Modern Engineering
Materials
Introduction
• Science and technology have made amazing developments
in the design of electronic and machinery using standard
materials.
• Traditionally, standard materials are divided into three
basic groups, namely metals, ceramics and polymers.
• Later three other groups of engineering materials are
added. They are composites, semiconductors and
biomaterials.
• Still there is a continuous search for materials with
enhanced properties such as strength, high stability, large
electrical conductivity etc.
• A number of materials such as amorphous metals, liquid
crystals, smart materials, biomaterials etc have been
discovered for high-tech applications.
AMORPHOUS METAL
An amorphous metal is a metallic
material with a disordered atomic-scale
structure. In contrast to most metals,
which are crystalline and therefore have
a highly ordered arrangement of atoms,
amorphous alloys are non-crystalline.
Materials in which such a disordered
structure is produced directly from the
liquid state during cooling are called
"glasses", and so amorphous metals
are commonly referred to as "metallic
glasses" or "glassy metals".
amorphous
crystalline
In addition to direct cooling, there are several other
ways in which amorphous metals can be produced,
including:
• physical vapor deposition
• solid-state reaction,
• melt spinning
• mechanical alloying .
Amorphous metals produced by these techniques are,
strictly speaking, not glasses; however, materials
scientists commonly consider amorphous alloys to be a
single class of materials, regardless of how they are
prepared.
In the past, small batches of amorphous metals have been
produced through a variety of quick-cooling methods.
For instance, amorphous metal wires have been produced by
sputtering molten metal onto a spinning metal disk. The rapid
cooling, on the order of millions of degrees a second, is too fast
for crystals to form and the material is "locked in" a glassy state.
More recently a number of alloys with critical cooling rates low
enough to allow formation of amorphous structure in thick layers
(over 1 millimeter) had been produced, these are known as bulk
metallic glasses (BMG). Liquid metal sells a number of titaniumbased BMGs, developed in studies originally carried out at
Caltech.
More recently, batches of amorphous steel have been produced
that demonstrate strengths much greater than conventional steel
alloys.
PROPERTIES of Metallic Glasses
• Amorphous metal is usually an alloy rather than a pure
metal.
• The alloys contain atoms of significantly different sizes,
leading to low free volume (and therefore up to orders of
magnitude higher viscosity than other metals and alloys) in
molten state. The viscosity prevents the atoms moving
enough to form an ordered lattice. The material structure
also results in low shrinkage during cooling, and resistance
to plastic deformation.
• The absence of grain boundaries, the weak spots of
crystalline materials, leads to better resistance to wear and
corrosion. Amorphous metals, while technically glasses, are
also much tougher and less brittle than oxide glasses and
ceramics.
• Thermal conductivity of amorphous materials is lower
than of crystals. As formation of amorphous structure
relies on fast cooling, this limits the maximum achievable
thickness of amorphous structures.
• To achieve formation of amorphous structure even during
slower cooling, the alloy has to be made of three or more
components, leading to complex crystal units with higher
potential energy and lower chance of formation.
• The atomic radius of the components has to be
significantly different (over 12%), to achieve high packing
density and low free volume.
• The combination of components should have negative
heat of mixing, inhibiting crystal nucleation and prolongs
the time the molten metal stays in super cooled state.
• The alloys of boron, silicon, phosphorus, and other glass
formers with magnetic metals (iron, cobalt, nickel) are
magnetic, with low coercivity and high electrical resistance.
The high resistance leads to low losses by eddy currents
when subjected to alternating magnetic fields, a property
useful for e.g. transformer magnetic cores.
• Amorphous alloys have a variety of potentially useful
properties.
• In particular, they tend to be stronger than crystalline alloys
of similar chemical composition, and they can sustain larger
reversible ("elastic") deformations than crystalline alloys.
• Amorphous metals derive their strength directly from their
non-crystalline structure, which does not have any of the
defects (such as dislocations) that limit the strength of
crystalline alloys.
Amorphous metallic alloys combine higher strength than
crystalline metal alloys with the elasticity of polymers.
Mechanical properties and performances of BMGs
Fracture toughness vs Young modulus for
different materials (reproduced with
permission from [25] ©2008, Nature Publishing
Comparative chart of fracture toughness
Group).
vs yield strength for BMGs and other
metallic materials (adapted from [26] ©
2010 JOM).
Fatigue endurance limit (stress-range based)
vs yield strength data for BMGs and other
metallic alloys (adapted from [26] © 2010
JOM).
PROCESSING OF METALLIC GLASSES
Virtually any liquid can be turned into a glass if it is cooled quickly
enough to avoid crystallization. The question is, how fast does the
cooling need to be?
Common oxide glasses (such as ordinary window glass) are quite
resistant to crystallization, so they can be formed even if the liquid is
cooled very slowly. For instance, the mirror for the 200" telescope at
the Palomar Observatory weighed 20 tons and was cooled over a
period of eight months, but did not crystallize.
Many polymer liquids can also be turned into glasses; in fact, many
polymers cannot be crystallized at all.
