ME 0307 MATERIAL TECHNOLOGY
Unit I : ELASTIC AND PLASTIC BEHAVIOUR
Elasticity in metals and polymers - Mechanism of plastic deformation - Role of yield stress, shear
strength of perfect and real crystals - Strengthening mechanisms, work hardening - Solid solutioning,
grain boundary strengthening, particle, fibre and dispersion strengthening - Effect of temperature,
strain and strain rate on plastic behaviour - Super plasticity - Deformation of non-crystalline material.
TEXT BOOKS
Dieter, G. E., Mechanical Metallurgy, McGraw Hill, Singapore, 2001
Thomas H. Courtney, Mechanical Behaviour of Engineering materials, McGraw Hill, Singapore, 2000
REFERENCE BOOKS
Flinn, R. A. and Trojan, P. K., Engineering Materials and their applications, Jaico, Bombay, 1989
Budinski K.G. and Budinski, M. K., Engineering Materials Properties and selection, Prentice Hall of
India Private Limited, New Delhi, 2004
ASM Metals Hand book, Failure analysis and prevention, Vol: 10, 14th edition, New York, 2002
Mechanism of Elastic Deformation
Elastic deformation is reversible. Once the applied forces are removed, the object returns to its
original shape. Elastomers and shape memory metals such as Nitinol exhibit large elastic deformation
ranges, as does rubber. However elasticity is nonlinear in these materials. Normal metals, ceramics
and most crystals show linear elasticity and a smaller elastic range.
The initial straight line (OP)of the curve characterizes proportional relationship between the stress and
the deformation (strain). The stress value at the point P is called the limit of proportionality:
σp= FP / S0
This behavior conforms to the Hook’s Law .i,e, σ = E*δ
Where E is a constant, known as Young’s Modulus or Modulus of Elasticity.
The value of Young’s Modulus is determined mainly by the nature of the material and is nearly
insensitive to the heat treatment and composition. Modulus of elasticity determines stiffness resistance of a body to elastic deformation caused by an applied force. The line OE in the StressStrain curve indicates the range of elastic deformation – removal of the load at any point of this part
of the curve results in return of the specimen length to its original value. The elastic behavior is
characterized by the elasticity limit (stress value at the point E):
σel = FE / S0
For the most materials the points P and E coincide and therefore σel=σp.
Mechanism of Plastic Deformation
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Yield stress, Shear strength of Perfect and Real Crystals
Critical Resolved Shear Stress(CRSS).
Critical resolved shear stress is the component of shear stress, resolved in the direction of slip in a
grain. It is a constant for a given crystal. Since this is a threshold value, it is called critical; and being a
component of the applied stress, it is said to be resolved.
Crystalline materials tend to deform or fail by the relative motion of planes of atoms under the action
of stress. This motion is induced by the component of stresses acting across the slip planes. The
deformation process is a collective motion of adjacent slip planes. But all the planes do not start
deforming simultaneously. The first slip in a single plane occurs when the shear stress across the
plane exceeds CRSS.
Temperature and crystal geometry influence the minimum stress required to cause the shear.
A pure crystalline solid, when pulled along different orientations, requires different amounts of load in
each instance for the very first slip to occur though the stress required for the planes to slip remains
same. This implies that the Critical Resolved Shear Stress is greatly influenced by the orientation of
the slip plane with the tensile axis.
Schmid's Law states that the critically resolved shear stress (τ) is equal to the force applied to the
material (σ) multiplied by the cosine of the angle with the glide plane (φ) and the cosine of the angle
with the glide direction (λ).
Resolved shear stress is given by τ = σ cos Φ cos λ where σ is the magnitude of the applied tensile
stress, Φ is the angle between the normal of the slip plane and the direction of the applied force and λ
is the angle between the slip plane direction and the direction of the applied force.
Hence critical resolved shear stress is given by,
max
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Plastic Deformation and Fracture
When sufficient load is applied to a material, it will cause the material to change shape or deform. A
temporary shape change that is self-reversing after the force is removed, so that the object returns to
its original shape. The change in shape of a material at low stress that is recoverable after the stress
is removed is called elastic deformation. Elastic deformation involves stretching of the bonds (but the
atoms do not slip past each other or twin which requires breaking of bonds).
As shown in the graph above, when the stress is sufficient to permanently deform the metal, it is
called plastic deformation. Plastic deformation involves the breaking and remaking of atomic bonds.
