Precipitation hardening, also called age hardening or dispersion hardening, are heat treatment techniques used to strengthen malleable materials, especially non-ferrous alloys including most structural alloys of aluminium, magnesium and titanium. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material. The impurities, in fact, play the same role as matrix substances in composite materials. Just as the formation of ice in air can produce clouds, snow, or hail, depending upon the thermal history of a given portion of the atmosphere, precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging. Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. ... Strength of materials is materials science applied to the study of engineering materials and their mechanical behavior in general (such as stress, deformation, strain and stress-strain relations). ... Malleability is a physical property of matter, signifying its capability of deformation, especially by hammering or rolling. ... Ferrous in chemistry is a term used for the iron with an oxidation number +2. ... An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... Aluminum redirects here. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... General Name, symbol, number titanium, Ti, 22 Chemical series transition metals Group, period, block 4, 4, d Appearance silvery metallic Standard atomic weight 47. ... Solubility is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent. ... For other uses, see Temperature (disambiguation). ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. ... For other uses, see Crystal (disambiguation). ... Enargite crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... For other uses, see Plasticity. ... A cloth of woven carbon fiber filaments, a common element in composite materials Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties and which remain separate and distinct on a macroscopic level within the finished structure. ... Tempering is a heat treatment technique for metals and alloys. ...

Note that two different heat treatments involving precipitates can alter the strength of a material: solution heat treating and precipitation heat treating. Solution heat treating involves formation of a single-phase solid solution via quenching and leaves a material weaker. Precipitation heat treating involves the addition of impurity particles to increase a material's strength.^[1] Precipitation hardening via precipitation heat treatment is the main topic of discussion in this article. A quench refers to a rapid cooling. ...

Kinetics versus thermodynamics

This technique exploits the phenomenon of supersaturation, and involves careful balancing of the driving force for precipitation and the thermal activation energy available for both desirable and undesirable processes. The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. ...

Nucleation occurs at a relatively high temperature (often just below the solubility limit) so that the kinetic barrier of surface energy can be more easily overcome and the maximum number of precipitate particles can form. These particles are then allowed to grow at lower temperature in a process called aging. This is carried out under conditions of low solubility so that thermodynamics drive a greater total volume of precipitate formation. Bubbles in a soft drink each nucleate independently, responding to a decrease in pressure. ... In physical chemistry, chemical kinetics or reaction kinetics is the study of reaction rates in a chemical reaction. ... Surface energy quantifies the disruption of chemical bonds that occurs when a surface is created. ... It has been suggested that this article or section be merged with chemical equilibrium. ... Thermodynamics (from the Greek θερμη, therme, meaning heat and δυναμις, dynamis, meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ...

Diffusion's exponential dependence upon temperature makes precipitation strengthening, like all heat treatments, a fairly delicate process. Too little diffusion (under aging), and the particles will be too small to impede dislocations effectively; too much (over aging), and they will be too few and far between to interact with the majority of dislocations. diffusion (disambiguation). ... Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. ...

Alloy design

Precipitation strengthening is possible if the line of solid solubility slopes strongly toward the center of a phase diagram. While a large volume of precipitate particles is desirable, little enough of the alloying element should be added that it remains easily soluble at some reasonable annealing temperature. In physical chemistry, mineralogy, and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... For other uses, see Annealing. ...

Elements used for precipitation strengthening of typical aluminum and titanium alloys make up about 10% of their composition. While binary alloys are more easily understood as an academic exercise, commercial alloys often use three components for precipitation strengthening, in compositions such as Al(Mg, Cu) and Ti(Al, V). A large number of other constituents may be unintentional, but benign, or may be added for other purposes such as grain refinement or corrosion resistance. General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... For other uses, see Copper (disambiguation). ... General Name, symbol, number vanadium, V, 23 Chemical series transition metals Group, period, block 5, 4, d Appearance silver-grey metal Standard atomic weight 50. ... Grain refinement is a set of techniques used in metallurgy to ensure that the crystallites (grains) that make up a metallic object are sufficiently small, so as to increase its strength. ... For the hazard, see corrosive. ...

Many alloy systems allow the aging temperature to be adjusted. For instance, some aluminium alloys used to make rivets for aircraft construction are kept in dry ice from their initial heat treatment until they are installed in the structure. 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 require expensive post-assembly heat treatment. A rivetted buffer beam on a steam locomotive A rivet is a mechanical fastener consisting of a smooth cylindrical shaft with heads on either end. ... Small pellets of dry ice sublimating in air. ...

