Engineering Physics 2 - Metallic glasses: preparation, properties and applications (Unit 5)

Anna University

Engineering Physics 2

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Metallic glasses: preparation, properties and applications

( Unit : V )

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Introduction

Scientist at Yale have developed a new fabrication process using bulk metallic glasses (BMGs), which are "amorphous metals" that can avoid crystalizing when cooled in a specific way. The upshot is that the metal -- while seeming solid -- acts as a slow- flowing liquid, with no structure beyond the atomic level. The BMGs can therefore replace several steps in the chip-stamping process, since they're more durable than silicon, but are more pliable than normal metals. Right now the folks at Yale are making patterns as small as 13nm, with better processes to come.

Electron diffraction image of an enstatite grain before irradiation (left panel) and after complete amorphization (right panel)

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An amorphous metal is a metallic material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline. Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "glasses", and so amorphous metals are commonly referred to as "metallic glasses" or "glassy metals". However, there are several other ways in which amorphous metals can be produced, including physical vapor deposition, solid-state reaction, ion irradiation, melt spinning, and mechanical alloying.

In the past, small batches of amorphous metals have been produced through a variety of quick-cooling methods. For instance, amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk. The rapid cooling, on the order of millions of degrees a second, is too fast for crystals to form and the material is "locked in" a glassy state. More recently a number of alloys with critical cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimeter) had been produced, these are known as bulk metallic glasses (BMG). More recently, batches of amorphous steel have been produced that demonstrate strengths much greater than conventional steel alloys.

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Learning Objectives :

On completion of this topic you will be able to understand:

1. What is meant by metallic glasses.

2. The properties of metallic glasses

3. The various methods for producing metallic glasses.

4. Applications of metallic glasses.

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Metallic glasses are a class of metallic alloys in which the crystallization process has been "frustrated" in some way, to the extent that a liquid-like, disordered long range atomic structure exists at temperatures well below the melting temperature of the alloy. Unlike the early metallic glasses which formed only under rapid solidification conditions (>1000's K/s), the highly stable bulk metallic glass alloys form at unusually low critical cooling rates (~1-10 K/s) and require no special processing to obtain their extraordinary mechanical properties.

Computer simulation of the disordered atomic structure of a three- component metallic glass.

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Properties

Amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume in molten state.Therefore they have therefore higher viscosity than other metals and alloys. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.

Thermal conductivity of amorphous materials is lower than of crystals. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures.

Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.

One modern amorphous metal, known as Vitreloy, has a tensile strength that is almost twice that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension, which limits the material applicability in reliability-critical applications, as the impending failure is not evident.

Sample of Amorphous metal in the lab

Perhaps the most useful property of bulk amorphous alloys is that they are true glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by injection molding, in much the same way as polymers. As a result, 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 via high velocity oxygen fuel technique as protective coatings.

The fatal drawback of plastics is their viscoelasticity behavior at a room temperature. So, when plastics are used as structural materials, they are normally reinforced with fiber reinforced plastic (FRP). However, in metal glass, since the glass transition temperature at which a viscoelasticity behavior becomes significant is much higher than room temperature and so problems due to viscoelasticity do not arise. Bulk glassy alloys also provide the outstanding moldability and high corrosion resistance as like plastics. Furthermore, since it is a metal, various functional characteristics: magnetic property, high strength and high toughness, are available.

They exhibit extraordinary physical, chemical, as well as mechanical properties, which result from their unique, disordered, atomic arrangements. Now amorphous alloys can be fabricated into bulk forms, ranging from several millimeters to several centimeters in thickness, by slow-cooling multicomponent alloy systems.

In particular, amorphous alloys tend to have low coercivity because there are no boundaries between crystalline grains to impede the motion of magnetic domain walls and because there is no magnetocrystalline anisotropy.

In this way, metallic glass can be regarded as a new material which has high moldability, high hardness and high toughness, and it can be used to make the micro scale industrial parts which could not be done with existing common crystalline alloys. The absence of grain boundary and dislocations are unique features of bulk glassy alloys making them suitable for using micro-scale parts fabrication.

