Abstract The low density of magnesium alloys makes them attractive for light weight constructions. However, important aspects are the rather poor mechanical properties of the most commonly used Mg alloy AZ91 (approximately 9 wt.-% Al, 1 wt.-% Zn), which limits its use in, e. g., power train applications. Alloying AZ91 with Ca leads to a significant improvement of creep resistance. To investigate the influence of the Ca addition on the fatigue behavior, symmetric push pull tests have been performed on AZ91 alloys with an addition of 0, 1 and 3 wt.-% of Ca, respectively. At relatively high amplitudes, the Ca containing alloys show a decrease in fatigue life compared to Ca-free AZ91. However, with a decrease in amplitude, the fatigue life of the alloy with 3 wt.-% Ca are comparable with the Ca-free AZ91 alloy and with a further decrease in amplitude the alloy with an addition of 3 wt.-% Ca has an even higher fatigue life than the Ca-free alloy. All alloys contain pores due to the casting process.
The mechanical properties of ultrafine-grained metals processed by equal channel angular pressing is investigated by nanoindentations in comparison with measurements on nanocrystalline nickel with a grain size between 20 and 400 nm produced by pulsed electrodeposition. Besides hardness and Young’s modulus measurements, the nanoindentation method allows also controlled experiments on the strain rate sensitivity, which are discussed in detail in this paper. Nanoindentation measurements can be performed at indentation strain rates between 10-3 s-1 and 0.1 s-1. Nanocrystalline and ultrafine-grained fcc metals as Al and Ni show a significant strain rate sensitivity at room temperature in comparison with conventional grain sized materials. In ultrafine-grained bcc Fe the strain rate sensitivity does not change significantly after severe plastic deformation. Inelastic effects are found during repeated unloading-loading experiments in nanoindentations.
The low density of magnesium alloys makes them attractive for lightweight constructions. However, creep remains an important limitation of Mg alloys. To gain a more detailed understanding of the correlation between microstructure and creep properties in Mg alloys, creep tests have been performed on MRI 230D samples featuring various microstructures. For this purpose, the MRI 230D Mg alloy has been thixomolded into a plate with four steps of different height, which gives different microstructures in each step due to different cooling rates. With an increase in cooling rate (e.g., a decrease in step height) the interconnectivity of the eutectic phase increases at virtually constant volume fraction. The creep strength is found to decrease with decreasing interconnectivity of the eutectic phase. This implies that a eutectic phase morphology, which is highly interconnected, benefits the creep properties and should therefore be one goal in further developments for creep resistant Mg alloys.
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