Crystallography Flashcards
What is a space lattice? And what is a unit cell?
A space lattice is an infinite array of points in three dimensions with bonds in 3 dimensions. A unit cell is the smallest unit which when repeating will create the lattice.
How do we indicate a direction in a cubic unit cell? Why is it important?
When dealing with metals and alloys that have varying properties with different crystal orientations, it is important to indicate a direction by drawing a direction vector from the origin (usually a corner of the cubic cell) to the surface of the cell
How any space lattices are there? What are they called? What are the three basic configurations?
- Bravais lattices. Simple cubic (1atom per unit cell), body centred cubic (2), face centred cubic (4). We have all sorts of different shapes.
What are the four types of crystal imperfections?
Line, point, surface, and volume. There are real crystals
What are the crystalline properties of sand and glass?
Sand is highly crystalline and made of silica crystals. When it is heated up to make glass, it becomes amorphous and thus becomes clear.
Is concrete crystalline? How does it affect the strength of concrete?
Yes very! All the components of concrete (aggregates, cements, and anything else) are highly crystalline. This makes concrete very strong, durable and resistant.
What happens to sulphates the concrete at 32.5 degrees?
They change their crystal structure which affects the deterioration of concrete.
What are point imperfections? What are the three types
They are defects found in point like regions of the crystal. They have a very small diameter.
- vacancy: an atom is missing which weakens bonds and allows for crack propagation
- substitutional impurity: foreign atom=different bonds
- interstitial: extra atoms of different kind that pushes atoms apart and weakens bonds
What are line imperfections?
Also called dislocations. They cause the space lattice to not line up perfectly. They are created during solidification or by plastic deformation. They are one dimensional are either edge or screw dislocations.
What are edge dislocations?
Created when an extra half plans of atoms are inserted. They are represented by a t-symbol. Also called cutting dislocations (look at l4 page 10)
What are screw dislocations?
Formed when a shear stress is applied to a crystal that has been separated by a cutting plane. Resembles staircase of atoms
What are surface imperfections?
Found at external surface of crystal (Eg steel) and at grain boundaries of crystal. We get defects due to imperfect alignment
What is the grain boundary?
Created during solidification during formation of crystals. It’s a narrow region of atomic mismatch between two grains and its shape is determined by growth restrictions. IRON PYRITE can grow perfectly cubed if it doesn’t have any restrictions.
