January 23, 2019 0 Comment

In our nature there are plenty of materials present ,each having its own sets of merits and demerits. For example if we take materials such as plastic and steel.If we look at the advantages of steel then it is very strong but we cannot change the shape of steel materials easily. Whereas if we observe the plastic material then we can convert them into any shape we want but it is not a strong material ie. Plastic is a soft material. So, if we could create a material with both the advantages ie. A material with the strength of steel and we can easily convert them to any shape we want.So over the years scientist have been trying to make an engineered a material mention above and they were successful. This engineered material was known as Metallic glass.It is also known as Glassy Metal. There are two types of metallic glasses. One are Metal metallic glass and other is Metalloid metallic glass. In Metal metallic glass it is a combination of two metals for example Ni(nickel)-Nb(Niobium) , Mg(Magnesium)-Zn(Zinc). Whereas in case of Metalloid metallic glass it is a combination of metal and metalloid for example Fe(Iron)-Co(Cobalt)and Ni(Nickel). This was different from the window glass in many of the properties for example has a good electrical conductivity. Also, in this report we will find that they both are structurally different. In recent times alloys with low critical cooling rates low enough for the amorphous structure formation are produced, which are known as Bulk Metallic Glass. Most of the BMG’S currently avaiable are made from expensive metals like palladium and zirconium and they have to be really pure as they don’t have to crystallize.Metallic Glass were firstly discovered 50 years ago.Also, the metallic glass does not have any crystal defect.Due to these and many other properties of the Metallic glass has many application. In this, report you will find its unique properties and application below.

The first metallic glass was made in the year 1960.It was actually an alloy of Gold(Au) and Silicon(Si).The exact empirical formula was Au75Si25. It was produced at Caltech by
W.Klement (Jr.), Willens and Duwez.Most of the metals are crystalline solids. This means if we see the microscopic view then we would find that the atoms are arranged in an orderly repeating pattern, which is found throughout the material. Examples of crystalline materials are diamond , salt , ice etc.When we heat these materials enough(about 630 K) the atoms gets excited and move freely and randomly but when we cool them atoms reorganise themselves re-establishing the crystal.The formation of Metallic glass and Simple Glass is almost similar.In case of a Simple Glass it is given ample amount of time to cool down ie. The rate of cooling is less but in case of Metallic glass they are cooled down at a very high rate, of the order of one mega-kelvin per second,10^8K/s. Due to this the atoms are not able to arrange themselves in an orderly fashion leading to an Amorphous nature. Whereas, in case of Metals(Simple Glass) the atoms are cooled down slowly hence they arrange themselves in an orderly fashion. This much of high order of cooling rate is achived by the process called “splat-cooling”. The structure of Metallic Glass has its own benefit of lacking the grain boundaries which most metals posses.These are the spots where the metals are more likely to have scratches or corrosion.Hence from above we can say that making the first Metallic glass won’t be easy because as the metal atoms crystallizes rapidly so, they had to maintain a high rate of cooling as mentioned above (one mega-kelvin per second).The form of Metallic glass were of a limited such as ribbons, foils and wires. Even the thickness of the Metallic glass was of the order of the tens or hundreds of microns, which was much thinner for most of the application of Metallic glass.As the years passed the new inovation took place in it’s formation. In the year 1969 Metallic glass of the alloy of palladium,copper and silicon was formed with cooling rate between 100 to 1000K/s. Which is a huge decrement in the cooling rate when compared with year 1960. Further in the year 1976 thin ribbons of amorphous metals were being manufactured by using the fast-spinning wheel method, the material made was alloy of iron , phosphorus , nickel and boron. This alloy was used in the low power distribution transformers. This alloy formed was known as Metglas. Glassy ingots were produced in the early 1980s. Also as the years passed the limitation of the thickness of the metallic glass was overcome by using the boron oxide flux. The thickness then produces was of the order of centimetres. The further researches on the metallic glass lowered the cooling rates to the order of 1 to 100K/s.Moreover in the year 1988 Al-based metallic glass with Scandium had a mechanical strength of about 1500Mpa.
The BMG as above mentioned were discovered in the year 1990s. The first BMG discovered was the alloy of iron, copper, titanium , magnesium.These had a thickness of the order of centimetre. Metallic glass has its application in the aero-space field also and the alloy name
Vitreloy was found by NASA and Department Of Energy in the year 1992. Bulk amorphous steel was discovered in 2004.

As mentioned above in this report how the first metallic glass was produced and also we observed that by modification made in the preparation process the cooling rate was brought down drastically. So the main process in the formation of metallic glass is of the cooling process. Nowadays various cooling methods are used such as spraying, laser-deposition and spinning for the production of metallic glass. The most advantageous method for the production of metallic glass from the three methods above mentioned is spinning.Some of the advantages are as follows
1Inexpensive Tooling
2Higher strength
3Design of the material could be changed easily
4Minimal Scrap
5No seams and Welds
Hence in this report we would have a brief look on the melt spinning process technique.
The figure below depicts the melt spinning process technique.

