Understanding Precipitation Hardening: Coherent vs. Dispersion Mechanisms

[RPI_Quantum]

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]

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]

... 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).

 

[Schroding79]

going off the main question but I would say precipitation hardening is usually preferred in Al alloys because it leads to much greater hardness than that from dispersion/decoherent hardening.
But yes decoherent particles are also pretty thermodynamically unstable and there is often a room temperature effect

 

[mah65]

I have a related question ...

When precipitation hardening is in progress, there is a peak in hardening; that is, at first the hardness increases, but after a time, the hardness starts to decrease. Why there is a peak in hardening?
I don't think that the precipitates lose their coherency, since the precipitates are the same. Only on condition that the precipitates change, they may lose their coherency. I think loosing coherency is due to the fact that the precipitates start to coalescence, thus their quantity is declined and the space between the precipitates is increased. Is my opinion true??

 

[Schroding79]

I'm not sure if precipitates coalesce with aging (like Ostwald ripening?) so can't directly answer your question.

What I do know is that in Al alloys there are several meta-stable precipitate types that can be present. So it's not just a question of precipitate size and spacing.
e.g. with aging/over-aging it is possible that the strengthening precipitates (GPZ, theta'') dissolve and the ones that don't strengthen as much (theta or theta') end up dominating.

There's a nice figure showing evolution of phases in Bastow & Celotto, Acta Mater. 2003 (http://dx.doi.org/10.1016/S1359-6454(03)00299-4 ) but I can't find a corresponding hardness curve for the Al-Cu alloy they look at.

Also, because the over-aging effect in these alloys may be due to new phases forming, there a thermal activation involved so in some of the Al-Cu alloys it is very difficult to see evidence of over-aging at room-temperature for example (although this might just be because the amount of time required to see evidence of this outstrips the length of time associated with research grants).