When Apple introduced the iPhone 6s on September 9, 2015 they pointed out in their press release and on their iPhone design page that they had upgraded the aluminum to that of the 7000 series aluminum, the same alloy used in the aerospace industry. Apple further noted that "It's created from a unique composition of elements that make it the strongest alloy we've ever used in an iPhone. Our 2015 patent report covered Apple's original 7000 series aluminum alloys for the iPhone.

Eight years later, and the US Patent & Trademark Office has published a patent application from Apple that relates to aluminum alloys with improved material properties and cosmetic appeal for applications that include enclosures for electronic devices.

In Apple's patent background they note that many commercial 7000 series aluminum alloys have been developed for aerospace applications. Commercial 7000 series aluminum alloys are not cosmetically appealing when in consumer products. Even alloys designed for cosmetic purposes can result in chipping of the anodized surface, stress corrosion cracking (SCC), and mechanical failure.

Apple's latest invention provides 7xxx series aluminum alloys with improved abilities over known alloys.

In certain variations, the alloys disclosed can meet one or more properties and/or processing variables simultaneously. These properties include a surprising reduction in anodized surface chipping, a reduction in SCC, a substantial reduction in time to failure, and/or an equal or improved Charpy impact energy over other cosmetic alloys. 

In some variations, these properties can be achieved without changes in grain microstructure (e.g., average grain size, grain aspect ratio, and as-large-as grain size) or substantial loss of extrusion manufacturing capability.

Apple's patent FIG. 8 below depicts the percent failure as a function of soak time for Alloy 3 at A78 and A79 temper conditions as compared to the Reference Alloy at A76 temper conditions in a heat soak ASTM G30 U-bend test at 90% relative humidity (RH) at 65° C., according to illustrative embodiments.

The rest of Apple's patent, including its technical graphs, will only be truly appreciated and understood by engineers in this field. Though of course anyone could explore Apple's patent application 20230227947 here titled "Aluminum Alloys with High Strength and Cosmetic Appeal.