They are not especially good conductors, due to their disordered atomic structure and high levels of alloying elements. It is sometimes assumed that metallic glasses, like common oxide glasses, are transparent to visible light, but this is not so. Photons at visible wavelengths are strongly scattered and absorbed by the conduction electrons in metallic glasses.

Although metallic glasses are electrically conductive, their resistance to current flow is generally higher than that of crystalline alloys. This helps to minimize eddy-current losses that occur due to rapid magnetization and demagnetization of the material.

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Methods for producing Metallic glasses:

Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Physical vapor deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the material onto various surfaces. The coating method involves purely physical processes such as high temperature vacuum evaporation or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition.

Melt spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated. A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop metallic glasses that require extremely high cooling rates in order to form. The cooling rates achievable by melt-spinning are on the order of 104–107 Kelvin per second (K/s).

Rapid Quenching method: This method is used to extract heat from the melt by contact to a chill block. Cooling rate of the order of 106 K/s is required to form metallic glasses from the melt. Since fast extraction of heat is necessary, the dimensions of the solid obtained is limited to 50 µm. The molten liquid metal alloy is pushed out of a fine nozzle by gas pressure. A layer of rotating metallic disk is formed between the nozzle and the chill block. A mettalic glass ribbon about 1 cm wide, 30 µm thick can be thus produced.

Rapid quench flow instrument

Solid- State Amorphization: Bulk metallic glasses that cannot be obtained by rapid cooling can be produced without going through a liquid phase.

1. When alloys are subjected to radiation such as electrons or ions, amorphization occurs.

2. A two-phase crystalline material gets converted into metallic glass through inter- diffusion when annealed at a moderate temperature.

3. Mechanical alloying also results in amorphization.

An ion implantation system at LAAS technological facility

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Applications of Metallic glasses

Knife edges: The high hardness gives good wear resistance, and the lack of microstructure makes it possible to sharpen a very fine edge.

Springs: The combination of good resiliance and low damping makes metallic glasses exceptionally good springs. This may include some items that you don't necessarily think of as springs, including tennis rackets and golf clubs.

Fashion items: Metallic glasses can be polished to a fine finish, and good wear resistance means they don't scratch easily. This may make them attractive for jewelry and consumer electronics (such as cell phone cases). Plus, the perceived novelty is a selling point for many of these items.

Microelectromechanical systems (MEMS): These are highly miniaturized mechanical devices produced using specialized techniques originally developed for semiconductor processing. The high yield strain, low damping, and resiliance of metallic glasses makes them useful, for instance as hinges on micromirrors used for digital light processing technology.

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Check your understanding

State whether True or False

1. Photons at visible wavelengths are strongly scattered and absorbed by the conduction electrons in metallic glasses.

Ans : True

2. They are especially good conductors, due to their disordered atomic structure and high levels of alloying elements.

Ans : False

3. Amorphous metal is usually an rather than a pure metal.

Ans : Alloy

4. The highly stable bulk metallic glass alloys form at very low critical cooling rates of ___?

Ans : 1-10 K/s

5. Why does an amorphous alloy has a low coercivity value?

Ans : In particular, amorphous alloys tend to have low coercivity value because there are no boundaries between crystalline grains to impede the motion of magnetic domain walls and because there is no magnetocrystalline anisotropy.

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Summary

On completion of this topic you have learned

Metallic glasses are a class of metallic alloys in which the crystallization process has been "frustrated" in some way. The special features of metallic glasses has been projected in this topic. The various methods of preparing metallic glasses is described. Finally some of the applications are also listed out.

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Activity

Find out the demerits of metallic glases.

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Suggested Reading

1. Charles Kittel ‘Introduction to Solid State Physics’, John Wiley & sons, 7th edition, Singapore (2007)

2. M. Arumugam, ‘Materials Science’ Anuradha publications, Kumbakonam, (2006).

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