In this blog post, we’ll explore the meaning and applications of phase equilibrium diagrams—a key concept in materials engineering—by examining the changes in the state of water that we commonly encounter in everyday life.
While attending college, I often hear people ask, “What department are you in?” When I reply, “I’m in the Department of Materials Science and Engineering,” I observe a variety of reactions. While some people know what the department is, more often than not, they don’t understand what we study there or what we do with that knowledge. If we explain the meaning of materials engineering using the five elements, we can say it involves studying and working with materials such as wood (木), metal (金), and earth (土) to create things using the sun (日) and moon (月), as well as elements like fire (火) and water (水).
If we put the water we drink every day in the freezer, it turns into cool ice, and if we put it in our mouths and let it melt, it becomes water again. If we apply force to the ice, it cracks and shatters, and if we add sugar because we want sweet ice, the freezing point drops, so it may not freeze even at temperatures below 0 degrees. As such, many substances we encounter in daily life change their properties depending on the surrounding environment, and a phase diagram is a tool that allows us to visualize these changes.
A phase diagram is a graph that shows the phase in which a substance exists under specific conditions, with environmental factors such as temperature and pressure plotted on the x- and y-axes, while other environmental factors are held constant. Under conditions where two phases coexist, both phases can exist simultaneously along the boundary line where they meet. A prime example is the phase diagram for water: depending on pressure and temperature, water exists in three states—solid ice, liquid water, and gaseous water vapor. At the triple point, where all three phases are in equilibrium, ice, water, and water vapor can all coexist. At the boundary where water and ice coexist, a solid and a liquid coexist; at the boundary where ice and water vapor coexist, a solid and a gas coexist; and at the boundary where water vapor and water coexist, a gas and a liquid coexist. The phase equilibrium diagram for water is a phase equilibrium diagram that illustrates the states of a substance and the conditions of equilibrium for a single component. On the other hand, just as adding sugar to water lowers its freezing point, the properties of a substance composed of multiple components vary depending on the types and ratios of those components. Therefore, a phase equilibrium diagram for a complex substance must also illustrate how the material’s properties change based on the ratios of its components. A phase equilibrium diagram can be described as a graph that enables a comprehensive understanding and prediction of a substance’s physical properties, such as its boiling point, melting point, and regions of phase stability.
So far, we have examined single-phase equilibrium diagrams using water as an example. Through these examples, we learn how a material’s phase changes in response to environmental factors—such as temperature and pressure—as illustrated by phase equilibrium diagrams. When it is necessary to adjust the properties of a material to manufacture a product, phase equilibrium diagrams allow us to predict these changes without conducting individual experiments. They are like a map that points the way toward our goal and helps us avoid taking unnecessary detours. In other words, understanding how to read phase equilibrium diagrams—which can be considered a “map of materials”—will help save time and costs during product design and enable the efficient adjustment of material properties.
