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bloopernova

In materials science, are alloys as seemingly complex as "Al92Ti2Fe2Co2Ni2" commonplace?

How do we arrive at that kind of alloy? (If it was explained in the paper, I didn't understand it)

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As mentioned in the paper, this is a typical "medium-entropy" alloy.

Pure metals are soft, because all their atoms have the same size, so the atom layers can slide over the others.

Mixing metals with different sizes increases the strength, because now the atom layers are no longer smooth, but they have bumps, which prevent sliding.

It can be shown that mixing many different kinds of atoms, taking from each about the same quantity, can provide very good mechanical properties, because the bumps in the atom layer will be frequent and they will have varied sizes and a random distribution, which will prevent any alignment between bumps, which could facilitate the sliding of the layers. Think about how to design an anti-sliding shoe sole. Random bumps of random size would give the best result.

The so-called "high entropy" alloys contain at least 5 different metals, with about the same quantity from each.

However the alloys that contain almost equal quantities of each component are very expensive. In order to make a cheap alloy, one must have one or at most two components in a much larger quantity than the others, so that the abundant components can be chosen from the few cheap metals, e.g. iron, manganese or aluminum, while the other components, which are added in small quantities, can be chosen from expensive metals, like nickel and cobalt.

For this reason, the better "high-entropy" alloys are normally replaced by the cheaper "medium-entropy" alloys, which use 5 metals, like the "high-entropy", but which are used in quite different quantities, with larger quantities from the cheaper metals, if possible.

The use of "high-entropy" alloys and "medium-entropy" alloys has begun only relatively recently. They are used to replace the cheaper classic alloys only when they offer a decisive advantage that can justify their higher cost.

This case is one such example. From the classic aluminum alloys, some of the weaker alloys, like AlSi or AlMgSi can be easily 3D printed, but they have low strength. The classic high-strength aluminum alloys cannot be 3D printed. Therefore this was a clear case when a newer kind of alloy must be tried, if high strength is desired. They have experimented with certain kinds of medium-entropy aluminum alloys, to keep the cost acceptable (and also in this case the high content of aluminum keeps the density low and the conductivity high, which are frequently the reasons for choosing an aluminum alloy), and the results were good.

Nevertheless, this alloy is likely to be several times more expensive than AlSi or AlMgSi, so it will be used only when its high strength is necessary.

I get the sense that stronger alloys are more "brittle" and harder to do things like welding, as they'll crack instead of yielding from all the thermal stresses. This is probably the same sort of thing with laser melting and 3D printing: solidification under high thermal gradients. It seems this material is not only high-strength but also ductile enough to gracefully handle the thermal stresses.

It's more complex than that. A lot of the material properties depend on both the cooling and the tempering in aluminum alloys.

The phase diagrams for these types of alloys look wild (you often want to achieve a certain material phase during cooling to "lock" in to get certain characteristics), and it can be difficult to ensure that the smaller metals participate during cooling. Also difficult to dissipate these slightly during tempering, typically to increase ductility.

This is probably why 3d printing hasn't been done in earnest, you can't design something within tight tolerances with unknown material properties.

So you need to control the solidification process to plot a course through the phase diagram, spending the right amount of time in each region, and ending up in a good place. And this alloy has a phase diagram that is compatible with a method of 3d printing.

I think this is also why welding aluminum alloys was such a pain before the introduction of friction stir welding, which doesn't melt the metal. FSW was invented surprisingly late, in the 1990s.

3D printing of metals is being done in earnest, although the industry prefers the term Additive Manufacturing. Metal powder bed fusion is a stable, reliable process that is being successfully used commercially. It's generally confined to high-value applications that require extreme geometric complexity, but it can be invaluable in industries like aerospace, motorsport and medical. The range of viable materials is still somewhat limited, but covers a good range from titanium and aluminium alloys through to tool steels and heat-resistant super-alloys.

https://www.renishaw.com/en/metal-3d-printing--32084

https://www.trumpf.com/en_US/products/machines-systems/addit...

https://www.carpenteradditive.com/powderrange-metal-powders

This is not a medium entropy alloy, it's a standard alloy in terms of the ratio of components, which forms medium entropy intermetallic precipitates which gives the alloy it's properties. Intermetallic MEA is an odd term I'm not really familiar with and would want to look into more, but is a little suspicious. Furthermore, while MEAs (3-4 equal primary components) and HEAs (5+ equal components) do have good mechanical properties, I'd be wary of the atomic size argument, last time I've been involved in it, that argument has increasingly been questioned, as the atomic size of the elements in question are generally pretty similar.

It is not uncommon to have an alloy with 10 elements. Usually it is 1 to 3 main elements and some minor additives, and that’s the case here.

Steels can be very complex as well.

If you just look at stainless steels, there are many alloys with 6+ elements, example below (904L is also know as Rolesor, used for steel Rolex watches)

SS 904L: Nickel, Chromium, Carbon, Copper, Molybdenum, Manganese, Silicon, Iron

Tool steel alloys (used for machine tools, hand tools, knives, etc) have iron, carbon, tungsten, chromium, vanadium and molybdenum.

Carbon steel is the most basic alloy steel, it consists of iron and carbon (and impurities).

If you notice these alloy elements add up to 100. This alloy can be thought of more as 92% Al with 2% each of the other elements. Its a metal-metal matrix composite, primarily pure aluminum with localized, tiny grains of what would be thought of as a traditional alloy (various aluminum-titanium, aluminum-iron, etc. alloys)

They're telling you the mole fraction of each element of the powder. It sums to 100.

I'm not familiar with that format, but it's not rare for alloys to have half a dozen or more alloying elements.

That notation is used sometimes in the literature on model alloys. This does not survive contact with engineering, where they tweak the formula to a hundredth of a percent.

Even with a precise formula that's only 20% of the work. With these superalloys the hard part is getting them to crystalize correctly so that all of the elements fit in the right spots in the matrix and stay there while it cools. A lot of them require seed crystals to form, which complicates the problem.

That’s why this laser sinterable superalloy is really interesting.

Oh yes, of course. I only referred to the notation. Additive manufacturing adds tons of issues on top of the basic problem of getting the alloy to crystallise in the right form (which we’ve had to deal with for millennia).

But the field is developing rapidly and we are already talking about complex concentrated superalloys. There are spectacular advances happening right now at every level of alloy development. The fact that additive manufacturing is far out of equilibrium is a problem for now, but this could become an advantage instead with the right alloys.

This is not a superalloy (i.e. turbine blade material), just a strong normal aluminum alloy that can be additively manufactured.

What features exactly are you asking about on that "like"?

The paper has links for other work with all kinds of similarity.

I rephrased it a little, I was wondering if other materials were as complex as that alloy appears to be, or if an alloy with that number of elements was used widely.

Apologies for the poorly phrased comment.

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