SpaceX struts are made of steel.
Any specific reason why they chose steel over aluminum?
Steel *is* stronger; just the strength costs weight.
It also depends what mode you are using to load the material.
A solid rod in pure tension can be stronger in steel, especially iconel, for the same weight.
However, a beam that is designed to resist bending, especially if in multiple directions like a tube, can be stronger for the same weight in aluminium. This is because aluminium has a lower density, and so for the same wall thickness, an aluminium tube can be a much larger diameter. Stiffness and ultimate strength go up proportionally to the 4th power of the diameter:
MI for Solid Round Beams = (pi * (OD^4 - ID^4)) / 64
Deflection = (length3 * force) / (3 * E * MI)
Bending Stress = (force * length) / (MI / (0.5 * height))
So both stiffness and ultimate bending strength can be much greater in aluminium.
A guitar string works well in steel.
A rocket fuselage works well in aluminium.
So, for the strut, they chose steel.
Edited to add exponent symbols