What gives a metal tensile strength vs impact strength?



What gives a metal tensile strength vs impact strength?

In: Chemistry

The strength of a material is based largely on what happens to it at a molecular level when it receives a particular force, whether it be tension or compression (or other forms like torsion, but that’s not as relevant right now). Basically, it comes down to how the particles of a material are bonded together. The most common distinction in solids is amorphous vs. crystalline – in an amorphous solid, the particles are more random, jumbled, and bonded pretty much to any other particle near enough to them; in a crystal, they are arranged with a specific pattern (for example, they might lie in flat, sheet-like layers of squares or form small pyramids at a microscopic level).

Tensile strength – the ability to be pulled through tension and deform without breaking – causes a shift at the molecular level; basically, when you pull on a ductile material, some of the bonds break apart, the particles shift, and new bonds form before the particles actually break apart. Metals tend to be more ductile than brittle materials like ceramic, because their atoms form bonds very easily (the bonds are formed by sharing electrons, and metals tend to conduct electricity, so electrons can freely move to fill gaps and create new bonds before the material breaks). As a strict comparison between types of metals (e.g. varieties of steel),
crystalline forms are more ductile because the electrons can take a more direct, linear route rather than passing through the randomness in amorphous metal. This is the main goal in annealing – cooling a hot piece of metal very slowly allows internal crystals to form (such as pearlite, in steel), allowing an increase in ductile strength (and often electrical conductivity!).

Impact strength – the ability to withstand a large compression force without breaking – is largely based on the opposite effect. If the particles in a material slip past each other in the direction you hit it, the material would deform very easily on impact. Therefore, amorphous solids tend to have greater impact strength, since the bonds are more random and can’t easily all be broken in one direction. (Most plastics naturally have fairly high impact strength, because the molecules are very long chains that bond together in almost any direction.) Back to metals, though: this is one of the goals of quenching. When a metal is hot enough, the bonds start to break down and the particles start to move more freely, almost like in a liquid but still rigid enough to hold together. Cooling it very quickly allows the metal to retain that randomized, amorphous structure (like the “austenite phase” in steel), so it has a very high impact strength.
Last point (I promise!): metal workers will often quench a steel piece to form that amorphous, high-impact-strength austenite layer on the outside, but then temper it (heating it several times, slowly, to get the same effect as annealing but at a low temperature) to form pearlite on the inside. This allows it to withstand high impacts on the outside, distributing the force around the internal crystaline structure, while also allowing it to be slightly ductile and withstand tension and torsion.