DigiTimes has quoted a few Taiwanese sources as saying the next iPhone will use stainless steel in its construction instead of aluminium. There's a good chance that not much was said about the build because the assumption would have been that Apple would stick with the same material it's used since the iPhone 5.
DigiTimes' sources say Apple won't use Foxconn as a supplier for the iPhone 8 and has instead placed orders for forged stainless steel casings from Jabil instead. It wouldn't be the first time Apple has used the material, as the iPhone 4 and 4S used it to make two antenna bands that sandwiched the glass body. However for the iPhone 4, Apple used CNC machining, which is different forging.
Apple Insider says the forging method squeezed a metal alloy between two halves of a mould to give a finished casing. Forged components are said to offer greater structural rigidity than those that are made by CNC, but CNC methods may still be used in some way to keep the internal screw design.
With each iPhone release, Apple likes to shed a few millimeters wherever possible and a forged stainless steel design could help towards that, as well as being an ideal method to give the iPhone 8 its much-rumoured all glass design.