1. They can now print metal objects with features the size of a flu virus (150 nm)
2. This is accomplished by making a larger metal object suspended in a hydrogel and then baking the gel out, resulting in the metal shrinking down to the desired size
3. It has the added benefit that disordered atoms turn out to provide unexpected structural benefits. That’s because too much order leads to catastrophic failure, whereas the inconsistencies bring structural resilience.
just read the article, yeah they're talking about grain boundaries. Basically when you have that level of size control you start to engage the well-known materials science strengthening mechanisms for impeding dislocation slip.
EDIT2:
excerpt from the Hertzberg fracture book (ch2-sec2):
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> In 1934 Taylor, Orowan, and Polanyi postulated independently the existence of a lattice defect that would allow the cube in Fig. 2.1 to slip at much lower stress levels.
> By introducing an extra half plane of atoms into the lattice (Fig. 2.3), they were able to show that atom bond breakage on the slip plane could be restricted to the immediate vicinity of the bottom edge of the half pane (called the dislocation line).
> As the dislocation line moves through the crystal, bond breakage across the slip plane occurs consecutively rather than simultaneously as was necessary in the perfect lattice (Fig. 2.1).
> The consecutive nature of bond breackage is shown in Fig 2.4 where the extra half plane is shown at different locations during its movement through the crystal.
> The end result of the movement of this half plane is the same as shown in Fig 2.1---the upper half of the cube has been translated relative to the bottom half by an amount equal to the distance between equilibrium atomic positions vec(b).
> The major difference is the fact that it takes much less energy to break one bond at a time than all the bonds at once.
> This concept is analogous to moving a large floor rug across the room.
> If you have ever tried to grab the edge of the rug and pull it to a new position, you know that it is nearly impossible to move a rug in this manner.
> In this case, the "theoretical shear stress" necessary to move the rug is strongly dependent on the frictional forces between the rug and the floor.
> If you persisted in your task you probably discovered that the rug could be moved quite easily in several stages by first creating a series of buckles at the edge of the rug and then propagating them, one at a time, across the rug shuffling your feet behind each buckle.
> In this way you were able to move the rug by increments equal to the size of the buckle.
Since the only part of the rug to move at any given time was the buckled segment, there was no need to overcome the frictional forces acting on the whole rug.
> Since the lattice dislocation is a similar work-saving "device," one may reconcile the large errors between theoretical and experimental yield strengths (Table 2.1) by assuming the presence of dislocations in the crystals that were examined.
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EDIT 3:
And if the rug is creased to conform to a ramp or a wall, in order to continue moving the buckle would need to change direction, causing extra resistance.
That's basically how grain boundary strengthening works. It forces the dislocations to change direction or even split off into different directions in order to continue, costing extra energy.
EDIT 4:
Metallurgist have been trying to figure out for decades what is the best way of making grain sizes as small as possible -- through forging, alloying, heat treatment etc.
But fundamentally it mainly comes down to the solidification -- if your cooling rate is extremely fast, then you have lots of excess energy for initial nucleation centers, and grains grow out from the sites into each other. The more centers you have, the smaller the grains are.
And what determines the cooling rate far more strongly than anything? The bulk volume of the material being solidified.
What better way to minimize the solidification volume than to just cool a tiny bit of material at a time rather than the whole volume at once?
That's the main idea of the nanoscale 3D printing.
Note that disclocation glide is a yield mechanism. Ultimate Tensile/Shear Strength is still limited by the atomic bonding strength and the material flow geometry.
It’s not just grain size strengthening, it seems the surprising magic is a particle strengthening effect from dislocation pinning on nanoscale porosity
During my Physics PhD I essentially built a nanoscale 3D printer using a piezo actuator mounted in the vacuum chamber of a thermal evaporator. I made a “shadow mask” of a suspended silicon nitride film through which I punched a hole using a focused ion beam.
By moving the mask and varying the speed during the evaporation I could ‘draw’ out structures. I drew nanostructures of gold and maybe chromium too.
