Over the years that I have been in western Michigan, I’ve had a lot of opportunities to increase my knowledge of my technical field. I’ve also had the chance to teach at multiple colleges and universities, as well as customize material for particular topics in materials engineering or for particular companies. I’ve grown to appreciate the value of the concepts that were “beat into me” by my professors when I was in college. Of course, coming out of school I was (at least relatively speaking) pretty useless. Fortunately, there were, and still are, companies willing to give young engineers a chance!
So I thought I would share what I have come to call my “crash materials degree” with my Industrial Heating blog readers. Here we go with the key concepts.
1.) The essence of materials science and engineering is that every material must be made into a part by a process, which creates a multi-level structure. This results in a constellation of properties, characteristics or behaviors in a given environment.
Traditionally, we show a triangle with "process" on the bottom left, "structure" on the bottom right and "properties" on the top (Figure 1).
2.) If the characteristics or behavior differ in two materials that are nominally “the same,” then they are not the same. Either the composition is different or the processing was different, which (either way) resulted in a different structure and thus different properties. This is a direct consequence of #1. Sometimes differences are trivial, but sometimes differences in composition or processing would seem to be trivial but result in significant property differences. Sometimes a 5-10°F difference in a heat-treat operation can cause a problem.
3.) There are two main forces that create structure in a given material. The first is thermodynamic tendency. This means that given a group of atoms in specific environment the atoms will want to group themselves and position themselves in a specific way. If there is more than one type of atom present, there will be a preferred way in which the atoms either avoid or seek out those of the other “species.”
The second “force” that influences the structure and arrangement of the atoms within a material is kinetics. This has to do with how fast the atoms can move in the given environmental situation they find themselves.
A classical example in the ferrous metallurgy field is that carbon in iron would really rather be present as graphite. Think about it. Carbon is totally different in about every way (think diamonds and coal) from iron (think steel, which is mostly iron). But steel generally has the carbon present as carbides or as individual dissolved carbon atoms in between the iron atoms. Why is this? It is because of kinetic considerations. When processing ferrous materials, we don’t generally allow time for the carbon atoms to find each other and push all the iron atoms out of the way. It’s “easier” for the carbon to compromise and make carbides. Even in relatively high-carbon materials such as cast iron, we have to add silicon to convince the carbon atoms to stick together and make graphite.
So, we have thermodynamic desire fighting kinetic resistance, and we get what we get in a given alloy composition subjected to a given process history.
Mechanical action can strongly influence kinetics. The blacksmith does more than change the shape. The blacksmith changes the arrangements of the atoms within the alloy.
Traditionally, we have equilibrium diagrams that show us how the atoms prefer to group themselves at a given temperature and time-temperature diagrams that help us figure out the effects of the kinetic influences.