As to the aluminum axle, I and others went through something similar. The point to be made here is that strength and stiffness are separate qualities. Aluminum is 1/3 the stiffness of steel, no matter what alloy or heat-treat it may be. Imagine we make two one-inch holes in the face of a vertical concrete wall, each six inches deep. Into one we put a two-foot steel bar, one inch in diameter. Into the other, a similar one-inch bar of aluminum, heat-treated to the same 60,000-psi strength level as ordinary, nothing-special steel. We start to hang equal weights on the free ends of these bars, and the bars deflect elastically under the load. We notice, as we add load equally, that the aluminum bar is deflected three times as much as the steel. Finally, at some load, both bars yield. The initial elastic deflection is a measure of stiffness. The eventual yield is the measure of strength.