Download ENGINEERING PHYSICS II ADVANCED

ENGINEERING PHYSICS II
ADVANCED ENGINEERING MATERIALS
UNIT – V
ADVANCED ENGINEERING MATERIALS
5. 1 INTRODUCTION
New engineering materials such as metallic glasses, shape memory alloys, nano phase
materials, non linear materials etc. are the advanced materials, which are the integral part of our
life. All these materials are composite materials which are prepared by grouping one type of
material with the other. In this chapter we are going to discuss about the new engineering materials
along with their properties and the wide range of applications in this modern engineering world.
5. 2 METALLIC GLASSES (MET GLASSES)
Metallic glasses are new type of materials which share the properties of both metals and
glasses. In general, they are strong, ductile, malleable, opaque and brittle. They also have good
magnetic properties and corrosion resistance. They are also called as amorphous metals.
Properties of metals + Properties of glasses = Properties of metallic glasses
5.3 FORMATION OF METALLIC GLASSES
Metals are made into glassy state by increasing their rate of cooling to a very high level. At
that state the atoms are unable to arrange in a proper manner and thus form an new amorphous
state. These new type of materials which are formed by the rapid cooling technique are called
metallic glasses.
Glass Transition Temperature
The temperature at which the metals in the molten form transforms into glasses (i.e. from
liquid to solid) is known as glass transition temperature. It was found that the glass transition
temperature for metallic alloys varies from 200 C to 3000 C.
5. 4 TECHNIQUES FOR PREPARATION
There are several techniques available for the production of metallic glasses.
1. Melt spinning system
Molten alloy is made to impinge on a fast rotating roller to form metallic glasses.
2. Twin roller system
Molten alloy is made to pass through the two rollers rotating in opposite directions to form
metallic glasses.
3. Metal extracting system
Fast moving roller sweeps off molten droplet into a strip to form metallic glasses.
4. Sputtering
The sputtering gas is ionized and the atoms are made to fly towards the substrate to form
metallic glasses.
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5.5 PREPARATION OF METALLIC GLASSES
Principle
Quenching is a technique used to form metallic glasses. Quenching means rapid cooling
Atoms move freely in the liquid state. When the liquid is quenched (rapidly cooled) it results in an
irregular pattern, which results in the formation of metallic glasses.
Technique
The process involved in the formation of metallic glasses is melt spinning technique.
MELT SPINNING TECHNIQUE
Experimental setup
The setup consists of a refractory tube with fine nozzle at the bottom. The refractory tube is
placed over the rotating roller made up of copper. An induction heater is wounded over the
refractory tube in order to heat the alloy inside the refractory tube.
Preparation
The alloy is put into the refractory tube and the induction heater is switched ON. This heats
the alloy and hence the molten alloy is ejected through the nozzle of the refractory tube onto the
rotating roller and is made to cool suddenly. The ejection rate may be increased by increasing the
gas pressure inside the refractory tube. Thus due to rapid quenching a glassy alloy ribbon called
metallic glass is formed over the rotating roller. Hence this technique is used to develop materials
that require extremely high cooling rates in order to form metallic glasses. The cooling rates
achievable by melt-spinning method are in the order of 104–107 Kelvin per second.
5.6 TYPES OF METALLIC GLASSES
1. Metal - metal metallic glasses
They are formed by combination of metals.
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Example:
(i) Ni - Nb (Nickel & Niobium)
(ii) Mg - Zn (Magnesium & zinc)
(iii) Cu - Zr (Copper & Zirconium)
(iv) Hf - V (Hafnium & Vanadium)
2.Metal - metalloid metallic glasses
They are formed by combination of metals and metalloids.
Example:
Metals like Fe, Co, Ni and metalloid such as B, Si, C, P.
5.7 PROPERTIES OF METALLIC GLASSES
(i) Structural properties
 They have tetrahedral closely packed structure rather than hexagonal closely packed
structure
 They do not have any crystal defects
(ii) Mechanical properties
 They are strong in nature
 They have high corrosion resistance
 They posses malleability and ductility
(iii) Magnetic properties
 They can be easily magnetized and demagnetized
 They have narrow hysteresis loop
(iv) Electrical properties
 High electrical resistance
 Electrical resistance will not vary with temperature
 Low eddy current losses
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