For both oxides and polymers, the key to glass formation is that the
liquid structure cannot be rearranged to the more ordered
crystalline structure in the time available.
Design and fabrication of BMGs
重量7 mg
A promising material for microdevices and micro-manufacture courtesy by prof. Wei Hua Wang
BMG Medalof Institute of Physics (CAS)
Beijing -courtesy by prof. Wei Hua Wang
Zr-based BMG pieces obtained by TPF based
blow molding (adapted with permission from
[43] © 2011 Elsevier).
Engineering parts at micro-scale fabricated from
BMGs (adapted with permission from [38] © 2011
Elsevier
Kinetic Energy Penetrators ‘‘KEPs’’
(reproduced from [64]).
Images with spacecraft (artist rendering during collection phase) and BMG components for
Genesis Mission (image credit NASA/JPL – Caltech/USC [66,67]).
Shape Memory Alloys
Introduction
• Shape memory alloy (SMA) is a metallic alloy that
remembers its shape and can be returned to its initial
shape after being deformed, by applying heat to the alloy.
• A material that can remember its shape
• SMA also exhibits superelastic (pseudoelastic) behavior
• When shape memory effect is correctly used, the material
becomes a light weight, solid-state alternative to
conventional actuators such as hydraulic, pneumatic and
motor-based systems.
• SMAs have several applications in the medical and
aerospace industries.
• Some examples for SMAs are Ni-Ti alloy; Cu-Al-Ni alloy;
Cu-Zn-Al alloy; Au-Cd alloy; Ni-Mn-Ga alloy and Fe-based
alloys.
Two Phases
Austenite
Hard, firm
Inelastic
Resembles titanium
Simple FCC structure
Martensite
Soft
Elastic
Complex structure
Austenite
• High temperature phase
•Cubic Crystal Structure
Martensite
•Low temperature phase
•Monoclinic Crystal Structure
Twinned Martensite
Detwinned Martensite
• Principles of Shape Memory Alloys
– Shape Memory Alloys (SMA) are alloys that
exhibit the shape memory effect.
– The shape memory effect is the process of
restoring a deformed material back to an
initial shape through a thermally induced
crystalline transformation
– The crystalline transformation occurs
between a low temperature ductile
martensitic phase and a high temperature
high strength austenitic phase.
Advantages of SMA’s
• The main advantages of SMA’s for microactuation are:
– SMA’s are capable of producing a large actuation
force
– SMA’s are capable of producing large
displacements
– SMA’s are activated through thermal heating
Disadvantages of SMA’s
• The main disadvantages of SMA’s are:
– Sensitivity of material properties in fabrication
– Residual Stress’s developed in thin films
– Nonlinearity of actuation force
– Lower maximum frequency compared to other
microactuator devices
Characteristic temperatures of SMAs
• There are four characteristic temperatures describing
the phase transformation. They are
• (i) Martensite Start temperature, Ms
• It is the temperature at which material starts
transforming from austenite to martensite.
• The transformation proceeds with further cooling and is
complete at the martensite finish temperature, Mf.
• Below Mf, the entire body is in the martensite phase
and a specimen typically consists of many regions each
containing a different variant of martensite. The
boundaries between the variants are mobile under
small applied loads.
• (ii) Martensite Finish temperature, Mf
• It is the temperature at which the transformation is
complete and the material is fully in the martensite
phase.
• (iii) Austenite Start temperature, As
• It is the temperature at which austenite first appear in
the martensite, with heating.
• With further heating, more and more of the body
transforms back into austenite
• (iv) Austenite Finish temperature, Af
• It is the temperature at which the reverse phase
transformation is completed and the material is in the
austenite phase. Above Af , the material is in the original
undistorted state.
Shape Memory Alloy Effect
“When an SMA is cooled below its transformation
temperature, it can deform into any new shape which it
will retain, and when the material in new phase is heated
above its transformation temperature it recovers its
original shape. This effect is known as shape memory
effect”.
Let us consider a shape memory alloy in cubic austenite
phase. When it is cooled below a temperature, called
the martensite finish temperature Mf and the material
is applied a constant load, it gets deformed. On cooling,
the SMA goes into twinned martensite phase and when
it is loaded in this state, the SMA deformed in to the
detwinned martensite.
Suppose the material in detwinned phase is heated
above the temperature called the austenite finish
temperature Af, the SMA recovers its original shape i.e.,
cubic austenite phase. Following figure illustrates the
graphical representation of shape memory alloy effect.
• Heat supplied to the material is used to drive the
molecular rearrangement of the alloy.
• It is clear that an SMA undergoes phase changes while
remaining a solid.
• The phase changes occur below its melting point.
• The phase changes involve the rearrangement of the
position of particles within the crystal structure of solid.
• Thus, the alloy can retain its shape without melting.
• Under the transition temperature, the alloy is in the
martensite phase and can be bent in any shapes.
• To get the initial shape, the allo must be in position and
heated to about 500oC.
• This high temperature causes the atoms to arrange
themselves into a high symmetry cubic arrangement
called the austenite phase.
