How does dislocations affects metal properties during strenghtening of metals?

Answer 1

When you straighten out a piece of metal you are creating spots that are thinner than other areas, so they would be weaker. You are also weakening bends so they lose their tonsil strength, just take a bent piece of aluminum and bend it back and forth, it will eventually break in two.

Answer 2

The work hardening answer is correct. Dislocations can have other, more subtle, effects on metals as well. Dislocation density will affect the electrical resistivity and magnetic properties. If you have a stainless steel sink, you may find that some areas of the sink are magnetic and some are not. It is all the same composition but the metal working process (involving dislocations) changes the magnetic properties.

this is very interesting stuff if you are a metallurgist.

Answer 3

Work hardening, strain hardening, or cold work is the strengthening of a material by increasing the material's dislocation density. In metallic crystals, irreversible deformation is usually carried out on a microscopic scale by defects called dislocations,which are created by fluctuations in local stress fields within the material culminating in a lattice rearrangement as the dislocations propagate through the lattice. At normal temperatures the dislocations are not annihilated by annealing. Instead, the dislocations accumulate, interact with one another, and serve as pinning points or obstacles that significantly impede their motion. This leads to an increase in the yield strength of the material and a subsequent decrease in ductility.

Any material with a reasonably high melting point such as metals and alloys can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at normal ambient temperatures; for example indium, which has a low melting point. This makes indium suitable for manufacturing gaskets, which deform to fill gaps, for high-vacuum use.

Work hardening is often produced by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing hardness tests before and after a process.