METALLIC GLASSES
The Metallic glasses are materials which have the properties of both metals and glasses.
Metallic glass = Amorphous metal
In general, metallic glasses are strong, ductile, malleable, opaque and brittle.
They also have good magnetic properties and high corrosion resistance.
METHODS OF PREPARATION and Principle
The principle used in making metallic glasses is extreme rapid cooling of the molten alloy. The technique is called as rapid quenching.
The cooled molten alloys are fed into highly conducting massive rollers at high speeds to give ribbons of metallic glasses.
PREPARATION OF METALLIC GLASSES
Principle
The principle used in making metallic glasses is extreme rapid cooling of the molten metal alloy.
This technique is called as rapid quenching.
Melt spinning system
A melt spinner consists of a copper roller over which a refractory tube with fine nozzle is placed.
The refractory tube is provided with induction heater as shown in fig.
The metal alloy is melted by induction heating under inert gas atmosphere (helium or argon).
The properly super heated molten alloy is ejected through the fine nozzle at the bottom of the refractory tube.
The molten alloy falls on the copper roller which is rotated at high speed.
Thus, the alloy is suddenly cooled to form metallic glass.
In this method a continuous ribbon of metallic glass can be obtained.
TYPES OF METALLIC GLASSES:
Metallic glasses are classified into two types:
(i)Metal –Metal metallic glasses
They are combination of metals
Metals Metals
Examples: Nickel (Ni) – Niobium (Nb)
Magnesium (Mg) – Zinc (Zn)
Copper (Cu) – Zirconium (Zr)
(ii)Metal –Metalloid metallic glasses
These are combinations of metals and metalloids.
Examples: Metals Metalloids
Fe, Co, Ni – B, Si, C, P
PROPERTIES OF METALLIC GLASSES
Structural properties
- They do not have any crystal defects such as grain boundaries, dislocation etc.
- Metallic glasses have tetrahedral close packing (TCP).
Mechanical properties
- Metallic glasses have extremely high strength, due to the absence of point defects and dislocation.
- They have high elasticity.
- They are highly ductile.
- Metallic glasses are not work-harden but they are work –soften. (work harnening is a process of hardening a material by compressing it).
Electrical properties
- Electrical resistivity of metallic glasses is high and it does not vary much with temperature.
- Due to high resistivity, the eddy current loss is very small.
- The temperature coefficient is zero or negative.
Magnetic properties
- Metallic glasses have both soft and hard magnetic properties.
- They are magnetically soft due to their maximum permeabilities and thus they can be magnetised and demagnetized very easily.
- It exhibit high saturation magnetisation.
- They have less core losses.
- Most magnetically soft metallic glasses have very narrow hysteresis loop with same crystal composition. This is shown in fig.
Fig. Hysteresis loop of iron based alloy in crystalline and metallic glassy phase.
Chemical properties
- They are highly resistant to corrosion due to random ordering.
- It highly reactive and stable.
- They can act as a catalyst. The amorphous state is more active than the crystalline state from the catalytic point of view.
APPLICATIONS OF METALLIC GLASSES
Metallic – glasses also called as met glasses have found wide applications in different fields.
Structural application
- They posses high physical and tensile strength. They are superior to common steels and thus they are very useful as reinforcing elements in concrete, plastic and rubber.
- Strong ribbons of metallic glasses are used for simple filament winding to reinforce pressure vessels and to construct large fly wheels for energy storage.
- Due to their good strength, high ductility, rollability and good corrosion resistance, they are used to make razor blades and different kinds of springs.
Electrical and Electronics application
- Since metallic – glasses have soft magnetic properties, they are used in tape recorder heads, cores of high-power transformers and magnetic shields.
- They use of metallic glasses in motors can reduce core loss very much when compared with conventional crystalline magnets.
- Superconducting metallic glasses are used to produce high magnetic fields and magnetic levitation effect.
- Since metallic glasses have high electrical resistance, they are used to make accurate standard resistance, computer memories and magneto resistance sensors.
Metallic glasses as transformer core material
- Metallic – glasses have excellent magnetic properties. When they are used as transformer core, they give maximum magnetic flux linkage between primary and secondary coils and thus reduce flux leakage losses.
- In view of their features like small thickness, smaller area, light weight, high resistivity, soft magnetic property and negligible hysteresis and eddy current loss, metallic glasses are considered as suitable core materials in different frequency transformers.
Nuclear reactor engineering
- The magnetic properties of metallic glasses are not affected by irradiation and so they are useful in preparing containers for nuclear waste disposal and magnets for fusion reactors.
- Chromium and phosphorous based (iron chromium, phosphorous-carbon alloys) metallic glasses have high corrosion resistances and so they are used in iner surfaces of reactor vessels, etc.
Bio-medical Industries
- Due to their high resistance to corrosion, metallic glasses are ideal materials for making surgical instruments.
- They are used as prosthetic materials for implantation in human body.
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