The invention relates to biodegradable, magnesium alloys, compositions and composites, methods for their preparation and applications for their use as implantable medical devices in load-bearing conditions. The magnesium alloys are composed of alloying elements selected from yttrium, calcium, zirconium, zinc, and strontium, with the remainder being magnesium and impurities due to production, and are prepared by melting together the elements and casting the resulting melted mixture. In certain embodiments, the methods of preparation include solution treatment and hot extrusion.