Microstructural characterization & viscoelastic properties of AlZnMg & AlCuMg alloys
- Autores: José Ignacio Rojas Gregorio
- Directores de la Tesis: Daniel Crespo Artiaga (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2012
- Idioma: inglés
- Tribunal Calificador de la Tesis: Juan José Suñol Martínez (presid.), Eloi Pineda Soler (secret.), Marina Lorena Galano (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
The comprehension of the viscoelastic behaviour of metals is of high interest as these materials are subjected to dynamic loads in most of their structural applications, and also because it enables a deeper understanding of several technologically essential properties, like mechanical damping and yielding. Thus, research on this field is needed not only because it may lead to new potential applications of metals, but also because predictability of the fatigue response may be greatly enhanced. Indeed, fatigue is the consequence of microstructural effects induced in a material under dynamic loading, while the viscoelastic behaviour is also intimately linked to the microstructure. Accordingly, the characterization of the viscoelastic response of a material offers an alternative method for analysing its microstructure and ultimately its fatigue behaviour. This research is aimed at the identification, characterization and modelling of the effects of temperature, excitation frequency and microstructure/phase transformations (when present) on the viscoelastic behaviour of aluminium alloys AA 7075-T6 and AA 2024-T3, and of pure aluminium in the H24 temper. The identification of the mechanical relaxation processes taking place and the relation between the viscoelastic response of AA 7075-T6 and AA 2024-T3 and the fatigue behaviour will be attempted for all these materials. Finally, we intend to investigate possible influences of the dynamic loading frequency on fatigue, and especially the existence of a threshold frequency marking the transition from a static-like response of the material to the advent of fatigue problems. AA 7075-T6 and AA 2024-T3 were selected for this study because these alloys are key representatives of their important families and are highly suitable to a number of industrial applications in the aerospace sector and transport industry. Pure aluminium was selected because of the inherent interest of this metal, for comparison purposes and for discussing the phenomena observed for the alloys. To accomplish the objectives, the viscoelastic response of the materials was measured experimentally with a Dynamic- Mechanical Analyser (DMA). The results were combined with Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC). An analytical model was proposed which fits the storage modulus up to 300 ºC. The model takes into account the effect of temperature, the excitation frequency and the concentration of some precipitates for the alloys. This allows us to test models proposed for the reaction rates of the associated microstructural transformations, to determine their kinetic parameters and to characterize their influence on the viscoelastic behaviour, showing that the DMA is a good tool for studying the material microstructure, phase transformation kinetics and the influence of transformations on the viscoelastic properties of materials. The Time-Temperature Superposition (TTS) principle has been successfully applied to the DMA data, providing master curves for the storage and loss moduli. Also, it is proposed that the decrease of yield and fatigue strength with temperature observed in some aluminium alloys may be due to the internal friction increase with temperature. Finally, the existence of a threshold frequency is suggested, below which materials subjected to dynamic loading exhibit a static-like, elastic response, such that creep mechanisms dominate and deterioration due to fatigue may be neglected. A procedure to estimate this transition frequency is proposed.
