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Solid Solution Softening in Steel

  • 1.  Solid Solution Softening in Steel

    Pittsburgh Chpt Admin
    Posted 03-10-2021 08:42
    Hello Everyone!

    Last night, I learned of the mechanism of solid solution softening in steel during an evening lecture.  This was my first encounter with this phenomenon and am struggling to understand its cause.  I have found W.C. Leslie's lecture on substitutional solid solutions in Iron from his 1971 ASM lecture where he described the observed phenomenon in some detail, but is not able to produce a complete explanation of its cause.  I've attached a figure from that lecture below.

    I thought I might leverage the community to help me on this one.  Do we have an explanation for this phenomenon today?  I imagine such material behavior would be very important for steels used in space exploration applications and don't doubt that it has been looked into in detail.  Any suggestions for good review articles are also welcome!  

    Thank you!

    Stephen Rooney
    R&D Metallurgist
    Ellwood Materials Technologies

  • 2.  RE: Solid Solution Softening in Steel

    Posted 03-11-2021 09:01

    There is a well-known literature on solution softening by hydrogen in steel, but I regret that I can't point you to a review; you will have to search the literature. This phenomenon is doubtless similar to the observations in the figure that you show in your post. As I recall,  it is believed that the increase in yield stress with decreasing temperature in BCC metals reflects thermally-activated formation of double kinks on the screw dislocations that control low-temperature plastic deformation. The idea is that solutes affect double-kink formation in some way; details would require an atomistic approach because the effect would be local to the dislocation core.

    Bill Tyson

    William Tyson
    Scientist emeritus
    Ottawa ON
    (613) 725-3853

  • 3.  RE: Solid Solution Softening in Steel

    Posted 03-11-2021 12:00
      |   view attached
    Hi Stephen,
    Very interesting topic!
    From what I understand solid-solution softening alloys are alloys where the mechanism of "strengthening" by solid-solution implies the use of solute elements (alloying elements) that has a "smaller bonding environment". We are more familiar with the solid solution hardening where the alloying elements atomic radius is smaller, as C and N in Fe (interstitial SS). However this phenomena does not happen for Ni in Fe, softening occurs. This is a simplistic explanation given to make the phenomena easy to understand, but quantic physics has given a better and more reliable explanation.
    First of all, we have to state that Solid solution implies "No Precipitation", thus the amount of solute should be such that the 2 elements are miscible. See, this has something to do with the nature of the chemical bonding of the two elements. Tanabe suggested the use of what it is called the Atomic Bond Population (ABP) of elements to determine the nature of the solid solution they will form (hardening or softening). When 2 elements are bonded, the bond extends in the direction that lowers the energy (the basic nature law), thus the length of the bond is supposed to give the answer. Other explanations were given based on the dislocation theory that we are more familiar to.
    As Leslie's diagram you shared is showing, the phenomena is temperature dependent.
    I saw a paper where SSS is exploited for space nuclear applications to allow the formability of W. 
    I am attaching 2010 Tanabe open source paper that gives an insight on the phenomena with the ABP explanation.
    So much to say but I'll leave it at this point for now.
    Thank you for bringing this topic up.


    Nihad Ben Salah
    NBS- M&P Consulting


  • 4.  RE: Solid Solution Softening in Steel

    Posted 03-12-2021 03:38
    Edited by Paul Tibbals 03-12-2021 03:44
    Hi Nihad,
    Thank you for contributing to this discussion!  If convenient, could you discuss a couple of your comments?  Maybe it has been too long since school for me, but I don't get the Tanabe paper on a first read-through.  Possibly the translation is an issue as well (they seem to re-define "ionic" and "ionic-like" their own way conveniently).  Do you know if the mentions within Tanabe, for instance to "paper 29", are referring to the Proceedings it was published in?  The references listed at the end of the paper do not go that high.  While aluminum of that purity seems to me of academic/research interest only, stating that no work hardening occurs in 6-nines aluminum is a remarkable statement attributed to this paper 29; work hardening is certainly used in aluminum grade 1350, which is 2.5 to 3-nines purity.  So that would be an interesting read as well.  I would love to hear a reasoned debate between Tanabe/Yamamoto and a proponent of dislocation theory and alloy design by orbitals.  I might even be able to follow part of it.

    After writing the above, I did find a paper through a Google search,
    Work Hardening and Softening of 4-6N Aluminum in the Processing of Cold Rolling and Heat Treatments, authors Atsushi IKEDA, Kazuhiro YOSHIDA and Mahoto TAKEDA
    from the same Proceedings, which describes how these higher purity aluminums can have both work hardening and softening depending on degree of cold work, purity, and heat treatments.  If others are interested it is at  This paper doesn't make the assertions of Tanabe et al any more clear to me.  I welcome any comments or pointers to references for further reading!

