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Metal Fundamentals

  • Posted 10.09.07
  • NOVA

Despite its ubiquity in our lives, most of us know little about metal. Why does it bend when other hard materials like ceramic or stone don't? What is the difference—a crucial one, it turns out—between hardness and toughness in metal? And why does it get stronger the more defects it has? In this interview with Dr. Rick Vinci, a Stanford-trained materials scientist and engineer at Lehigh University, find answers to these and other basic questions about metals, including how it all relates to that wonder of steel technology, the samurai sword.

The tip of a sword

The blacksmiths who crafted samurai swords were wizards with metal, whose malleable nature helped to make this lethal yet alluring weapon a perfect blend of function and form. Enlarge Photo credit: © WGBH Educational Foundation

The stuff of metal

NOVA: Very simply, what are metals?

Rick Vinci: Metals are elements. They are made up of single kinds of atoms. Most of the elements on the Periodic Table are metals, even though we don't think of them as metals. The common ones that people are familiar with are things like iron, titanium, copper, gold, and all of those have simple characteristics: They have excellent conductivity of heat and electricity, and, perhaps more importantly, they have this ability to bend, take on new forms, and also to have their mechanical properties be controllable.

By adjusting their chemistry, by adding a little bit to them or taking something away, by controlling their heat treatment, their temperatures, and the rates that they are heated and cooled, you have a lot of control over their properties. This makes them very, very versatile and really dependable for the modern society that we live in.

What sort of chemical changes are you talking about?

Well, if you start with iron and then add a little bit of carbon, you get steel. Carbon gives the iron much more strength than it would have by itself. And you can add other things. For instance, if you add chromium atoms, you create stainless steel, and the presence of chromium gives it this ability to resist rusting. It is a little bit like being in a kitchen and adjusting the recipe by adding a pinch of salt, or maybe saying, "This could really use just a hint of lemon," and somehow that little bit really changes the taste of the food.

Screenshot of the

In our "Structure of Metal" interactive (see Editors' Picks), watch how metal behaves at the atomic level. Enlarge Photo credit: © WGBH Educational Foundation

It seems paradoxical to me, but if you add defects to a metal, it gets stronger, right?

Yes. The great thing about metal is that its atoms are able to rearrange relatively easily, using defects as a way of moving around, and this allows us to change the metal's form. And if we start to build up more and more defects, they start to distort the structure, and the atoms no longer move as easily. The result is we build up the strength. So this is another way that we can tune up the properties of the material, by controlling the number of defects that exist inside.

So the number of defects actually determines how strong it is?

Yes. If I take a piece of copper tube, for instance, it's relatively easy to bend it the first time. And if I were to install it in a home or business, I could straighten it out as needed. But if I make a mistake and try to change the shape of it again, it gets much more difficult to straighten it out the second time and even more difficult the third time. Because every time I bend it, I create lots of microscopic defects inside, and the more defects I have, the stronger the material gets.

When metals fail

So are defects related to why metal structures fail?

Many disasters that people are familiar with are due to design, quite often because whatever failed was not designed appropriately for that particular situation. One of the best-known failures due to an actual metal problem is the Titanic failure. Friends of ours in the metallurgical community are fond of saying "Why did the Titanic sink? Well, because it hit an iceberg." Ultimately, that is the reason why it sank. But why it sank so quickly is another issue, and the National Institute of Standards and Technology has shown quite clearly that it was the quality of the metal that led to the rapid failure.

A hammer head

They may look identical, but the metal on the striking face of a hammer is heat-treated differently than that on the rest of the head. Hardness at the front is traded for toughness at the back. Enlarge Photo credit: © Sondra Paulson/istockphoto.com

Before quality-control processes were improved to the state of the art we find today, it was quite possible to create different batches of steel that would have different properties from one batch to the next. Especially if the impurity content was too high—if, for instance, you had too much sulfur in your steel. Sulfur is a embrittling agent, so rather than strengthening the steel like carbon does, sulfur atoms actually cause it to crack more easily.

So if you have rivets holding your ship together and there is too much sulfur content in there, when the rivets undergo some stress, say from the ship hitting an iceberg, rather than stretching and bending in order to absorb the energy, they just shear off quite easily. The Titanic is an example in which poor toughness due to lack of control of the materials processing unfortunately led to the rapid sinking of the ship.

