Precipitation hardening
Heat treatment technique that increases yield strength via particle precipitation.
Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels, stainless steels, and duplex stainless steel. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength. The technique 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.
- field
- Metallurgy / Materials Science
- known_for
- Increasing yield strength of alloys via precipitation of fine particles
- type
- Heat treatment technique
- applies_to
- Aluminium, magnesium, nickel, titanium, steels, stainless steels, superalloys
Lore & Background
Precipitation hardening 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 most commonly accepted theory behind precipitation is classical nucleation theory (CNT). Nucleation occurs at a relatively high temperature 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 ageing.
Reader's Guide
Precipitation hardening is significant because it enables the strengthening of many structural alloys that cannot be effectively hardened by traditional methods. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place; this time delay is called 'aging'. The technique is critical in aerospace and other high-performance applications, as seen in aluminium alloys used for aircraft rivets that are kept in dry ice until installation and then age at room temperature. The process requires careful control: too little diffusion (under ageing) produces particles too small to impede dislocations, while too much (over ageing) yields particles too large and dispersed. The addition of large amounts of nickel and chromium in stainless steels makes traditional hardening ineffective, but precipitates of chromium, copper, or other elements can strengthen the steel similarly. Recent technology focuses on additive manufacturing due to the higher amount of metastable phases obtainable from fast cooling.
Did You Know?
- Precipitation hardening is also called age hardening or particle hardening.
- The technique relies on changes in solid solubility with temperature to produce fine particles that impede dislocation movement.
- Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place.
- Some aluminium alloys used for aircraft rivets are kept in dry ice from initial heat treatment until installation, then age at room temperature.
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