Abstract
Two cast Mg–Al–Ba–Ca alloys, DieMag633 and Die-Mag844, are the focus of this study. Their microstructure consists of a eutectic Ca-rich structure, the lamellar Al2Ca, and of a hard, compact and brittle interdendritic Ba-rich phase, the Mg21Al3Ba2 phase. Both phases have a much higher hardness than the a-Mg matrix and strongly influence the crack propagation under quasi-static and fatigue loading. Mostly interdendritic crack propagation has been found—the crack growth follows the coherent interdentritic network of second phases. Transgranular cracking within the compact Ba-rich phase is very pronounced, showing many microcracks within this phase. The phases strain harden in a different amount. Transdendritic cracks are found only under cyclic fatigue loading, and the cracks transit from the second phases into the a-Mg dendrites. The lower crack growth rate and the stress increase at the interface to the dendrites seem responsible for the transdentritc crack propagation. The influence of the chemical composition of the alloying elements and the fatigue stress ratios are discussed.
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