Chapter 4 Flashcards
What is solidification?
Initiated by the casting of molten (liquid) material. First the nuclei of the cold phase form, then crystals grow until their boundaries meet each other, and the crystals become grains.
What shapes can grains have?
- Equiaxed (roughly the same dimension in all directions, due to rapid cooling)
- Columnar (elongated in one direction, often due to slow cooling and heat flow)
What are grain boundaries?
Regions between grains that have crystallographic misalignment, and atoms near the boundaries have high mobility and chemical reactivity.
What is a crystalline defect?
A lattice irregularity with dimensions on the order of an atomic diameter.
What are the three types of imperfections?
- Vacancies, interstitial atoms, substitutional impurity atoms (point defects 0D)
- Dislocations (linear defects 1D)
- Grain boundaries (interfacial defects 2D)
What are vacancies?
Vacant atomic sites (holes in atomic lattice) that cause inward distortion.
What are self-interstitials?
Atoms that are positioned in interstitial sites (the small gaps between atoms) that cause outward distortion.
What is the relationship between vacancies and temperature?
The number of vacancies increases as temperature increases.
What is Boltzmann’s constant k?
A thermal energy constant having the value of 1.38 × 10−23 J/atom ⋅ K or 8.62 × 10−5 eV/atom ⋅ K.
What are the two types of solid solutions?
- Substitutional
- Interstitial
What is a host atom?
The primary atom in a solution also called a solvent
What is a solute?
An atom of lower concentration in a solution, dissolved in the solvent
What are second phase particles?
Different compositions/structures within a solid solution (often when there is a greater solute concentration).
What are the conditions for Hume-Rothery rules?
- difference in atomic radius < 15%
- proximity in the periodic table (electronegativity)
- same crystal structure for pure metals
- valences (metals tend to dissolve other metals of higher valences)
What are dislocations and what types are there?
One-dimensional defects around which atoms are misaligned. There are three types: edge, screw, and mixed.
What is an edge dislocation?
There is an extra half-plane of atoms inserted into a crystal structure, Burger’s vector is perpendicular to the dislocation line.
What is a Burger’s vector b?
A measure of lattice distortion
What is a screw dislocation?
A spinal planar ramp resulting from shear deformation, Burger’s vector is parallel to the dislocation line.
What is an alloy?
A metallic substance that is composed of two or more elements.
When does permanent (plastic) deformation occur?
From dislocation motion caused by applied stresses.
What are interfacial (planar) defects?
Grain boundaries, twin boundaries/planes, and stacking faults
What is weight percent?
A concentration specification on the basis of weight (or mass) of a particular element relative to the total alloy weight (or mass).
What are twin boundaries/planes?
Mirrors that reflect atom positions from one side of a twin plane to the other.
What are stacking faults?
Errors in the planar stacking sequence
Ex: in an FCC metal, the normal sequence is ABCABC, so a stacking fault would be ABCABABC
What is a catalyst?
Something that increases the rate of a chemical reaction without being consumed. Catalytic reactions often occur at surface defect sites.
What is optical microscopy?
Uses light, effective up to 2000x magnification. Potentially requires polishing to remove surface features and etching to change reflectance based on grain orientation.
Why do grain boundaries appear as dark lines under microscopes?
They are more susceptible to etching.
When is polarized light used?
In metallographic scopes to increase contrast or with transparent samples.
What is a scanning electron microscope (SEM)?
A microscope that produces an image by using an electron beam that scans the surface of a specimen; an image is produced by reflected electron beams. Examination of surface and/or microstructural features at high magnifications is possible.
What is a scanning probe microscope (SPM)?
A microscope that does not produce an image using light radiation. Rather, a very small and sharp probe raster scans across the specimen surface; out-of-surface plane deflections in response to electronic or other interactions with the probe are monitored, from which a topographical map of the specimen surface (on a nanometer scale) is produced.
What is a transmission electron microscope (TEM)?
A microscope that produces an image by using electron beams that are transmitted (pass through) the specimen. Examination of internal features at high magnifications is possible.