Plastic deformation may take place by slip, twinning or a combination of both methods.
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Dislocations and Strengthening Mechanisms
Plastic deformation –Dislocations
Permanent plastic deformation is due to shear process–atoms change their neighbors. Inter-atomic
forces and crystal structure plays an important role during plastic deformation. Cumulative movement
of dislocations leads to gross plastic deformation. Edge dislocation move by slip and climb, while
screw dislocation move by slip and cross-slip.
During their movement, dislocations tend to interact. The interaction is very complex because of
number of dislocations moving over many slip systems in different directions Dislocations moving on
parallel planes may annihilate each other, resulting in either vacancies or interstitials. Dislocations
moving on non-parallel planes hinder each other’s movement by producing sharp breaks–jog (break
out of slip plane), kink(break in slip plane) . Other hindrances to dislocation motion–interstitiall and
substitutional atoms, foreign particles, grain boundaries, external grain surface, and change in
structure due to phase change.
Material strength can be increased by arresting dislocation motion
Plastic deformation mechanisms -Slip
Mainly two kinds :slip and twinning.
•
•
•
•
Slip is prominent among the two. It involves sliding of blocks of crystal over other along slip
planes.
Slip occurs when shear stress applied exceeds a critical value.
Slip occurs most readily in specific directions(slip directions)on certain crystallographic
planes.Feasible combination of a slip plane together with a slip direction is considered as a
slip system.
During slip each atom usually moves same integral number of atomic distances along the slip
plane
Extent of slip depends on many factors-external load and the corresponding value of shear stress
produced by it, crystal structure, orientation of active slip planes with the direction of shearing
stresses generated. Slip occurs when shear stress applied exceeds a critical value. For single crystal,
Schmid defined critical shear stress,which can be expressed as:
Both factors τ and σ are measured in stress, which is calculated the same as pressure by dividing
force by area. φ and λ are angles usually measured in degrees.
In a poly-crystalline material, individual grains provide a mutual geometrical constraint on one other,
and this precludes plastic deformation at low applied stresses. Slip in polycrystalline material involves
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generation, movement and (re-) arrangement of dislocations. During deformation, mechanical integrity
and coherency are maintained along the grain boundaries. A minimum of five independent slip
systems must be operative for a polycrystalline solid to exhibit ductility and maintain grain boundary
integrity–von Mises. On the other hand, crystal deform by twinning.
Strengthening mechanisms of material can be increased by hindering dislocation, which is
responsible for plastic deformation.Different ways to hinder dislocation motion/Strengthening
mechanisms:
In single-phase materials
-Grain size reduction
-Solid solution strengthening
-Strain hardening
In multi-phase materials
-Precipitation strengthening
-Dispersion strengthening
-Fiber strengthening
-Martnsite strengthening
Strengthening by Grain size reduction
It is based on the fact that dislocations will experience hindrances while trying to move from a grain in
to the next because of abrupt change in orientation of planes. Hindrances can be two types: forcible
change of slip direction, and discontinuous slip plane. Smaller the grain size, often a dislocation
encounters a hindrance. Yield strength of material will be increased. Yield strength is related to grain
size(diameter, d)as Hall-Petch relation:
Grain size can be tailored by controlled cooling or by plastic deformation followed by appropriate heat
treatment
Solid solution strengthening
Impure foreign atoms in a single phase material produces lattice strains which can anchor the
dislocations. Effectiveness of this strengthening depends on two factors –size difference and volume
fraction of solute. Solute atoms interact with dislocations in many ways:
-elastic interaction
-modulus interaction
-stacking-fault interaction
-electrical interaction
-short-range order interaction
-long-range order interaction
Elastic, modulus, and long-range order interactions are of long-range i.e. they are relatively
insensitive to temperature and continue to act about 0.6Tm
Yield point phenomenon
Localized, heterogeneous type of transition from elastic to plastic deformation marked by abrupt
elastic-plastic transition–Yield point phenomenon. It characterizes that material needs higher stress to
initiate plastic flow than to continue it.
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The bands are called Lüders bands/Hartmann lines/stretcher stains, and generally are approximately
45 to the tensile axis.
Occurrence of yield point is associated with presence of small amounts of interstitial or substitutional
impurities. It’s been found that either unlocking of dislocations by a high stress for the case of strong
pinning or generation of new dislocations are the reasons for yield-point phenomenon. Magnitude of
yield-point effect will depend on energy of interaction between solute atoms and dislocations and on
the concentration of solute atoms at the dislocations.
