Jay
Your understanding is sound and I appreciate the descriptions.
My only suggestion is to replace your term "elasticity" with resilience or toughness. Like Elarson shared and you understand, the difference between yield strength and ultimate tensile strength is a measure of ductility. This can indicate "toughness", or ability to absorb overstress without fracturing.
Fatigue resistance will be more about ultimate tensile strength and less about toughness but both are important for a crank. As discussed in this thread, there are MANY ways a crank can fail.
I would expect a forged 4340 crank to outperform a cast crank in every way. Every property follows microstructure, which in turn follows composition, fabrication history, and heat treatment. "Dirty" material will have more inclusions (sulfides are typical). Poorly-processed material will have those inclusions agglomerate and form large chunks instead of being dispersed.
Being a dinosaur metallurgist, I started work for a giant vertically-integrated corporation when US manufacturing was robust, and then "transitioned" to dealing with materials and parts sourced from all over. I expect vintage GM-manufactured cranks to be highly-engineered, high-quality, and consistent parts. I don't buy into any general claims of foreign material being inferior but I do know it costs money for a supplier to both "know" what they want and to make sure they get it. If you want to know what the microstructure looks like, somebody somewhere has to get paid to section the part and look at it. GM did this. Others today... maybe not so much...
Having zero experience with automotive parts, I enjoy and value all the posts in this and any thread that talks to materials and failures. You guys know your stuff!
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