Question

Describe what happens, on the atomic scale, to cause differences in recovery in materials stressed within the elastic region and in materials stressed to an amount exceeding their yield strength

Answer 1

on the atomic scale:

when materials stressed within the elastic region, phenomenon occurs due to the segregation of impurity solute atoms(C and/ or N in Fe) around dislocations so as to reduce the strain energy associated with the distorted atomic arrangement. This additional stress required to free the dislocations and set them in motion needed for plastic deformation is called the upper yield point. Once dislocations have been freed, the stress needed for their motion drops abruptly and is called the lower yield point. There comes slight variation in its value due to the interaction of moving dislocations with the impurity solute atoms obstructing their paths.

when materials stressed to an amount exceeding their yield strength, For most metallic materials, the elastic deformation region is relatively small. At some point, the strain is no longer proportional to the applied stress. At this point, bonds with original atom neighbors start to break and reform with a new group of atoms. When this occurs and the stress is relieved, the material will no longer return to its original form, i.e., the deformation is permanent and nonrecoverable.