Explain what the effect is of reducing the grain size on sig

Explain what the effect is of reducing the grain size on sigma_y and %EL, and briefly discuss why gain size reduction causes these effects.

Solution

Decrease in grain size results in increase in both yield strength

Given enough stress and thermal energy, dislocations will easily move throughout the crystalline grains, resulting in permanent distortion of the grain itself. However, once a dislocation reaches a grain boundary, it has nowhere to go. In other words, grain boundaries stop dislocations . Thus, an easy way to improve the strength of a material is to make the grains as small as possible, increasing the amount of grain boundary. Smaller grains have greater ratios of surface area to volume, which means a greater ratio of grain boundary to dislocations. The more grain boundaries that exist, the higher the strength becomes.

Small grains also improve the overall strength of a metal another way. Within each grain, there are preferred planes where the atoms (in the form of dislocations) are free to move across each other. These are known as slip planes. If the applied stress coincides with a slip plane, the dislocations can move easily. If the applied stress is perpendicular to the slip plane, it would be extremely difficult for the dislocations to move. Therefore, each grain is weaker in certain directions than in others