Deck 13-Looking inside materials Flashcards
What is an amorphous structure?
A disordered structure of atoms in a solid which resembles a disordered arrangement in a liquid.
How do you get a material to have an amorphous structure?
Rapid cooling tends to trap particles in an amorphous state, which happens because the particles do not have time to rearrange themselves.
What is a crystalline structure?
A highly ordered array of atoms.
How do you get a crystalline structure?
Slow, controlled cooling can produce a single crystal.
Give an example of a crystalline material and describe what its uses are.
A single crystal material is high-purity silicon which is used for making microchips.
What is a polycrystalline material?
A material which doesn’t have an amorphous structure but it neither has a crystalline structure.
How do you get a poly-crystalline material?
When cooled, each crystal grows out into remaining liquid until it runs into neighbours. The result is a patch work of tiny crystals or grains. Where the grains meet, the interface is known as the grain boundary. Therefore a polycrystalline material consists of a number of grains all orientated differently relative to one another but with an ordered structure within each grain.
What is average glass made from?
Silica with various metal oxides.
Is glass amorphous?
Yes
What structure do metals have?
Crystalline.
Why does glass shatter easily?
When you bend glass, cracks open up easily which then act as stress raisers making the stress around the crack up to 1000 times stronger.
Why can dislocations in a metals structure make a metal stronger?
The dislocations can help spread the stress around the crack.
Write the step by step process from bending a piece of glass to shattering it.
- When you bend a piece of glass it becomes strained elastically. It stores strain energy.
- At the tip of the crack, two neighbouring atoms are pulled apart. Work is done in breaking the bond between the atoms.
- Then the next two atoms are pulled apart, and the next two, and so on. The crack moves through the material like a zip being undone. The energy required to do this is known as fracture energy
- Once the piece of glass is broken, it no longer stores strain energy. The energy has been used up in breaking bonds, in sending fragments of glass flying and making atoms vibrate.
What is toughness a measure of?
A measure of the energy needed to deepen and extend cracks, creating a new fracture surface in cracks.
What will happen if the energy available from the stresses in a material is larger than the energy required to extend a crack?
The crack will propagate and the material will fail. This is why glass can break almost explosively.