"In steel, there's always a tradeoff between hardness and toughness."

How so exactly?

When the ship hit the iceberg, a crack developed, and rather than that crack going a small distance and letting a little bit of water in, leading to the eventual sinking of the ship, instead the crack grew very, very quickly because of the brittleness of the metal. All of the energy from hitting the iceberg went into growing this huge crack, which then breached multiple bow sections of the ship, allowing water to flow into compartments that the designers thought would withstand such an impact and keep the ship afloat long enough to get people safely evacuated.

You mentioned toughness. What's the difference between hardness and toughness in a metal? They sound similar to me.

Well, consider the common hammer. We really need the striking face of the hammer to be quite hard, so that when we're hitting it against another hard object like the head of a nail we don't develop permanent dimples on the face of the hammer. But back where the wooden shaft joins the body, first of all we don't need such high hardness because it's not a striking surface, but second of all it would actually be quite dangerous to have it be as hard as the face.

Gassan Sadatoshi preparing metal for a samurai sword

Gassan Sadatoshi, one of Japan's most renowned contemporary swordsmiths, prepares to forge the two types of steel that will eventually constitute the blade of a new samurai sword. Enlarge Photo credit: Courtesy of Parthenon Entertainment Ltd./Attention Films

In steel, there's always a tradeoff between hardness and toughness. If I made the back-of-the-striking-face part of the hammer very, very hard, it would also not have very much toughness. As a result, if I used it in a manner for which it was not intended and it started to crack back there, it could fly apart, causing various parts to hit the user and potentially causing serious injury. Instead, this hammer is heat-treated differently on the striking face than on the middle part, to get the hardness you need at the end while retaining the toughness you need behind the end for safety.

Despite its hardness and toughness, metal can bend, unlike ceramic or stone. Why?

Metals have this inherent capability in which when you apply a stress, the atoms can move, and they can swap locations within their general arrangement. They don't mind being in this position or that position, and for that matter, when they move from one position to another, they really don't mind temporarily being in between.

Many of the other hard materials, such as ceramic, might be made up of two different kinds of atoms. For instance, you might have a metal atom and an oxygen atom together, and they are arranged in a very specific way. They don't want to switch positions, and even if they did, being in the intermediate position on the way to switching requires an immense amount of energy. If you apply stress to it, the atoms will start to move, and there will be greater and greater resistance to the atom movement, until ultimately it is easier for the material to fracture than to change shape.

So if you drop a metal coffee mug, it will typically dent when it lands. But if you drop a ceramic mug, it really doesn't have an ability to dent. Instead it cracks, and you have a broken mug.

A cracked windshield

Like cracks in a windshield, which are stopped from going all the way through the windshield by a layer of plastic placed between the two layers of glass, the core of a samurai sword blocks the path of defects that start on the sharp outer edge. Enlarge Photo credit: istockphoto.com/asterix0597

The science of swordsmithing

Getting to the samurai sword, how is it an example of engineering metals?

The sword is an excellent example of people, first of all, understanding the requirements of the particular application. Understanding that the cutting edge of the blade must have a certain sort of characteristic—hard so that it retains its sharpness and its ability to penetrate—while the back edge must have a different sort of characteristic—it must be tough in order to withstand damage and fracture. And then the ability to understand how to achieve these properties through the right choice of materials and the right processing.

It's an excellent analogy of what people do when they build a complicated, modern engineered structure. You look at different parts of the structure, and you first answer the question, "What is the requirement of this part of the structure?" and then second, "How do I achieve the requirement by choosing the right material and the right processing to get the particular set of properties that I need?"

"One can mass-produce swords, but one cannot mass-produce swords that are also objects of art."

And so all the steps we see in making the samurai sword—mixing elements and such—is about controlling such properties.

Exactly. The person designing the sword, or for that matter designing a girder to build a building, has to know exactly what the requirements are for that particular piece of metal. For certain applications, he might need it to be both very hard and very strong. For another application, he might want it to be very ductile, very reformable. And for a third application, maybe somewhere in between. Maybe you need corrosion resistance, because it will be built into a ship that will sail on the salty oceans.

