WHAT 3

study

  • read chapters 4 and 5
  • suggested problems
    • chapter 4: 2, 3, 4
    • chapter 5: 1, 2, 4, 6
  • If you have a polymer or composite, you should go through the “Polymer” tutorial.
  • If you have a metal, there are many tutorials that may be of interest. Go through at least one.
    • For aluminum or titanium alloys, you have the “Light alloys” tutorial.
    • If you want to understand your phase diagram better, there is a tutorial on “Phase equilibria” that will step you through metal phases and one practical implication, welding.
    • If your part was manufactured through casting, “Casting and Recrystallization” has got you covered.
    • “Dislocations” describes the fundamental mechanism for plastic deformation in all metals (which we’ll talk more about later).
    • Of course, if you think one of the other tutorials is interesting to your application, go for it (for example, “Fracture and Fatigue” if your component is subject to cyclic stresses).

Tutorials referenced above can be found here.

tasks

  • Send me a description of one piece of evidence you would like to submit for materials science. If I offered to send information to to help with this, I will not be offended if you remind me to do that.
  • Research your product. See the posted project deliverable description.

We will not do new work on the project this week. However, if you did not figure out the structure of your material last week, you should do so. You want a “cartoon”-description, here. Something that will allow you to qualitatively reason about your material. If you have an alloy of some sort, start with the structure of the base material–you’ll go into more detail about the alloying elements later in the course.

a materials scientists take on the readings

In chapter five we learn about crystallography, a topic many folk spend years studying. Fortunately for us, Ashby and Jones have done an exceptional job of pulling out the important information in a concise manner. Nonetheless, you should still brace yourselves for a heavy dose of MatSci. And get excited, because the stuff in this chapter will allow you to communicate like a legit materials scientist.

So the basic deal with this chapter is that we have to figure out how to pack atoms into crystals or grains. In order to do this for crystalline materials (metals and ceramics) we first make a few simplifying assumptions: atoms are hard spheres subject to non-directional bonding. So essentially, imagine a bunch of billiards balls and then try to pack them into three-dimensional structures.

Much like when we did with the mechanical properties of materials, we use a normalized convention to refer to the crystal structures of materials: the unit cell. And we use plane indices ((1,1,1) and {1,1,1}), and direction indices ([1,1,1] and ) to describe planes and atoms within the unit cell.

Now, atoms do not always pack into the same crystal structure. There are some atoms that pack in a close-packed arrangement and some that do not. Close-packed structures can either be face-centered cubic (fcc) or close-packed hexagonal (cph); close-packed structures have close-packed planes. Materials that have a close-packed crystal structure have a high density, as the atoms are packed in as tightly as possible. Another common structure is the body-centered cubic (bcc), a structure that has closed-packed directions but not closed-packed planes. As a result, materials that have a body-centered cubic crystal structure are not as dense. Materials with this type of structure tend to have directional bonding that prevents the atoms from becoming close-packed.

Fortunately for us, metals and ceramics are not the only types of materials available to us. There are also polymers and inorganic glasses (and composites, but we’ll just ignore those for now). In these materials, the atoms are not arranged into neatly ordered crystals. Instead, they feature a disordered structure that is amorphous. In polymers, the atoms are strung together into chains through covalent bonds and chains interact with each other through secondary bonding. Depending on the size of the atoms involved and the types of bonding present, polymers can form some crystalline sections where the chains are ordered, leading to a semi-crystalline (or partly crystalline) structure. In inorganic glasses, we also find atoms arranged into amorphous structure. As a rule of thumb, polymers that are clear are amorphous; those that are not are semi-crystalline. Why do you think this is? Can you think of any other ways to identify whether a material has a crystalline or an amorphous structure?

As a side note, according to one of my friends, Jon Stolk (which is American for “Jan Stolk”), this is what a conversation between materials scientists might look like:


Jeong: I’m trying to deposit an epitaxial copper film on a silicon wafer, but the film stress is too high and the films keep failing. I want this layer to be fairly thick, so maintaining ductility is important.

Pam: You may have too much lattice mismatch between the Cu (100) and the Si (100). Have you tried the Cu orientation?

Jeong: No, but I’ll give it a try. Will the lower surface energy of the Cu (111) cause any problems with deposition?

Pam: I don’t think so, and you’ll definitely get a Cu layer with lower stress. But if you have time for more experiments, I’ve heard that Cu can be successfully grown with the (111), and (001) planes oriented to Si (111), so you may want to give that a try. Can you get your hands on some Si (111) wafers?

Jeong: Yeah, no problem. Hey, I was also considering molybdenum layers. What do you think?

Pam: I thought you said you wanted ductile films! Moly is bcc, and it has much higher bond energy compared to Cu, so I wouldn’t expect a ductile film with that approach. And geez, have you even looked at the lattice parameters for Mo? I think you’re going to have a heck of a time getting an epitaxial layer to grow, and you might even see some reactions at the Mo and Si interface. Stick with the fcc metals if you’re looking for ductility. And pay attention to those lattice parameters.

Jeong: Speaking of lattice parameters, do you know that song “You Spin Me Round (Like a Record)” by Dead or Alive? For some reason, that song always of reminds me of thin films research.

Pam: Yeah, that song is awesome, and I totally know where you’re coming from.


My goal for you guys this semester is not to be able to sound like our friends Jeong and Pam, but to at least know what they are talking about, and to be able make use to the proper terminology when possible. Also, if you see the molecular composition of a material, it would be awesome if you could guess its structure.