The implant itself is made up of combinations of metal. For TKA and Oxfords:
The metal components on the femur and tibia are titanium and cobalt and chromium with small amounts of nickel. The plastic bearing between the metal components is polyethylene that is specially treated to minimize wear.
For THA:
The metal components are titanium for the femoral stem and the acetabular cup.
The femoral head uses ceramic instead of the standard cobalt chrome head which can last longer and minimize the small risks of corrosion wear between the stem and head which occurs mostly just in other implant designs.
The plastic liner is polyethylene like the knees with all the special treatments that make them last longer than in the past and avoid wear issues that cause bone breakdown.
Titanium is basically always used for the cup and stem but there have been other options around for the head and liner – metal on metal, or ceramic on ceramic.