Materials 4: Defects Flashcards
Identify the Types of defects that can occur in materials
Point Defects - 0D
atoms missing or in irregular places in the lattice (lattice vacancies, substitutional and interstitial impurities or self-interstitials).
Linear Defects - 1D
Groups of atoms in irregular positions (e.g. screw and edge dislocations)
Area Defects - 2D
The interfaces between homogeneous regions of the material (e.g. grain boundaries, stacking faults or external surfaces).
Volume Defects - 3D Extended defects (pores or cracks).
Identify the different types of Point defects
Vacancies: a vacant atomic site within a structural lattice.
Note: Vacancies are always present in crystals and they are particularly numerous at high temperatures, when atoms are frequently and randomly change their positions leaving behind empty lattice sites
Self-interstitials: Extra atoms positioned between atomic sites.
Additionally there are interstitial impurities and substitutional impurities. these impurities are significantly smaller or larger atoms that can be with in order with the lattice structure or between atomic sites. these impurities will cause stress to the surrounding atoms.
What is the term given for a materials ability to dissolve
Solubility
Identify the factors for high solubility
• Atomic size factor - atoms need to “fit” ⇒ solute and solvent atomic radii should be
within ~ 15%
- Crystal structures of solute and solvent should be the same
- Electronegativities of solute and solvent should be comparable (otherwise new intermetallic phases are encouraged)
- Generally, in metals, more solute goes into solution when it has higher valency than solvent
Describe how solid solutions are made and describe the parts involved
Solid solutions are made of a host (the solvent or matrix) which dissolves the minor component (solute).
➢ Solvent: in an alloy, the element or compound present in greater amount.
➢ Solute: in an alloy, the element or compound present in lesser amount.
➢ Solid Solution:
✓ homogeneous
✓ maintain crystal structure
✓ contain randomly dispersed impurities (substitutional or interstitial)
Describe Line defects or dislocations.
Line defects or dislocations are imperfections or distortions within a crystal lattice structure. There are two primary types, edge dislocations and screw dislocations which can
be combined to form a variety of different defects. Dislocations are caused by the application of shear stress to a crystal lattice. Measurement of dislocations using the Burgers vector can determine the magnitude and direction of distortion that has occurred.
Describe Burgers vector
Burgers vector: the length of the dislocation of the crystal lattice caused by a single dislocation is known as the
Burgers vector, and this will be constant for any one slip system.
Describe edge and screw dislocations
Edge dislocations are an additional half lattice plane embedded within the crystal structure. it moves in response to shear stress and results in bulk atomic movement.
Screw dislocations are centered on a dislocation line, displacement occurs over one lattice distance resulting in a twisting effect
In screw dislocations, the Burgers vector and dislocation line are they are parallel.
What is a Slip plane and Slip direction.
The plane in which the dislocation moves is called the slip plane, and the direction in which it moves is called the slip direction
What is dislocation density
The number of dislocations in a material is expressed as the dislocation density - the total dislocation length per unit volume or the number of dislocations intersecting a unit area. Dislocation densities can vary from 10^5 cm^-2 in carefully solidified metal crystals to 10^12 cm^-2 in heavily deformed metals.
Define Grain boundaries
Polycrystalline material comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. Motion of dislocations can be impeded by grain boundaries.
Grain Boundaries • regions between crystals • transition from lattice of one region to that of the other • slightly disordered • low density in grain boundaries – high mobility – high diffusivity – high chemical reactivity
Explain what is dislocation and their types
Line defects or dislocations are imperfections or distortions within a crystal lattice structure. There are two primary types, edge dislocations and screw
dislocations which can be combined to form a variety of different defects. Dislocations are caused by the application of shear stress to a crystal lattice. Measurement of dislocations using the Burgers vector can determine the magnitude and direction of distortion that has occurred.
Edge dislocations are an additional half lattice plane embedded within the crystal structure.
Screw dislocations are centered on a dislocation line, displacement occurs over one lattice distance resulting in a twisting effect. In screw dislocations, the Burgers vector and dislocation line are they are parallel.
