Electrical interaction arises from the fact that solute atoms always have some charge on it due to the dissimilar valences and the charge remains localized around the solute atoms. The solute atoms can then interact with dislocations with electri­cal dipoles. This interaction contributes negligibly compared to the elastic and modulus interaction effects. It becomes significant only when there is large differ­ence in valence between the solute and matrix and the elastic misfit is small.

There were different theories proposed and debated over several decades. Some have found that the strength increment varies as the square root of the solute atom fraction (c1/2) or c1/3, or even c. One of the examples of solid solution strengthening is the martensite strengthening. Martensite phase in steels has a body-centered tetragonal (BCT) lattice structure and is considered as a supersaturated solid solu­tion in the host lattice of iron. Carbon atoms present in the lattice strain the crystal creating nonspherical (tetragonal) distortion, which affects the dislocation move­ment severely. This is one of the reasons why the martensite phases are so hard and brittle. Figure 4.34 shows the variation in the yield strength of iron as a func­tion of different solute concentrations. Note that the solid solution strengthening imparted by the interstitial elements like carbon and nitrogen is much higher than that imparted by substitutional solutes.

4.4.4