First up is an analysis of the aluminum used for the shell. As expected, the aluminum contains a higher presence of zinc pointing to 7000 series aluminum. Apple says its new alloy of 7000 Series aluminum is 60 percent stronger than standard alloys.
It appears that the iPhone 6S (and likely the iPhone 6s Plus) will be made out of a completely different alloy than the iPhone 6. Based on the evidence gathered it's likely that the upcoming iPhone 6S and 6S Plus will utilize a stronger 7000 series aluminum. This change will likely produce a more durable iPhone but it may be more costly to produce.
When comparing the bendability of the iPhone 6 rear shell and the iPhone 6s rear shell, the iPhone 6 back plate bent with just 30lbs of pressure but the alleged iPhone 6s shell was able to handle nearly 80lbs of pressure before bending. That's a significant improvement.
Take a look at the video below...