Fracture is the separation of a single body into pieces by an imposed stress. Information about plastic
deformation and fracture is given in this article. As polycrystalline material is made up of many grains
which may have second phase particles and grain boundaries. It is therefore easier to study plastic
deformation in a single crystal to eliminate the effects of grain boundaries and second phase particles
Plastic deformation mechanisms –Twinning
It results when a portion of crystal takes up an orientation that is related to the orientation of the rest
of
.
The important role of twinning in plastic deformation is that it causes changes
in plane orientation the untwined lattice in a definite, symmetrical way so that further slip can occur.
Twinning also occurs in a definite direction on a specific plane for each crystal structure
Deformation by Slip.
If a single crystal of a metal is stressed in tension beyond its elastic limit, it elongates slightly, a step
appears on the surface indicating relative displacement of one part of the crystal with respect to the
rest, and the elongation stops. Increasing the load will cause movement on another parallel plane,
resulting in another step. It is as if neighboring thin sections of the crystal had slipped past one
another like sliding cards on a deck. Each successive elongation requires a higher stress and results
in the appearance of another step. With progressive increase of the load, a stage is reached which
causes the material to fracture.
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Sliding occurs in certain planes of atoms in the
crystal and along certain directions in these planes. Thus the mechanism is a new type of flow that
depends upon the perfectly repetitive structure of the crystal . It allows the atoms in one face of a slip
plane to shear away from their original neighbors in the other face, to slide in an organized way along
this face carrying their own half of the crystal with them, and finally to join up again with a new set of
neighbors as nearly perfect as before. The movement in the crystal takes place along planes having
highest atomic density with greatest distance between similar parallel planes and in the direction of
close-packed atoms. This can be simply stated as: Slip occurs on planes that have h ighest planer
density of atoms and in the direction with highest linear density of atoms.
Slip occurs in directions in which the atoms are most closely packed since this requires the least
amount of energy. As shown in the figure, close-packed rows are vertically further apart from each
other (d1) than rows that are not close-packed (d2), therefore they can slip past each other with less
interference.
Black bars between atoms show the slope of the path of atoms. Since the slope is less steep in case
of closed-packed rows, less force is required for a given horizontal displacement. In addition, less
displacement (D1 < D2) is required by the atoms to move from one stable position (before slip) to
other stable position (after slip).
To get a better grasp of this, think of gluing ping pong balls to two boards in a widely spaced pattern
as shown in above figure. Put the boards together, ping pong balls to ping pong balls, and begin to tilt
the bottom board. You will have to go to a very steep angle before the top board will slip from its
position of the nestled ping pong balls. Now do the experiment again, but this time gluing the balls
very close together. The top board will now slip at a much lesser angle.
Strain Hardening (Work Hardening)
Methods have been devised to modify the yield strength, ductility, and toughness of
both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability
to tailor the mechanical properties of materials to suit a variety of different applications. For example,
the favorable properties of steel result from interstitial incorporation of carbon into
the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared
to its constituent metals due to solution strengthening. Work hardening (such as beating a red-hot
piece of metal on anvil) has also been used for centuries by blacksmiths to introduce dislocations into
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materials, increasing their yield strengths. The ability of a metal to plastically deform depends on the
ability of dislocation to move.
Restricting or hindering dislocation motion renders a material harder and stronger.
Phenomenon where ductile metals become stronger and harder when they are deformed plastically is
called strain hardening or work hardening. Increasing temperature lowers the rate of strain hardening.
Hence materials are strain hardened at low temperatures, thus also called cold working. During
plastic deformation, dislocation density increases. And thus their interaction with each other, resulting
in increase in yield stress.
Dislocation density (ρ) and shear stress (τ) are related as follows:
During strain hardening, in addition to mechanical properties physical properties also change:
-a small decrease in density
-an appreciable decrease in electrical conductivity
-small increase in thermal coefficient of expansion
-increased chemical reactivity (decrease in corrosion resistance).
Deleterious effects of cold work can be removed by heating the material to suitable temperatures–
Annealing. It restores the original properties into material. It consists of three stages–recovery, recrystallization and grain growth. In industry, alternate cycles of strain hardening and annealing are
used to deform most metals to a very great extent
Precipitation & Dispersion hardening
Foreign particles can also obstruct movement of dislocations i.e. increases the strength of the
material. Foreign particles can be introduced in two ways – precipitation and mixing -andconsolidation technique. Precipitation hardening is also called age hardening because strength
increases with time. Requisite for precipitation hardening is that second phase must be soluble at an
elevated temperature but precipitates up on quenching and aging at a lower temperature.
