotheses done about the microstrucutural changes produced at different HT temperatures and show clear evidence of the presence of precipitates Ti2Ni. At the same time, a quantitative analysis of the texture, microstrain and presence of phase B2, B19' and Ti2Ni for all HT temperatures was also carried out analyzing the diffractograms from SXRD. The characterization of the termomechanical behaviour at different temperatures was studied through stress-strain tests at different temperatures for the whole range of working temperatures. After that, a comparative study of the main parameters for the different HT temperatures was carried out. The study of the tension/compression stress-strain asymmetry through the elastic and transformation zone was carried out on tubes with stacked rosette strain gauges. At the same time, the strain ratio (relationship between transverse and axial strains) evolution was studied. In order to study the thermomechanical cycling stability (to simulate the most common actuators working conditions) different tests conditions were applied: constant tension tests with thermal cycling at different stress levels (CS); and isothermal tests (below Mf) at a constant strain level and subsequent thermal recovery after each cycle without load (IT). Hence, constant stress with thermal cycling performed superior results for actuation purposes with high recovery strains and stabilization. Nevertheless, isothermal tests at a certain stress level showed degradation that lead to an unstable behaviour that was not reliable for actuator purposes. The results from CS tests showed that stress-temperature behaviour presented different tendencies in function of the stress level and there were significant differences between the behaviour of the initial cycle and the stabilized one at high stress levels (from 30 MPa). The microstructural changes due to thermomechanical cycling have been analyzed through microhardness increase measurements and SXRD. Hardness tests showed correlation between hardness increase and thermomechanical cycling. SXRD analysis also showed correlation between the quantity of retained martensite (B19 (orthorhombic) and B19' (monoclinic)) at 120ºC and different thermomechanical cycling conditions.