It's been quite a long time since I studied this, but the primary problem with materials in implants is the quality of being bioinert. Many titanium alloys are not bioinert. Any combination would have to be tested separately.
This. Anytime you have metal in the body, you want to make sure it's not going to leach into the bloodstream and cause heavy metal toxicity, like mercury or amalgam fillings do.
Not only leach. Even titanium alloys used in implants have narrow specifications. For example, the Ti6Al4V that is used in implants differs from the used in airplanes by the amount of Iron on it. From 0.05% to 0.5% makes your body react to it and reject the implant.
Mercury/amalgram fillings isn't the best example, but biocompatibility is still the primary constraint.
Mayo clinic has a good overview and timeline regarding the discovery process for cobalt-chromium, which was previously thought to be a good all-around implant material: