Dislocations are a condition in materials, where an atom is knocked out of place in the crystal lattice. Curiously, strings of dislocations actually increase the strength of metals. take steel-as cast, steel can be rather soft. However, hammering and forging the steel (which produces these dislocations) increases the strength by a huge factor. One would think that breaking bonds in a crystal lattice would reduce strength-so why do these dislocations have this effect?
Single-crystal metals (metallic whiskers) are immensely strong, and they contain no dislocations-seems like a contradiction to me.
SWAG… it reduces the possibility of cleaving?
forging packs the atoms closer and allows more bonding.
Look into the Hall-Petch relationship or more generally Work Hardening. I haven’t taken a course in materials science in years and it’s not my field so I don’t think I could summarize it better than Wikipedia or any other source if you google those terms.
From Wiki:
[QUOTE=Wikpedia Work Hardening]
Before work hardening, the lattice of the material exhibits a regular, defect-free (no dislocations) pattern. The defect-free lattice can be created or restored at any time by annealing. As the material is worked hardened it becomes increasingly saturated with new dislocations, and more dislocations are prevented from nucleating (a resistance to dislocation-formation develops). This resistance to dislocation-formation manifests itself as a resistance to plastic deformation; hence, the observed strengthening.
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But you knew that. You’re asking why dislocations do that.
[QUOTE=Wiki]
In grain-boundary strengthening the grain boundaries act as pinning points impeding further dislocation propagation. Since the lattice structure of adjacent grains differs in orientation, it requires more energy for a dislocation to change directions and move into the adjacent grain. The grain boundary is also much more disordered than inside the grain, which also prevents the dislocations from moving in a continuous slip plane. Impeding this dislocation movement will hinder the onset of plasticity and hence increase the yield strength of the material.
Under an applied stress, existing dislocations and dislocations generated by Frank-Read Sources will move through a crystalline lattice until encountering a grain boundary, where the large atomic mismatch between different grains creates a repulsive stress field to oppose continued dislocation motion. As more dislocations propagate to this boundary, dislocation ‘pile up’ occurs as a cluster of dislocations are unable to move past the boundary. As dislocations generate repulsive stress fields, each successive dislocation will apply a repulsive force to the dislocation incident with the grain boundary. These repulsive forces act as a driving force to reduce the energetic barrier for diffusion across the boundary, such that additional pile up causes dislocation diffusion across the grain boundary, allowing further deformation in the material. Decreasing grain size decreases the amount of possible pile up at the boundary, increasing the amount of applied stress necessary to move a dislocation across a grain boundary. The higher the applied stress to move the dislocation, the higher the yield strength. Thus, there is then an inverse relationship between grain size and yield strength, as demonstrated by the Hall-Petch equation. However, when there is a large direction change in the orientation of the two adjacent grains, the dislocation may not necessarily move from one grain to the other but instead create a new source of dislocation in the adjacent grain. The theory remains the same that more grain boundaries create more opposition to dislocation movement and in turn strengthens the material.
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In simple terms, the crystal structure of metals means the atoms are in uniform layers. That means that the ‘gaps’ between them are inline. This allow one layer to easily slide along the next. The more you work the metal, the more those gaps no longer line up, making it more difficult for the layers to slide along each other.
This is also the reason they add carbon to iron to make steel. The carbon locks the iron layers to each other, preventing them from moving.
So my cleaving guess was accurate?
Yes, in a sense. It’s about the ability of the layers to slip past each other.
Just like how they carefully created that little bump on Massachusetts so that it doesn’t slide into the ocean. Little do these folks know what they are messing around with. They are trying to remove the dislocation, but that would put their northerly neighbors in peril and weaken the integrity of our nation.
But seriously, what others have said about dislocations and impurities of different sizes follows my understanding about this. In nuclear power school we studied this because one nice way of banging atoms out of smooth ranks is to shoot neutrons at them. Over time, the material hardens, putting reactor pressure vessels and associated paraphernalia at risk of brittle fracture.
Here’s an article talking about embrittlement of pressure vessels due to neutron bombardment. They say, “The primary mechanism of embrittlement is the hardening produced by nanometer features that develop as a consequence of irradiation.”