DM308 Lecture 2: Crystals, Defects, and Interfaces

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Questions and Answers

What is a crystal?

A crystal is a solid material in which the atoms, molecules, or ions are arranged in an ordered and repeating three-dimensional pattern extending throughout the entire structure

What are the two elements that comprise a crystal structure?

The two elements are the lattice and the basis.

What are the unique minimal spacings between the lattice points known as?

The unique minimal spacings between the lattice points are known as the lattice parameters.

Which materials are semi crystalline?

An example of semi-crystalline materials is polymers/elastomers such as polyethylene. Signup and view all the answers

What are some examples of amorphous materials?

Examples of amorphous materials include glasses and bulk metallic glasses. Signup and view all the answers

What are spherulites, and what do they do to the polymer structure?

Spherulites are semi-crystalline structures that greatly increase the polymer density, opaqueness, and strength. Signup and view all the answers

What are the two possible ways to stack hexagonal layers in close-packed crystals?

Hexagonal close-packed (HCP) and Face-centred cubic (FCC) Signup and view all the answers

What is the smallest possible unit cell known as?

Primitive unit cell Signup and view all the answers

What are the three types of crystallographic defects?

Point defects, line defects, planar defects Signup and view all the answers

What is responsible for the plastic deformation of crystalline material?

Dislocation glide Signup and view all the answers

What is the displacement vector needed to close a circuit around the dislocation line called?

Burgers vector Signup and view all the answers

What is the minimum stress required to initiate the glide of a dislocation called?

Critical resolved shear stress (CRSS) Signup and view all the answers

What are the three possible types of dislocations?

Edge, screw, mixed Signup and view all the answers

What is responsible for the plastic deformation of a material?

Dislocations Signup and view all the answers

What is the term for the breaking and reforming of bonds by dislocation movement due to an applied force?

Plasticity Signup and view all the answers

What occurs because the dislocations that are continually generated act as obstacles to one another?

Work hardening or strain hardening Signup and view all the answers

How can work hardening be exploited to strengthen ductile materials?

By increasing the yield strength at the cost of ductility Signup and view all the answers

Explain Incoherent inter phase interfaces between dissimilar crystals

The two lattices do not mesh and the interface between them comprises a dense network of geometrically necessary dislocations. Signup and view all the answers

What type of strengthening occurs when a dissolved impurity atom generates a stress field that interacts with the stress field around a dislocation and inhibits its glide through the lattice?

Solute strengthening Signup and view all the answers

What is the critical resolved shear stress due to a dissolved impurity given by?

shear strength of solute square of atom/particle radius solute concentration shear modulus of solute Signup and view all the answers

How is order strengthening achieved when a dislocation moves through a chemically ordered crystal?

By creating an antiphase boundary that opposes the motion of the dislocation Signup and view all the answers

What type of strengthening occurs when fine particles with a stronger/harder crystal structure are formed within the prior parent crystal?

Precipitation hardening Signup and view all the answers

How do dislocations interact with numerous fine particles formed within a material during precipitation hardening?

They must either cut through the particles or bypass them via the Orowan bowing mechanism Signup and view all the answers

Why are most manufactured materials polycrystalline?

Because polycrystalline materials are stronger and easier to produce than single crystals Signup and view all the answers

What is the term for the regions with distinct chemistry and crystal structure that can co-exist in the same material?

Phases Signup and view all the answers

What happens when Organic (carbon-based) polymers can form semi-crystalline structures known as spherulites.

This greatly increases the polymer density, opaqueness and strength. Signup and view all the answers

What are the two main steps involved in the process of refining the grain structure of a material?

Plastically deform the material and then anneal the deformed material Signup and view all the answers

What is the purpose of annealing the deformed material?

To begin the process of recovery, recrystallization, and grain growth Signup and view all the answers

What occurs during the process of recrystallization?

Dislocation networks start to form cells within the deformed grains, which gradually evolve into finer sub-grains and eventually give a grain structure that is finer than the original Signup and view all the answers

What follows once recrystallization is complete?

Grain growth Signup and view all the answers

What will occur if a material is not deformed prior to annealing?

Only grain growth will occur Signup and view all the answers

Why does grain growth occur after recrystallization?

To minimize the overall grain boundary area, which is thermodynamically unfavorable Signup and view all the answers

What are grain boundaries composed of, and how do they affect the transfer of plastic strain?

Grain boundaries are networks of dislocations, which act as obstacles to dislocation glide, limiting the transfer of plastic strain between the grains. Signup and view all the answers

In Grain boundary strengthening, Explain the relationship between grain size and the strength of a material.

The smaller the size of the grains in the material, the higher its strength. Signup and view all the answers

What role do dislocations play in grain boundary strengthening?

