A theorem of creep strengthening was derived from two axioms of creep for the polycrystalline Ni–20 massÿCr–X (X = Nb, Ta, Mo, W) solid solutions on the
basis of internal stress concept. The general equation describing the minimum creep rate can be divided into two terms: one term is determined by the creep testing
condition and the other term with no dimensions represents the creep strength. In the core–mantle model in dislocation creep, the central core region sustains the
internal stress during creep and the peripheral mantle region along grain–boundaries is free from the internal stress. The creep strength for the polycrystalline solid
solutions is improved by the enhancement of core intensity and core fraction. The strengthening techniques in high–temperature creep for the polycrystalline solid
solutions rely on this simple principle: Enhancing core region renders a material stronger.