# What is the strongest known *metallic* material which can be used on Earth?

The discovery of nanotubes and graphene has pushed the limit of material strength high enough that building a space elevator could be managable. But most of the new materials are created from carbon or other non-metallic substances (silicon, boron).

What I would like to know: What is the strongest known metallic component and what advancements had been made in this regard ?

In the former question I was too imprecise because I erronously assumed that it may be deduced what I mean with strongest metallic material. So you get know my complete list of conditions:

• After I perused the literature, I decided now that I want the material with the highest "tensile toughness" meeting the other conditions (in J m^-3). This solves elegantly the problem of desired ductility because strong brittle and weak ductile materials are ruled out.

• While I suppose the first condition will leave only metals, in case it does not: Allowed are alloys and half-metals (B,Si etc.) as long as the end product shows the characteristic metallic luster if polished.

• The material does exist and it can be provably synthesized on Earth. Theoretical materials are not allowed.

• The material exists at standard NIST conditions: 293.15 K and a pressure of 101 325 Pa. It exists in Standard Dry Air.

• It is also stable during a human lifetime, it does not dissolve, break apart or lose more than 5% of its strength when stored in Standard Dry Air.

• Given unlimited resources (money and processing) I expect that I would get a 10 g ingot of the material within a week.

What are the material parameters (Yield strength, compressive strength, shear strength, tensile strength) in contrast to e.g. V2A steel ?

• It would be interesting to see how Hydrogen fits your requirements. Temperature of operation may be a problem. And, yes, it is a metal :-). – Russell McMahon Jul 4 '15 at 23:59
• If you assume the von Mises criterion for yielding failure, which is virtually always correct for isotropic, ductile materials, then shear strength is approximately 1/sqrt(3) times the axial stress-strength, or about 0.577 times the tensile strength wiki. The reason is that all plastic deformation in isotropic, ductile metals is the result of shear deformation, i.e. slip. For metals, compressive strength is not often a useful metric, though it is generally higher than tensile strength. – wwarriner Jul 5 '15 at 15:51
• Additionally, you may want to clarify your metric because tensile strength almost always comes at the expense of ductility, and vice-versa, perhaps having some ductility lower-limit such as x% elongation at failure. Yield strength can also almost always be increased at the expense of ductility by applying cold-work. A metric which more clearly distinguishes between metals, irrespective of cold work, is ultimate tensile strength. However, some alloys use metastable microstructure features for strength, such as TRIP and TWIP steels using retained austenite to increase both stength and ductility. – wwarriner Jul 5 '15 at 16:05
• I guess what I'm saying is: material selection is complicated, so it would help to know what problem you're trying to solve. – wwarriner Jul 5 '15 at 17:18
• Purely hypothetical questions are very hard to answer. – hazzey Jul 5 '15 at 18:29

I'm going to assume by "tensile toughness" that the questioner means "integral of the space under a tensile-test curve." However, lacking straightforward access to that data for most materials, I will instead substitute the product of engineering strain at failure and engineering ultimate tensile strength. Two candidate materials, both steels:

 Class     Grade      UTS (MPa)  STF (%)    Toughness (J m^-3)
---------- ---------- ---------- ---------- ------------------
Eglin      EG-5       ~1850       ~16.5     305
TWIP       *see below ~1000       ~62       620


For the Eglin steel, the properties are reported for low strain-rate. The alloys were invented for high-ground-penetration bomb noses. They tend to have slightly higher toughness at higher strain rates, but for comparison purposes we chose the lower strain rate here. See this pdf for more information. Eglin steels are designed to retain strength at higher temperatures, as demonstrated in the table in the pdf linked above. Their composition is 85-90% Fe, 5% Ni, 1.5-3.5% Cr, 1.25% Si, and small quantities (<1%) of C, Mn, Mo, W, V, and Cu. For these steels it is critical that there is minimal P, S, and N, and there must be as close to zero oxygen as possible. Even very small quantities of these embrittling elements can reduce properties considerably.