E.g.:Al-alloys ,Cu-Be alloys, Mg-Al alloys, Cu-Sn alloys
If aging occurs at room temperature–Natural aging; If material need to be heated during aging–
Artificial aging. In dispersion hardening, fine second particles are mixed with matrix powder,
consolidated, and pressed in powder metallurgy techniques. For dispersion hardening, second phase
need to have very low solubility at all temperatures.
E.g.: oxides, carbides, nitrides, borides etc.
Dislocation moving through matrix embedded with foreign particles can either cut through the particles
or bend around and bypass them. Cutting of particles is easier for small particles which can be
considered as segregated solute atoms. Effective strengthening is achieved in the bending process,
when the particles are submicroscopic in size
Fiber strengthening
Second phase can be introduced into matrix in fiber form too.
Requisite for fiber strengthening:
Fiber material – high strength and high modulus
Matrix material – ductile and non-reactive with fiber material
E.g.: fiber material – Al2O3, boron, graphite, metal, glass, etc; matrix material – metals, polymers
Mechanism of strengthening is different from other methods.
Higher modulus fibers carry load, ductile matrix distributes load to fibers. Interface between matrix
and fibers thus play an important role. Strengthening analysis involves application of continuum, not
dislocation concepts as in other methods of strengthening
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Cold work will lead to:
•
Increase of Yielding Strength
•
Increase of Tensile Strength
•
Reduction of Elongation
•
Material becomes stronger but more brittle
Plastic deformation occurs when large numbers of dislocations move and multiply so as to result in
macroscopic deformation. In other words, it is the movement of dislocations in the material which
allows for deformation. If we want to enhance a material's mechanical properties (i.e. increase the
yield and tensile strength), we simply need to introduce a mechanism which prohibits the mobility of
these dislocations. Whatever the mechanism may be, (work hardening, grain size reduction, etc.) they
all hinder dislocation motion and render the material stronger than previously.
The stress required to cause dislocation motion is orders of magnitude lower than the theoretical
stress required to shift an entire plane of atoms, so this mode of stress relief is energetically
favorable. Hence, the hardness and strength (both yield and tensile) critically depend on the ease with
which dislocations move. Pinning points, or locations in the crystal that oppose the motion of
dislocations, can be introduced into the lattice to reduce dislocation mobility, thereby increasing
mechanical strength.
Dislocations may be pinned due to stress field interactions with other dislocations and solute particles,
creating physical barriers from second phase precipitates forming along grain boundaries. There are
four main strengthening mechanisms for metals, each is a method to prevent dislocation motion and
propagation, or make it energetically unfavorable for the dislocation to move. For a material that has
been strengthened, by some processing method, the amount of force required to start
irreversible (plastic) deformation is greater than it was for the original material.
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In amorphous materials such as polymers, amorphous ceramics (glass), and amorphous metals, the
lack of long range order leads to yielding via mechanisms such as brittle fracture, crazing, and shear
band formation. In these systems, strengthening mechanisms do not involve dislocations, but rather
consist of modifications to the chemical structure and processing of the constituent material.
The strength of materials cannot infinitely increase. Each of the mechanisms explained below
involves some trade-off by which other material properties are compromised in the process of
strengthening.
Solid solutioning, grain boundary strengthening
Commonly used strengthening methods include (A) strengthening via solid solution, whereby solute
atoms strain the matrix to impede the motion of a dislocation (red line) through the lattice; via
precipitates or dispersed particles that interact with mobile dislocations, leading to overall
strengthening of the material; or via elastic interactions between intersecting dislocations (blue and
red lines), as well as geometry changes and subsequent obstructions to slip (as, for example, through
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the formation of sessile dislocation segments) associated with such encounters. GB strengthening (B)
is another commonly used method in which dislocation (red ⊥ symbol) motion is blocked by GB
(whose incoherent structure is schematically shown on the right) so that a dislocation pile-up is
formed. A higher stress is needed to deform a polycrystalline metal with a smaller grain size d (more
GBs). (C) Nano scale TB strengthening is based on dislocation-TB interactions from which mobile
and/or sessile dislocations could be generated, either in neighboring domains (twin or matrix) or at
TBs. Gliding of dislocations along TBs is feasible because of its coherent structure [the right panel in
(C) denotes a Σ3 TB]. Higher strength and higher ductility are achieved with a smaller twin thickness λ
in the nanometer scale.
