Why do good materials operate in non-equilibirium conditions?

Question

If we look at the majority of useful or industrial materials surrounding us, like metallic alloys, glasses, ceramics, or plastics, it is often the case that these materials went through really hard times or difficult stages of their life during synthesis and processing.

For example this could be the heating of a metal to an extreme temperature and abruptly putting it in a cold environment to quench it, thus typically resulting in good mechanical resistance.

Perhaps the question may be not so precise, but is there some abstract reason why good material properties (mechanical, thermal, electrical, and so on...) are typically obtained through procedures that really drive the materials out of their native solid stable states? Can this somehow be related to the difference in phase flow topology between very stable states (liquid, solid, gas) and the "non-equilibrium" states obtained through extreme conditions (like heat-treatment and so on)?

Answer 1

For example this could be the heating of a metal to extreme temperature and abruptly putting it in cold environment to anneal it, thus typically resulting in good mechanic resistance.

The main reasons for the fragility of materials are imperfections of their crystalline structure and impurities. Heating iron allows the impurities to migrate out of it (especially during forging or a similar process, where the iron is squeezed to expel the impurities). Heating also allows the crystal bonds to break and find a potentially more stable configuration. Suddenly cooling the hot iron fixes its state - preventing it from getting new impurities with the dust, and also fixing its required shape.

The same principles are at work elsewhere. For example, a piece of graphite would not transform by itself into diamond or carbon nanotubes - it needs to be heated - possibly even evaporated - and allowed to recrystallize under high pressure and other suitable conditions.

Answer 2

The induced stresses experienced during processing of engineering alloys at high temperatures and strain rates is intended to improve the mechanical properties of the alloy by diffusing beneficial alloy constituents (including carbon) into certain positions in the grain structure and crystal lattice of the base metal (iron in this case). Once "cooked" by heat treatment into position, those alloy constituents are then "locked in place" by quenching the material down to a temperature where the diffusional mobility of those constituents is shut down.

Note that heat treatment of iron does not expel carbon, it mobilizes its diffusion. Iron is rid of carbon by blowing oxygen through it while molten in a blast furnace, yielding pure, soft iron with no carbon in it. Carbon is then added back in precisely-measured amounts to yield steel which, because of the carbon diffused into it, is far stronger than pure iron.

Answer 3

Frame Challenge: What are "good" materials?

There are quite a few things that can make a material "good". Like hardness (diamonds), ductility (copper), compressive strength, tensile strength (graphene), etc. pp. Note that most of the record holder materials are pure in some sense: The strength of Diamonds and graphene relies on their perfect crystalline structure, and even the ductility of copper is an effect of its regular crystal structure (the difference being that carbon atoms form covalent bonds while copper is a regular crystal of ions in a sea of electrons).

However, we don't use these record holders in many circumstances for various reasons. For instance, while we can put diamonds to good use in grinding blades, we cannot create cutting edges from them because we cannot work them like other materials. And even if we could do that, we would generally not want that because those edges would be quite brittle.

And that brings me to my main point: Good materials are generally compromises, often more than their constituent parts, and over all not simple. A good tool is neither made from the hardest material available, nor from the strongest. It uses compromise materials everywhere. Even when you have a chisel made from a single rod of steel, the tip will receive a different treatment, forming different crystal structures than the shaft. Engineers use the materials that provide the best combination of features for the job, not materials that excel in a single feature.

And there is the effect that a combination of different materials frequently outperforms the sum of its parts. Carbon fiber parts are an excellent example: The carbon fiber tissue itself has little strength, and the matrix material is easy to break. But together, they form a compound of excellent strength. And the same happens in any odd piece of steel that you get your hands on: There are grains of different crystalline structure in there, each structure with its own strengths and weaknesses, but together they stop each other's weaknesses from leaving the confinement of their respective grains. (Roughly speaking, I'm not very precise here.) So, together, their strengths add up while their weaknesses are compensated. Other examples are multi-layer paint systems used on cars, glass panes laminated with a plastic foil to stop shard formation, glass panes that have their bulk layer under high tensile stress to provide crack resistance at the surface where the glass is under strong compression (google "Prince Rupert's Drop" for an excellent show case of the principle). Many of the high-tech materials are actually combinations of different materials, sometimes created by chemical, mechanical, or thermal treatment, sometimes explicitly combined together. And the more complex the material, the more elaborate you should expect the creation process to be.