Theory

The primary species of precipitation strengthening are second phase particles. These particles impede the movement of dislocations throughout the lattice. You can determine whether or not second phase particles will precipitate into solution from the solidus line on the phase diagram for the particles. Physically, this strengthening effect can be attributed both to size and modulus effects, and to interfacial or surface energy.

The presence of second phase particles often causes lattice distortions. These lattice distortions result when the precipitate particles differ in size from the host atoms. Smaller precipitate particles in a host lattice leads to a tensile stress, whereas larger precipitate particles leads to a compressive stress. Dislocation defects also create a stress field. Above the dislocation there is a compressive stress and below there is a tensile stress. Consequently, there is a negative interaction energy between a dislocation and a precipitate that each respectively cause a compressive and a tensile stress or vice versa. In other words, the dislocation will be attracted to the precipitate. In addition, there is a positive interaction energy between a dislocation and a precipitate that have the same type of stress field. This means that the dislocation will be repulsed by the precipitate.

Precipitate particles also serve by locally changing the stiffness of a material. Dislocations are repulsed by regions of higher stiffness. Conversely, if the precipitate causes the material to be locally more compliant, then the dislocation will be attracted to that region.

Furthermore, a dislocation may cut through a precipitate particle. This interaction causes an increase in the surface area of the particle. The area created is

where, r is the radius of the particle and b is the magnitude of the burgers vector. The resulting increase in surface energy is

where is the surface energy. The dislocation can also bow around a precipitate particle.

Governing Equations

There are two equations to describe the two mechanisms for precipitation hardening:

Dislocations cutting through particles:

where "tau" is material strength, "r" is the second phase particle radius, "gamma" is the surface energy, "b" is the magnitude of the Burgers vector, and "L" is the spacing between pinning points. This governing equation shows that the strength is proportional to r, the radius of the precipitate particles. This means that it is easier for dislocations to cut through a material with smaller second phase particles (small r). As the size of the second phase particles increases, the particles impede dislocation movement and it becomes increasingly difficult for the particles to cut through the material. In other words, the strength of a material increases with increasing r. In materials science, a dislocation is a linear crystallographic defect, or irregularity, within a crystal structure. ...

Dislocations bowing around particle:

where "tau" is the material strength, "G" is the shear modulus, "b" is themagnitude of the Burgers vector, "L" is the distance between pinning points, and "r" is the second phase particle radius. This governing equation shows that for dislocation bowing the strength is inversely proportional to the second phase particle radius r. Dislocation bowing is more likely to occur when there are large particles present in the material. In materials science, a dislocation is a linear crystallographic defect, or irregularity, within a crystal structure. ...

These governing equations show that the precipitation hardening mechanism depends on the size of the precipitate particles. At small r, cutting will be the dominant strengthening mechanism, while at large r, bowing will be the dominant strengthening mechanism. Image File history File links No higher resolution available. ...

Looking at the plot of both equations, it is clear that there is a critical radius at which max strengthening occurs. This critical radius is typically 5-30 nm.

Some precipitation hardening materials

• 2000-series aluminum alloys (important examples: 2024 and 2019)
• 6000-series aluminum alloys
• 7000-series aluminum alloys (important examples: 7075 and 7475)
• 17-4PH stainless steel (UNS S17400)
• Maraging steel
• Inconel 718
• Alloy X-750
• Rene 41
• Waspaloy

The Unified Numbering System (UNS) is an alloy designation system widely accepted in North America. ... Maraging steel is an iron-based steel alloy which is known for possessing superior strength without losing malleability. ... Inconel® is a registered trademark of Special Metals Corporation referring to a family of austenitic nickel-based superalloys. ...

See also

• Strength of Materials
• Strengthening mechanisms of materials
• Metallurgy
• Superalloy

Understanding how to strengthen materials is crucial to a variety of disciplines. ... Georg Agricola, author of De re metallica, an important early book on metal extraction Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their compounds, which are called alloys. ... A superalloy, or high-performance alloy, is an alloy able to withstand extreme temperatures that would destroy conventional metals like steel and aluminum. ...

Related reference

• Project aluMatter includes an illustration

References

1. ^ W.D. Callister. Fundamentals of Materials Science and Engineering, 2nd ed. Wiley & Sons. pp. 252.