Solid solution strengthening is a type of alloying that can be used to improve the strength of a pure
metal. The technique works by adding atoms of one element (the alloying element) to the crystalline
lattice of another element (the base metal). The alloying element diffuses into the matrix, forming
a solid solution. In most binary systems, when alloyed above a certain concentration, a second phase
will form. When this increases the strength of the material, the process is known as precipitation
strengthening, but this is not always the case
.
Depending on the size of the alloying element, a substitutional solid solution or an interstitial solid
solution can form. In both cases, the overall crystal structure is essentially unchanged and remains
the same.
Substitutional solid solution strengthening occurs when the solute atom is large enough that it can
replace solvent atoms in their lattice positions. According to the Hume-Rothery rules, solvent and
solute atoms must differ in atomic size by less than 15% in order to form this type of solution.
Because both elements exist in the same crystalline lattice, both elements in their pure form must be
of the same crystal structure. Examples of substitutional solid solutions include the Cu-Ni and the AgAu FCC binary systems, and the Mo-W BCC binary system.
When the solute atom is equal to or slightly smaller and can fill the interstices of the solvent atoms,
an interstitial solid solution forms. The atoms crowd into the interstitial sites, causing the bonds of
the solvent atoms to compress and thus deform.Elements commonly used to form interstitial solid
solutions include H, Li, Na, N, C, and O. Carbon in iron (steel) is one example of interstitial solid
solution
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The strength of a material is dependent on how easily dislocations in its crystal lattice can be
propagated. These dislocations create stress fields within the material depending on their character.
When solute atoms are introduced, local stress fields are formed that interact with those of the
dislocations, impeding their motion and causing an increase in the yield stress of the material, which
means an increase in strength of the material. This gain is a result of both lattice distortion and
the modulus effect.
When solute and solvent atoms differ in size, local stress fields are created. Depending on their
relative locations, solute atoms will either attract or repel dislocations in their vicinity. This is known as
the size effect. This allows the solute atoms to relieve either tensile or compressive strain in the
lattice, which in turn puts the dislocation in a lower energy state. In substitutional solid solutions, these
stress fields are spherically symmetric, meaning they have no shear stress component. As such,
substitutional solute atoms do not interact with the shear stress fields characteristic of screw
dislocations. Conversely, in interstitial solid solutions, solute atoms cause a tetragonal distortion,
generating a shear field that can interact with both edge, screw, and mixed dislocations. The
attraction or repulsion of the dislocation centers to the solute particles increase the stress it takes to
propagate the dislocation in any other direction. Increasing the applied stress to move the dislocation
increases the yield strength of the material.
The energy density of a dislocation is dependent on its Burgers vector as well as the modulus of the
local atoms. When the modulus of solute atoms differs from that of the host element, the local energy
around the dislocation is changed, increasing the amount of force necessary to move past this energy
well. This is known as the modulus effect. Meanwhile, in the specific case of a lattice distortion, the
difference in lattice parameter leads to a high stress field around that solute atom that impedes
dislocation movement.
Surface carburizing, or case hardening, is one example of solid solution strengthening in which the
density of solute carbon atoms is increased close to the surface of the steel, resulting in a gradient of
carbon atoms throughout the material. This provides superior mechanical properties to the surface of
the steel.
Grain boundary strengthening
In a polycrystalline metal, grain size has a tremendous influence on the mechanical properties.
Because grains usually have varying crystallographic orientations, grain boundaries arise. While
undergoing deformation, slip motion will take place. Grain boundaries act as an impediment to
dislocation motion for the following two reasons:
1. Dislocation must change its direction of motion due to the differing orientation of grains.
2. Discontinuity of slip planes from grain one to grain two
Grain-boundary strengthening (or Hall–Petch strengthening) is a method
of strengthening materials by changing their average crystallite (grain) size. It is based on the
observation that grain boundaries impede dislocation movement and that the number
of dislocations within a grain have an effect on how easily dislocations can traverse grain boundaries
and travel from grain to grain. So, by changing grain size one can influence dislocation movement
and yield strength. For example, heat treatment after plastic deformation and changing the rate of
solidification are ways to alter grain size
The stress required to move a dislocation from one grain to another in order to plastically deform a
material depends on the grain size. The average number of dislocations per grain decreases with
average grain size (see Figure 3). A lower number of dislocations per grain results in a lower
dislocation 'pressure' building up at grain boundaries. This makes it more difficult for dislocations to
move into adjacent grains. This relationship is the Hall-Petch relationship and can be mathematically
described as follows:
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where
is a constant,
is the average grain diameter and
is the original yield stress.