Could do features as small as 10 nm this way. Probably could get smaller with further optimisation.
Reminds me somewhat of roman concrete; archeologists were under the impression that roman concrete was poorly mixed due to various hydroreactive additives, that were thought to have been accidentally added during the mixing process.
It turned out these additives grant self healing properties to the structure, as when cracks appear revealing the additives, they expand to fill the crack.
I demonstrated 3D printing of metals using dynamic stencil deposition in a thermal evaporation chamber during my PhD about 18 years ago. Separate comment has details.
> “But when the material is full of defects, a failure cannot easily propagate from one grain boundary to the next. That means the material won't suddenly fail because the deformation becomes distributed more evenly throughout the material.”
Disordered structure gives glass its strength but also its transparency. I wonder if this stuff is transparent or if the fact that it is made out of metal would prevent that.
* refractive index determines the split between reflected and refracted light vs angles.
* diffuse albedo determines the absorption of refracted light.
* specular albedo determines the absorption of reflected light.
* opacity determines the scattering of refracted light.
* roughness determines the scattering of reflected light.
In PBR, metals are modeled with a purely black diffuse albedo (with very high refractive index), while dielectrics are modeled with a purely white specular albedo.
A transparent dielectric means all the refracted light goes straight, while a opaque white dielectric means all the refracted light are scattered (but not absorbed!)
For metals, it doesn't really matter whether the refracted light is scattered or not, if they're all gonna be absorbed anyways.
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EDIT:
laying out the parameters like this, I just realized why Unreal Engine's PBR felt "off" at times -- their "metalness" workflow does not allow you to adjust the refractive index of metals!
I'm guessing they just assume that metals have infinite refractive index and so the refraction-reflection split is 0%-100% with no angle dependence.
This means that they are generally overestimating how shiny metals should look with no Fresnel effect at all.
> But when the material is full of defects, a failure cannot easily propagate from one grain boundary to the next.
That's also an argument for the strength of a society of independent thinkers rather than a more uniform hive mind. Or against monocrop agriculture or large scale cloning.
Yet that metaphor itself isn't very robust since a diamond is both so regular and strong. But a diamond slightly less regular at the nano scale could be harder to cut than a pure crystal. I don't know if learning more about this would make the pattern more or less relevant to social structures.
It probably needs several military applications first. For example, you very rarely see transparent aluminum (also known as ALON®) in windows 30 years after its invention, but we wouldn't even be making it, for example for IR missile domes, if its development hadn't been funded by the U.S. Department of Defense.
The next step would then be special medical applications. For example, as a synthetic material for implants. And there you wouldn't want ultra strength, but a certain bounciness and relatively low density.
I am guessing yes. It just would be really, really slow and expensive to build, so you might as well switch to something more cheap, like diamonds..
Edit: but it is not clear to me, whether this technic can be applied to macroscopical objects at all, or if you would have to do it in steps - and then have structural weaknesses at the connection points.
In principle, I think so, but there a lot of technical barriers to learning how to actually make the structures you’d need to do it. (I haven’t read the actual paper because of the paywall)
From my understanding, there are (at least) 3 factors responsible for the high strength
- nanoscale geometry, basically the can 3D print nanoscale truss structures
- nanocrystalline base metal. There are established methods for fabricating bulk nanocrystalline metals [0], but doing it in an additive manufacturing process at bulk scale I think is a totally open question
- they’re getting extra strength from residual porosity in the metal grains (the pores pin dislocations, stopping one of the principal deformation mechanisms [1]). This is a well known phenomenon, but getting uniform nanoscale pore structure sounds really hard, especially in a bulk metal, and extra especially in an additive setting
1. They can now print metal objects with features the size of a flu virus (150 nm)
2. This is accomplished by making a larger metal object suspended in a hydrogel and then baking the gel out, resulting in the metal shrinking down to the desired size
3. It has the added benefit that disordered atoms turn out to provide unexpected structural benefits. That’s because too much order leads to catastrophic failure, whereas the inconsistencies bring structural resilience.
Pretty cool achievement!