Experiments with the Bubble Model of Metal Structure 1952 - Sir Lawrence Bragg, W.M Lomer, J.F. Nye
Summary
TLDRThis 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
- đŹ Metals like aluminium have atoms arranged in regular patterns, forming a crystalline structure that can be revealed by etching with acid.
- đ” 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.
- đ 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.
- đČ 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.
- đ 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.
- đ 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.
- đ 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.
- đ The Burgers vector is used to quantify dislocations, defining the magnitude and direction of the displacement in a crystal lattice.
- đ Dislocations can interact with each other, sometimes canceling out or combining to form new dislocations, which can be observed in the bubble model.
- đ§ The presence of impurities or defects can significantly affect dislocation movement, acting as barriers or points of interaction within the crystal structure.
- đ 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
đŹ 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.
đ 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.
đ 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.
đ 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.
Mindmap
Keywords
đĄCrystalline
đĄEtching
đĄClose Packing
đĄSurface Tension
đĄCrystal Boundaries
đĄPlastic Deformation
đĄDislocations
đĄBurgers Vector
đĄVacancies
đĄImpurities
đĄPlastic Movement
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
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