Without trying to sound like a Specialized apologist, M5 alloy is more than a marketing name. It is a known 4-digit aluminum alloy, yes - - but it has been further alloyed with other specific metals to give it the properties Specialized was looking for.

It began with M2 but they found that they pretty much could only make basic round-tube frame shapes with it. So they went looking for a more malleable mix, hence M4. M4 was something they could butt, squeeze, draw and otherwise manipulate more and still retain or increase strength in key areas.

Starting with their road bike line, they wanted more. To quote them: "M4 worked (and continues to work) very well. But, to make the frames even lighter, we needed a material with higher tensile strength and one that would retain more of its integrity after welding. Working with Columbus, E5 was developed. E5 made its first appearance under Moreau and Casero last year in the form of the S-Works SL."

From E5 was born M5. To quote from another source: "Specialized engineers [had] a clear objective - design the lightest mountain frame possible. What they learned with E5 lead them to the M5 alloy which shares the outstanding strength (and hardness) and elongation characteristics of its cousin. Specifically:

• M5 has exceptionally high tensile strength so less material can be used without sacrificing strength requirements.
• M5 retains substantially more of its strength (hardness) after welding than other alloys, again enabling less material use while maintaining high strength.
• M5 has fantastic elongation, which means it can be radically manipulated, allowing a truly optimized tubeset."

Other info I turned up shows that the alloying agents in M4 are: silicium, copper, magnesium, vanadium (making it a modified 600 series); and for M5 are silicium, copper, manganese, magnesium, zinc (a cross between 6000 and 7000 series).