Question

a. How do dislocation junctions contribute to strengthening? b. The addition of C to Fe greatly increases the room temperature strength of the alloy, but an equal amount of C added to Ag has little effect. Why?

Answer 1

This type of strengthening is also known as strain hardening. Let us take an example of any ductile material. We know the ductile materials show significant plastic deformation before it fails.

Now If we perform deformation operation of ductile material at lower than the average melting temperature, the material strength increases for subsequent deformation. That means the force which we need to apply has to be increased for subsequent deformation. This is due to strain hardening effect.

The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one.

Addition of carbon to iron has a great effect on strength as the added carbon molecules sit at the interstitial position of the Iron that increases the strength of the iron.

Silver has a low melting point compared to carbon elements. When we decrease temperature, carbon particle solidifies first and then comes silver. So carbon elements won't sit at the interstitial position, obviously, this results in less in strength.