For the TWIP (TWinning-Induced Plasticity) steel, I had difficulty finding a uniform table of grades and properties, so I am using data from page 121 of this pdf. The values given here are for lower strain rates, unlikely to be seen in their intended automotive energy-absorption use, but useful for comparison. The grade is not known to me, but the composition is in the article. TWIP steels have quite impressive mechanical properties, and have low material cost, requiring about 75-80% Fe, 15-25% Mn, ~ 0.5% C, and varying Al and Si. They are not particularly corrosion resistant and have fairly low heat tolerance. If the temperature gets close to or above the austenitization temperature (~730 °C), the retained-austenite microstructure will eventually be lost, removing the special properties.

TWIP steels can have a UTS*STF of over 600 J m^-3, and Eglin steels can have UTS over 1800 MPa with STF over 15%.

Aside from theoretical results of very recent ICME work worldwide (well over 2000 MPa, but no production process yet), these two metals are apex examples of high damage tolerance.

It is worth noting that I haven't considered shape-forming processes. Some materials may not be formable into arbitrary shapes. There are castable Eglin steels, but TWIP requires a precise working process to form the microstructure, so it is not castable or even generally forge-able. It is not clear if Aermet, as noted in the comments, is castable, but it is age-hardened. Age-hardened materials have limited service temperatures, and in the case of Aermet that would seems to be somewhere below 500 °C.

• Steel ?! I really expected some alloy of exoticum and unobtainium. At first: You are correct with the integral of the space under a tensile-test curve. After you have given a reference point (had no idea which material is a good starting point), I could search. If we look at the usual suspects like molybdenium, vanadium and e.g. tantalum: Pure tantalum has according to this table if I read right, a maximum tensile strength of 1400 MPa and a ductility of 50 % which would put its toughness to 700 Jm^3 ? – Thorsten S. Jul 5 '15 at 20:36
• Eglin is a substitute to other, more expensive steels according to Wikipedia. One of this is Aermet 100 Alloy which has a fracture toughness of 115 MPa/sqrt(m) while tantalum has a maximum of 150 ?. While the values are not directly related to toughness, I begin to wonder if steel alloys are really the best possible material. But thank you because at least I get an overview what steel is able to do. – Thorsten S. Jul 5 '15 at 20:44
• My toughness method is a rough estimate, the shape of the curve would of course dictate a more precise value. On that table for tantalum there is also a minimum value listed for each, it is not clear why the UTS would vary significantly (though a variable yield point is sensible). Perhaps it is related to crystallographic orientation. To produce a significantly large, useful component from pure tantalum may very well require most of those unlimited resources! \$50/oz and powder processing, which has significant shape limitations. – wwarriner Jul 5 '15 at 22:17
• Also beware comparing apples and oranges. The metric we've supplied for toughness is very different from fracture toughness. Fracture toughness is a measurement of ability to resist crack propagation as a function of stress times square-root of crack length. Our metric is a measure of volumetric strain energy required to cause rupture. The former is more applicable when a crack is already present and is generally used for brittle materials though can be useful for ductile materials, the latter is a more raw measurement of a bulk sample of ductile material. – wwarriner Jul 5 '15 at 22:25
• Could it be that material scientists have concentrated their effort so much on steel (because iron is abundant and quite flexible with carbides and alloy components) that many possible, more exotic alloys are not investigated (very expensive, hard to work with) ? Is steel in fact so optimized that it is already near the maximum you can get with metallic components or is it an open question ? – Thorsten S. Jul 7 '15 at 0:44

Bulk metallic glasses can often be even stronger than that. These are materials that can take 2% strain (steels are typically around 0.5% (this is elastic strain)) and have UTS north of 1800 MPa. Some of the iron-based ones are supposed to be north of 2200MPa.

The commercially used ones are Zr-based (Zr-Ti-Cu-Ni-Al (in the exact right proportions) is most common).

There are also some Zr-based versions containing Be that are ductile and have absolutely monstrous stress-strain curves. With a lower density.