I think the rubber ball analogy is simply wrong. Plasticity as far as I am aware is the ability of a material to undergo deformation when subject to force and to remain in its new shape after the force is removed. That is not the rubber ball. I'm perfectly clear on what ductility and malleability are; it is the relationship with plasticity that concerns me. I think, in fact, I have answered my own question, as malleability and ductility seem to be subdivisions of plasticity. As far as I know, the opposite of brittle is toughness.