Precipitation hardening

Precipitation hardening, also known as age hardening or particle hardening, is a heat treatment method that boosts the yield strength of malleable materials. These include most structural alloys of aluminum, magnesium, nickel, and titanium, as well as some steels, stainless steels, and duplex stainless steel. In superalloys, this process creates a yield strength anomaly, which gives them excellent strength at high temperatures. The technique works by taking advantage of how solid solubility changes with temperature. This allows fine particles of an impurity phase to form, which then block the movement of dislocations—defects in a crystal's lattice. Since dislocations are the main way materials deform plastically, blocking them hardens the material. These impurities act like the particles in particle-reinforced composites. Just as the formation of ice in air can produce clouds, snow, or hail depending on the atmosphere's thermal history, precipitation in solids can create particles of many different sizes, each with very different properties. Unlike ordinary tempering, alloys must be kept at an elevated temperature for hours to allow precipitation to happen. This waiting period is called "aging." The combination of solution treatment and aging is sometimes shortened to "STA" in metal specifications and certificates.

Two different heat treatments involving precipitates can change a material's strength: solution heat treating and precipitation heat treating. Solid solution strengthening creates a single-phase solid solution through quenching. Precipitation heat treating adds impurity particles to increase strength.

The technique relies on supersaturation and requires a careful balance between the driving force for precipitation and the thermal activation energy available for both desired and undesired processes. The most widely accepted theory for precipitation is classical nucleation theory (CNT). Nucleation happens at a relatively high temperature, often just below the solubility limit, so the kinetic barrier of surface energy is easier to overcome and the maximum number of precipitate particles can form. These particles are then allowed to grow at a lower temperature in a process called aging. This is done under conditions of low solubility, so thermodynamics drive a greater total volume of precipitate formation. Because diffusion depends exponentially on temperature, precipitation strengthening, like all heat treatments, is a delicate process. Too little diffusion, called under-aging, leaves particles too small to block dislocations effectively. Too much diffusion, called over-aging, makes particles too large and spread out to interact with most dislocations.

Precipitation strengthening is possible when the line of solid solubility slopes strongly toward the center of a phase diagram. While a large volume of precipitate particles is desirable, only a small enough amount of the alloying element should be added so it remains easily soluble at a reasonable annealing temperature. Although large volumes are often wanted, they are wanted in small particle sizes to avoid a decrease in strength. Elements used for precipitation strengthening in typical aluminum and titanium alloys make up about 10% of their composition. While binary alloys are easier to understand as an academic exercise, commercial alloys often use three components for precipitation strengthening, such as Al(Mg, Cu) and Ti(Al, V). Many other constituents may be unintentional but harmless, or may be added for other purposes like grain refinement or corrosion resistance. An example is adding Sc and Zr to aluminum alloys to form FCC L12 structures that help refine grains and strengthen the material. In some cases, such as many aluminum alloys, an increase in strength comes at the cost of corrosion resistance. More recent technology focuses on additive manufacturing because it can produce more metastable phases due to fast cooling, whereas traditional casting is more limited to equilibrium phases.

Adding large amounts of nickel and chromium for corrosion resistance in stainless steels means traditional hardening and tempering methods are not effective. However, precipitates of chromium, copper, or other elements can strengthen the steel by similar amounts compared to hardening and tempering. The strength can be adjusted by changing the annealing process, with lower initial temperatures leading to higher strengths. Lower initial temperatures increase the driving force for nucleation. More driving force means more nucleation sites, and more sites mean more places for dislocations to be disrupted while the finished part is in use. Many alloy systems allow the aging temperature to be adjusted. For instance, some aluminum alloys used for rivets in aircraft construction are kept in dry ice from their initial heat treatment until they are installed. After this type of rivet is deformed into its final shape, aging occurs at room temperature and increases its strength, locking the structure together. Higher aging temperatures would risk over-aging other parts of the structure and would require expensive post-assembly heat treatment, because a high aging temperature makes the precipitate grow too readily.

There are several ways a matrix can be hardened by precipitates, which can also differ for deforming precipitates and non-deforming precipitates. For deforming particles, or weak precipitates, coherency hardening occurs when the interface between the particles and the matrix is coherent. This depends on parameters like particle size and how the particles are introduced.