It is an amazing frame. Normally not too keen on aluminium but this feels really special
M5 is an unusual one, it's not exactly aluminum anymore. They added some fancy stuff to it to change the properties.
"M5 is a specific aluminum alloy (not a metal matrix composite) containing silicon, copper, manganese, magnesium, and zinc (Five alloying metals, hence "M5". Get it?)
Specialized uses it because it has an exceptionally high ultimate tensile strength (about 60K PSI vs. about 45K PSI for 6061-T6 aluminum) and M5 retains substantially more of its strength and hardness after welding. Together, these properties allow the use of less material, resulting in a lighter frame.
M4 is also an aluminum alloy, this one using silicon, copper, magnesium, and vanadium. Four alloying metals, hence "M4".
Neither M4 nor M5 is a common aluminum alloy. They're not 6061, nor 7005, nor 7075"
From this thread:
https://www.mtbr.com/threads/what-is-m5.364577/page-2
Whole bunch of chest thumping nonsense in the rest of the thread, but there's at least the one decent response on page 2. I've read a history on the development of the first few generations of Metal Matrix material, but I can't find them. I remember there being ceramic strands in the earlier versions which added to the strength and rigidity of the frame.