FIG. Explaining the melt spinning process technique.

For the preparation of the metallic glass firstly a suitable combination of either metal-metal or metal-metalloid is taken in a proper bits stoichiometric ratio. This alloy is taken into a refractory tube which consists of a nozzle at the bottom of it.Now this nozzle is just place over the spinning wheel/disc.The spinning disc is made up of copper. For melting the alloy present in the refractory tube an induction heater is attached to the refractory tube. The temperature of this melt is kept above the melting point of the alloy till the time it gets converted into a homogeneous mixture. For ejecting the melt present in the refractory tube an inert gas such as helium is made to flow through it. So,the melt get ejected from the nozzle on to the spinning disc. The ejected melt is cooled at a faster rate with the help of spinning disc. The spinning disc is kept at a lower temp for the porous of cooling the melt. The desired rate of cooling can be achieved by changing the speed of the spinning wheel. For changing the ejection rate of the melt from the nozzle we can vary the pressure of the inert gas ie. If we increase the pressure of the inert gas the ejecting rate of melt will increase. Hence if we continue this process then a glassy alloy ribbon gets start to form on the spinning wheel. Alteration in the speed of the spinning disc could change the thickness of the glassy ribbon.

In case of the metallic glass they don’t have a crystalline structure rather the atoms in a metallic glass are arranged randomly. This gives them some unique properties than the metals as metals have a crystalline structure. Due to amorphous nature of the metallic glass it has better mechanical, magnetic and unique chemical properties in comparison to the metals as they have crystalline structure. Also in case of metallic glass it is a combination of materials(ie. alloy) due to this there is difference in the sizes of the atoms in the metallic glass leading to low free volume in molten state. In the past when scientists were discovering and researching on the metallic glass they found that the pressure could change the materials property for example its volume or its electronic behaviour. So, researchers long thought that as the atoms are densely packed so they couldn’t convert metallic glass into more denser state. But the experiments in 2007 showed that cerium metallic glass became denser at high pressure. Due to this the theorist suggests that electrons become de-localized at high pressure causing the bonds between atoms to shrink. Below are some of the useful properties mentioned which has some important application.
1Metallic glass posses shiny and opaque looking nature which is similar to the metals. But in terms of strength metallic glass are far more better than the metals as they can withstand more load in comparison to the metals. Also they withstand a lot of force without bending or denting which could be used for making ultra-sharp scalpels and ultra string electronics cases, screws, hinges and more.
2Metallic glass have an ability to store and release elastic energy.This property could be beneficial in making of the sports goods such as golf stick,racquet,skis etc.
3Metallic glass also has the ability to take any shape or rather complex shapes with a mirror like surface in single moulding step. This is possible because of the property that at higher temperature ie about 600 degree Celsius they often get softer in nature. Due to softening they tend to be moulded in any shape. Moreover when they are cooled back they regain their strength.
4The structure of the metallic glass add-on some of the property such as it improves its strength and durability.
5Metallic glass also make high efficient magnets, as they posses the soft magnetic properties.
6Also metallic glass shows lower eddy loss in accordance with its magnetic property.As a crystal possessing imperfection would make it difficult to change its magnetization in an external field. This requires energy and leads to energy loss in the switching process.
7Metallic glasses are corrosion resistance due to their structure as they do not possess grain boundaries.

Fig. Corrosion resistance property of metallic glass

8 Thermal conductivity of crystalline metals is lower than the metals.
9 The strength of the metallic glass is around 1500-2500 MPa whereas the elastic limit is around 2%. Mathematically it is found that ? ?E/50 where ? is the tensile strength and E is the Young’s modulus. This property of high elastic strain limit and high yield strength and the fracture toughness makes metallic glass most suitable material for storing a high density elastic energy

Fig. Typical strengths and elastic limit for various materials
10Metallic glass have lower ductility and fatigue strengths.
11More on the Electrical property of metallic glass:-
For metallic glass its electrical resistivity is as high as the nichrome alloys, the nichrome alloys are used as a resistive element in the circuits.Also the experiments have showed that the electrical resistivity of metallic glass doesn’t change with the change in temperature.
But for all other general devices resistivity decreases with the increasing temperature. So, metallic glass could be used in the electrical circuit where the circuit has to be insensitive to the temperature.
The super conducting metallic glass are much more stable than the simple metallic glass. These superconducting glasses are used in the high field electromagnets which would contain the high temperature plasma in the fusion reactors.

Metallic glass can be used in the motors as they have amorphous structure and provide high magnetic efficiency. Also using metallic glass in the motor could reduce the core loss upto 90% in comparison to the crystalline magnets.

Metallic glasses are also used in the cryothermometers, computer memories and magnetoresistance sensors as they possess high electrical resistivity and nearly zero temperature coefficient of resistance.
The nuclear waste disposal containers are made up of metallic glass as metallic glasses are not affected by the irradiation.
Metallic glasses are also used for the preparation of magnets for fusion reactors and magnets for levitated trains.