The strength of metals is improved by impeding the motion of dislocations through the metals. One approach to achieving this improvement is to form 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.
The particles are formed by precipitation, which involves a series of 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 solid structure formed by the array of atoms of the main element. For Al-Cu alloys, the copper atoms dissolve into the array of aluminum atoms. The dissolved structure is then retained at ambient temperatures 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.
Regardless of the aging process, 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 of the precipitates has a direct influence on the strength of the alloy. For artificially aged alloys, this is controlled by the aging temperature and time. At any particular aging temperature, there is an aging time at which the alloy will reach its maximum strength. This maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging for a time that is 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. 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.