Phase and metal hardening
Phase and metal hardening
(OP)
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....
Thanks.





RE: Phase and metal hardening
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
RE: Phase and metal hardening
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.
RE: Phase and metal hardening
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&...
http://www.timet.com/images/document/technicalmanu...
http://c.ymcdn.com/sites/www.titanium.org/resource...
http://www.georgevandervoort.com/images/Metallogra...
RE: Phase and metal hardening
RE: Phase and metal hardening
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