Embodiments are directed to radiopaque implantable structures (e.g., stents) formed of cobalt-based alloys that comprise cobalt, chromium, tungsten, and nickel, and methods for their manufacture. Tungsten is present above its solubility limit (about 15%) at ambient temperature, but is still only present as a super-saturated, primarily single-phase material exhibiting an FCC microcrystalline structure.