A metal alloy was a compound metal created by combining two or more metals. The First Doctor cited molybdenum as an example of an alloy used in steel. (TV: The Sensorites)
Cupronickel was an alloy of copper and nickel. (PROSE: All I Want for Christmas)
Skaronium was so strong a metal that the Thals were forced to render it into an alloy to manipulate it. Their network of ionising towers was made of the Skaronium-alloy. (AUDIO: Return to Skaro)
Rehabilitators were made of a dense metal alloy that was highly addictive to humans. This alloy was of extra-terrestrial origin, and Torchwood Three were unable to identify it. (PROSE: Kaleidoscope)