Over the past four parts of this series, we've talked about different levels of structure. We started with atomic structure and went on to microstructure. We've talked about how process influences microstructure. We discussed the important concept of boundary energy.
Boundary energy is one of the main conceptual foundations of understanding how microstructures evolve when exposed to different environments. The carbides in alloy-steel pipes used in high-temperature applications, such as steam lines in power plants, tend to spheroidize over time. This makes the material softer and not as strong. The pressure that was once carried with ease gradually causes the working stresses to approach the yield strength at the working temperature. If the power-plant personnel have not monitored the changes in the microstructure over time, they may not realize that the piping is compromised. The wall also thins due to oxidation over time. Eventually, a disastrous rupture may occur.
Microstructure is intentionally attained in many different materials by the choice of processing parameters. This includes hot rolling, forging and extruding, cold rolling, cold forming and heat treating. All of these fall into the category of "thermomechanical" processing. Microstructure changes can happen at room temperature for some alloys. Some grades of aluminum will experience enough diffusion at room temperature to change its properties. Cold forming that disrupts the crystal structure is also used to modify the characteristics of materials.
Figure 1 shows a metallographic mount of a piece of low-carbon steel that has been etched to reveal the grain boundaries. This is a stamped part. Note how the sheet-metal surface appears to have a structure that responded differently to the etchant. This may be due to the fact that the part was zinc plated, and the plating protected the underlying steel from the effects of the acid etchant. But we can clearly see that the grains in area A are blockier than the ones in area B. This is due to the deformation that happened as a result of the forming process to a greater extent in area B.
We would expect area B to be harder than area A. (If you are not a metallurgist, and you are wondering why, I did not really explain this!) Because the etch was ineffective along the surface, we can't tell what is going on there. Room-temperature deformation usually makes the material harder and often makes it stronger. Room temperature deformation locks energy into the volume of the material.
As we see here, in this area that is just over 1.4 mm wide in reality, the microstructure varies a lot. If we assume this is a bracket and the dark area at the top is the plastic mounting compound and the service stresses tend to open the angle, we might possibly expect that a crack could start at the middle (vertical) of the three arrows if the stresses are too high and grow into the core of the steel. Because there is a gradient of deformation, the crack may wander to the left (as shown in Figure 1) because that material is softer and probably weaker. Cracks start and propagate when and where the stress exceeds the strength. We see in this image that the microstructure and resultant strength can vary quite a bit from nearby location to another nearby location. That's why, to understand fracture, we need to understand the microstructure at the crack-initiation area. That's also why we need to be quite confident that we find the initiation area.