There are very good reasons that aluminum is the best option for a mobile phone, rather than titanium. The most important is environmental. Aluminum is more abundant than titanium, it is easier and friendlier to extract/process. It can be recycled (very important!) and it is cheaper. Aluminum also has much better technical qualities. It is much lighter weight and easier to machine. It is softer, which means the casing will absorb most of the force from an impact when you drop your phone. And as others have pointed out; your screen or battery will break long before the structural casing. I have personally never been bothered with scratches on the casing of my iPhone. I'm much more worried about the overall environmental impact of the device.