In this lesson we're going to be talking about the development of high strength alloys. Over the several lessons that we'll be covering, we're going to be looking at how do we improve the properties of iron carbon or steels. And how do we improve the properties of materials like aluminum, copper alloys that precipitation harden. The first alloy system that we're going to be looking at will be steels. And in particular, we're going to be focusing on what we refer to as plain carbon steels. Plain carbon steel is just essentially iron and carbon. Now, it turns out that when you take an iron and carbon steel and you quench it from an elevated temperature, and we'll see more exactly what we mean by that in the next lectures. But, we go to a temperature where the Fe-C phase is stable, or the austenite, and we quench it down below a particular temperature that we refer to as the eutectoid temperature. When we go below the eutectoid temperature, we have two phases that are present. We have a phase of ferrite, which is essentially iron in the form of body-center cubic iron and the other is cementite or Fe3C and iron-carbide. Now it runs out that those two phases are present below the equilibrium line of the eutectoid, except they come in a couple of different morphologies depending upon the temperature to which you quench. The first is, the first set of two phases of ferite and cementitie is referred to as perlite. If we go to a lower temperature, the second is referred to as bainite. But again, these are two phase regions which contain Fe3C and alpha iron. Now, these phases form as a result of a process of diffusion, which controls both the nucleation and the growth processes of the phases that are forming out of the austenite. The lower temperature to which the materials are quenched, the finer the structure. Perlite form at a high temperature, whereas bainite forms at a low temperature. When the austenite is quenched to even lower temperatures, a single phase microstructure results. And we call this single phase microstructure, martensite. This phase does not form by a diffusional process, but rather by a diffusionless transformation and that occurs by shear. Now if we look at the section of the iron carbon phase diagram where we focus around the eutectoid temperature, which is 727, and what we're going to do is to look at compositions along that red line. And we're going to start out in the single phase field in which we have the austenite, or the gamma phase, stable. So that's up here in a temperature in that single phase field. Now, the first thing that we're going to do is we're going to quench it down to lower temperatures and when we do that, what begins to occur is something that's referred to as the decomposition of austenite. So austenite is going to transform to another phase or phases depending upon the temperature. Now, if we quench down to where the blue line is, what we find is we're going to form that two phase microstructure that we refer to as perlite. Again, this is ferrite plus cementite. When we quench down to the second blue circle, we're going to be quenching down to a lower temperature, again we form ferrite and cementite. But this time it's in a morphology that's referred to as bainite. So again, we have the two two phase regions. Now, if we quench down to a low temperature and we do it fast enough and suppress the formation of either perlite or bainite, we produce a structure that we refer to as martensite. And I have separated that amp by color indicating the fact that martensite is in fact different than perlite and bainite in the fact that not only is it a non-diffusional process, but it is a single phase microstructure. So, the development of microstructure in a precipitation hardening alloy is a little bit different. As you recall, the way we formed a microstructure that contains two phases and develop strength from those two phase microstructures, is by eliminating the austenite and replacing the austenite with these new phases. When we talk about the development of high strength alloys that are based upon precipitation hardening, in aluminum alloys what we do is, we are not completely replacing the phase which starts out. But we're going to develop a second phase that comes out. So, aluminum-copper microstructures develop when you quench an aluminum copper alloy in a solid solution range and you quench down to a temperature below the solvus temperature. Then depending upon the temperature, these alloys will, and not only temperature but time, those will determine the scale of the microstructure and hence the strength. And remember, finer is always better in terms of producing strength. So, here is a section of our aluminum-copper phase diagram where we are heat treating the material to put it in a single phase field, is the alpha rich region that's indicated by the arrow on the diagram. The second phase that forms is called Al2Cu, or an aluminum copper phase of theta. So, if we take the material and quench it along that red line, what's going to happen is we started out with a single phase of alpha. When we quench it down to a low temperature, what we've done is to produce a super saturated solution of copper that's been quenched into the alpha aluminum. And so by quenching from above the solvus down to the single phase field, what we wind up doing is producing a microstructure that can lead to a distribution of fine particles. And the strength of the material will depend upon the size of the particles that result. So, these are two different approaches that are in two different alloy systems, the steels. Meaning, we replaced the austenite with another two phases or single phase. And in the case of the aluminum copper alloys, we don't completely replace the alpha phase, but we introduce a second phase. And in the case of aluminum copper it's the theta phase. Thank you.
