High entropy alloys: ¡Long life to chaos¡

When we study the materials present on earth we realize that these are the balanced result after millions of years of constant transformations. Metallurgy will represent a set of unnatural seeking techniques to reverse the existing balance in order to obtain different metals or metal alloys tailored to our own needs. 

This “crime” performed by metallurgists requires as well a high use of energy and the development of ordered novel structures, as the first step in the genesis of a new chaos. Once converted the different metals into pure state, actual alchemists start playing to find the diverse combinations that will allow to find the philosopher's stone of modern times.

These philosopher's stones as very light materials or materials that are able to withstand extreme environmental conditions, or even outer space missions, keep on disturbing our rest and encouraging our daily life. The combination between elements is not a random outcome and could be easily explained by the laws of a branch of physics called thermodynamics, which are able to show an already open secret in our society. After a hard day of work the most likely situation when arriving home is to lay down on the couch to watch TV and let the clothes "more or less set in place". Without realizing it, we are experiencing one of the principles of thermodynamics related to the spontaneity of chemical reactions and thus contributing to the balance of the universe (I say this not to feel so guilty). The spontaneity of a chemical reaction is therefore defined by the GIBBS free energy. Without going into detail, it tells us how the systems are much more spontaneous when the combination of the chemical elements generate a greater release of energy. This means that we tend to the lowest energy and to a greater disorder.
 

Within the world of metals and metallic alloys, the metallic bond is the main driver and responsible for their characteristics. Electrons from this bond are shared between all the atoms giving as a result joints that are stretched or deformed almost infinitely before separating forever. However, the characteristics of pure metals are not ideal for a society in which power ambitions, leisure or exploration have introduced new rules of the field of knowledge.

Beginning of plastic deformation (what is known as yield stress) is required to the maximum possible force, even close to the fracture point. Imagine for a moment that you are driving a car and that the spring of the shock absorber is permanently deformed with the first bump from the road. Additional bumps or further road impacts would end up absorbed by the car chassis, and consequently by ourselves. Metallurgy has found various ways to get a pure metal to develop tailored properties that make him the perfect candidate to be used in manufactured goods with very specific properties.

The principles associated with the transformations that make a metal move from a decorative element into a high added value alloy for society, are the following:

• Solid solution of a metallic or semi-metallic element, that becomes the base metal from a specific alloy. The location or substitution of an atom in the network or interstitial space avoids the displacement of crystallographic planes that is prevented by the presence of these distortions in the network.
• Hardening by precipitation, where different compounds generated in the solidification of the alloy during subsequent small heat treatments are distributed in the base metal structure, preventing the free movement of atoms under certain applied efforts. 

• Precipitation of harder phases in the matrix of the base metal that generate a higher surface hardness and lower elongation properties of the material.

The metallurgy linked to the above mentioned principles has been always defined for a base metal with the addition of further elements that condition its final properties. The solid solution of metals, as a concept, has been limited primarily to pairs of metals, and/or to concentrations in narrow ranges compared to the major base metal. The increase or the introduction of an additional metal varying the concentration levels cause almost immediate development of stable phases (as shown in the lower figure), which quite often generate undesirable fragility or corrosion phenomena in the alloy.

               Fragile phases in a complex alloy (σ phase)

Recent studies have questioned this theory since it has been observed how metal combinations in similar proportions generated in certain cases solid solutions without stable phases. Thermodynamics provide some hint in this direction and help to explain that this situation is such, because the clutter or the chaos generated is greater than the energy released by the phases formed.

This new research line has led to the beginning of the so-called “high entropy alloys”, some of which might be able provide much lighter alternatives to the super-alloys actually used in extremely demanding sectors such as in the air and space industry. These new alloys seek to achieve superior characteristics of hardness and high temperature resistance, or a better ratio density vs. mechanical properties, compared to the light alloys known up to date.

A new hope in the world of metallic alloys is therefore open, bringing fresh air and challenging opportunities so lacking in the past. That is why I take this opportunity to shout out “long life to chaos”, but I hope that my son does not read this blog, because after this I will have no arguments to make him keep his room neat and tidy.