According to the search results, the iPhone 15 Pro models have a housing made of grade-five titanium, which is the most premium material ever used in an iPhone enclosure. The titanium mid-frame replaces the stainless steel on the iPhone 14 Pros, and it is a brushed metal that reduces the amount of fingerprints that show on it. The titanium used in the iPhone 15 Pro and iPhone 15 Pro Max is 1mm thick, and it is thermally bonded to the aluminum. Therefore, the titanium is only present on the outside of the phone, while the inside is made of aluminum. In summary, only the housing and mid-frame of the iPhone 15 Pro models are made of titanium.