If one assumes that amorphous metal solidifies from different places of the same melt and coalesce in later stages of solidification than I would expect that the cooling rate is differing in the regions where different “grains” are coalescing. Does this result in variations in density in these regions and if so how would they be called?
Hello Markus, I think the cooling rate is so fast to avoid crystallization that it makes almost no difference in density if there is different cooling rates between the surfaces and the bulk, for example. Therefore, I would say there are no "grain" boundaries in these materials because these density differences may be negligible.
Hallo Markus,
as you know, in crystalline metals plastic deformations are mediated by moving dislocations, that can collectively arrange and form a (low angle) grain boundary. In an amorphous metal, the prototypical microscopic defect is the so-called Shear Transformation Zone (STZ). Further, the formation of Shear Bands (SB) during plastic deformation of metallic glasses is typically understood as liked with super-sized STZs. See for instance:
Article Size distribution of shear transformation zones and their ev...
In this picture, you may understand a Shear Band (SB) in a metallic glass as a kind of a grain boundary, that appears upon external deformation. However, it should be clearly mentioned that SBs can also be observed in crystalline materials. See for instance:
Article Orientation dependence of shear banding in face-centered-cub...
The „solidification“ of a metallic glass proceeds in a fundamentally different manner than the crystallization of i.e. a usual polycrystalline material.
When a crystal forms based on its liquid melt, there is a substantial difference in the configurational entropy between the highly ordered crystal and the amorphous liquid. This difference in the configurational entropy between the solid and the liquid phase is the atomistic origin of the enthalpy of fusion or also called the latent heat. This comparatively large amount of energy has to be released at the solidi/liquid interface during the proceeding solidification, which acts against the necessary interface cooling and quite often causes interface instabilities, such as the Mullins-Sekerka instability. See for instance:
Article Sharp phase-field modeling of isotropic solidification with ...
The formation of a glass is fundamentally different in this regard. The glass is rather the original liquid just with a frozen out amorphous structure, where all major, motional, atomistic degrees of freedom are energetically inaccessible any more. Thus, there is a vanishing difference in the configurational entropy between the glass and the liquid. Consequently, there should be no latent heat release and no such interface instabilities, as known from crystallization.
Hi Francisco Cavilha Neto, I would have thought the same thing, but when meeting somebody at a conference, the person was talking about a critical cooling rate of 20 K/s, which started making me wonder about that.
Article Structural deviations of bulk metallic glasses in downfacing...
Nice to hear from you, Michael Fleck, and thank you for giving a comprehensive overview and setting the proper context for this question. One thing that I still ask myself is if the liquid that is "frozen" to amorphous glass by falling below the glass transition temperature is the same, independent of the solidification conditions.
When we are looking at the glass transition temperature isotherm that is traveling in the same material via different thermal gradients when cooling down, the liquid, in both cases, reduces its density depending on the temperature. I would expect that this decrease in density is accompanied by changing the short-range order of the involved elements in the liquid phase.
So one possibility I could imagine how we could end up with different material properties is if the short-range reordering process becomes time-dependent and results in a different short-range order for different cooling rates that are "frozen" into the metallic glass. Does this sound reasonable?
Thank you, Martin E. Stiehler, for providing these articles; they sound promising. I will have a look.
Amorphous metals (when become a solid from a liquid) are formed due to high viscosity of the liquid as the materials cool. If kinetics of crystallization is delayed (which surely does start at different places in the liquid)and the temperature is reduced, the viscosity becomes so high that crystallization process is altogether absent. So, this has to be fully amorphous in absence of crystallization. However, there are observations that some small clusters form (which cannot be termed as crystals) which has short range ordering.
Very interesting question and many reasons have been nicely summarized. i will add up some information on the discussion.
Amorphous or Glassy metals are strongly composition dependant and one of the essential requirement is the negative enthalpy of mixing between solvent and solute atoms in the molten state. This contributes to formation of SRO clusters or Chemical SROs. Depending on the composition, we may have free volume or Glue atoms between these clusters.
During undercooling, the SROs will be retained at ambient temperature freezing similar SRO arrangements and allows for little change in density. The critical cooling rate or critical diameter sets the upper limit in amorphization. Indirectly, retention of molten structure.
Minor spatial change in SROs across the sample is common and again a function of alloying elements. (Example B-centered clusters in Fe-B alloy, Fe-B,Fe-P, Fe-Zr clusters in Fe-B-P-Zr alloys).
The concept of grain boundary-like or glassy phase separation behaviour is pronounced during thermal de-vitrification or heating the amorphous metals. During heating, relaxation, annihilation of free volume or viscosity changes or phase separation occurs , contributing to minor two phase regions.
I hope the information helps a little.
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