specific stiffness
Specific stiffness is simply stiffness per unit weight — how much rigidity a material gives you for each kilogram. It explains a surprising fact: for a simple lightweight, stiff tie rod, steel, aluminum, and magnesium come out almost dead even.
The measure is E / rho (Young's modulus divided by density). Steel: E=200 GPa, rho=7.8 g/cm^3, so the ratio is about 25.6. Aluminum: 70 / 2.7 is about 25.9. Magnesium: 45 / 1.74 is about 25.9. All three are nearly identical! Carbon-fiber composite (CFRP), by contrast, sits far higher, around 100 or more.
The consequence is that for a simple tension member, swapping one metal for another buys you almost nothing; to really beat them you must go to a composite or change the geometry. Honest caveat: for a beam in bending the relevant specific stiffness is E^(1/2)/rho, and there low-density materials such as wood and composites pull dramatically ahead of the metals.
For a lightweight stiff panel you might expect steel to lose to aluminum, but on specific stiffness E/rho they are almost tied (25.6 versus 25.9). What actually beats both is carbon-fiber composite, at roughly 100 or more — several times higher — which is why aircraft and racing bicycles moved to CFRP even though it costs far more per kilogram.
Why swapping one metal for another barely helps, but switching to a composite does.
Nearly equal specific stiffness among metals is for tension (E/rho); for bending or buckling the relevant index is E^(1/2)/rho or E^(1/3)/rho, where low-density materials like wood and foams pull ahead.