Please explain precisely why the yield point apprears prematurely on unloading the sample.
Yes, but more simply, like a valley between two mountains B_____->__A. At first step - gliding dislocations are stuck on initial barriers . You apply stress to start plastic strain when dislocations start moving and reach the ultimate barrier A, which is currently maximal for many of them and they cannot overcome it. Many dislocations start movng not at the same time, hence we need to involve more and increase stress to move them to their barriers with stress level of A. On the second step - they move simultaneously in the opposite direction B. These dislocations called "unpinned" thus can easily move back from A to B, because they are pushed by the barrier in exactly the opposite direction. So the barrier A helps to move them back through the valley to B.
If they overcome the barrier A at the first step - there will be no effect on the second step as they would need to overcome the barrier again from the opposite side.
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