dahermit, each company uses their own alloy; and some use different alloys for different cartridges.
Some of the alloys, metallurgically, don't even qualify as 'cartridge brass'.
For a read on just a few different brands, take a look at this article:
X-Ray Spectrometry of Cartridge Brass
Note that, of the seven types tested, only 2 use an alloy worthy of being called 'Cartridge Brass'.