Types of Shape Memory Alloy Effects
There are two types of SMA effects, namely :
(i) One-way shape memory alloy effect and
(ii) Two-way shape memory alloy effect.
One-way shape memory alloy effect:
• This SMA exhibits shape
memory alloy effect only
on heating. On cooling, the
SMA retains the shape
that it had before heating.
• A schematic view of the
one-way shape memory Fig. One-way memory effect: (a) material in
martensite phase, (b) material deformed, (c)
alloy effect is given fig.
sample heated and (d) sample cooled again
• When a SMA is in its cold state (i.e., below As), the
metal can be bent or stretched into different new shapes
and will hold in that shape until it is heated above the
transition temperature. Upon heating, the shape changes
back to its original shape. When the metal cools again it
will retain in the hot shape, until deformed again. Note
that the shape recovery is achieved only during heating.
• With one way effect, cooling from high temperature
does not cause a macroscopic shape change. A
deformation is necessary to create the low-temperature
shape. On heating, transformation starts at As and is
completed at Af (typically2 to 20oC or hotter, depending
on the alloy or the load conditions).
Two-way shape memory alloy effect:
• The mechanism of SME describe above, only the shape
of the austenitic phase is remembered. However, it is
possible to remember the shape of the martensitic
phase under certain condition. This behavior is a
common property of SMA, it is called Two Way Shape
Memory Effect (TWSME).
• TWSME is not intrinsic property (as the OWSME) to
SMA, but it can be exhibited after specific thermomechanical treatments known as training procedures.
• TWSME refers to the reversible and spontaneous shape
change of materials with thermal cycling, in other
words, this property permitted to SMA a spontaneous
shape change on both heating and cooling.
• TWSME refers to the reversible
and spontaneous shape change
of materials with thermal cycling,
in other words, this property
permitted to SMA a spontaneous
shape change on both heating
and cooling.
• Once that the material has
learned the behavior, it is
possible to modify the shape of
the material, in a reversible way
between two different ones and
without applied stress or load,
only by changing of temperature
across Af and Mf.
• A schematic representation of
the macroscopic observed
behavior is reported in Fig.
Fig. Two-way memory effect: (a) material in
martensite phase, (b) material deformed, (c)
sample heated and (d) sample cooled again
• At microscopic level, the reason for which a specimen
remembers the shape is explanation as follows.
• Upon heavy deformation in martensitic phase,
dislocations are introduced so as to stabilized the
configuration of martensitic phase.
• These dislocation exist even in the parent phase after
reverse martensite upon heating. In particular, there
are three key microstructural forms for SMA.
• This key microstructure is preferred in which becomes
learned by the alloy during training process. Therefore,
this structure promotes the TWSME.
Shape Memory Alloy Qualities
Ability to “remember” its austenite phase
As the metal is cooled to the martensite phase, it
can be easily deformed. When the temperature is
raised to the austenite phase, it reforms to the
original shape of the material.
Pseudoelasticity
When the metal is changed to the martensite phase
simply by strain. The metal becomes pliable and can
withstand strains of up to 8%.
A mix of roughly 50% nickel and 50% titanium is the
most common SMA. Also CuZnAl and CuAlNi are
widely used.
Pseudoelasticity
Pseudoelasticity (superelasticity) occurs when
the alloy is above the martensite temperature, but
there is a load strong enough to force the austenite
into the martensite phase. The alloy will not return to
the austenite phase until the loading is decreased or
there is a large enough change in temperature.
The figure shows load versus
temperature on an SMA.
The figure below shows NiTi’s
ability to change its shape along
phase planes. Other metals, as
we know, slide along slip planes
when there is an induced stress.
The above figure shows the Martensitic
transformation and hysteresis (= H)
upon a change of temperature. As =
austenite start, Af = austenite finish, Ms
= martensite start, Mf = martensite
finish and Md = Highest temperature to
strain-induced martensite. Gray area =
area of optimal superelasticity.
Biological Applications
Bone Plates
Memory effect pulls bones together to promote
healing.
Surgical Anchor
As healing progresses, muscles grow around the
wire. This prevents tissue damage that could be
caused by staples or screws.
Clot Filter
Does not interfere with MRI from nonferromagnetic properties.
Catheters
Retainers
Eyeglasses
Aircraft Maneuverability
Nitinol wires can be used in
applications such as the
actuators for planes. Many
use bulky hydraulic systems
which are expensive and
need a lot of maintenance.
USAF Aircraft Pictures
Typical actuator in the wing of a plane.
Picture of wing with SMA wires.
The wires in the picture are used to replace the actuator.
Electric pulses sent through the wires allow for precise
movement of the wings, as would be needed in an
aircraft. This reduces the need for maintenance, weighs
less, and is less costly.
Problems With SMAs
Fatigue from cycling
Causes deformations and grain boundaries
Begin to slip along planes/boundaries
Overstress
A load above 8% strain could cause the SMA to
completely lose its original austenite shape
Difficulty with computer programming
More expensive to manufacture than steel and
aluminum
Relatively new