Precipitation Hardening

**RPI_Quantum** · Sep17-05, 12:01 PM

I'm trying to understand the mechanisms behind precipate hardening, and I am not able to find a good source to explain the differences in coherent precipitates and dispersion hardening. I understand that the crystal structure of the precipitate is different in dispersion hardening (that's what I think at least). How does the mechanism of strengthening differ with the structure of the precipitate?

**Astronuc** · Sep17-05, 04:52 PM

Precipitation hardening means that a second phase such as a carbide or intermetallic compound is precipitated in the alloy. This means the constituent is precipitated from a supersaturated solid solution, e.g. excess C in and Fe-alloy matrix. The process by which this is accomplished is aging the metal, so the specific result is age hardening.

aging (heat treatment) - from the ASM Metals Handbook

A change in the properties of certain metals and alloys that occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench aging in ferrous alloys, natural or artificial aging in ferrous and nonferrous alloys) or after a cold-working operation (strain aging). The change in properties is often, but not always, due to a phase change (precipitation), but never involves a change in chemical composition of the metal or alloy. See also age hardening, artificial aging, interrupted aging, natural aging, overaging, precipitation hardening, precipitation heat treatment, progressive aging, quench aging, step aging, and strain aging.

Dispersion strengthening of a metal or alloy is accomplished by incorporating chemically stable submicron size particles of a nonmetallic phase (ususally an oxide such as Al₂O₃) that impede dislocation movement at elevated temperature. Nonmetallic phase(s), such as Al₂O₃, MgO, SiO₂, CdO, ThO₂, Y₂O₃, or ZrO₂ may be used singly or in combination. An example would be Y₂O₃ dispersed in nickel-chromium superalloys used for gas turbine components.

Here is a good article on hardening process in steel - The Strengthening of Iron and Steel

Strengthening mechanisms in alloy steel

In fact one will find the whole site very useful.

http://www.key-to-steel.com/Articles.htm

**PerennialII** · Sep17-05, 05:55 PM

... to further elaborate on the decoherent (pretty much 1-1 dispersion) to coherent (precipitate) aspect, a good example would be for example the precipitation (age) hardening of an aluminum alloy. During the aging at a suitable temperature the supersaturated solid solution forms a dense 'array' of coherent particles (particles which have a continuous lattice with the 'matrix' metal lattice), which will provide the desired strengthening effect by distorting the lattice and impeding dislocation movement. With excessive time and/or temperature the particles will (when reaching towards the stable thermodynamical state) grow and decohere from the matrix (decoherent particles, dispertions - 'problem' of making too big of a particle to fit the lattice), which typically have strength wise a lower strengthening effect .... thus precipitate hardening is usually preferred (in "normal" temperature applications for one).