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Optimising Precipitate Shape and Habit for Strengthening in Magnesium Alloys
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
Mg-Zn-Zr (ZK) alloys are widely employed in weight critical applications due to their high specific strength properties. Zinc and zirconium are added into the alloy for the purpose of forming precipitation hardening and grain refinement, respectively. However, both elements are likely to form some intermetallic compounds and may have effect on strengthening of the alloys. The purpose of this work was to explore this interaction to understand how to optimise the composition and processing of ZK alloys. In the present study, ZK60 with composition of 6 wt.%Zn and 0.6 wt.%Zr was heat treated by solutionizing and aging at various temperatures and times. The resultant microstructure was characterised by using optical microscope, X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and the age hardening response determined by using Vicker's microhardeness testing. The results show that the interaction between zinc and zirconium causes stable compound formation, including hcp (Zn_2Zr) Laves phase and cubic Zn_2Zr phase. These compounds are distributed unevenly, causing beta'_1 strengthening precipitate to form inhomogeneously during aging. This reduces the strengthening effect of aging, e.g. low age hardening response. It has been shown that the areas containing Zn_2Zr particles are essentially free of strengthening beta'_1 precipitates, an undesirable microstructure. The aged hardening and strengthening of ZK60 alloys accounting for the interaction between Zn and Zr has been explored through a new model for ZK alloys. The simulated age hardening strengthening in ZK60 is similar to the combination of the grain size strengthening and solution strengthening. The prediction has shown that the benefit of adding zirconium, which is grain boundary strengthening improvement by refining grain size, is balanced by a depletion in age hardening response. The total strength is estimated to increase by 15% if the grain size, that is normally obtained by zirconium alloying, could be achieved by another grain refinement method without the loss of zinc solute. The experiments and the models have identified the importance of considering the interaction between Zn and Zr when optimising the processing and composition of ZK magnesium alloys. That had never been emphasised and very well presented. In particular, when ZK alloys are used in wrought applications, control of grain size by recrystallisation rather than Zr may be beneficial in enabling a greater age hardening response.