I'd be very surprised to read that the aft lug that cracked (and the bearing it contained) were made of aluminum. They were almost certainly steel or Inconel.

Wouldn't that be true of all cast metals objects?

Or are some metals impervious?

No; roughly, yes. Based on the crystal structure of the metal, fatigue works differently.

> The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure.[1] Some metals such as ferrous alloys and titanium alloys have a distinct limit,[2] whereas others such as aluminium and copper do not and will eventually fail even from small stress amplitudes.

https://en.wikipedia.org/wiki/Fatigue_limit

I know Veritasium gets posted here a lot, but a few days ago he posted a deep-dive into the the engineering of jet engine turbine blades. Turns out they're made from a single crystal of a superalloy that entangles itself at a molecular level such that it actually gains strength as it's heated, only losing strength above 1200 degrees C / 2200 degrees F. Below that temperature, as long as the strain on the part is below the plastic deformation threshold, it's not really losing any strength at all over time.

https://www.youtube.com/watch?v=QtxVdC7pBQM