Of course this question is loaded with sub-questions so feel free to take your answers wherever you feel inclined.
For me I am interested in solution annealed, 300 series austenitic stainless steels (FCC, polycrystalline, isotropic, ~100micron grain size, low stacking fault energy, SFE), undergoing plasticity, creep and low cycle fatigue at temperatures of 450-650°C.
DSA is commonly observed at these temperatures (particularly 500-550°C) at the sorts of strain rates we use for mechanical testing (0.5 to 0.01%/s).
Ductilities are typically low at the same temperatures, so the thought in my mind is do the dislocation mechanisms which occur during DSA (repeated dislocation glide followed by pinning with solute atmospheres) result in damage. For example do persistent slip bands result in dislocation pile-ups at grain boundaries and thereby create grain boundary steps. Can these grain boundary steps (or indeed any other microstructural changes that might occur during DSA) be considered as "damage"?
Dear Michael, as such they may not be cınsidered as "damage", but if we cınsider the "golden rule" for fatigue as "any inhomogenity is a potential site for fatigue initiation, one may say yes, they may be regarded as "damage" in the absence of other more effective initiators.
Best wishes
Thanks Ali
I agree although I was actually thinking about initiation sites for creep cavities. Nevertheless, as persistent slip bands on surfaces contributing to extrusions and intrusions they would act as nucleation sites for fatigue cracks.
Is there any evidence that fatigue cracks nucleation more readily at temperatures and strain rates at which DSA occurs?
Thanks again Mike
Hi Michael,
This is an interesting question.
I’m not an expert in the specific issue of damage potentially caused by DSA. However, I would like to share some comments on base of my past experience of damage in metals due to creep and LCF. Hopefully, they could trigger some ideas for further research, experimental testing, reading, …. !
So, in the frame of your question, I understand damage in relationship with a time accumulation factor, thus long term damage. This is because you allude to creep and/or LCF. Damage would mean that the metal has reached a level where it has been irreversibly harmed and has lost part of its residual life under creep and/or LCF operating conditions.
Now, does DSA cause damage in relationship to LCF ? I feel in line with attached (old) article that DSA would enhance inhomogeneity of deformation during LCF due to solute locking of the slow moving dislocations between the slip bands. DSA would favour partitioning of cyclic strains into separate regions characterized by high and low amplitudes of dislocation movement. When dislocations moving at higher velocity in specific slip planes would intersect grain boundaries, this would lead to formation of pile-ups and consequent development of long range back stresses. The result could then be intergranular brittle decohesion and cracking. However, the risk would be reduced for the cases you mention with fine grain size (~100 µm) compared to medium grain size (300 µm). Grain boundary steps of « critical height » (50 – 100 nm ?) would participate to crack initiation.
Concerning creep, DSA could again enhance partitioning in regions with varying amplitude of dislocation movement. Or local stresses in some regions could be higher than a threshold stress allowing the dislocations to be pulled away from the solute atmosphere. In both cases, steps would form at grain boundaries where vacancies would migrate and merge more easily than in the case of creep without DSA. This would enhance initiation of microvoids alignment and cracking.
So in both cases, LCF and creep, I would conclude that additional damage specific to DSA is possible, even if in a limited amount.
But of course to know for sure what actually happens at these nano/micro-meter levels, one should submit laboratory or service aged samples to testing and metallographic analysis (TEM, SEM, ….).
Thanks Guibert
Useful paper which gives some evidence for reduced life to fatigue crack nucleation in the presence of DSA. Yes I also agree that internal stresses will be very important. I wonder if there is a difference between when (I) slip planes are only active in specific grains and when (iI) slip bands form across many grains and indeed across whole cross sections. The latter being more likely in anisotropic materials. Does stacking fault energy play a role in the formation of slip bands?
Hi Michael,
Yes, in FCC metals, SFE would probably play a role in the formation of slip bands. With low SFE, screw dislocations divide into partial dislocations and form local stacking fault zones. The movement of other dislocations across these zones is then rendered difficult. This would have a negative influence on the formation of slip bands. However, at higher temperatures one could expect that partial dislocations recombine under stress and thermal fluctuations. Also whole dislocations pass over when there is sufficient number of slip systems in the grains. So slip bands should form in spite of the low SFE. But maybe less at 500°C than 550°C. I doubt that slip bands would form across grain boundaries and other grains in your isotropic austenitic steels.
There appears some comments here every now and than, they are so detailed yet so elegant, uncomplicated and highly teaching. The answers by Guibert Crevecoeur is exactly one such.
Ali Arslan Kaya, thanks for your kind words. I just wanted to answer the question !
However as seen from the files enclosed to the answer of Osama Mohammed Elmardi Suleiman there is indeed interesting additional stuff which is not directly related to the question but could have indirect influence (depending on the actual situation) : things like twin formation, interstitial solute concentration, prior deformation, Portevin-Le Châtelier effect, ….
In line with such extensions, I join an article which gives results for AISI 316L tested under low strain rates at 650°C and 700°C : thus a bit outside the ranges mentioned in the question. The article shows that DSA occurs at both 650°C and 700°C (serrated yielding during plastic deformation – Portevin-Le Châtelier effect). There are indeed interactions of slip bands and slip planes with the grain boundaries. The dominating fracture micromechanism is voids or small cracks in the grain boundaries due to embrittlement from precipitates depending on the strain rate.
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