Yield point phenomenon in mild steel

During the plastic deformation of mild steel a phenomena called the yield point phenomenon occurs. This phenomenon is also called discontinuous yielding. This phenomenon explains about the upper and lower yielding happening in the mild steel. The reason behind this phenomenon is basically the presence of interstitial atoms like carbon and nitrogen atoms in the alloy. Now as the reason for plastic deformation is dislocations, they are now locked to these interstitial atoms and hence are not dislocated. Now as the plastic deformation occurs, the number of dislocations increases as a result of cottrell atmosphere formation. Therefore the density of dislocations which are locked with the interstitial atoms causes this phenomenon. Further, as the stress increases too much the dislocations gets free and as a result of that stress starts decreasing. But the as dislocations' density increases, stress again increases.

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