The strength of metals is improved by impeding the motion of dislocations through metals. One approach to achieving this improvement is precipitation strengthening - forming a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy.
Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.
An example of a precipitation strengthened component is the tubing used for bicycles with aluminum frames. The alloy is 6061.
The particles are formed using a series of precipitation heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the crystal structure of the main element. For Al-Cu alloys, the copper atoms dissolve into the aluminum crystal. This is called a solid solution. The solid solution is then retained at room temperature by cooling the alloy rapidly, such as by water quenching.
After cooling, precipitates are formed either by natural aging or artificial aging. With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged. Aluminum alloys are examples of alloys that can be naturally and artificially aged.
Regardless of whether an alloy is naturally or artificially aged, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage influences alloy strength. For artificially aged alloys, this is controlled by the aging temperature and time.
For a particular aging temperature, there is an aging time at which the alloy will reach maximum strength. Maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging times that are too short or too long will result in less than maximum alloy strength.
For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. Time to reach maximum strength decreases as the aging temperature increases. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.
Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.
More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.