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The electrochemical corrosion mechanisms of Mg alloys were extensively studied in previous investigations of different chemical compositions, modified surface states and various electrolyte conditions. However, recent research focused on the active state of Mg dissolution, leading to unresolved effects of secondary phases adjacent to a stable α-solid solution passive layer. The present study investigates the fundamental electrochemical corrosion mechanisms of three different Laves phases with varying phase morphologies and phase fractions in the passive state of Mg-Al-Ca alloys. The microstructure was characterized by (transmission-) electron microscopy and synchrotron-based transmission X-ray microscopy. The electrochemical corrosion resistance was determined with a standard three-electrode setup and advanced in-situ flow cell measurements. A new electrochemical activity sequence (C15>C36>α-Mg>C14) was obtained, as a result of a stable passive layer formation on the α-solid solution. Furthermore, nm-scale Mg-rich precipitates were identified within the Laves phases, which tend to inhibit the corrosion kinetics.

The effect of varying Al concentrations on the electrochemical corrosion resistance of binary Mg-Al solid solutions thin films under alkaline immersion conditions was investigated via a combination of in-situ flow-cell, scanning vibrating electrode technique and microscopy analysis. These spatially resolving characterization techniques are employed along the Al concentration gradient of the combinatorically grown thin films enabling efficient screening of the Al concentration dependent electrochemical corrosion behaviour. The analysis revealed an increasing corrosion resistance with increasing Al concentration, as a consequence of Al induced hydroxide reinforcement. Specifically, the addition of >4 wt.% Al decreases the corrosion current density in the range of 70–90 % compared to pure Mg.

The electrochemical corrosion rate of magnesium depends on the stability of the surface layer formed. Based on the Mg substrate, the oxide structure comprises a dense MgO/Mg(OH)2 mixture underneath a porous plate-like Mg(OH)2 layer. While the kinetics of the anodic partial reaction have been mainly attributed to MgO, recent studies have showed an effect of the Mg(OH)2 layer thickness on the cathodic partial reaction. A thinner Mg(OH)2 layer has been associated with higher kinetics of the oxygen reduction reaction. In the present study, the mechanism has been further investigated via in situ respirometric measurements using Mg-Al-Ca solid solution in electrolytes with different pH values (pH = 8.0–13.0). The results indicate an additional effect based on the structure of the surface layer for Mg-Al-Ca alloys in the passive state. Furthermore, two different Al-enriched interfaces were observed and discussed in terms of their thermodynamic stability under alkaline immersion conditions.

Mg-Al-Ca alloys with Laves phase reinforcement are suitable for structural applications. The composition, crystal structure, and distribution of Laves phases can be tuned by the alloy composition and heat treatment, which subsequently influence their mechanical properties. In this study, three model alloys Mg-6Al-2Ca, Mg-5Al-3Ca and Mg-4Al-4Ca were investigated, which include C15, C36, and C14 Laves phases. The as-cast alloys have interconnected Laves phases that form a skeleton structure. After annealing, they became more rounded particles, while the metastable C36 phase was transformed to C15. The Laves phases in different crystal structures exhibit distinct ranges of chemical compositions and lattice parameters. Well defined orientation relationships were observed between small C15 platelets and the Mg matrix (Mg(0002) // C15(111), Mg[112̅0] // C15[112̅]). Another pair of parallel orientations was found between Mg(11̅01) and the c-plane of hexagonal C36/C14. Nevertheless, most coarsened Laves phases have incoherent interfaces with the matrix and hinder dislocation slip transfer. The annealed alloys have lower yield strength than their as-cast counterparts, but higher ductility or ultimate tensile strength. The yield strengths of as-cast alloys are correlated to the interconnectivity of the skeleton, whereas those of annealed alloys are related to the spacing between Laves phases.

In this paper Split Hopkinson pressure bar (SHPB) was used for dynamic testing of nickel coated polyurethane hybrid foams. The foams were manufactured by electrodeposition of a nickel coating on the standard open-cell polyurethane foam. High strength aluminium alloy bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. Experiments were observed using a high-speed camera with frame-rate set to approx. 100-150 kfps. Precise synchronisation of the high-speed camera and the strain-gauge record was achieved using a through-beam photoelectric sensor. Dynamic equilibrium in the specimen was achieved in all measurements. Digital image correlation technique (DIC) was used to evaluate in-plane displacements and deformations of the samples. Specimens of two different dimensions were tested to investigate the collapse of the foam structure under high-speed loading at the specific strain-rate and strain.