Pure Tungsten metal is selected primarily for its high-temperature performance and thermal stability, but it is very brittle and hard to machine. Heavy metal Tungsten alloys, by contrast, are engineered for density rather than temperature resistance and consist of Tungsten combined with a Nickel-iron or Nickel-copper matrix, containing 90-97.5% tungsten, and are very machineable. While both are Tungsten-based, they serve very different mechanical and functional purposes.