The colder it gets, the tougher it gets! What material is this?

Audit expert: Luo Huiqian

Associate Researcher, Institute of Physics, Chinese Academy of Sciences

Toughness is a word we have heard since childhood. Toughness means firmness and toughness means flexibility, which represents a firm spirit and courage in the face of external challenges such as dangers and disasters. At the same time, in engineering, this characteristic is both strong and flexible, with strong resistance to deformation and good resistance to fracture. Therefore, the toughness of materials is an important indicator in the engineering field.

Source: pexels

But in fact, the strength and ductility of materials are difficult to be compatible. In the process of engineering implementation, concessions and compromises are often required. The optimal solution is made after careful consideration of the importance of both and the adaptability to the project. But recently, scientists have discovered a new type of material and measured the highest toughness ever . Next, let's take a look at what kind of material this is.

The toughest material

A recent paper published by British and American scientists in the international authoritative magazine Science stated that they measured the strongest toughness to date when studying CrCoNi (chromium-cobalt-nickel alloy), an alloy composed of three metals: chromium, cobalt and nickel. And the test was conducted in a low-temperature environment, so the material is expected to play an important role in low-temperature fields (such as aerospace).

Source: science.org

This experiment used high-end technologies such as transmission electron microscopy to test the initial fracture toughness of the alloy CrCoNi and another alloy containing manganese and iron, CrMnFeCoNi, at a low temperature of 20 Calvins (i.e. -253.15°C). The CrCoNi alloy showed a toughness of up to 495 MPa·m^1/2 (megaPascals times the square root of meters). As the fracture diameter increased to 2.25 mm, the toughness increased to 540 MPa·m^1/2, indicating that the energy required for the fracture of this material is very high.

Microscope image showing the path of fracture and accompanying crystal structure deformation in a nanoscale CrCoNi alloy during stress testing at 20 Kelvin.

Source: Robert Ritchie/Berkeley Lab

The data alone may not give you a clear idea, so let's find some examples in life for comparison. The toughness of silicon is only 1 MPa·m^1/2; the toughness of aluminum used for aircraft fuselages is 35 MPa·m^1/2; and the toughness of some of the best-performing steels is around 100 MPa·m^1/2, which is only one-fifth of that of this alloy.

High carbon steel with strong toughness

Source: Internet

Normally, the higher the strength of a material, the lower its toughness. Hard materials are usually brittle. For example, diamond is one of the hardest substances currently available. It is difficult to leave a mark on diamond, but it can be crushed. However, CrCoNi alloy is unusual. It does not need to make a trade-off between the two. The greater its strength, the greater its toughness.

At the same time, it has another unique feature, which is the effect of temperature on the material. Generally speaking, when the temperature of the material decreases, the toughness will also decrease and become more brittle, and the fracture resistance will decrease. However, when the temperature of the CrCoNi alloy material decreases, the hardness and ductility will increase rather than decrease.

These properties make it possible to play a role in low-temperature and cold environments, such as spacecraft in space. In these fields, the alloy has unlimited potential, but the production cost of the material is very high and it cannot be promoted for the time being.

Lattice - the source of strength and toughness

To understand why this alloy has such high strength and toughness, we need to first understand a concept called "crystal". It refers to a structure in which a large number of microscopic particles (such as atoms, ions, molecules, etc.) are arranged in order according to certain rules. Due to the differences in their structural particles and forces, they are divided into four types: atomic crystals, molecular crystals, ionic crystals, and metallic crystals .

Green fluorite crystals

Source: Wikipedia

In order to vividly represent the arrangement patterns of microscopic particles in various crystals, we can regard atoms as points, and then connect these points with lines to form a regular space grid, which is the lattice .

Crystal structure of sodium oxide

Source: Wikipedia

What determines the strength and toughness of a material is often its crystal lattice . We mentioned four types of crystals above. Their physical properties are different, which are closely related to their crystal lattices. For example, metal crystals have good ductility and mechanical strength, which is closely related to their crystal lattices. This type of crystal lattice is composed of metal ions and free electrons. There is a strong metallic bond between the metal ions and the free electrons, and the metal bond is non-directional and non-saturated, which allows the metal lattice to be densely stacked, so the hardness of the metal is generally higher. The stronger the metal bond, the higher the hardness.

When metal is subjected to external force, the layers of the crystal will slide, but the arrangement will remain unchanged. At this time, the free electrons moving freely in the crystal can act as a lubricant, allowing it to maintain this interaction and not easily break, so metals generally have good ductility . For example, when CrCoNi alloy is deformed, its structure will change, resulting in amazing fracture resistance.

Scanning electron microscopy images showing fracture in CrCoNi at 293K and 20K

Source: Robert Ritchie/Berkeley Lab

The hardness is often related to the spatial structure of the lattice and the bond energy between the elements that make up the lattice . For example, graphite and diamond are both composed of carbon atoms, but the hardness of graphite is significantly lower than that of diamond. We can see that the carbon atoms in diamond are arranged relatively tightly in three dimensions. The arrangement of graphite is just a layered structure. Its strength on the two-dimensional plane is still relatively high, but it is very fragile in the third dimension.

Diamond lattice

Source: Wikipedia

The layered structure of graphite is held together by intermolecular forces, which are much weaker than the covalent bonds between carbon atoms in diamond. Therefore, if a material is to be strong, it must have strong hardness in every direction. It cannot be "partial" but must be a "generalist".

Graphite lattice

Source: Wikipedia

As research progresses, we also expect that this tough new material, CrCoNi alloy, will help humans go further in their journey to explore the stars and the sea in the future.