Existence of upper and lower yield points is typical for alloyed steels. The reason for that is so called "solute drag", namely: solute atoms' preference to displace in the vicinity of the dislocations (due to an interaction of their stress fields with those of around dislocation core). Thus, when an external stress applied to a sample reaches the vicinity of a yield stress of the material, initially existing dislocations, surrounded (caught) by solute (impurity) atoms, need to additional stress to come off them. This "additional" stress developed onto the loaded sample, is seen as a "higher" yield point. When the majority of dislocations overcome their interaction (attraction) with solute atoms, the yield point "falls" to its "lower" level. This mechanism works both for interstitial and substitutional solid solutions.

Regarding superalloys, their phase formation should be discussed in details for each specific case separately. For example, for multi-principle alloys (like high-entropy alloys), no "solute" atoms may be defined and then they normally do not demonstrate higher and lower yield points. For intermelallic or precipitation hardened superalloys, the majority of alloying elements form inermetallide (or oxide, nitride, silicide, etc.) phases, so no "free" solute atoms exist and then these materials also have no higher and lower yield points.

Thanks so much, Dr. Katz-Demyanetz and Professor Raabe, for your time and for giving a thorough answer. These answer the question I had in mind. I will definitely take a look at the attached articles.

Interruption in stress-strain curve due to yield point phenomenon is precisely known for annealed plain carbon steel. It is basically understood by carbon diffusion mechanism and it gradually disappears with any other conditions of steel specially with alloying and cold working.

This is due to friction between cross-head and specimen.

Comment of Mr. Shaho Smaelzade is not understood clearly.