Experiments with the Bubble Model of Metal Structure 1952 - Sir Lawrence Bragg, W.M Lomer, J.F. Nye

Summary

This educational video script explores the crystalline structure of metals, using aluminum as an example. It explains how atoms are arranged in regular patterns, often revealed through etching with acid. The script introduces a model using spheres to represent atoms, and a soap bubble raft to simulate crystal formation and deformation. It delves into the concept of dislocations within crystals, their movement, and how they contribute to plastic deformation. The script also discusses the role of dislocations in restoring order in metals and the limitations of the bubble model in representing the three-dimensional structure of metals.

Takeaways

• 🔬 Metals like aluminium have atoms arranged in regular patterns, forming a crystalline structure that can be revealed by etching with acid.
• 🔵 The close packing of atoms in metals is often represented by models such as spheres or table tennis balls, illustrating the uniform forces between them.
• 💭 Bubbles in a soap solution can mimic the behavior of atoms in a metal, showing how they interact due to surface tension and internal pressure.
• 🔲 The crystalline pattern in metals can be visualized through the formation of a crystalline raft of bubbles, which aligns in a regular, three-dimensional pattern.
• 🔄 When a metal rod is deformed, it can return to its original shape elastically, but excessive deformation leads to permanent changes, analogous to the movement of bubbles in the model.
• 📏 Dislocations in a metal's crystal structure are like defects that can move and cause plastic deformation; they can be visualized in the bubble model as dark lines moving in specific directions.
• 🔄 The movement of dislocations can alter the shape of a crystal, with the strain required for their movement being less than if the entire atomic row shifted at once.
• 🔍 The Burgers vector is used to quantify dislocations, defining the magnitude and direction of the displacement in a crystal lattice.
• 🔗 Dislocations can interact with each other, sometimes canceling out or combining to form new dislocations, which can be observed in the bubble model.
• 🔧 The presence of impurities or defects can significantly affect dislocation movement, acting as barriers or points of interaction within the crystal structure.
• 🔄 The restoration of order in a distorted metal crystal involves the movement and combination of dislocations, which can be accelerated by external forces, as demonstrated in the bubble model.

Q & A

What is the essential feature of a crystal?

The essential feature of a crystal is that its atoms are arranged in regular patterns.

How can the regular patterns of atoms in a metal be revealed?

The regular patterns of atoms in a metal can be revealed by etching it with an acid, which forms pits that reflect light and show the crystal structure.

What is the characteristic form of the regular pattern of atoms in a metal?

The characteristic form of the regular pattern of atoms in a metal is generally one of the close packing models.

How does the model made of spheres (like table tennis balls) relate to metal atoms?

The model made of spheres, such as table tennis balls, demonstrates the close packing of atoms in a metal, where small bubbles floating on a solution experience forces similar to those between metal atoms.

What is the purpose of the bubble model in understanding metal crystals?

The bubble model serves as a visual representation to understand the crystalline structure of metals, where bubbles form a crystalline raft that mimics the regular pattern of atoms in a metal.

How does the bubble model demonstrate the concept of crystal boundaries?

The bubble model demonstrates crystal boundaries by showing how distinct patterns of bubbles meet at certain points, similar to how different crystal patterns meet in a metal.

What is the significance of dislocations in the context of metal deformation?

Dislocations are significant in metal deformation because they represent the atomic-scale movement that leads to plastic deformation. They allow the closely packed planes of atoms to slip over each other, causing the metal to change shape.

What is a Burgers vector and how does it relate to dislocations?

A Burgers vector is a vector that defines the magnitude and direction of the displacement associated with a dislocation. It is used to quantify the movement of dislocations within a crystal lattice.

How can dislocations be created in a crystal?

Dislocations can be created in a crystal through rapid crystallization, which may leave them as growth accidents, or from groups of vacancies where atoms are missing, leading to the formation of dislocation pairs.

