Entropy is the degree of disorder and chaos that exists in the universe. The second principle of thermodynamics tells us that the universe around us becomes more disordered, deteriorates and heads towards total chaos.
The ancient Greeks already used the word entropy to designate an evolution or transformation. Today, this beautiful word is used as a physical measure of the disorder of a system. With regard to the science of materials, it can be said that a solid material has a much smaller entropy than the same material in a liquid or gaseous state, with its atoms in constant movement and in full disorder. But what does all this have to do with the development of the new materials of the future?
To better understand it, Papel De Periodico will bring you back to the end of the Neolithic period, where the first pure metals began to be used in the Near East. Over the years, these pure metals began to melt and very small quantities of other elements were added to them (increasing entropy, which was still very low), creating the first alloys.
Historically, alloys have consisted of mixtures between a principal element and one or more elements added in small concentrations to improve the properties of the principal element.
For example, steel is based on the main element iron (solvent) with the addition of one or more minor elements such as carbon (solute). Although the possibility of adding more principal elements into the mixture has always existed, there has also been a fear: that this would lead to multi-stage alloys that would make it very difficult to control the metallurgy of the alloys.
In conventional alloys, the atoms of the main element retain their crystalline structure and accommodate the mixed elements in their microstructure, until reaching the limit of solubility that causes the appearance of new minority phases with properties and compositions different from the main phase. In most cases, this effect increases strength and hardness, but greatly impairs ductility.
What was unknown to date is the effect of alloying several elements in sufficient quantities for the “chaos” to be sufficient to prevent the formation of a main phase based on the main element. It should be noted that alloys designed by the traditional method improve the properties of the pure element through the addition of new components, but still maintain properties intrinsic to the pure material.
With the advent of the 20th century, and the new demands of humanity to build higher, fly faster, create new devices that meet our social needs, travel into space, etc. this fear of research into new metallic materials that are not based solely on one main element was lost once and for all.
Revolutionized the field of physical metallurgy, since, against all odds, disordered solid solutions were stabilized, instead of fragile intermetallic compounds. These new microstructures presented a combination of mechanical and physical properties that no other alloy had been able to present to date.
This opened a new field in materials engineering, since the number of High Entropy Alloys to be investigated can be said to be almost infinite, as around 1078 possible alloys have been estimated.
In this way, the scientific community was launched to the discovery of new materials based on the design of high entropy. Scientific production in this field increased exponentially,
Four effects were identified as the main “culprits” of the new discovery:
- High entropy of mixture: contrary to what could be expected, alloying a greater quantity of components causes an increase in the atomic disorder of the structure when they are close to the melting point and, consequently, fewer phases than expected due to the increase in the solubility of the components and the decrease in the formation of intermetallic compounds, which are generally fragile structures.
- Slow diffusion: in a crystalline network composed of several different elements, the atoms will find a very different potential energy when diffusing. It might well be the case then that the potential energy in that new position is much higher and that the atom returns to its original place. This slow diffusion justifies that they have excellent properties at high temperature.
- Distortion of the crystalline grid: The wide distortion of the crystalline grid comes from the different sizes of the atomic radius of the components. Among the properties obtained from a structure with a wide network distortion are the increase in hardness and resistance, the reduction of thermal and electrical conductivities and a lower dependence on these temperature parameters.
- Cocktail effect: this effect was described for the first time as “a synergistic mixture where the final result is unpredictable and the results obtained are better than the results obtained separately”.