The plot below shows the increase in critical resolved shear

The plot below shows the increase in critical resolved shear stress, tau_CRSS for steel as a function of concentration for a number of solid solutions. Explain how dissolved solute atoms prevent dislocation motion. Explain why increasing the concentration of solute (i.e. the number of solute atoms per unit volume) increases the critical resolved shear stress. Explain why carbon has a stronger effect on the critical resolved shear stress than Ni.

Solution

When solute atoms increase in size, local stress fields are created. Depending on their relative locations, solute atoms will either attract or repel dislocations in their vicinity. This is known as the size effect. This allows the solute atoms to relieve either tensile or compressive strain in the lattice, which in turn puts the dislocation in a lower energy state. In substitution solid solutions, these stress fields are spherically symmetric, meaning they have no shear stress component. As such, substitution solute atoms do not interact with the shear stress field’s characteristic of screw dislocations. Conversely, in interstitial solid solutions, solute atoms cause a tetragonal distortion, generating a shear field that can interact with both edge, screw, and mixed dislocations. The attraction or repulsion of the dislocation centers to the solute particles increase the stress it takes to propagate the dislocation in any other direction. which in turn prevent dislocation as plastic deformation is very low When concentration of solute atoms increases, the enhanced concentration of solute atoms pin dislocation more strongly & thereby increase critical resolved shear stress which can be explained as solute atom segregate to the dislocations to decrease stacking fault energy. In this case, if dislocation moves, it has to leave solute atoms behind which changes core composition of dislocation & increases energy of dislocation. This it increases critical resolved shear stress in form of back stress on dislocation.