What role do impurities play in the movement of dislocations?

Impurities, represented by bubbles of different sizes in the model, can interfere with the movement of dislocations, acting as centers of strain and significantly affecting the metal's plastic deformation.

How does the bubble model illustrate the process of plastic deformation and recovery in metals?

The bubble model illustrates plastic deformation by showing dislocations moving and combining under stress, and recovery by demonstrating how dislocations tidy up areas of strain and create nearly perfect crystal regions, which then adjust their boundaries to reduce their energy.

Outlines

00:00

🔬 Understanding Metal Crystallography

This paragraph introduces the concept of metal crystallography, explaining that metals like aluminum have a crystalline structure with atoms arranged in regular patterns. The use of acid etching reveals these patterns, which are typically close-packed. A model using spheres, akin to table tennis balls, illustrates this arrangement. The paragraph also discusses how small bubbles on a soap solution can mimic the behavior of metal atoms, forming a crystalline raft under constant pressure. This raft, with its regular pattern, mirrors the structure seen in etched metals. The discussion then moves to the deformation of metals, suggesting that the atomic rearrangement during deformation is similar to the movement of bubbles in the soap solution, with 'dislocations' playing a key role in plastic deformation.

05:03

📏 The Role of Dislocations in Crystal Deformation

The second paragraph delves into the concept of 'dislocations' within a crystal lattice. It describes how dislocations, which can be thought of as lines where the regular pattern of atoms is disrupted, facilitate the movement of atoms under stress, requiring less energy than if the entire row of atoms moved. The 'Burgers vector' is introduced as a way to quantify dislocations. The paragraph further explains how dislocations can be created during rapid crystallization or from groups of vacancies where atoms are missing. It also discusses how dislocations can move, combine, and cancel each other out, and how they can be influenced by impurities. The bubble model is used to visually demonstrate these concepts, showing how dislocations can start at points of high stress, such as cracks or notches.

10:16

🔄 Interactions and Movements of Dislocations

This paragraph explores the dynamics of dislocations, including their interactions and movements within a crystal lattice. It describes how dislocations can be in equilibrium with each other and how similar dislocations can attract or repel based on their positions. The concept of combining Burgers vectors to form resultant vectors is introduced, and the paragraph shows how dislocations can jump from one plane to another, removing vacant sites in the process. The bubble model is used to visually represent these interactions, including the movement of dislocations and the formation of complex vacancy structures. The paragraph also touches on the role of impurities, represented by bubbles of different sizes, which can significantly disrupt dislocation movement and act as pinning centers.

15:17

🌐 Crystal Rearrangement and Plastic Movement

The final paragraph discusses the processes involved in the rearrangement of crystals and the restoration of order in a distorted lattice. It explains how dislocations can tidy up areas of strain and create nearly perfect crystals, which then adjust their boundaries to minimize their energy. The paragraph also describes how the type of boundary can vary from simple rows of dislocations to more complex structures at larger angles of disorientation. The bubble model is used to illustrate the movement of dislocations and boundary sliding, which are key processes in plastic deformation. The paragraph concludes by acknowledging the limitations of the bubble model, particularly its two-dimensional nature and the inability to simulate heat motion, but emphasizes its value in providing a visual and conceptual understanding of what happens within a metal during deformation and annealing.

Keywords

💡Crystalline

Crystalline refers to the orderly and repeating arrangement of atoms in a solid material, forming a regular pattern. In the context of the video, the crystalline structure of metals such as aluminum is highlighted. The video explains that while this pattern is not always visible, etching the metal with acid reveals the regular patterns, which are essential for understanding the material's properties. The script uses the analogy of a model made of spheres, like table tennis balls, to illustrate the close packing characteristic of metal atoms.

💡Etching

Etching is a chemical process used to reveal the underlying structure of a material by selectively removing material from its surface. The video script describes how etching a piece of aluminum with acid brings out the regular patterns of its crystalline structure, allowing the light to glint on these patterns and make them visible. This technique is crucial for studying the atomic arrangement in metals.