    You also mentioned a situation where "SSS is exploited for space nuclear applications to allow the formability of W".  Wow, that is an interesting statement.  I did not work with refractory metals but am unaware of tungsten being employed structurally, much less being formed for such.  Is that process/procedure available in the open literature?  Was the W being used for high energy x-ray/gamma shielding?

    It's true that in the nuclear field things do get repurposed.  I was once called by a client who was apparently researching how to get the heaviest possible material for a balancing counterweight, and wanted to know if I could help them find depleted uranium (!).  This is an example of how dangerous non-materials people can be when turned loose with a handbook of values!  I suggested that sintered W, just a couple of more letters down the alphabet, would be far more suitable for the usage, was similarly dense, and would not require contacting the military for permits.  I didn't even get a chance to address the radiological and health issues of DU before the client decided I was not being helpful enough and ended the exchange.  And I never got to find out whether they were successful!

    Paul Tibbals

  • 5.  RE: Solid Solution Softening in Steel

    Posted 03-12-2021 02:07
    The comments to your question have been fascinating, thank you for posting it here.

    As far as usefulness in space however, my guess is that the solid solution phenomena (either softening or strengthening) would not be meaningful in steels.  Steel wins out when some combination of strength, cost, stiffness, etc. is superior.  Taking the example of a 787 aircraft, one reference shows it to be less than 10% steel by weight, and I would guess this to be the landing gear; spacecraft are likely to be even less than 10% "steel" which would exclude stainlesses.  But even based on the graph shown, the changes in strength from solid solution additions are only present in sub-ambient temperatures.  The part of a space mission envelope when a structure is stressed highly is often at ambient temperature launch.  And for cryogenic temperatures, BCC irons aren't used due to brittleness in any case.

    Also, changing alloy properties by solid solution additions is relatively inefficient compared to the changes in properties that can be obtained in steels through heat treatment, carbide/precipitate formation, etc.  Relative to pure iron, such enhancements can be on the order of 500 to 1000 percent, nearly an order of magnitude better than SSS would provide.

    Paul Tibbals

  • 6.  RE: Solid Solution Softening in Steel

    Pittsburgh Chpt Admin
    Posted 03-12-2021 09:20
    I'm glad I'm not the only one who finds this interesting!

    WIlliam - Leslie does make mention of the double-kink on screw dislocation idea in the paper I referenced, and I've heard this referenced elsewhere.  I'm afraid that I would need to deepen my understanding of dislocation mechanics to fully appreciate how that would accomplish such an effect.

    Nihad - Thank you for the reference!  That there are other theories for hardening/softening behavior in metals other than dislocation theory is interesting.  I do wonder how the theory presented by Tanabe and Yamamoto would explain the minimum in softening shown in the graph I attached, especially for Si and Mn substitutional alloying.  From what I understand, this softening effect can be altered or mitigated altogether by an adjustment in the strain rate of the tensile test, which points me down the road of a dislocation based mechanism for the softening.  But even though I, and many of us, have been trained in a dislocation-based framework, I accept that the behavior of the bonding between atoms is a more fundamental explanation.

    I am, as a rule, skeptical of claims made in papers where all of the references include one of the authors, but this is certainly something worth looking further into.

    Paul - Ah, yes, space structures were the first application I could think of where these sorts of temperatures would be relevant, but I agree that the greatest demands of strength are before it is exposed to anything close to what we see in the graph.

    As far as steel content by weight goes, I have seen publications that SpaceX plans on increasing the amount of stainless steel in their designs.  I don't know by how much, and I believe these are austenitic alloys in any case.

    Stephen Rooney
    R&D Metallurgist
    Ellwood Materials Technologies

  • 7.  RE: Solid Solution Softening in Steel

    Posted 03-12-2021 16:51
    Great observation about the authors being part of all of the references.

    Yes, I think I saw the same article about SpaceX.  Might have been here!  Anyway, they did give some technical justification for going to "stainless steel", grade unspecified, rather than carbon fiber, grade also unspecified.  In the summary I saw they claimed significant strength at 1600F, which you don't get in the alloys that cost $3/lb (sorry for all of the non-SI units but that's quoting the article ;-)  The interview with Musk is from Popular Mechanics magazine, unfortunately behind a paywall at their site.  I'm sure that he or at least his engineers are knowledgeable about the past materials uses in spacecraft design and can make good choices. And they may be able to use short-term tensile properties, rather than designing for years or even days at design temperature. 

    But as I mentioned before, weight is such an overriding factor for spacecraft design that I will be amazed if his concern for cost isn't misplaced.  They have been successful innovators, so it will be interesting to see what they accomplish.

    Paul Tibbals