The metals involved in a bimetallic strip can vary in composition so long as their thermal expansion coefficients differ. The metal of lower thermal expansion coefficient is sometimes called the passive metal, while the other is called the active metal. Copper, steel, brass, iron, and nickel are commonly used metals in bimetallic strips. Metal alloys have been used in bimetallic strips as well, such as invar and constantan. Material selection has a significant impact on the working temperature range of a bimetallic strip, with some having a temperature limit up to 500°C, with others only reaching 150°C before failing.