The MSE Paradigm as a Framework for Science and Engineering

Hopefully, 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 actually performing 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 complex materials engineering questions systematically using the paradigm, allowing us to formulate a so-called Systems Design Charts (SDCs). This is an advanced topic - our majors usually don't construct one of these until their capstone projects, but we believe 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, working for any material system
3. it will be extremely useful for you if you decide to write a Materials Vignette at term'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 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. We quantify the air cells by their size, shape, and overall volume. The starch and gluten form a the cake's structural matrix - but we'll simplify this to a fraction of the matrix that is solid. 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 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 affect 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.

This description above can be graphically represented in a so-called System Design Chart (SDC) - a model that serves as a powerful tool in the design and optimization of materials. 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 a an SDC, in which we've organized columns by the processing, structure, properties, and performance of the cake. Note that this is a highly simplified version of an actually 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 the homogeneity. Each of these structures can be quantified. For example, we can measure the size, shape, and distribution of air cells or interconnectedness of 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, but the strength and importance of these connections is always up for debate among the experts at this stage in the process. To fully understand how strong these connections are, we need to identify connections of interest: (e.g. mixing time and homogeneity) and then devise and experiment (or theory or simulation) to measure whether mixing does indeed affect the homogeneity of the baked cake. Indeed, we might discover that it isn't that important, or we might be able to directly correlate a mixing time with a measure of the homogeneity of the cake. The job of the materials scientist or engineer is to 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 Fig. Figure 1.5.1, we've identified only strong, well-understood connections: The mixing process will greatly affect the homogeneity because the how much one mixes controls how effectively the ingredients are mixed. Similarly, mixing incorporates air into the cake based 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 appreciable 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, does determine the size, distribution, and 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 rotocraft and auto racing.

Once we have the SDC, the next step is 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 helps scientist and engineers identify questions of interest and design experiments.

So - cake is a material. So is aluminum, steel, polyurethane, concrete, or 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: to make the connections between materials processing, structure, properties, and performance, and to consider how you might control processing and structure to get the properties and performance that you want.