The MSE Paradigm as a Framework for Science and Engineering

Hopefully, even after only a few hours thinking about the topic, the MSE Paradigm makes sense as an underlying organizational principle of the field. In this section we'll explore its utility as a framework for practicing material science and engineering. As will become clear soon, the relationships between the aspects of the paradigm (processing, structure, properties, and performance) can become very complex quite quickly. To approach these complex problems, we can organize approaches to materials engineering questions systematically using the paradigm, allowing us to formulate a so-called Systems Design Chart (SDC). This is an advanced topic—MSE majors don't construct one in full detail until their capstone projects, but we believe that introducing a systems approach to materials is useful in the first week of any introductory MSE course because:

1. You see the ultimate destination in the training of a materials engineer
2. It's a great organization model to use when considering the application of materials in any material system
3. It will be extremely useful for you if you decide to write a Materials Vignette at the quarter's end.

So, let's use an example that most people are familiar with: cake. Yes, cake is indeed a material: it is a solid material that we use in a specific application: dessert (or breakfast, sometimes). It can also be highly engineered.

Let's say you've decided on your ingredients (sugar, flour, eggs, butter, baking powder, etc.), and it is now time to process the cake. Let's assume a very simplified processing procedure with just two steps: 1. mixing the batter and 2. baking.

By changing the way we do these processing steps, we can influence our structure. If we don't mix the cake batter well enough, we may have an inhomogeneous mixture with clumps of flour, sugar, and butter. If we don't bake long enough, we may not develop air cells in the cake, our gluten may not set, or our starch molecules may not gelate.

The results of these processing steps result in structures that can be quantified. Air cells are quantified by their size, shape, and overall volume. Starch and gluten form the cake's complex structural matrix—including crosslinked networks of gluten proteins and gelatinized arrangements of starch molecules. We won't get that detailed right now, but for those interested, van der Sman and Renzetti (2020) provide a general, but detailed, description of the structural features that arise during baking, with good structural depictions.

These structural features affect the mechanical properties of the cake: without the air cells, our cake would be too dense and hard, and without sufficient starch gelation, the cake would remain runny. If it is inhomogeneous, the cake may be too gooey (viscous) in spots and incohesive (crumbly) in others. There would be no homogenous connective matrix holding the cake together. Similar issues arise if the starches don't gelate or the glutens don't set.

Ultimately, the combination of these properties affects the performance of the cake—leading to a "good" or "bad" cake. Here, we'll highlight the "eatability" and "aesthetic" of the cake: you want the cake with the right mechanical response when chewing (spongey, not too crumbly, which are affected by both the mechanical response and the cohesion of the cake particles), and you want it to look and feel right when cut: it shouldn't crumble into pieces.

The Materials Paradigm to the System Design Chart

The description above lists processing steps, structures, properties, performances, and components of the Materials Paradigm. We can take this a step further, though, and graphically represent about how these paradigm features affect each other. This representation is called System Design Chart (SDC), and we use it as a powerful model of the materials system that aids us in the design and optimization of materials.

So, the paragraphs above constitute a so-called "Materials Summary." It describes the material and its application in plain language. Figure 1.5.1 shows the translation of the Materials Summary into an SDC, in which we've organized columns by the processing, structure, properties, and performance aspects of the cake. Note that this is a highly simplified version of an actual SDC. It includes only some processing steps (excluding, for example, cooling or the details of baking, like humidity and temperature control). The processes shown, however, are directly related to the important structures of the cake: the porosity, matrix structure, and homogeneity. Each of these structures can be quantified. For example, we can measure the size, shape, and distribution of air cells or the degree of interconnectedness of the gluten matrix. In turn, the properties of the cake that are critical to performance are connected to the relevant structures and performances.

The lines indicate strong connections within the system, but the strength and importance of these connections is always up for debate among the experts. To fully understand just how strong these connections are, we need to identify connections of interest (e.g., mixing time and homogeneity) and then devise an experiment (or theory or simulation) to measure whether the process does indeed influence the structure. That is, does mixing indeed affect the homogeneity of the baked cake? We might discover that this connection really isn't that important, or we might be able to directly correlate a mixing time with a measure of the homogeneity of the cake—a valuable piece of data for a cake-maker. The job of essentially any materials scientist or engineer is to identify and explore these connections.

Now, while a brief introduction to MSE and some basic knowledge allows a person to translate a simple Materials Summary into a System Design Chart, understanding the details of the connections between processing-structure and structure-properties requires domain expertise. In the example in Figure 1.5.1, we've identified only strong, well-understood connections: The mixing process will greatly affect the homogeneity because how much one mixes controls how effectively the ingredients are mixed. Similarly, mixing incorporates air into the cake, which is dependent on the speed at which the mixing occurs. Therefore, mixing directly influences both the structural features of porosity and homogeneity. Baking, on the other hand, doesn't appreciably affect the homogeneity of the cake because the ingredients will not change position much during baking time. The time, temperature, and humidity of the baking settings, however, do determine the size, distribution, and shape of the air cells.

In this way, we can construct full system maps connecting important processing steps, structures, properties, and performance. In Figure 1.5.2 we show a more complex diagram used in the design of a gear steel by Evanston company QuesTek. The details regarding the steel's metallurgy are well beyond what we'll address in this class, but if you continue your journey in MSE, you may one day work with a system like this! This SDC was used to produce a new steel (Ferrium C64) that is now used in rotorcraft and auto racing, two extremely high-performance applications.

Once we have the SDC, the next step of a materials sciencist or engineerg to consider the connections (e.g., mixing speed $\rightarrow$ porosity and porosity $\rightarrow$ cohesive strength) and figure out how to explore these connections. Perhaps we'll do this empirically by systematically changing mixing speed while holding other factors constant, then quantify the porosity with a microscope. Then, we could use a computational technique to predict how the porosity affects the cohesive strength. This approach helps scientists and engineers identify questions of interest and design experiments.

So—cake is a material. So are aluminum, steel, polyurethane, concrete, and wood. In each case we must consider how we process the material to yield a specific structure. These structures give us properties, which combine to give us performance. In this class you will start thinking like a materials scientist or engineer: making the connections between materials processing, structure, properties, and performance, and considering how you might control processing and structure to get the properties and performance that you want.