Systems engineering in materials design

Every few years, international political tensions disrupt the global supply of rare earths, and every time this results in a scramble to find more ore or to build refineries faster.¹ China controls around 70% of rare-earth processing, but even if you solved that bottleneck tomorrow, you would still have a problem: rare earths are not interchangeable commodities.

Neodymium, for example, is central in permanent magnets, which drive wind turbines and electric motors. The problem is that this element cannot be swapped with a different element, because that would force you to redesign the corresponding alloy. A different alloy needs different processing. Different processing produces a different structure, different magnetic properties, and different performance in the final device. One substitution, and the whole application doesn’t work anymore. Why?

The cascade from composition through processing through structure to performance is not specific to magnets. All materials work this way. We tend to think of a material as a property on a datasheet, but it is really a web of cause-and-effect relationships that goes from atoms to applications. There is a framework that maps these relationships as a system: process-structure-properties-performance, or PSPP. The rest of this post is about what this framework looks like and where it comes from.

What is a material? #

Before exploring how systems thinking enters materials science, I think it helps to be precise about what a material is. However, when I turned to standard textbooks looking for a definition, I couldn’t find one.^{2 3} They classify materials, describe their properties, discuss their history, but never say what the word actually means.

So I searched further for a working definition of “material”, and to my surprise, I found that defining the term is not at all straightforward.^{4 5 6} As I understand the problem, three factors make this harder than I initially assumed:

1. The word “material” is treated as a primitive — a word everyone understands well enough that it doesn’t need a formal definition.
2. The field is interdisciplinary, and no single discipline has felt the need to own the definition.
3. There is no clear boundary between a “chemical” and a “material.” The distinction depends on length scale, but where do you draw the line? Is a single molecule of polyethylene a material? What about a monolayer or a colloidal suspension?⁷

Eventually I found two definitions that I could work with. In Materials Chemistry, Fahlman defines a material as:⁸

Nature Materials gives a similar definition, but with a broader scope:⁹

These definitions, together with the broader discussion cited earlier, point to three features that seem to be common to all materials:

1. They are condensed-state substances.
2. Their properties emerge from the interactions, assembly, and defect structure of their constituent subunits.
3. They are created or designed for a specific purpose.

In other words, a material is not raw matter sitting in the ground: it is matter that someone has processed with an end in mind. The definition itself encodes a relationship between processing, structure, and purpose, and this entanglement is not just a matter of semantics, because the concept cannot be reduced to a single property or a single scale. If the definition is this entangled, perhaps the right answer is not a sharper definition but a better framework to think about materials.

Three questions and a chain #

The study of materials is often divided into three disciplines: materials science, materials engineering, and materials design. However, this division is cleaner on paper than in practice. A more useful way to think about them is as three questions, each building on the last.

The first question is why. Why does this material behave the way it does? This is materials science, which investigates the relationships between a material’s structure and its properties. A materials scientist synthesizes a new material and asks what gives it its characteristics.

The second question is how. How do we make a material behave the way we want? This is materials engineering: designing or manipulating the structure to produce a predetermined set of properties. Where the scientist asks why something works, the engineer asks how to produce it and make it work differently.

The third question is which. Which material and which process satisfy a specific need? The answer takes different forms depending on the starting point:

• Materials discovery screens all possibilities.
• Materials design explores a specific region of the design space, often adding new materials or properties to it.
• Materials selection picks the best candidate from a space that is already known.

All three sit close to the application and depend on the answers to the other two questions.

A material’s properties depend on the spatial organization of its constituents at every hierarchical level, from atomic arrangement up to macroscopic form. But structure is not static. At any moment, a material carries structural features that have not yet reached thermodynamic equilibrium. These features reflect not only what it is made of, but how it was made and what it has endured in service. Processing stays with the material long after it is produced.

This causal chain — processing determines structure, structure determines properties, properties determine performance — is the central paradigm of materials science and engineering, and it is known as process-structure-properties-performance (PSPP).^{10 11} The diagram below shows how it works:

Materials science typically works the chain from left to right: you start with a process and ask what properties come out. This is the forward problem. You pick apart individual links and examine them in isolation, like how does this heat treatment change the grain size, and how does grain size affect hardness? Each link gets its own experiments, its own models.