💡Close Packing

Close packing is a geometric arrangement where spheres (or atoms) are packed together in a way that minimizes the empty space between them. The script mentions that the regular pattern of atoms in a metal is generally one of close packing, which is a characteristic form that maximizes the density of the material. This concept is illustrated using a model made of spheres, akin to table tennis balls, to represent the efficient arrangement of atoms in a metal.

💡Surface Tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules. In the video, surface tension is discussed in relation to small bubbles floating on a solution, which are drawn into contact with each other by this force. This phenomenon is used as an analogy to explain how atoms in a metal interact, with the internal pressure of the bubbles setting a limit to their approach, similar to how atoms in a metal are packed closely together.

💡Crystal Boundaries

Crystal boundaries are the interfaces where different crystal structures or orientations meet within a material. The script describes how in a larger raft of bubbles, which serves as a model for a metal's crystalline structure, the rows of bubbles are in different directions in neighboring 'crystals,' indicating the presence of crystal boundaries. These boundaries are significant in materials science as they can influence the material's mechanical properties and deformation behavior.

💡Plastic Deformation

Plastic deformation refers to the permanent change in shape a material undergoes when subjected to an external force. The video explains that a metal rod can deform elastically, returning to its original shape when the force is removed, but if deformed beyond a certain point, it undergoes plastic deformation and does not return to its original shape. This concept is crucial for understanding the limits of a material's elasticity and the onset of permanent structural changes.

💡Dislocations

Dislocations are line defects in the crystal lattice of a material, where the orderly arrangement of atoms is disrupted. The video script uses the bubble model to illustrate dislocations, showing how they can move and interact within the crystal structure. Dislocations play a key role in the deformation and strength of materials, as they facilitate the movement of crystal planes over one another, allowing for plastic deformation to occur.

💡Burgers Vector

The Burgers vector is a quantitative measure of the magnitude and direction of a dislocation in a crystal lattice. The video script describes how the Burgers vector can be visualized by drawing a circuit around a dislocation; if the circuit does not close as it would in a perfect crystal, the Burgers vector defines the displacement. This vector is essential for understanding how dislocations contribute to the deformation of materials.

💡Vacancies

Vacancies refer to the missing atoms in a crystal lattice. The script mentions that dislocations may arise from groups of vacancies, where atoms are missing. The video uses the bubble model to demonstrate how the collapse of rows on either side of missing atoms can create a pair of dislocations. Vacancies are important in materials science as they can act as sources or sinks for atoms during processes like diffusion and can influence the material's properties.

💡Impurities

Impurities are foreign atoms or molecules that are present in a material and can affect its properties. In the context of the video, impurities are represented by bubbles of the wrong size in the model, which interfere with the movement of dislocations. The script explains that impurities can act as barriers to dislocation motion, thereby influencing the material's resistance to deformation and its overall mechanical behavior.

💡Plastic Movement

Plastic movement refers to the permanent deformation of a material that occurs without a change in volume. The video script describes how plastic movement in a crystal can occur through the movement of dislocations or by boundary sliding. This concept is illustrated in the bubble model, where the movement of dislocations and the sliding of crystal boundaries are shown to restore order to a distorted structure. Understanding plastic movement is vital for predicting how materials will respond to applied forces.

Highlights

Metals like aluminium have atoms arranged in regular patterns, forming a crystalline structure.

Etching a metal with acid reveals the regular patterns of atoms, similar to the surface of a soap bubble raft.

A model made of spheres, like table tennis balls, represents the close packing characteristic of metal atoms.

Soap bubbles in a solution demonstrate the forces between atoms in a metal, influenced by surface tension and internal pressure.

A crystalline raft of bubbles forms under constant air pressure, illustrating the uniformity and regularity of metal crystals.