Dislocations act as obstacles to dislocation glide, limiting the transfer of plastic strain between the grains. Signup and view all the answers

Explain "Coherent" inter phase interfaces between dissimilar crystals

The two lattices mesh perfectly, but the difference in lattice parameters results in a misfit strain. Signup and view all the answers

Explain "Semi Coherent" inter phase interfaces between dissimilar crystals

The two lattices mesh well, but some dislocations are formed between coherent areas. Signup and view all the answers

Explain what phases are in Interfaces between dissimilar crystals

Regions with different crystal structures and chemical composition can co-exist in the same material. These regions with distinct chemistry and crystal structure are known as phases. Signup and view all the answers

For controlling the average grain size of a material grain size, what is the first step?

First we need to plastically deform the material to increase the number density of dislocations in it. The more deformation, the greater the dislocation number. (E.g. cold rolling, cold extruding, cold forging, cold swagging) Signup and view all the answers

For controlling the average grain size of a material grain size, what is the second step?

Anneal the deformed material by raising its temperature and holding it in the heated state for a period of time. (I.e. put it in a furnace and let it sit there) Signup and view all the answers

In annealing explain the recovery stage

The dislocations will start to migrate and react with one another either annihilating or combining into dislocation networks. Signup and view all the answers

In annealing, explain the recrystallisation. stage

The dislocation networks will start to form cells within the deformed grains, that will gradually evolve into finer sub-grains and eventually giving a grain structure that is finer that the original Signup and view all the answers

In annealing explain the grain growth stage

Larger grains will grow at the expense of while small ones which will shrink and disappear This process occurs to minimise the overall grain boundary area. If the material is not deformed prior to annealing, only grain growth will occur. Signup and view all the answers

How are new dislocations formed via the Frank-Read mechanism

A dislocation segment in which the ends are immobilised (“pinned”) will act a source for multiple new dislocations. The segment bows out under applied stress until it forms a dislocation loop and a newly reformed segment. The cycle repeats generating more loops. Signup and view all the answers

What are Amorphous materials

Amorphous materials are materials that lack a crystalline structure. the atoms in amorphous materials do not exhibit a long-range, repeating pattern. Resulting in them being brittle Signup and view all the answers

What happens when When dislocations with opposing Burger’s vectors meet?

The net Burgers vector becomes zero, and becomes a perfect crystal Signup and view all the answers

What are defects featuring only atoms from the original crystal called, and name the type of defects

Intrinsic. Vacancy, self interstitial Signup and view all the answers

What are defects from impurity atoms are called called, and name the type of defects

extrinsic. Anti site defect, substitution impurity, interstitial impurity Signup and view all the answers

Study Notes

A crystal is a combination of a lattice and a basis. The lattice is a spatial grid of points, and the basis is the object or objects that sit on those points.
Crystals are formed by placing atoms or molecules on each lattice point. The unique minimal spacings between lattice points are lattice parameters.
Not all materials are crystalline; some are amorphous, like glasses or bulk metallic glasses, while others are semi-crystalline, such as polymers.
Semi-crystalline polymers can form spherulites, which increase density, opaqueness, and strength.
Crystals are described using repeat elements called unit cells. The smallest possible unit cell is a primitive unit cell.
There are 14 non-equivalent Bravais lattices that can fill space. These are the simplest unique lattice structures from which more complex structures can be formed.
Crystals come in different structures, such as face-centered cubic (FCC), rock salt, body-centered cubic, hexagonal close-packed, perovskite, and diamond.
Close packing in 2D is achieved with a hexagonal arrangement, and it can be stacked in two ways: hexagonal close-packed (HCP) and face-centered cubic (FCC).
Crystals have directions indexed using vectors and crystallographic planes described by their normal vectors.
Defects in crystals disrupt periodicity or symmetry and can be point, line, or planar defects.
Point defects include intrinsic defects, like vacancies or interstitials, and extrinsic defects, like impurities or dopants.
Line defects are dislocations, which can be edge, screw, or mixed.
Dislocations move through crystals by breaking and reforming interatomic bonds (dislocation glide) and can interact with other stress fields.
Dislocations can form sources, such as Frank-Read sources, which generate more dislocations via the Frank-Read mechanism.
Dislocations are responsible for plastic deformation in crystals, which is the breaking and reforming of bonds.
Elasticity is the stretching of bonds by an applied force, while plasticity is the breaking and reforming of bonds by dislocation movement due to an applied force.
Work hardening or strain hardening is the increase in yield strength due to the generation and multiplication of dislocations.
Planar defects include interfaces between dissimilar crystals or phases, and they can be coherent, semi-coherent, or incoherent.
Solute atoms with different sizes can interact with dislocation stress fields, forming beneficial stress fields and increasing the material's strength.

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Description

Test your understanding of crystals, defects, and interfaces as discussed in Dr. Vassili Vorontsov's lecture on production techniques. Explore the concepts of lattice, basis, crystal structure, and the formation of crystals.