Metallic Glasses

METALLIC GLASSES

In the year 1970, scientists discovered metallic glasses which are a new class of materials. Their important characteristic is non-crystalline structure. But, the normal metal alloys have crystalline structure. They are also called as amorphous metals.

Metallic glass = Amorphous metal

Generally, glass is an amorphous, brittle and transparent solid. We know that the metals are malleable, ductile and exhibit crystalline properties. The metallic glasses have the properties of both metals and glasses.

It is found that the metallic glasses are strong, ductile, malleable, opaque and brittle. They have good magnetic properties and high corrosion resistance. leiretam

Metallic glasses are usually prepared by cooling a metallic liquid (which has a disordered structure) so rapidly such that there is no enough time for ordered crystalline structure to develop.

During the solidification, there is essentially no change in spatial atomic configuration. Thus, a glass may be considered as a solid frozen in liquid structure.

 

Glass transition temperature

Temperature at which transition from liquid to solid occurs is known as glass transition temperature (T_g).

The change of state from molten liquid to metallic glasses is shown in fig. 4.4.

Case (1): In fig. 4.4, the curve ABDE shows the change of state from molten liquid to crystalline solid at the temperature called melting point temperature (T_m).

This occurs due to normal rate of cooling with the decrease in its specific volume. Thus it forms a normal crystalline material.

Case (2): The curve he curve ABC shows the the change of state from a molten liquid to metallic glasses due to rapid cooling without decrease in its specific volume at the temperature called glass transition temperature (T_g). Thus, it forms materials with crystalline property.

The glass transition temperature for the metallic glasses is about 20°C to 300°C instead of several hundred degrees as in glasses. Upon heating, the metallic glasses show a reversible glass liquid transition at T_g

 

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 systems

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. 4.5.

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

Example:

Nickel (Ni) - Niobium (Nb)

Magnesium (Mg) - Zinc (Zn)

Copper (Cu) - Zirconium (Zr)

(ii) Metal - Metalloid metallic glasses

These are combinations of metals and metalloids.

Example:

Metals:    Metalloids

Fe, Co, Ni       B,Si C, P

 

Properties of Metallic Glasses

In view of high degree of atomic disorder, metallic glasses show unusual and remarkable structural, mechanical, electrical, magnetic and chemical properties.

Let us discuss the different 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 hardening 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. Thus, they can be magnetised and demagnetised very easily.

• They 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 4.6.

Chemical properties

• They are highly resistant to corrosion due to random 918 yedi nedW

• They are highly reactive and stable.

• They can act as a catalyst. The amorphous state is more floa active than the crystalline state from the catalytic point of view.

 

Applications of Metallic Glasses

Metallic glasses are also called as met glasses. They have found wide applications in different fields.

Structural applications

1. They possess 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.

2. Strong ribbons of metallic glasses are used for simple filament winding to reinforce pressure vessels and to construct large fly wheels for energy storage.

3. 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 applications

1. Since metallic glasses have soft magnetic properties, they are used in tape recorder heads, cores of high power transformers and magnetic shields.

2. The use of metallic glasses in motors can reduce core loss very much when compared with conventional crystalline magnets.

3. Superconducting metallic glasses are used to produce high magnetic fields and magnetic levitation effect.

4. Since metallic glasses have high electrical resistance, they are used to make accurate standard resistance, computer memories and magneto resistance sensors.

5. 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, iniog sidy smaller area, light weight, high resistivity, soft magnetic property and negligible hysteresis and eddy current loss, metallic glasses are considered as core materials in different frequency transformers.

Nuclear reactor engineering applications

1. The magnetic properties of metallic glasses are not affected by irradiation. So, they are useful in preparing containers for nuclear waste disposal and magnets for fusion reactors.

2. Chromium and phosphorous based (iron chromium, phosphorous - carbon alloys) metallic glasses have high corrosion resistances. Hence they are used in inner surfaces of reactor vessels.

Bio-medical Industries applications

1. Due to their high resistance to corrosion, metallic glasses are ideal materials for making surgical instruments.

2. They are used as prosthetic materials for implantation in human body.