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No. 989
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Part 3(c): Making metals stronger
In this section I will describe some techniques used to increase the overall lattice resistance of metals, and hence stop dislocations from moving. The movement of dislocations is the mechanism by which slip planes move, and on a macroscopic scale is 'bending' which is often undesirable.
Solid solution strengthening: Is basically 'alloying'. Consider this: with water, you can dissolve sugar into it and get a solution of water with sugar molecules interspersed through it. With metals, the same is true: you can 'dissolve' molten zinc into molten copper and receive a 'solution' of both.
How does this inhibit the movement of dislocations? Solid solution strengthening introduces impurity atoms into the crystal lattice. These impurities have different atom size compared other atoms, and so they distort the crystal lattice. Dislocations need more force/energy to move past these distortions. Two main factors are responsible for the increase in intrinsic lattice resistance: difference is atom size and the amount of impurities added. Copper and zinc have fairly similar atom size, so there needs to be a mixture of about 30% Zinc to 70% copper. Carbon and iron have a huge difference in atom size, so only 0.3% of carbon needs to be added to the Iron to get the same effect.
Precipitation hardening: Let us carry our analogy with sugar and water further. If you add lots of sugar to hot water and dissolve it all, you may find out that some of the sugar recrystallises out of the solution when it cools down. The same thing occurs with metal alloys. Some impurity atoms will 'precipitate', forming small particles of impurities inside the matrix of the alloy.
These precipitate particles impede dislocation movement. Dislocations are forced to either go around, or cut through these particles. If they go around the particle, the dislocation tends to leave traces of itself around the particle called 'Orowan loops', which themselves impede dislocation movement and get bigger and bigger with each passing dislocation.
Work hardening: This is basically working the metal to deliberately introduce more dislocations, and create a forest of dislocations to stop dislocation motion. The metal can be hammered, rolled, bent, d
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