Stiffness and brittleness USUALLY go together but it is really dependent on the materials. What is wanted in this use is a materials property called 'toughness' (Yes, that is real materials term...). The materials used in a forging are quite different than a casting; the forged crank material HAS to be able to withstand a lot of distortion, because the forging process involves literally 'pushing' the material around in some areas to get it into the final shape and it gets a crapload of force and movement going on within portions of the original casting. So the forged crank materials would seem to NEED to possess better toughness to even survive the forging process alone.
