Now that we have a sound foundation to understand the structure of atoms and atomic bonds, it's time to move on to the concept of microstructure.
Microstructure is associated with relatively large groupings of atoms – large enough to see in an optical microscope, when the specimens are properly prepared. See one of my old blog posts for a refresher on how to prepare metallographic specimens for microstructure evaluation.
So how does the microstructure develop in a given component? Part 1 of this series addressed the twin concepts of thermodynamic driving force and the kinetic reality. Thermodynamic driving force is what makes a given type of atom want to seek out its fellows or, alternatively, avoid them in preference to other types of atoms. Thermodynamic tendency has one other very important effect. In any system that has areas of more than one composition, there will be boundaries between the two compositions. There may also be boundaries between two areas of similar composition. We call these grain boundaries.
Getting back to solid materials with at least two different compositions, the boundaries take energy to create and maintain. For example, in most steels, we have an iron-rich matrix, which would be of one composition and particles of carbides, oxides, sulfides, etc. Each of these "-ides" is of a different composition. There is a boundary between the matrix that contains these particles and the particles themselves. Boundaries take energy to create and maintain.
Think fences, whether to keep the dog in or to keep the deer out. Think households. I live alone now in the same house that used to house a family of up to five, as far as I am aware. Maintaining my household includes heating the place in the winter, paying the taxes, cleaning, repairing the roof, etc. The amount of PER CAPITA energy that is devoted to maintaining boundaries is much higher when only one person inhabits the property. On the other hand, I'm a little grouchy. So it also takes energy to live in peace with someone else. It's a trade-off. There's no free lunch, as "they" say.
Now let's think about national borders, which really take a lot of energy to maintain, especially when the parties on either side of the boundary are unfriendly to each other. The point is that different types of boundaries take different amounts of energy to maintain.
The boundaries between areas of differing composition may require more or less energy to maintain. Now we get to the kinetics. If there is enough energy available, it may be used to minimize the surface area of the boundaries. How is this done? One way is by diffusion of the atoms that are in the smaller particles toward the larger particles. Another way is by changing the shape of flat or pointy particles into rounded ones. The sphere has the lowest ratio of surface-to-volume area of any shape.
So, heat (or thermal energy) allows faster movement toward the low-energy (lazy) state that thermodynamics wishes.
Figure 1 shows a piece of annealed steel. The small protruding particles visible in the flat matrix are carbides. Most of the smaller ones have become quite rounded due to exposure to a suitable "spheroidizing-anneal" temperature. Some of the larger particles, seen at the white arrows, still have shapes that deviate strongly from spherical or spheroidal. Note that the upper white arrow shows a smaller particle than the lower. Or does it? We always need to remember that we are viewing a cross section. What we are seeing might be a slice through a narrow area of a larger particle.
A longer time at the suitable spheroidizing annealing temperature would allow the larger particles to become more spheroidal as well.
Note also that increasing grain size is another way for metals to reduce their energy, even if they need temporary infusions of energy to accomplish the change. The small grains get absorbed into the larger grains, leaving less grain-boundary area per unit of volume.
How do the atoms "know" whether they are in a small grain and need to move to a larger one? That is a big question that we probably won't answer here!