TY - THES

T1 - Development and characterisation of an ultrafine-grained Al-Mg-Si alloy for space applications

AU - Gonzaga Hernandez, Sandra

N1 - no embargo

PY - 2024

Y1 - 2024

N2 - This work explores alloys in the Al-Mg-Si system for application in space. To be considered suitable for space applications, an alloy must meet requirements that include, not exhaustively, high strength-to-weight ratio, high thermal performance, optimal radiation shielding and resistance. Due to its lightweight and high-strength, aluminium and its alloys are considered suitable options. The ability to achieve increased strength via age-hardening, sets aluminium alloys as strategic materials for future space programs. Age-hardening promote precipitation that prevents dislocation motion, thus increasing strength. However, when subjected to space, energetic particle irradiation from the Sun can cause degradation of aluminium alloys as it induces both dissolution of precipitates and formation of extensive networks of dislocation loops. To mitigate the deleterious impact of energetic particle irradiation, one solution has been the development of new ultrafine-grained (UFG) aluminium alloys, where the large amount of grain-boundaries promote the fast-annihilation of radiation damage. Nevertheless, developing an UFG aluminium alloy with high thermal stability against recrystallisation has been a major challenge. For these reasons, this work addresses on the synthesis of an UFG AA6061 alloy using a severe plastic deformation (SPD) technique known as high-pressure torsion (HPT). After demonstrating the feasibility of synthesis, the research investigates the precipitation behaviour in the UFG AA6061 alloy compared with its coarse-grained (CG) counterpart through a combination techniques such as differential scanning calorimetry (DSC) and scanning transmission electron microscopy (S/TEM). Thermal stability of the UFG AA6061 alloy was also investigated via in-situ TEM experiments. The findings indicate that the phenomenon of precipitation hardening is highly dependent on the average grain size of the two CG and UFG AA6061 alloys. The phenomenon of precipitation was also demonstrated to be accelerated in the UFG alloy and this was reflected by the fact that intermediate metastable phases such as the beta'' were not observed in the UFG AA6061 alloy, and that the equilibrium phase beta, rapidly formed at lower temperatures. It was found via analytical S/TEM mapping techniques that the UFG AA6061 alloy presents limited precipitation ability taking place mainly at intra-granular positions and with low volumetric densities when compared with the CG AA6061 alloy. These characteristics were responsible for recrystallization of UFG microstructure at around of 180°C when tested via in-situ heating. This thesis highlights the potential of UFG aluminium alloys for application in sapce, but also underlines the difference between the UFG AA6061 alloy and the UFG crossover aluminium alloys, the AlMgZnCuAg, which due to its high volume density of T-phase precipitates and low (highly negative) enthalpy of formation for precipitation (compared to beta precipitates in AA6061 alloys) restricts grain boundary movement and delays recrystallisation via intra-granular precipitation and growth.

AB - This work explores alloys in the Al-Mg-Si system for application in space. To be considered suitable for space applications, an alloy must meet requirements that include, not exhaustively, high strength-to-weight ratio, high thermal performance, optimal radiation shielding and resistance. Due to its lightweight and high-strength, aluminium and its alloys are considered suitable options. The ability to achieve increased strength via age-hardening, sets aluminium alloys as strategic materials for future space programs. Age-hardening promote precipitation that prevents dislocation motion, thus increasing strength. However, when subjected to space, energetic particle irradiation from the Sun can cause degradation of aluminium alloys as it induces both dissolution of precipitates and formation of extensive networks of dislocation loops. To mitigate the deleterious impact of energetic particle irradiation, one solution has been the development of new ultrafine-grained (UFG) aluminium alloys, where the large amount of grain-boundaries promote the fast-annihilation of radiation damage. Nevertheless, developing an UFG aluminium alloy with high thermal stability against recrystallisation has been a major challenge. For these reasons, this work addresses on the synthesis of an UFG AA6061 alloy using a severe plastic deformation (SPD) technique known as high-pressure torsion (HPT). After demonstrating the feasibility of synthesis, the research investigates the precipitation behaviour in the UFG AA6061 alloy compared with its coarse-grained (CG) counterpart through a combination techniques such as differential scanning calorimetry (DSC) and scanning transmission electron microscopy (S/TEM). Thermal stability of the UFG AA6061 alloy was also investigated via in-situ TEM experiments. The findings indicate that the phenomenon of precipitation hardening is highly dependent on the average grain size of the two CG and UFG AA6061 alloys. The phenomenon of precipitation was also demonstrated to be accelerated in the UFG alloy and this was reflected by the fact that intermediate metastable phases such as the beta'' were not observed in the UFG AA6061 alloy, and that the equilibrium phase beta, rapidly formed at lower temperatures. It was found via analytical S/TEM mapping techniques that the UFG AA6061 alloy presents limited precipitation ability taking place mainly at intra-granular positions and with low volumetric densities when compared with the CG AA6061 alloy. These characteristics were responsible for recrystallization of UFG microstructure at around of 180°C when tested via in-situ heating. This thesis highlights the potential of UFG aluminium alloys for application in sapce, but also underlines the difference between the UFG AA6061 alloy and the UFG crossover aluminium alloys, the AlMgZnCuAg, which due to its high volume density of T-phase precipitates and low (highly negative) enthalpy of formation for precipitation (compared to beta precipitates in AA6061 alloys) restricts grain boundary movement and delays recrystallisation via intra-granular precipitation and growth.

KW - Aluminium

KW - Al-Mg-Si-Legierungen

KW - Raumfahrtanwendungen

KW - Aluminium

KW - Al-Mg-Si alloys

KW - space applications

U2 - 10.34901/mul.pub.2024.240

DO - 10.34901/mul.pub.2024.240

M3 - Master's Thesis

ER -