[AdamHeard]

Material alone isn't enough to answer the question; the overall geometry is very important. You could use wooden dowels for axles and be fine, and use very high quality 4340 steel and shear them every match.

We use a lot of 7075-T6 Aluminum (and rarely any steel), but we generally always do the analysis to see if these shafts will fail (and go to a higher quality steel if necessary, or increase diameter, change how they're loaded, etc...).

If you have an engineer on the team, or someone willing to give you some time, they should be able to do these calculations for/with you (it would be difficult to teach students to handle it for all cases, but for some specific repeatable ones they could). If that's the case, I'd say go ahead and look at aluminum as an option. For someone with an engineering degree, this should be trivial.

If analysis isn't an option for you, I wouldn't trust anyone's anecdotal evidence really. I would stick to steel, preferably something real tough and strong like the chromoly series.

EDIT: A useful trend to know, even if you can't do the actual calculations, is that torsional strength of a shaft goes with diameter^3, shear/tensile strength of a shaft goes up with diameter^2 and bending strength of the shaft goes up with diameter^4 (someone please verify these relationships for me, been a while since materials). Using this you can quickly realize/show that increasing diameter is one of the best things you can do for shaft strength. Bending is also the most likely failure mode for FRC shafts as well.