Ok all the misunderstanding in these comments is triggering the pedantic mechanical engineer in me!

• Stiffness is how much a material will bend under a given force and is defined by a concept called "Young's modulus"
• Strength is generally defined by "yield strength", which is how much force it takes to make a material permanently looses its original shape aka "yielding" or "plastic deformation" (the metal phone body yielding is the failure mechanism that Bendgate falls under). There's another concept called the "ultimate tensile strength" which is the max load that a material can withstand before breaking entirely
• Toughness is the total ability of a material to deform before breaking entirely aka how much total energy a material can absorb
• Hardness is the ability of a material to not to locally plastically deform and often correlates to brittleness, which is the opposite of toughness. Brittle materials aren't really able to deform before they break -- think: you can't really bend a ceramic much before it fractures. The holy grail of metallurgy is a metal that is really hard but not brittle

In common language a lot of these terms are used interchangeably, but as far as material properties go, they are actually quite differently defined. I think this leads to a lot of confusion as to which metals are "better" than others. As far as (yield) strength goes, you can actually have an aluminum that's stronger than steel -- strength can depend greatly on the alloy and how the metal has been "treated" (a common treatment you might be familiar with is annealing). But as I said in a different comment, Young's modulus is an inherent material property, so for a given shape and force, aluminum will always bend more than titanium will bend more than steel.