Question

Quesiont is

'Explain Luder's band in detail. Make reference to the yield point, dislocation motion and solute atoms'

Answer 1

Some polycrystalline metals, such as mild steel, display a discrete yield point type of behavior where a higher stress is necessary to initiate plastic flow than to continue it. Thus, there exists a localized, heterogeneous type of transition from elastic to plastic deformation which produces a yield point in the stress-strain curve i.e. elastic-plastic transition is very well demarked and occurs abruptly in what is called yield-point phenomenon. During loading the load increases steadily with elastic strain, drops suddenly at the upper yield point where plastic deformation gets initiated. Continued deformation fluctuates slightly about some constant stress value – lower yield point. Subsequently stress increase with increasing strain. Elongation that occurs at constant load is called the yield-point elongation. Yield strength for metals with this phenomenon is taken as average of lower yield point, thus it is not necessary to employ strain offset method.

At the upper yield point, deformed part of metal forms a discrete band usually visible to eye at a stress concentration. Many bands may form at the same time at different stress concentration sites. Each band then propagates along the length of the specimen, causing yield-point elongation. These bands are called Luders bands / Hartmann lines / stretcher stains, and generally are approximately 45 to the tensile axis. Occurrence of yield point is associated with presence of small amounts of interstitial or substitutional impurities. It’s been found that either unlocking of dislocations by a high stress for the case of strong pinning or generation of new dislocations are the reasons for yield-point phenomenon. Thus when the dislocation line is pulled free from the influence of solute atoms, slip can occur at a lower stress. On the other hand, when dislocations are strongly pinned, new dislocations will generate followed by drop in flow stress. This is the origin of upper yield stress. Released dislocations tend to pile-up at grain boundaries, producing stress concentration which in addition to applied stress unlocks sources in next grain. This is the way band propagates. Magnitude of yield-point effect will depend on energy of interaction between solute atoms and dislocations and on the concentration of solute atoms at the dislocations.