Phase and metal hardening 2

[richard4556]

Hi. This is not coursework, I'm trying to understand the subject.

I read that one method of hardening a metal is through strengthening with a second phase.

I'm not entirely sure I understand the phrase "strengthening with a second phase".

Can the phrase "strengthening with a second phase" be shown on a phase diagram?

I seem to have convinced myself that what amounts to a second phase, in this context, is when the result of cooling a metal results in a metal containing a mix of alpha and beta - a heterogeneous material. Not sure if I'm right though.

Phase diagram:

https://en.wikipedia.org/wiki/File:Eutektikum_new.svg

Thanks.

 

[EdStainless]

This usually means one of two things, either a secondary phase that forms on cooling from high temperature (martinsite in steel) or one that forms with an aging process after processing (17-4PH).
The phases are usually intermetallics or carbides and they are very hard and strong. They act as pinning against slip and prevent the movement of dislocations.
Look on the Fe-C phase diagram. You can tell that after reheating above the austenitizing temperature there is a partitioning of the C.

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P.E. Metallurgy, Plymouth Tube

 

[richard4556]

I'm newbie on this BTW. In the phase diagram as per the link, when the volume of beta in the composition is above a certain point, when cooling, the point is reached when the solid alloy contains alpha and beta. Both alpha and beta I presume are different types of crystallization. Are we saying then, that a crystallization involving alpha within the alloy, and crystallization involving beta within the alloy - are called phases?

In other words, are we saying that strengthening is the result of beta crystallization - which is (perhaps) known as a secondary crystallization or a second phase - developed within an alpha crystallization - as particulates? Beta being the particulates? Thus resulting in a heterogeneous alloy rather than a homogeneous one - a dual phase alloy rather than a single phase alloy.

The text does talk about an aging process. And it talks about the introduction of small particles of a second phase. And I'm wondering if this "second phase" is see-able on a phase diagram, and how.

 

[TVP]

Richard,

Yes, different the phase diagram that you linked to shows an alloy that has two phases, alpha and beta, which would have different crystal structures. An example of this is the workhorse titanium alloy Ti-6Al-4V. Titanium has a BCC crystal structure at high temperatures and a HCP structure at low temperatures. By using specific alloy elements and thermomechanical processing (deformation and heat treatment), one can create microstructures composed of the alpha and the beta phases together. Titanium is complicated by the fact that there is a martensitic transformation as well, which means that equilibrium and non-equilibrium microstructures can be produced, all of which can be shown on phase diagrams (conventional binary, ternary, etc.). Here are some links for more information, including images of two-phase microstructures:

http://www-eng.lbl.gov/~shuman/NEXT/MATERIALS&COMPONENTS/Pressure_vessels/titanium_4311_chap01.pdf

http://www.timet.com/images/document/technicalmanuals/TIMETAL_6-4_Properties.pdf

http://c.ymcdn.com/sites/

http://www.titanium.org/resource/resmgr/Technical_Library/RMI_Metallography_brochure.pdf

http://www.georgevandervoort.com/images/Metallography-Specific/titanium_IndustrialHeating.pdf

 

[richard4556]

Good links. I think I got it now. The main issue is diffusion. In particulate hardening, one metal cannot diffuse, or entirely diffuse into the other, and has to participate out when the alloy is cooled. So, although it might start out as a single phase alloy when in a liquid state, which is homogeneous, the end result of cooling is two separate types of crystalites or grains. Therefore, the alloy cannot be a homogeneous mass, but heterogeneous. This is shown in the phase diagram in the region a + b. My understanding.

 

[EdStainless]

The diffusion is driven by solubility limits.
Some phase changes are also, but some (martensite) don't involve diffusion. It might just be related to phase stability at a particular temperature.


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P.E. Metallurgy, Plymouth Tube