Materials design goes the other way. You start with what you need the material to do and work backward: what properties would deliver that performance, what structures would produce those properties, and what processing would create those structures? This is the inverse problem, and it only becomes tractable if you can hold the full chain in view at once.

Materials as systems #

So far, PSPP looks like a causal chain where one thing leads to the next. But that picture is too simple. PSPP treats a material as a system, and that word carries specific meaning.

A system is an arrangement of elements, interconnections, and a function that work together to fulfill a purpose.¹² The elements can be tangible (atoms, grains, phases) or abstract (a model, a design constraint). The interconnections define how elements influence each other. The function is what the system is designed (or has evolved) to do.

A system’s behavior over time is tracked through stocks and flows. A stock is a measurable quantity that accumulates. Flows are the rates at which a stock changes: inflows add to it, outflows subtract. When inflows and outflows balance, the system reaches dynamic equilibrium. In a material, structure is the central stock. Every processing step — every heat treatment, every deformation pass, every hour in service — is a flow that deposits or removes structural features. The microstructure you observe at any moment is the cumulative record of everything the material has experienced.

Consistent behavior patterns in a system arise from feedback loops: circular relationships where the system’s output influences its own behavior. Materials are full of them. Grain growth during annealing slows as grains coarsen, because larger grains have less boundary energy driving further growth. Crack propagation accelerates as a crack lengthens, because the stress concentration at the tip increases with crack size. These loops (some stabilizing, some reinforcing) are what give materials their characteristic responses to time and environment.

What makes a system more than the sum of its parts is emergence: the interactions between components produce behaviors that no single component exhibits alone. A pile of iron atoms is not steel. The arrangement matters, and the properties that result from that arrangement cannot be deduced by studying iron and carbon in isolation.

A material is a hierarchically organized system composed of subsystems, each operating at a different length scale. At the atomic scale: bonding and crystal structure. At the microstructural scale: grain size, phase distribution, and defect populations. At the macroscopic scale: geometry, surface condition, and manufacturing history. Each level has its own characteristic structures and its own models, but none is self-contained, and changes at one scale propagate up and down the hierarchy.

From Smith to systems engineering #

The idea that materials behave as systems did not arrive all at once. In his cornerstone article on materials design, Olson traces the origins of this idea to the metallurgist and historian of science Cyril Stanley Smith, who was among the first to describe materials as hierarchical systems.¹³ Clarence Zener extended Smith’s picture by adding the path dimension: processing history. In any real material, some structural features have not had time to equilibrate. The system is never at rest, and it carries the memory of how it was made.

Morris Cohen then introduced the principle of reciprocity between structure and properties: properties can be rationalized in terms of structure, but structure can equally be rationalized in terms of the properties it produces. If the structure–property link runs both ways, then tools built to explain can also be turned around to design.

What was still missing was a way to integrate these insights into something operational. That came from systems engineering. The first step is analysis to identify the problem and decompose it into component subsystems and their interactions. This step is followed by synthesis: develop models for each subsystem, integrate them to generate candidate solutions, build prototypes, and test. Feedback from testing drives iterative redesign until the objectives are met. Applied to materials, each scale becomes a subsystem with its own models, its own experiments, and its own interface to the scales above and below.

Reading the chain backward #

For most of its history, materials development advanced through testing compositions, adjusting processes, and accumulating know-how. Serendipity played a significant role in this process, and still does.^{14 15} If we wanted a better magnet, we made magnets and tested them until something worked.

But if the links between processing, structure, properties, and performance are causal in both directions, then the chain can be read backward. Start with a need. Translate it into performance requirements, then into target properties, then into the structures that would produce them, and finally into the processing routes that would create those structures.

This changes what the rare-earth problem actually is. Without PSPP, a supply disruption is a procurement problem: find more ore, build more refineries, secure the chain. With PSPP, it is a design problem: identify which scales are coupled to the element you are losing, which feedback loops constrain substitution, and which processing routes are available for candidate replacements. Those are different questions, and they lead to different responses and different timescales.

The practical tools for carrying out this inversion — computational modeling, materials informatics, integrated computational materials engineering — are subjects for future posts. What matters here is the shift in how we ask the question.