So the scientist or engineer or blacksmith needs to understand those sorts of requirements and then adjust all the treatments and the chemistry of the material to match that particular set of requirements. They have a whole toolkit of options open to them, and they choose from that toolkit—which particular processes, which additives they want to put into the steel, to get the ideal properties for the application they have in mind.

A samurai and his sword

In the hands of a warrior, the samurai sword can be a fearsome weapon. But it is also an object of beauty and a potent symbol of the samurai code of honor. Enlarge Photo credit: Courtesy of Parthenon Entertainment Ltd./Attention Films

Ideal properties such as hardness and toughness, in a samurai sword.

Right. Inside the sword we have lower-carbon steel, and its primary property is that it is quite tough. If there is a small crack introduced in the hard blade, as is probably inevitable in the course of battle when you are hitting armor, instead of that crack running through all the blade the way it would on a single piece of glass, it goes a little way and then the core steel arrests or stops the crack.

Your car windshield is a nice example. A windshield is actually made of two layers of glass with a sheet of plastic in between. If you get a crack in the outer part of the glass, it will go through that layer and hit the plastic, which is tough, and the crack will stop. This is why when you get a small rock dent on your windshield, your entire windshield doesn't immediately fall into your lap.

Objects of beauty

Can we make a sword today that's as good as the samurai sword of yore?

There is no mystery at this point about exactly what the metals are, so yes. It is understood what makes up a samurai sword, and there is no inherent problem with reproducing it using industrial techniques. But what, of course, would be lost is the artistry of adjusting each individual sword and making it a unique item. One can mass-produce swords that will certainly very effectively act as cutting instruments, but one cannot mass-produce swords that are also objects of art.

So you think samurai swords are beautiful.

I do think they are beautiful, beautiful pieces of art, and part of their beauty is the functionality associated with them. They are devices that are at once artistic and extremely purposeful. There are many different ways that one could make a cutting implement, but the artists involved in creating the samurai sword opted not to take the easy path, to just make something long with a sharp edge. Instead they created an elegant device that has both a very serious purpose and a serious beauty to it as well.

Rick Vinci

"They created an elegant device that has both a very serious purpose and a serious beauty to it as well," says Rick Vinci of the Japanese smiths who fashioned the samurai sword. Enlarge Photo credit: © WGBH Educational Foundation

And would you say it also embodies ideals valued by the Japanese?

Yes. The samurai sword is not just a killing implement. It also embodies all sorts of concepts like honor and trustworthiness and responsibility. The people who carried these were supposed to have personal characteristics and attitudes that were perhaps different from what an ordinary soldier might have when he carried a spear or something of that nature. These people had to have an unusual set of qualities, and the sword is the representation of those.

"The ancient samurai sword is a very impressive piece of technology."

Even if we can make a sword as good as the ancient swords today, are you still impressed by what the swordsmiths of early Japan were able to do?

Absolutely. These were people who could tell by looking and feeling and smelling exactly what was happening inside the steel. They had the ability from experience to make minute changes in order to adjust imperfections in the process or in the original starting material. Now we use computers and highly automated systems to do these things for us. We achieve the same goal, but we do it with the benefit of many, many decades of science and technology behind us. They did it on an individual person basis and with the benefit of a few years to decades of personal experience and practice.

And were you impressed by the sword made in the film?

I couldn't do it! [laughs] What always amazes me is the degree of complexity involved in making something that is as simple ultimately as a sword. What is it at its heart but a stick of metal that cuts things? But it is so much more than that, and it requires so many different steps, and if any one of those steps goes wrong the entire sword would be ruined.

So even with all your scientific knowledge, you are still impressed?

Absolutely. The ancient samurai sword is a very impressive piece of technology. It required some very deep understanding of what's going on inside the metal in order to achieve a rather sophisticated set of properties.

This feature originally appeared on the site for the NOVA program Secrets of the Samurai Sword.

Interview of Rick Vinci conducted on April 25, 2007 by Doug Hamilton, producer of "Secrets of the Samurai Sword," and edited by Peter Tyson, editor in chief of NOVA Online

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