Heat Treatments and Recovery

As we just saw in Section 12.6.7, metals can be strengthened by "cold working" them to add more dislocations which then repel one another, making it harder for dislocations to move. This can be reversed by heating up the material, a process known as annealing. When you heat up the material, diffusion increases and dislocations have more energy to move. Some of them will migrate to the surface of the material and annihilate there. Others will move towards dislocations of opposite sign and annihilate as we saw in Video 12.6.1. This process reduce the overall strain energy in the lattice and allows the remaining dislocations to move more easily. Substantial recovery will occur starting at around half of the melting temperature of a material $T=0.5T_M$.

Annealing can also affect other strengthening mechanisms as well. For example, we saw in Section 12.6.5 that reducing grain size will generally strengthen a material. Annealing will cause some grains to grow and some to shrink and eventually disappear, thus affecting the strength of the material. Annealing can also cause the sizes and distributions of precipitates to change.

Exercise 12.7.1: Plumbing the Depths of Understanding
Not Currently Assigned

Lead is a metal with an interesting combination of properties: it is dense, ductile, and inert. It is also relatively easily extracted from galena and as such, inexpensive. It also has a low melting temperature $T_{\text{M}} = 600\,\text{K}$. Take 2-3 minutes on this.

  1. Let's say you've been living under a rock for 80 years, and you decide to install lead pipe in your house. Which of the following techniques could you use to strengthen the pipe prior to installation? Explain why each would/wouldn't work.

    • Annealing to remove dislocations.
    • Strain-hardening the pipe.
    • Alloying lead with antimony to create a solid solution.
    • Alloying the lead with copper to acquire a Cu-rich precipitate.