The stationary members inside the casing are called diaphragms. The diaphragm includes a diffuser for the gas as it leaves the impeller, and a channel to redirect the gas through the return bend and return channel into the next stage. Diaphragms can be either cast or fabricated, with cast diaphragms normally made of iron. Normally, diaphragms are not exposed to high pressure-differentials, and therefore are not highly stressed. Diaphragms should be made of steel where high-differentials may exist (such as back-to-back impellers).