What About the Metals we Use

Metals are strange stuff. Everyone has bent a bit of wire or sheet metal back and forth until it has broken – that's what you do when you need to cut a piece off but have no cutting pliers about you. But how does it work?

The reason metals appealed so much to their ancient discoverers was that, unlike tools made of flint or other stony materials, metals can bend rather than just snap under stress. They can also be hammered, bent, or otherwise forced into desired shapes – especially with application of heat.

Metal atoms are held to each other by electrical forces, mediated by outer shell electrons so lightly bonded that they really constitute and 'electron gas' in the metal. This being so, a particular metal atom, when forcibly moved, can bond into a new location as well as it did in its original one. The atoms of stony materials, by contrast, are bonded by exchange of electrons between particular atoms. When those bonds are broken, they are not self-repairing. And so metals yield while stony stuff snaps.

The mechanism of yielding involves the shifting of ranks of atoms over each other – a process made easier by the occasional out-of-place atom called a 'dislocation'. When stress is applied, motion begins more easily at the out-of-place atom and spreads to its neighbors. Bump-bump, the ranks of atoms move across each other, and dislocations move as well. The dislocations within it are driven, eventually forming tangles that resist further movement. This is why, as we bend the wire back and forth, it resists more strongly just before it breaks. The metal has become harder. Metallurgists say the metal has been 'work-hardened'. Since the tangled dislocations can't move any further, the material stiffens. When we keep on bending the material, it can't bend any more so it cracks and breaks in two.
People of my father's generation, seeing a metal part that had broken were likely to say: 'It crystallized.' Nothing could be further from the truth, because all practical metals consist of a jumble of randomly orientated crystals of various sizes. All metals are crystalline. In a crystal, atoms are arrayed in a regular, repeating pattern, but the atoms of a given metal may be able to arrange themselves in different patterns. This is the subject of crystallography – a subject that come into being only once we humans discovered X-rays and shone them into solids. Reflections of the X-ray beam from a crystal's various planes of symmetry from a mysterious-looking array of dots on X-ray film or on the screen of an electronic detector. From the locations and intensities of these reflected spots, a mind-breaking mathematics allows adepts to decode the structure of the crystal.

When iron or steel first solidifies from molten form, its crystalline form is called austenite. As it cools further, it progressively transforms to another structure that is more stable at lower temperatures, martensite. The iron cylinder liners of older engines are made austenitic because that form has a higher coefficient of thermal expansion, closer to that of the aluminum pistons. This made seizure less likely and allowed use of closer, less noisy cold clearance between cylinder and piston. Parts requiring high strength are heat-treated to transform austenite into the stronger martensite. Metallurgists devise or discover ways to achieve desired combinations of metal properties through means such as heat-treatment, alloying, or cold working.

A recent technique is the manufacture of connecting-rods from metal powders. This can control crystal grain size and allows parts to be made which are very close to 'net shape' - the shape of the finished part. This is valuable as every machining or grinding operation adds costs. An unfinished powder part can be broken by hand like a biscuit, but once its substance is sintered or fused by heat into solid form, it becomes strong enough to be used to make the connecting-rods or modern high-revving motorcycle engines.

When most rods were either cast or forged, the separate big-end cap, to the retained by bolts or studs, had to be produced separately, then fitted to the rod for final machining. More costs! Someone had the bright idea of finish-machining the rod in one piece, then fracturing the cap off it with sudden force. Small strategically placed notches act as crack-initiation points, and when an impactor applies sudden force to the cap, it is broken free of the rod in a predictable way. The irregularities of the fracture surface act to key cap and rod together, preventing relative movement at the joint.

Connecting-rods for the highest-performance applications continue to be machined from forgings. Specialized metals with novel properties continue to be developed, while many elements of today's machines continue to be made from tried-and-true metal alloys from decades ago.
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