Polyalloy

From Memory Alpha, the free Star Trek reference

A polyalloy is a material used in structures which need to withstand extreme stress. Data's upper spinal support was made from a polyalloy. (TNG: "The Chase")

An alloy is typically a homogeneous mixture or solid solution of two or more metals. A polyalloy is likely a combination of two or more alloys that, because it is not homogeneous, is not itself a further alloy.