What is Precipitation Hardening?
Precipitation hardening is a heat treatment method which increases the yield strength of many alloys. It is also known as age hardening or ageing. The strengthening is a result of particles being distributed into the metals grain structure, hindering dislocation movement. Precipitation hardening relies on heating the metal to a temperature which alters its solubility, allowing the creation of solid impurities or precipitates which impede the movement of dislocations.
Three Stages of Precipitation Hardening
Age hardening has three fundamental components.
1. Solution Treatment
Initially, the metal is heated to a temperature which increases the solubility of the material. The increase in solubility allows the alloying elements to dissolve into the structure. After a designated period, the solution becomes a single-phase and impurities removed.
2. Quenching
Next, the metal is cooled rapidly by quenching until the solubility limit is exceeded. The solubility limit is the maximum amount of a substance that can be dissolved in a solvent at a specified temperature and pressure. Typical quenching agents are water and oil. The lowered temperature from quenching prevents any of the excess solute diffusing out of the microstructure.
3. Ageing
The final stage sees the metal heated to an intermediate temperature and held at this temperature for a period time (this is where the name age-hardening originating from). During this process, the microstructure changes its arrangement and second phase solid impurities or precipitates form. Once held at the required temperature for the desired time the metal is either air-cooled or quenched to achieve the desired results.
If the metal is held at the intermediate temperature for too long, the material hardness will decrease. This phenomenon is known as over ageing. Over ageing occurs due to the precipitates or solid impurities breaking down into smaller fragments while also being spaced out. As a result, they are not able to inhibit dislocation movement. This phenomenon can also be experienced, by ageing the material at a temperature higher than required.
Cooling Rate Effect On Precipitation Hardening
The cooling rate will affect the size of the precipitates seen in the microstructure. For example, a slow cooling rate for aluminium-copper will result in large precipitates primarily on grain boundaries. On the other hand, a rapid cooling rate will give small precipitates and distributed evenly. Depending on its application, the cooling rate is manipulated to give the desired structure for the metal.
The size of the particles will have a contributing factor to the strength of the metal. However, it is not a case of the bigger the precipitates the stronger the metal.
The size of the particles will have a contributing factor to the strength of the metal. However, it is not a case of “the bigger the precipitates, the stronger the metal”.
The size of the particles will determine how the dislocations in the structure travel past/through the precipitates.
The image on the left shows that at smaller precipitate sizes, cutting is more predominant, and at larger precipitate sizes, bowing is predominant.
The image on the right shows how cutting and bowing occurs on the relatively sized particles.
Cutting is a result of the dislocation overcoming the precipitates strength and penetrating the particles. At larger particle sizes, this is not possible. The precipitate is strong enough to prevent dislocation penetration, however, the dislocation will bow around the particle instead. Dislocation bowing is also known as Orowan strengthening.
The graph shows there is a critical size for the precipitate particles, which prevents a dominant dislocation mechanism. The critical size of the precipitates is where the maximum strength of the precipitates occurs. The critical radius is typically between 5-30 nm.