As mentioned in section 4.10 the crystal lattice of a solid solution will be in a state of strain. Solute atoms will be either larger or smaller than the atoms of solvent, creating either a positive or negative strain in the lattice. Yielding and plastic deformation in metals is due to dislocations being moved through the crystal lattice under the action of an externally applied force. The strained areas of lattice caused by the presence of solute atoms will hinder the movement of dislocations and so the force necessary to move dislocations will be increased. In other words, the presence of solute atoms will cause an increase in the yield strength of the metal. The amount by which the yield stress is raised will depend on the total amount of strain developed in the lattice and this, in turn, is related to the amount of solute present and the magnitude of the difference between the atomic diameters of solvent and solute atoms.
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