If done properly it lets you build a lighter and/or stronger bike than steel would. And it's cheaper than carbon fibre or titanium.
Magnesium, particularly in an aluminum-magnesium alloy makes for light, stiff frames.
For bike frames titanium can be used unalloyed.
5056 aluminum
Yes. Several aluminum alloy's particularly 7075 series
A mountain bike frame is used in the construction of a working mountain bike. The frame is usually the metal tubes or bars that other parts connect into.
Aluminum extrusions are used to turn aluminum alloy into other materials for cross sectional profiles. It is extracted from other metals containing aluminum and squeezed out.
Simply aluminium doesn't rust!!
an alloy
5056 aluminum
Aluminum alloy or cast iron
It would be a magnesium alloy, most likely specified by the designer of the bike.
Alloy wheels are generally made of aluminum so they are actually the same thing. There are also alloy wheels made of magnesium that are used sometimes used for race cars. Aluminum wheels will give you a smoother ride than regular steel wheels.