The fact that the yield strength increases with decreasing grain size is accompanied by the caveat
that the grain size cannot be decreased infinitely. As the grain size decreases, more free volume is
generated resulting in lattice mismatch. Below approximately 10 nm, the grain boundaries will tend to
slide instead; a phenomenon known as grain-boundary sliding. If the grain size gets too small, it
becomes more difficult to fit the dislocations in the grain and the stress required to move them is less.
It was not possible to produce materials with grain sizes below 10 nm until recently, so the discovery
that strength decreases below a critical grain size is still finding new applications.
The concept of dislocation pile up and how it effects the strength of the material. A material with larger
grain size is able to have more dislocation to pile up leading to a bigger driving force for dislocations
to move from one grain to another. Thus, less force need be applied to move a dislocation from a
larger, than from a smaller grain, leading materials with smaller grains to exhibit higher yield stress.
Grain growth
Grain growth follows complete crystallization if the material is left at elevated temperatures. Grain
growth does not need to be preceded by recovery and re-crystallization; it may occur in all polycrystalline materials. In contrary to recovery and re-crystallization, driving force for this process is
reduction in grain boundary energy. Tendency for larger grains to grow at the expense of smaller
grains is based on physics. In practical applications, grain growth is not desirable. Incorporation of
impurity atoms and insoluble second phase particles are effective in retarding grain growth. Grain
growth is very strongly dependent on temperature.
Super plasticity
Superplasticity is the capability to deform crystalline solids in tension to unusually large plastic
strains, often well in excess of 1000%. This phenomenon results from the ability of the material
to resist localized deformation much the same as hot glass does.
Such a state is usually achieved at high homologous temperature. Examples of superplastic
materials are some fine-grained metals and ceramics. Other non-crystalline materials (amorphous)
such as silica glass ("molten glass") and polymers also deform similarly, but are not called
superplastic, because they are not crystalline; rather, their deformation is often described
as Newtonian flow. Superplastically deformed material gets thinner in a very uniform manner, rather
than forming a "neck" (a local narrowing) that leads to fracture. Also, the formation of microvoids,
which is another cause of early fracture, is inhibited.
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In metals and ceramics, requirements for it being superplastic include a fine grain size (less than
approximately 20 micrometres) and a fine dispersion of thermally stable particles, which act to pin the
grain boundaries and maintain the fine grain structure at the high temperatures and Existence of Two
Phases required for superplastic deformation. Those materials that meet these parameters must still
have a strain rate sensitivity (a measurement of the way the stress on a material reacts to changes in
strain rate) of >0.3 to be considered superplastic.
The mechanisms of superplasticity in metals are still under debate—many believe it relies on
atomic diffusion and the sliding of grains past each other. Also, when metals are cycled around
their phase transformation, internal stresses are produced and superplastic-like behaviour develops.
Recently high-temperature superplastic behaviour has also been observed in iron aluminides with
coarse grain structures. It is claimed that this is due to recovery and dynamic recrystallization.
As high elongations are possible, complex contoured parts can be formed in a single press cycle
often eliminating the need for multipart fabrications. Thus materials with superplastic properties can
be used to form complex components in shapes that are very near the final dimension. Superplastic
forming also
enhances design freedom, minimizes the amount of scrap produced, and reduces the need for
machining. In addition, it reduces the amount of material used, thereby lowering overall material
costs.
Differences between Crystalline and Non-crystalline Materials
• In crystalline materials, permanent deformation is generally related to identified defects such as
dislocation, atom diffusion involving voids, vacancies, etc.
• The macroscopic deformation is similar in both crystalline and non-crystalline (i.e. behave in a
brittle manner under high strain rate and low temperature)
• In non-crystalline materials, permanent deformation is often related to localized slip and/or viscous
flow (low stress or high temperature)
• The non-crystalline arrangement is thermodynamically stable above material’s Tm
• The crystalline just opposite (below the Tm is more stable)
• Crystalline arrangement is more ordered ⇒ less molar volume
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