This blog post examines the principles of metal diffusion technology and explores what changes might have been possible if it had been applied to the ship’s hull design at the time, using the sinking of the Titanic—a symbol of early 20th-century technology—as a case study.
On April 10, 1912, a ship departing from Southampton, England, struck an iceberg and sank, its hull split in two. The tragic protagonist of this disaster, which claimed the lives of 1,500 of the 2,200 passengers as they sank with the ship, was none other than the famous Titanic. The Titanic was considered the pinnacle of technology at the time, boasting the title of ‘unsinkable’. Yet, despite this reputation, the Titanic sank on its maiden voyage. While people at the time may have viewed this as a simple accident, the sinking of the Titanic starkly revealed the limitations of modern technology.
What if the Titanic’s hull had been stronger back then? Perhaps the tragedy of colliding with an iceberg and sinking could have been avoided. Moreover, the movie Titanic, starring Leonardo DiCaprio, might never have existed. This film remains a masterpiece of the century to this day, deeply moving countless people. Without the sinking of the Titanic, we wouldn’t have the diverse emotions and memories evoked by this film. This is a prime example of how a page in history can influence art and culture.
However, if the person who built the Titanic’s hull had understood the ‘diffusion’ of metals well, the ship might not have sunk. Had they understood diffusion and properly adjusted the metal’s strength, the tragedy of the Titanic as we know it today might never have occurred.
When we hear ‘diffusion,’ we often picture perfume scent spreading through the air or a single drop of ink mixing in water. Indeed, some dictionaries define diffusion as the phenomenon where molecules spread from areas of higher to lower density or concentration within a gas or liquid. While this explanation provides an important foundation for understanding diffusion, diffusion in solids like metals involves a more complex mechanism. The fact that diffusion occurs in solids—materials that appear rigid and do not flow like water or air—may seem somewhat unfamiliar. Yet diffusion in metals happens frequently, albeit not visibly fast, and it is a crucial factor determining the properties and performance of the metal products we use.
Car motors, gears, and the steel plates used in aircraft and ship hulls are made of alloys. Even stainless steel kitchenware found in kitchens nationwide—many metals we know—are made of alloys rather than pure metals to enhance strength. One widely used method for creating these alloys is metal ‘diffusion’. Metal diffusion is the movement of atoms that occurs across the interface between two different materials when they are in contact. During this process, new alloys form, altering and strengthening the metal’s properties.
There are two primary mechanisms for metal diffusion: Vacancy Diffusion and Interstitial Diffusion. First, let’s examine Vacancy Diffusion. Metals are aggregates of atoms connected by metallic bonds between them. Even metals that appear smooth and solid on the surface contain empty spaces, or vacancies. Vacancy Diffusion refers to atoms diffusing through these empty spaces. When a metal atom moves into an adjacent vacancy, the position it vacated becomes empty again, prompting another neighboring atom to move into that new vacancy. This process occurs relatively slowly but induces significant changes in the metal’s microstructure.
The second mechanism, Interstitial Diffusion, differs from Vacancy Diffusion and is frequently observed in metals where atoms have significantly different sizes. It involves smaller atoms moving into the spaces between larger atoms. Compared to Vacancy Diffusion, where the probability of movement is low due to fewer vacancies relative to the number of atoms, Interstitial Diffusion generally occurs at a faster rate. Imagine a room nearly filled with golf balls and ping pong balls of similar size; this represents Vacancy Diffusion. A room filled with bowling balls and ping pong balls of markedly different sizes represents Interstitial Diffusion. Visualize the movement of the balls in each scenario. Interstitial Diffusion, occurring between atoms with large size differences, is primarily observed at the interface between gas and solid.
Diffusion is a phenomenon influenced by time. Therefore, the behavior over time can be divided into two cases: Steady-State Diffusion and Nonsteady-State Diffusion. The distinguishing factor between Steady-State Diffusion and Nonsteady-State Diffusion is the diffusion flux. Here, diffusion flux refers to the amount of mass diffusing per unit time and per unit area perpendicular to the interface between metal and metal or between metal and gas. Suppose metals A and B are in contact. If the amount of atoms from A moving toward B equals the amount of atoms from B moving toward A during the same time period, resulting in a net diffusion flux of zero, this is called Steady-State Diffusion. In Steady-State Diffusion, the amount of atom movement in each direction is equal. While diffusion actually occurs, it appears as if no diffusion is happening. Conversely, Nonsteady-State Diffusion describes the state we most commonly observe in nearly all situations. The diffusion of atoms in one direction dominates, meaning the diffusion flux value is not zero even when accounting for the diffusion of atoms in the opposite direction. For example, if atoms of A move toward B at a rate of 3 atoms per unit time and per unit area, while atoms of B move toward A at a rate of 5 atoms per unit time and per unit area, the diffusion flux value would be +2 atoms toward A. Outwardly, it would appear that only atoms of B are moving toward A at a rate of 2 atoms.
We have now explored how diffusion occurs in metals. Metals are indispensable in our lives, and their importance continues to grow. From steel plates used in large ships, airplanes, and automobiles, to everyday items like smartphone cases and kitchen utensils, metals permeate nearly every aspect of our lives. Isn’t it fascinating that metals, which appear solid and static, are actually actively diffusing? Even at this very moment, metals are constantly diffusing to transform into stronger, new alloys. Understanding this dynamic characteristic of metals not only deepens our comprehension of daily life but will also play a crucial role in future technological advancements.
