Creep resistant zinc alloys: Towards thermodynamic and mechanical stability by microalloying
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
The project has investigated the underlying phase formation and deformation mechanisms in Zn based alloys, as well as explored the possibility to overcome the drawbacks of Zn alloys related to their low room temperature ductility and low creep resistance/mechanical instability through micro alloying with Mg and Ti. The investigations of the mechanical deformation have shown that the local mechanical properties of the individual microstructural components are crucial for the properties of bulk Zn alloys. The primary Zn phase deforms predominantly by dislocation slip and {1012}[1011] mechanical twinning, while the eutectic and eutectoid structures deform mainly by grain and phase boundary sliding. Further, during the deformation of bulk ZnAl4Cu1 based alloys, mainly the primary Zn phase contributes to the bulk deformation at RT, while both, primary Zn and eutectic / eutectoid structures contribute to the global deformation at 85°C. These combination results in the observed work hardening and brittleness of bulk ZnAl4Cu1 based alloys at low temperatures and/or high strain rates, and work softening and ductile deformation of these alloys at elevated temperatures and / or low strain rates. Further, the creep behaviour of ZnAl4Cu1 based alloys was found to be controlled by dislocation movement in the primary η-Zn phase. Dilute alloying of Mg and Ti alter the microstructure of ZnAl4Cu1 based alloys and modify the morphologies of the individual microstructural components. Dilute Mg alloying causes an improvement in the strength and ductility of ZnAl4Cu1 alloys, however, it also leads to an accelerated creep rate of ZnAl4Cu1 alloys. On the other hand, dilute alloying of Ti has the same effect as Mg in terms of local mechanical properties and plasticity deformation mechanisms of the individual microstructure components. To perform reliable computational thermodynamics for understanding the microstructure formation of the Zn alloys, we first developed a consistent thermodynamic description of the Zn-Al-Cu-Mg quaternary system by combining the thermodynamic description of the Zn-Al-Cu system developed in a previous project, with the description of Zn-Al-Mg system obtained from literature, and revised description of the Zn-Cu-Mg and Al-Cu-Mg systems in the present work. The phase equilibria and thermodynamic properties of the Zn-Ti system were investigated by experiments, first principles calculations and Calphad assessment. Differential scanning calorimetry measurements and microstructure characterization confirmed that the Zn-richest eutectic reaction occurs at 691.3 ± 0.4 K with a Ti content of about 0.27 at.% in the liquid. DFT calculations of the heat capacity of the Ti-Zn intermetallic phases were applied for Calphad assessment for the first time. A comprehensive comparison of the Calphad calculated Zn-Ti phase diagram with experimental data was established. The presently developed Calphad description, improved from previous publications, produces phase diagrams which are in good agreement with experimental data, especially on the Znrich side. Thermodynamic description of the Al-Ti-Zn system was developed based on the newly developed Zn-Ti binary system in combination with descriptions of the Al-Ti and Al-Zn systems from previous publications. Finally, the Zn-Al-Cu-Mg-Ti system was constructed by implementing the Al-Ti- Zn system into the Zn-Al-Cu-Mg-Ti system. Together, the joint study of thermodynamics and deformation mechanisms has improved our understanding of the role of individual microstructural components, their properties and how they can be affected by purposeful alloying and solidification conditions. We performed computational thermodynamic analysis applying both Scheil and equilibrium simulation for low alloyed Zn-4Al-0.6CuxMg samples investigated in this project and high alloyed Zn-4Al-3Cu-xMg samples dedicated experimentally investigated in the literature. Contradictions concerning the variation of apparent fraction of primary (Zn) phase, the eutectic structure and visibly larger fractions of eutectoid structure with increasing Mg-content can be resolved by these detailed calculations. The viability of this computational thermodynamic approach is demonstrated by providing a detailed analysis going beyond the explanations and interpretations in the original publications. The coverage of a larger Zn-alloy composition range suggests a predictive capability of this approach.
Publications
- Creep behaviour of eutectic Zn-Al-Cu-Mg alloys. Materials Science and Engineering A, 724 (2018) 80-94
Z. Wu, S. Sandlöbes, Y. Wang, J. S.K.-L. Gibson, S. Korte-Kerzel
(See online at https://doi.org/10.1016/j.msea.2018.03.068) - Data on measurement of the strain partitioning in a multiphase Zn-Al eutectic alloy. Data in Brief, 20 (2018) 1639-1644
Z. Wu, S. Sandlöbes, J. Rao, J.S.K.L. Gibson, B. Berkels, S. Korte-Kerzel
(See online at https://doi.org/10.1016/j.dib.2018.09.010) - Local mechanical properties and plasticity mechanisms in a Zn-Al-Cu-Mg alloy. Materials and Design, 157 (2018) 337–350
Z. Wu, S. Sandlöbes, J. Rao, J.S.K.L. Gibson, B. Berkels, S. Korte-Kerzel
(See online at https://doi.org/10.1016/j.matdes.2018.07.051) - Phase equilibria of the Zn-Ti system: Experiments, first-principles calculations and Calphad assessment, Calphad, 64 (2019) 213-224
S.-M. Liang, H.K. Singh, H. Zhang, R. Schmid-Fetzer
(See online at https://doi.org/10.1016/j.calphad.2018.12.009)
