Metals - structure and deformation Flashcards
State three common structures metallic structures
Face centred cubic
Hexagonal close-packed
Body centred cubic
Define polymorphism
Polymorphism (also known as allotropy) is the ability of the structure of metals to change as a function of temperature. Structures that exhibit polymorphism are known as allotropes or polymorphs
State Bragg’s law
Describe a common form of phase diagram
A common phase diagram plots temperature against pressure for a given structure
Describe what a phase diagram tells us about a structure
Phase diagrams reflect the thermodynamic stability of a structure under different conditions but doesn’t tell us about the speed of the transformation
Define an alloy
An alloy is a mixture of metals and metals or metals and non-metals, resulting in a new metal with modified properties
Describe substitutional alloys and give an example
Two or more elements are combined, and while the basic crystal structure remains the same, essentially you swap one atom for another. Atoms are distributed randomly in proportion to their mole fraction.
Are also known as substitutional solid solutions.
Brass is an example of a substitutional alloy, formed of copper and zinc.
Describe the necessary conditions for substitutional alloys to be formed
The sizes of the atoms must be similar (within approx. 15%)
The atoms must have similar redox properties to ensure there is no electron transfer. If the metals differ significantly in electronegativity, you get electron transfer which leads to intermetallic compounds.
Describe an intermetallic compound
Intermetallic compounds occur when there is a big difference in electronegativity between the elements. Electrons are transferred from one element to the other. Intermetallics have regular structures.
Describe interstitial solid solutions.
Interstitial alloys are ones in which atoms of one type are inserted into gaps in the structure of another. Normally the interstitial atom is very small and may be distributed either randomly or regularly.
Describe the grain structure of metals
Metals are not made from a single crystal but instead from a mass of grains - tiny crystallites locking together
Describe why metals are formed of grains.
Metals are formed by heating an ore and then cooling the molten mass. As the mass cools, it crystallises, but not uniformly. The mass will start to cool from a seed point that the crystal then grows out of, in the form of a dendrite. The dendrites eventually meet, forming grains with boundaries between them.
Give Hooke’s law and describe an example
For example, a wire of length l that is being pulled on one end (ie uniaxial stress). If this pulling force is removed, the wire will go back to the original shape - there is an elastic deformation because the stretch is completely reversible.
Describe the elastic region
There is a limit to how much you can deform a material. Beyond the elastic region, if the metal is deformed it won’t be able to return to its original shape. This is known as plastic deformation and is the basis of the ductility of metals.
Describe the ductility of metals
Due to plastic deformation, metals can be reformed and reshapes by bending, pressing and twisting the original material.
Define stress
Stress is the force applied to a material divided by the material’s area
Define strain
Strain is known as the fractional deformation, relating the original length to the change in length
Define Young’s modulus
Young’s modulus is the ratio of stress to strain and is a measure of how stiff the material is
Define the elastic modulus, μ
The applied stress, σ, leads to a displacement of the plane by x, compared to a unit cell distance b.
Note that we are representing the arrangement of atoms as if planes of atoms are stacked on top of each other. When stress is applied, one plane moves relative to the other.
Describe slip
If you look at a wire that has been pulled beyond its elastic limit under a microscope, you will be able to see a series of steps that relate to the planes of atoms that have moved. This process is referred to as slip and the skipping occurs along slip lines.
Describe mechanical twinning
This process is where the slippage leads to the crystal structure flipping across a particular plane.
Describe stacking faults
one way to think of the difference between the fcc and the hcp structure is to think of the sequence of the close-packed planes. The first goes ABCABC… while the other one repeats ABAB…Now the difference in energy between these two often isn’t that large and you can imagine that if you start to deform a metal that is ABC you could get an A plane to move ending up at B as shown in the middle picture above. If so then there is an “error” in the structure. We call that a stacking fault.
Describe edge dislocation
An extra sheet of atoms sits within the lattice but only extends part way through the crystal. Now there is a line of mismatched atoms running though. Stress is applied to the material and the atoms at the dislocation jump across and link up with the row below.
Describe screw dislocations
There is a twist in the rows of atoms such that the rows become misaligned through the crystal.