Crystal boundaries in a metal are where distinct crystal patterns meet, shown by the different directions of rows in neighboring crystals.

Metal rods can elastically return to their original shape after limited deformation, similar to the behavior of bubbles in a raft.

Plastic deformation in metals is believed to be due to the movement of dislocations within the crystal structure.

Dislocations are lines of atoms that have slipped, and they can be visualized by the movement of bubbles in the model.

The Burgers vector is used to define a dislocation quantitatively, representing the movement of atoms in a crystal.

Dislocations can be created by growth accidents during rapid crystallization or from groups of vacancies where atoms are missing.

The movement of dislocations can be influenced by the presence of impurity atoms, which can act as barriers to their motion.

Dislocations can combine or cancel each other out, changing the overall strain within the crystal.

The interaction between dislocations can lead to their alignment or repulsion, affecting the crystal's overall structure.

The plastic movement in a crystal can occur through the movement of dislocations or by boundary sliding.

The bubble model, despite its limitations, provides a visual representation of dislocations and their role in metal deformation.

The model suggests that restoring order in a distorted metal involves the movement and combination of dislocations.

The process of crystallization in metals can be impeded by impurity atoms, which cause local imperfections.

Transcripts

all metals like this piece of aluminium

a crystalline the essential features of

a crystal being that its atoms are in

regular patterns this is not always

apparent in an ordinary piece of metal

but if we etch it with an acid regular

pizza formed and the light glinting on

these bits shows the crystals the

regular pattern of atoms in a metal is

generally one of the close packing this

model made of spheres actually a table

tennis balls is a characteristic form

small bubbles floating on a solution

have very closely the same forces

between them as the atoms in a metal the

surface tension draws the bubbles into

contact and the internal pressure sets a

limit to their approach as if small

balloons were being pressed together so

we can make a model of a metal these

bubbles which crystallizes itself a tank

is filled with soap solution

air supplied by this aspirator is blown

under constant pressure from this dip

set just beneath the surface of the

solution

the bubbles produced form a kind of

crystalline raft which is quite regular

because the bubbles are so uniform in

size here is a latest stage in the

formation of such a raft the pattern has

rows in three directions it's just like

the model made of Spears when the raft

of bubbles forms it's generally in

portions of pattern which are not

parallel or as we should say are

distinct crystals these meet at crystal

boundaries it can be seen in this raft

that the rows are in different

directions in neighboring crystals this

pattern of crystals in this larger raft

is very similar to the pattern of

crystals seen in an etched metal

a metal rod deformed to a limited extent

springs back elastically to its original

shape when released the same is true

over after bubbles noticed information

at the places where bubbles are missing

if the rod is pushed too far it gets a

permanent deformation and does not come

completely back what happens on the

atomic scale when this plastic

deformation takes place we believe that

the bubble model supplies the answer

dark lines are seen dashing about in

certain directions along with respect

rose these are the district Asians which

are going to study in detail by their

movement the general shape of the

massive bubbles alters their its

structure remains regular and

crystalline

from the geometry of the deformation

it's clear that the sheets of atoms

glide over each other the closely packed

planes slipping like a pack of cards but

all the atoms in a sheet do not jump

from one position of packing to the next

simultaneously as they're doing here a

dislocation stops at one end and runs to

the other like a letter in a stocking

the dislocation can be thought of as a

place where slip is terminated in midair

there's a row about which is not

partnered by row below you've just seen

the rows and here is the corresponding

arrangement of bubbles

the strain required to make a district

ation run across the crystal is far less

and that which would be necessary if a

whole row jumped at once

we must have some way of defining a

district ation quantitatively and this

is the purpose of the burgers vector we

would draw a circuit which would close

in a perfect crystal

if the circuit includes a vacant lettuce

site it'll still close exactly if it

includes a dislocation it does not close

can visit you is the burgers vector it's

equal to an inter Tomic distance however

the circuit is drawn the result is the

