Polyduranide is a metallic material used in 24th century construction.
Polyduranide was used aboard Galaxy-class starships as part of the spaceframe, computer control surfaces, warp nacelles and antimatter generators. (TNG reference: Star Trek: The Next Generation Technical Manual)
Polyduranide was used to create baseball bats, although the Cestus III baseball league decided to use wooden bats when they established their team in 2371. Benjamin Sisko asked Kasidy Yates whether the teams used polyduranide or wood. (DS9 episode: "Family Business")
In 2368, energy conduits in a cargo bay on the USS Enterprise-D ruptured and ignited the polyduranide inside a bulkhead, creating a plasma fire that threatened the lives of Geordi La Forge and Beverly Crusher. (TNG episode: "Disaster")