Precipitation Hardening: Definition & Process

Instructor: Nichole Miller

Nichole is a research scientist with a PhD in Materials Science & Engineering.

This lesson defines precipitation in solid materials and explains how precipitation hardening increases the hardness of materials by impeding dislocation motion.

Precipitation

When you hear the word 'precipitation', you might think of the weather. Precipitation occurs when water vapor in the atmosphere condenses and falls to the ground as rain, snow, sleet, or hail.

Just as water vapor in the atmosphere can condense to form liquid water, a solid dissolved in a liquid can condense to form a solid. This process, called precipitation, is the opposite of the process of dissolving (dissolution). A solid can dissolve in a liquid, but it can also do the opposite and 'emerge' from that liquid as a solid.

Say you stir sugar into boiling water. The sugar will dissolve in the water. So what happens when boiling water containing the maximum amount of dissolved sugar is cooled to room temperature? Since water at room temperature cannot dissolve as much sugar as boiling water, the extra sugar precipitates, or emerges as a solid from the liquid.

Precipitation also happens in solids. For example, aluminum can dissolve some copper at high temperatures. When the temperature is decreased, the extra copper precipitates by forming small copper-rich regions in the aluminum. These small copper-rich regions are called precipitates.

More generally, precipitates are small impurity regions that form in a material when it is no longer able to dissolve the impurity.

Dislocations and Hardness

Before discussing precipitation hardening, we have to understand the important role of dislocations in the deformation of materials.

In a perfect crystal, the atoms arrange in a perfect pattern. Dislocations are lines of atoms where the perfect pattern of atoms is broken.

When you bend a paperclip, you rearrange the atoms into a new shape. Whether you know it or not, you also cause dislocations inside the paperclip to move. If you somehow make it more difficult for these dislocations to move, you also make it more difficult to deform the paperclip and therefore make the paperclip harder.

Precipitation Hardening

One way to harden a material is by adding something that blocks or slows down the movement of dislocations. Precipitates can do just that. Precipitation hardening is the hardening of a material due to the growth of precipitates that impede dislocation motion.

Basically, this process involves heating a mixture to a high temperature, then cooling, then heating to a medium temperature, and finally cooling again. Here's a more detailed overview of the precipitation-hardening process:

1. Bring a mixture of two or more components to an elevated temperature, where they mix completely.
2. Cool the material very quickly to lock in the completely mixed state.
3. Bring the material to an intermediate temperature, often called the 'aging temperature'. The aging temperature must be high enough that diffusion can occur rapidly, but low enough that one of the components can no longer dissolve the other so that precipitation occurs.
4. Cool the material to room temperature after the precipitates have grown to the desired size. Since room temperature is too cool for diffusion to occur rapidly, the precipitates will stop growing.

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