same the dislocation runs in a direction

which relieves the strain if the strain

is reversed it runs the other way after

it's passed the crystal is perfect as

before now that we've seen what a

dislocation is we can look again at the

picture of dislocations moving in all

directions as the crystal is deformed

it's still not certain how dislocations

begin in a crystal they may be left in

the crystal as accidents of growth

during rapid crystallization as can be

seen in the massive bubbles which is

building itself up here they may also

arise from groups of vacancies where

atoms are missing the line diagram shows

how pair of dislocations might be

created in this way each parallelogram

encloses an atom which is going to be

taken away the rows and either side of

the missing atoms collapsed together to

make a pair of dislocations

the bubbles show the same thing

happening a few bubbles are burst with a

hot wire and two dislocations start from

the holes so created dislocations are

most likely to start where the stress is

greatest at cracks or notches like this

two or more burgers vectors may be

combined to give a resultant vector this

will be seen in the following series

where a circuit is run first round each

dislocation separately and then around

both

the combinations will now be shown

taking place in the bubble model you

will see first two equal and opposite

dislocations in the same row canceling

each other

their second day two in neighboring rows

combining but leaving a vacancy thirdly

dislocations leaving a complex of

vacancies

finally you will see two dislocations

the resultant is not zero approaching

each other and turning into a third

resultant dislocation which runs away in

another direction if a dislocation jumps

from one river to another it may remove

a vacant site the line diagram shows how

this takes place

now you will see it actually happening

in the bubble model watch the vacancies

in the bottom left-hand corner

since the region around a dislocation is

in a state of strain that are forces

between neighboring dislocations first

we shall see two dislocations in

relative positions of equilibrium if one

is displaced it returns to position two

similar dislocations on neighboring

planes attract one another these two

refuse to part company a number of

similar district Asians attract in the

same way and line up an outsider cannot

join the party if it's on the same plane

as one in the row but if we move it

sideways onto another row by bursting a

few bubbles it joins up here's the whole

row moving together and a late comer

having to join their company to

repelling dislocations are reluctant to

pass one drives the other before it an

impurity atom can be represented by a

bubble of the wrong size it interferes

greatly with a movement of dislocations

and it's a center of travel

the type of boundary may be anything

between two extremes if the angle of

disorientation is small it's like a row

of dislocations at larger angles the

nature of the boundary is more complex

with small angles a plastic movement

corresponds to an advance of the line of

dislocations as illustrated here here we

see this movement in the bubble model

with larger angles the sliding may take

place along the disordered boundary here

is the corresponding movement in the

bubble model finally the plastic

movement is here taking place both by

moving dislocations and by boundary

sliding

all these processes we have seen play a

part in restoring order to a much

distorted route the distortions can be

affected by stirring up the raft with

gas rods this corresponds to a violent

deformation of the metal the

dislocations travel and combine so to

tidy up the areas of greatest strain and

create a mass of nearly perfect crystals

these then adjust their boundaries says

to reduce their links as far as possible

the readjustment is at first very rapid

but it soon slows down and in these

shots it's much speeded up by the camera

the black spots which suddenly appear

are due to bubbles bursting some small

crystals disappear in this cleaning up

process crystallization is impeded by

impurity atoms this small bubble causing

the local imperfection was difficult to

burst the bubble model is far from being

a complete representation of the middle

structure one serious disadvantage is

its two-dimensional character whereas a

middle is of course three-dimensional it

is possible to make three-dimensional

respect bubble masses each bubble lies

between three others above and three

below as well as six around it as these

close-ups show but the three-dimensional

packing is hard to obtain in sypt in a

layer of a few sheets and even it were

thicker one could not see what happens

inside another disadvantage is that one

cannot set the bubbles in vibration and

so simulate the agitation of heat motion

nevertheless the model suggests many

ideas and when we see the dislocations

dashing about we can form a mental

picture of what is happening in the

metal itself