Metallic Glasses

CRYSTAL

• Regular atomic arrangements.

• Slip planes for Plastic deformation.

• Metals and alloys are naturally crystalline.

• Have high strength and low hardness.

•No periodic arrangement.

•Density same as crystal.

•Have high hardness.

•Plastic flow is difficult.

GLASS

CRYSTAL

• High strength

• Low hardness

GLASS

•High hardness

METALLIC GLASS

•High strength •High hardness•Quasi brittle fracture (at room temperature)

• An Amorphous metal is a metallic material with a disordered atomic scale structure.

•Formed by rapid cooling of metallic liquids.

•Have disordered structure just like glass and composed of metallic atoms.

•Much harder than any glass and twice as strong as an alloy like stainless steel.

METALLIC GLASSES (AMORPHOUS METAL)

HOW TO MAKE A GLASS

•A glass is formed by avoiding crystallization on cooling.

•To avoid crystallization rapid cooling is done directly from liquid state.

•The density of glass depends upon the cooling rate.

•Pure metals does not form glasses their structure crystallizes on cooling the liquid when cooling is done at a slow rate.

History

In 1957 Pol Duweg produced first metallic glass alloy (Au80Si20) at Caltech, these alloys had to be cooled rapidly(106 Ks-1) to avoid crystallization.

In 1976 H. Liebermann and C. Graham manufactured Metglas which is an alloy of iron , nickel, phosphorous and boron.

The research in Tohoku University and Caltech yielded multicomponent alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper and titanium with cooling rate between 1Ks-1 – 100Ks-1

Many amorphous alloys are formed by exploiting a phenomenon called the “Confusion Effect”

WHAT IS THIS

CONFUSION EFFECT

We know that the metallic glass is an alloy of different elements (often a dozen or more) that upon cooling at faster rates the constituent atoms does not coordinate themselves into equilibrium crystalline state before there mobility is stopped and in this manner the random disordered state of the atoms is “locked in”.

Methods

The various methods to produce amorphous alloys are:

• Physical vapour deposition• Melt spinning• Suction casting• Solid state reaction

Physical vapour deposition

• Source material gets heated up in the tube furnace and forms vapours.

• The vapours are carried by the carrying gas and are made to get deposited on the substrate.

•The vapours get deposited on the substrate in the form of a disordered atomic scale structure.

•Hence the name given as physical vapour deposition and the vapours deposited on the substrate will solidify to form the glassy structure.

Single roller melt spinning

In this process, the alloy is melted (typically in a quartz tube) by induction heating.

Then it is forced out through a narrow nozzle onto the edge of a rapidly rotating chill wheel(usually made up of copper)

At least one dimension must be very small, so that heat can be extracted quickly enough to achieve the necessary cooling rate.

As a result, the early glass-forming alloys could only be produced as thin ribbons (typically around 50 μm thick), wires, foils, or powders.

Single roller melt spinning (cont..)

Suction Casting

An ingot in the upper chamber under an inert atmosphere is melted with an electric arc (much like in arc welding) and then sucked into a mold when the lower chamber is opened to vacuum.

One of the potentially useful properties of metallic glasses is that they do not melt abruptly at a fixed temperature.

Instead, like ordinary oxide glasses, they gradually soften and flow over a range of temperatures.

By careful control of temperature, the viscosity of the softened glass can be precisely controlled.

This ability can be used to form metallic glasses into complex shapes

Suction casting (cont..)

Properties

• Alloys rather than metals.

• Low free volume.

• High viscosity, low shrinkage and resistance to plastic deformation.

• Better resistance to wear and corrosion due to absence of grain boundaries.

• Thermal conductivity lower than crystals.

• They are not ductile in room temperature and tends to fail suddenly when loaded in tension ( quassi brittle fracture )

• Therefore, they are bombarded with high energy ions in order to make them ductile in nature.

Amorphous metallic alloys combine higher strength than crystalline metal alloys with the elasticity of polymers

VITRELOY

• Vitreloy was the first commercial amorphous alloy in 1992.

• Developed at Caltech as a part of Department of Energy and NASA research of new aerospace materials. • Tensile strength that is almost twice that of high grade titanium.

Applications

• Amorphous alloys have been commercialized for use in sports equipment, medical devices, and as cases for electronic equipment.

• Thin films of amorphous metals can be deposited on substances via high velocity oxygen fuel technique as protective coatings.

•Used in injection moulding since they melt and soften over a range of temperature.

•They are used in producing various complex shapes.

Equipments

Why metallic glasses is used as one of the component in Sports equipment

• Consider an experiment where in you have two cylindrical tubes.

• One is covered with a metal at the bottom and the other with a metallic glass.

• A ball bearing is made to fall on both the metal and glass surface.

Conclusions from the experiment

From the experiment we can conclude that

• The energy transmitted by the ball bearing to the titanium metal is more which is the reason why it bounces to a small altitude and comes to rest.

• The energy transmitted by the ball bearing to the liquid metal alloy is less so as a result it keeps on bouncing at a fixed height for a longer period of time.

• This is the reason why golf sticks and other sports equipments are made with metallic glass material so that the energy transferred from the ball to the stick is less which enables the ball to travel greater distances.

Observations from the experiments

• The ball bearing when it falls on the titanium metal, we observe that the ball will come to rest after a span of around 3-4 seconds.

• But when the ball falls on the liquid metal alloy we observe that the ball keeps on bouncing at a constant height for certain period of time say about 8-10 seconds.

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 done.

Common oxide glasses are quite resistance to crystallization so they can be formed even if the liquid is cooled very slowly.

The earliest metallic glasses (discovered at Caltech in 1950) required rapid cooling around 1 million degrees celsius per second to avoid crystallization.

Processing of Metallic Glasses (cont..)

Bombardment of high energy ions

•The kinetic energy of high energy ions will make the atomic structure present in the glass to reorient themselves in such a manner that they become ductile in nature.

[1]http://www.sciencedaily.com/releases/1998/03/980331074950.htm

[2] M. Telford, Materials Today, March 2004, 36-43 [3] http://www.its.caltech.edu/~vitreloy/development.htm [4]

http://www.nanonet.go.jp/english/mailmag/2004/014a.html

[5] http://en.wikipedia.org/wiki/Metallic_Glass#Properties [6] http://science.nasa.gov/ssl/msad/dtf/under1.htm [7] http://www.liquidmetal.com/technology/ [8] W.H. Wang, C. Doug, C.H. SHek, Materials Science and

Engineering, R44 (2004), 45-89

References