Hello to all the members of the world of science!
It's a tensile-related question and kind of a basic one; but I'd really like to know the answer. So here we go:
Notwithstanding the fact that for most of the iron-based alloys, a rapid fall from upper yield point to lower yield point is witnessed (in addition to the Lüders bands), some iron-based "superalloys" exhibit an almost straight, fall-free form of curve. What is the reason?
Thanks in advance!
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.
I am very much obliged Dr. Katz-Demyanetz and Professor Raabe! I appreciate your generosity in spending time and giving a thorough answer, for they indeed accomodate the question I had in mind. I will definitely read those precious attached articles. Best of luck and may success be with you!
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.
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