Even steel has a fatigue life, it is just a ton longer than aluminum, so much better that it is basically a non factor. A pile o junk volvo can go a million miles and the rods are almost as strong as new. All metals have a certain number of cycles before they fail, aluminum is within the realm of posibility in a conecting rod, steel is not, now a piston has even less stress than a rod so it can be made of aluminum and basically never fail from cycles unless pushed beyond it's elastic point by severe detonation or too hot, steel pistons would probably never melt in an internal combustion engine. I would bet money however that they do have aluminum alloys with longer fatigue life now than they did in the 60s.