This is a view of the same material as in Phase Transitions 1. What has happened? Well, there was an intermediate stage. The lines visible in that one are maybe just cracks, but I don’t know. The material is cooling and that is putting it under some tension:
But the third stage (the picture above) is different. What has happened there? The material has been cooling and then it underwent another phase transition, this time from one solid state into another solid state. The molecules in the crystals arranged themselves into another configuration. This is called a lattice transformation. Energy is released in this process. It is happening at a certain temperature during the cooling process.
The problem is that the angles of the crystals are changing during this transformation. As a result, the crystals don’t fit again into their places between the other crystals. They are put under tension. This is causing them to “zig-zag” in a process called “twinning”. You can see this at the border of the crystal. The lattice of molecules goes in one direction, then flips into its mirror image in the next section. The resulting stripes appear very quickly (with something like the spee of sound in the material). They appear not all at once but several of them might appear at the same time (as far as you can see). This changes the pattern of tension, so another set of stripes appears a moment later, and so on, until a relatively stable configuration is reached.
The result is a complex pattern of stunning beauty, with a mix of order and disorder.
The yellow section down in the center is a harbinger of things to come, but you will have to wait until next time before I am going to post that. And I am probably going to post some other stuff in between. So that is the cliff hanger here :-) .
The following pictures might illustrate what is happening in a crystal during twinning. This is, of course, a very simplified model (these models where built by Svend Keller for his material science lessons). The molecules in the organic crystals are not simple spheres, they have more complex shapes. But what you can see here are two octaedral crystals consisting of spherical “atoms” (plastic spheres), The third “crystal” in the middle shows an intermediate stage. The models consists of stacked layers of shperes. The spheres in each layer are glued together. If you push the blue sphere to the left, the middle “crystal” will “flip” from one configuration to the other. If you flip between the second and third image, you may get an idea what happens during twinning. This is also called lattice shearing.
This is also called lattice shearing.
All pictures: Svend Keller