In this blog post, we’ll explore why aluminum foil and steel spoons—even though they’re both metals—have such different properties, and examine the principle of precipitation hardening, which makes metals stronger.
Every year on Liberation Day, you can see the movie ‘Liberation Day Special Envoy’ airing on TV. This film tells the comedic story of two prisoners who escape from prison the day before they are scheduled to be released as part of a Liberation Day amnesty, causing all sorts of chaos along the way. The protagonist, Mu-seok, played by Cha Seung-won, spends a whopping six years preparing for his escape, and his method alone is enough to make people laugh: he digs a tunnel using a spoon.
Interestingly, there have actually been real-life cases of escapes using this very method. In places like Surabaya, Indonesia; Breda, the Netherlands; and Alcatraz Prison in San Francisco, USA, spoon-digging escapes straight out of a movie have taken place, and these incidents have captured the public’s attention. One interesting point here is that most people who hear about these cases are simply amazed by the escapees’ incredible determination, without paying much attention to the properties of the spoons that actually made it possible.
Spoons made of stainless steel. People take it for granted that spoons are very hard and can withstand the immense stress of scraping against a wall for a long time. The material that makes up the spoon—steel—is, of course, a metal. What image comes to mind when you hear the words “metal,” “iron,” or “steel”? You probably think of something hard and sturdy first. This is because most metal products we see around us are very hard. However, among the items we frequently use in our daily lives, there is one that is completely different from this general image of metal. It is aluminum foil, which is as soft as tofu. What secret lies behind the fact that aluminum foil and spoons—both metals—exhibit such vastly different properties? Could it be that aluminum foil, if subjected to a certain process, could become hard enough to dig a tunnel, just like a spoon?
There’s a saying that “a woman’s transformation is innocent.” It’s not uncommon for people to become the talk of the town because their appearance changes drastically before and after applying makeup. The metallic materials around us also exist in “transformed” forms. Pure metals that haven’t undergone any transformation are extremely soft and easily deformed, so they’re far removed from the image of metal we typically imagine. A classic example is a pure gold ring that even a one-year-old child can easily bend. Pure iron, with a purity close to 100%, is also too soft, limiting its range of applications; it’s mainly used for research or experiments. Aluminum foil can also be considered a “raw” metal that hasn’t undergone this transformation process.
So how can aluminum foil—which is merely soft—be transformed into something as hard as a spoon capable of piercing a wall? While there are several methods, we’ll focus here on precipitation hardening, which is relatively easy to understand. Interestingly, just as the process of applying women’s makeup may be complex but the underlying principle is simple, the basic principle of precipitation hardening is also relatively easy to grasp.
You’ve probably heard of supersaturation. This phenomenon occurs because the amount of solute a solvent can dissolve varies with temperature. Generally, when a solid dissolves in a liquid, a higher temperature allows for greater solubility, whereas for gases, solubility tends to decrease as temperature rises. If you dissolve as much salt as possible at a high temperature and then lower the temperature, the amount of salt that water can hold decreases, causing the remaining salt to precipitate as crystals. The formation of fog at dawn follows a similar principle: as the temperature drops, the amount of water vapor the air can hold decreases, causing the remaining water vapor to condense.
This same principle applies to precipitation hardening. However, because both the solvent and the solute are metals—which are solids—the process cannot be explained as simply as it is for liquids or gases. This is because, while particles in liquids and gases move actively, allowing precipitates to form easily, particle movement in solids is highly restricted. Therefore, for precipitation to occur smoothly in a metal solid solution—that is, a state where two or more types of metal atoms are uniformly mixed—several steps are required.
First, the metal is solution-treated at a high temperature to ensure that the alloying elements are mixed sufficiently and uniformly. Next, rapid cooling (quenching) is performed to maintain a supersaturated solid solution state. Although precipitation should occur in this state, it does not proceed easily at low temperatures because atomic movement is very slow. Therefore, an aging treatment is performed, in which the metal is reheated to an appropriate temperature and held there for a certain period of time. During this process, as atoms move, extremely fine precipitates form and become densely distributed throughout the metal.
Let’s imagine taking a piece of aluminum foil, shaping it into a spoon, and then “transforming” it in this way. What principle allows it to become hard enough to dig a tunnel? The secret is as follows. Breakwaters are installed along coastlines to block waves. Without breakwaters, waves would surge onto land without much resistance. Conversely, if breakwaters are installed very densely, the waves cannot advance easily.
Now, let’s think of the movement of waves as the deformation occurring inside the metal. The fine precipitated particles act like breakwaters. Since pure metals have almost no such obstacles, deformation occurs easily—which is why the aluminum foil we use is so soft. In contrast, metals that have undergone precipitation hardening contain a large number of fine precipitates that hinder deformation. When viewed under a microscope, countless obstacles are densely packed throughout the material, effectively suppressing deformation. Ultimately, this prevents the material from deforming easily, resulting in high strength. This is the secret behind how aluminum foil, which is as soft as tofu, can become as hard as a steel spoon.
As such, precipitation hardening is a highly useful technology and is still widely used in various aluminum alloys today. If a product is labeled as made of “aluminum alloy,” it is likely a material whose strength has been enhanced using this principle. A prime example is duralumin. Duralumin is lightweight and highly strong, and has long been used in various fields, including aircraft structural components; even today, it is widely used in the aerospace, transportation, and industrial sectors in the form of advanced aluminum alloys. In addition, various precipitation-hardening aluminum alloys are used in products such as bicycles and laptops that require both light weight and high strength.
Precipitation hardening is not a technology exclusive to aluminum. Some precipitation-hardening stainless steels possess much higher strength than standard stainless steels and are utilized in various fields, including the aerospace industry, automotive parts, and industrial equipment. Furthermore, beryllium copper—used in various industrial applications such as wrenches, screws, fasteners, and molds—is also a representative precipitation-hardening alloy. As such, precipitation hardening is one of the core technologies that plays a crucial role in achieving the desired mechanical properties in today’s metal industry.
