Mechanical behaviour and deformation mechanisms of Zn-Al-Cu-Mg alloys

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (xii, 129 Seiten) : Illustrationen (2018). = Dissertation, RWTH Aachen University, 2018
Zn-Al based alloys are widely used as structural and decorative parts as well as machinery and equipment with a complex geometry or equipment needing a high manufacturing precision, particular in the die casting industry. However, Zn-Al based alloys suffer from low creep resistance and long-term mechanical instability. This thesis therefore aims to investigate the underlying physical mechanisms, as well as to explore possible methods to overcome these drawbacks. To this end, three eutectic ZnAl4Cu1 alloys with different Mg contents (0.04 wt.%, 0.21 wt.% and 0.31 wt.%) were comprehensively investigated from the macroscopic scale to the microscopic scale in terms of their mechanical properties, microstructures and deformation mechanisms using macroscopic ex-situ / in-situ tensile tests and micromechanical test in conjunction with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and atom probe tomography (APT). Chapter 4 introduces the macro mechanical response and mechanisms of the three eutectic ZnAl4Cu1 alloys investigated. Dilute Mg alloying caused an improvement of the yield strength and ductility of ZnAl4Cu1 base alloys. Uniaxial tensile tests revealed two distinct deformation regimes of the alloys: (i) work hardening and brittle fracture at low temperatures and / or high strain rates with basal slip and {10-12}[10-1-1] twinning as predominant deformation mechanisms of the primary η-Zn phase; (ii) work softening and ductile failure at elevated temperatures and / or low strain rates where the predominant deformation mechanisms are deformation twinning, dislocation motion in the primary η-Zn phase and grain / phase boundary sliding in the eutectic and eutectoid structures. The creep behaviour of the bulk ZnAl4Cu1 alloys as well as the local creep behaviour of the individual microstructural constituents are presented in chapter 5. Tensile creep tests in the temperature range of 25 to 105°C and stress range of 61 to 130 MPa revealed stress exponents of 6.9 – 8.0 and creep activation energies of 93 – 104 kJ/mol in the bulk ZnAl4Cu1 alloys, suggesting that the creep behaviour of these alloys is controlled by dislocation movement. Primary η-Zn phase showed the highest creep resistance, and η-Zn + α-Al eutectoid structures containing Mg2Zn11 precipitates showed the lowest creep resistance of the microstructural constituents during nanoindentation creep tests. Moreover, dilute Mg alloying caused an accelerated creep rate of ZnAl4Cu1 alloys due to increased grain / phase boundary activities and reduced geometrical constraints of eutectic and eutectoid colonies.Furthermore, the local mechanical properties and deformation mechanisms of the individual microstructural constituents in alloy ZnAl4Cu1Mg0.31 were systematically studied using nanoindentation tests at room temperature (25°C) and 85°C, presented in chapter 6. Estimation of the strain rate sensitivities and activation volumes from nanoindentation strain rate jump tests suggested predominant deformation by dislocation-mediated mechanisms in the primary η-Zn phase and by grain / phase boundary sliding in the eutectoid structures. This was later verified using SEM-EBSD and AFM. Chapter 6 further provides information on the role of individual microstructural constituents in alloy ZnAl4Cu1Mg0.31 during bulk deformation obtained using quasi in-situ micro digital image correlation (µ-DIC) in the SEM during tensile deformation at 85°C. µ-DIC showed that eutectic / eutectoid colonies carried higher strain than the primary η-Zn phase grains, and confirmed strain transfer across Zn-Al phase boundaries and eutectic / eutectoid colony boundaries. Chapter 7 is focussing on the understanding of the local precipitation behaviour in alloy ZnAl4Cu1Mg0.31 and its influence on the mechanical properties. The precipitation and decomposition phenomena in alloy ZnAl4Cu1Mg0.31 were investigated using TEM and APT. A non-equilibrium ZnxAl1-x (x≥0.7) transition phase, which dissolved during deformation at 85°C, was identified and structurally as well as chemically characterised. The partial dissolution of these precipitates was proposed to contribute to the lack of long-term mechanical stability of Zn-Al alloys that currently poses one of the major drawbacks to their application.
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