2. General description of metallic glasses

According to atomic arrangement, we can categorize the existing and man-made solid materials into two main groups: crystalline and amorphous. When there is a proper ordered arrangement of atoms then we say it is a crystalline material. If there is a random arrangement of atoms, then the material is called amorphous. The atomic arrangements of crystalline and amorphous materials are shown in Figure 1. To get such randomness, the sizes of the atoms are very important. Much difference in the atomic radius of the components leads to more randomness in the atomic arrangement. Glass forming is majorly concerned with the study of crystallization of materials in order to avoid crystallization. When metallic alloys are cooled at a very fast rate, possibilities of getting an ordered arrangement are poor [5].

The glass transition temperature (Generally denoted as "Tg") characterizes amorphous/glass nature of materials. This is more easily understood in the case of a polymer. If we cool a polymer from its liquid state, initially it undergoes cooling and it gets a rubbery state and then after crossing the Tg, it becomes brittle. This kind of phenomenon occurs in amorphous metals too. In case of metallic glasses, we can say that Tg is the temperature at which material gets soft from hard upon heating or get hard upon cooling. This definition for polymers and metals looks similar but it is restricted to amorphous and semicrystalline metals only. The best way to explain the process of getting an amorphous metal or metallic glass is by supercooling the metal from its liquid state. In Figure 2 Tf is the freezing temperature. During cooling, the liquid goes beyond the freezing point and is known as supercooled metal which can have an amorphous structure [6]. In this way, we can get a metallic glass. In the absence of supercooling, the liquid has a tendency to crystallize [6].

During the formation of glass, the material should avoid the route of crystallization. Crystallization happens during the cooling of material below its liquidus temperature. The difference in Gibbs free energy between liquid and crystalline state is an important factor for the ability of a metal to crystallize or to become amorphous. Whenever there is a transformation between liquid to the solid-state of a material,

successfully decreased its percentage beyond 20% [7]. As described earlier atomic size, the heat of mixing is also considered for making metallic glasses and used for classification of these materials. The properties based on base metal and categories of metallic glasses are provided in Tables 1 and 2, respectively. This type of glasses

is more often used in commercial applications.

Metallic Glasses: A Revolution in Material Science DOI: http://dx.doi.org/10.5772/intechopen.90165

Periodic table showing metals, metalloids and non-metals.

Base metal Properties based on the base metal Fe-based Soft magnetism (glass, nanocrystal)

Co-based Soft magnetism (glass, nanocrystal)

Cu-based High strength, high ductility (glass, nanocrystal)

Ni-based High strength, high ductility

Pd-based High strength

The basic properties of different LTM-based BMGs [8].

Hard magnetism (nanocrystal) High corrosion resistance

Hard magnetism (nanocrystal) High corrosion resistance

High corrosion resistance High hydrogen permeation

High corrosion resistance

High corrosion resistance

High endurance against cycled impact deformation

High endurance against cycled impact deformation

High fracture toughness, high fatigue strength

High fatigue strength, high fracture toughness

Figure 3.

Table 1.

15

Figure 2. Principle of supercooling.

the phase transformation at constant enthalpy gives a crystalline material. If enthalpy varies in that process, then the material escapes from the crystalline route and becomes amorphous. As we can see in Figure 2, a supercooling does not exist in the case of getting a crystalline material. On the other hand, in the case of supercooling, the enthalpy of transformation changes gradually. Therefore, during the manufacturing of the metallic glasses kinetics of the supercooling has a great impact on the quality of the glasses.

Considering the periodic table, the metallic glasses are mainly divided into two categories: metal–metal and metal–metalloid. In Figure 3 elements which are metals and metalloids are shown in different colors for the better understanding of their selection for making metallic glasses.

As shown in the periodic table, metals are the elements starting from Lithium with atomic number 3. These are placed from group IA to VIA and shown by the yellow color in Figure 3. Some of the examples of metal–metal type metallic glasses are Ni–Nb, Mg–Zn, Hf–V, Cu–Zr, etc. Metals can be alkali, alkaline and rare earth metals etc. In metal–metal type, the atomic percentage of individual constituents can be up to 50%.

In the case of metal-metalloid type, one constituent is a metal and the other one is a metalloid. Metalloids like B, Si, Ge can be mixed with metals like Fe, Ni, Co etc. Metalloids in the periodic table are positioned in a step-like manner and colored with peach color. Metalloids have properties that are intermediate to both metals and non-metals. The composition percentage of metalloids in this category is lower than percentage of metals. After the discovery, the compositions of metalloids in the glasses were generally up to 20% but gradually people work on that problem and successfully decreased its percentage beyond 20% [7]. As described earlier atomic size, the heat of mixing is also considered for making metallic glasses and used for classification of these materials. The properties based on base metal and categories of metallic glasses are provided in Tables 1 and 2, respectively. This type of glasses is more often used in commercial applications.


#### Figure 3.

the phase transformation at constant enthalpy gives a crystalline material. If enthalpy varies in that process, then the material escapes from the crystalline route and becomes amorphous. As we can see in Figure 2, a supercooling does not exist in

the case of getting a crystalline material. On the other hand, in the case of

impact on the quality of the glasses.

selection for making metallic glasses.

can be up to 50%.

14

Figure 2.

Metallic Glasses

Principle of supercooling.

supercooling, the enthalpy of transformation changes gradually. Therefore, during the manufacturing of the metallic glasses kinetics of the supercooling has a great

Considering the periodic table, the metallic glasses are mainly divided into two categories: metal–metal and metal–metalloid. In Figure 3 elements which are metals and metalloids are shown in different colors for the better understanding of their

As shown in the periodic table, metals are the elements starting from Lithium with atomic number 3. These are placed from group IA to VIA and shown by the yellow color in Figure 3. Some of the examples of metal–metal type metallic glasses are Ni–Nb, Mg–Zn, Hf–V, Cu–Zr, etc. Metals can be alkali, alkaline and rare earth metals etc. In metal–metal type, the atomic percentage of individual constituents

In the case of metal-metalloid type, one constituent is a metal and the other one is a metalloid. Metalloids like B, Si, Ge can be mixed with metals like Fe, Ni, Co etc. Metalloids in the periodic table are positioned in a step-like manner and colored with peach color. Metalloids have properties that are intermediate to both metals and non-metals. The composition percentage of metalloids in this category is lower than percentage of metals. After the discovery, the compositions of metalloids in the glasses were generally up to 20% but gradually people work on that problem and Periodic table showing metals, metalloids and non-metals.


#### Table 1.

The basic properties of different LTM-based BMGs [8].


These structural motifs arise from the strong tendency to form as many bonds as possible between unlike species because of the large negative heat of mixing which is usual in good glass formers. The size of the cluster and its type depend on the relative size of the solvent and the solute. The replacement of Pt solute in Figure 4 by much smaller Be reduce the number of Zr neighbors which can be accommodated around the solute, and the solute concentration in the alloy would be correspondingly much higher. The medium-range order and dense packing in threedimensional space can be possible by the overlapping of the cluster via various

Model of a simple binary metallic glass: Interpenetrating quasi-equivalent clusters sharing faces, edges, or vertices in the atomic packing configuration of a Zr-Pt metallic glass. The blue balls represent the solvent Zr

The adaptability of the metallic glass in the real world applications is spread in various fields, such as striking face plate in golf clubs, frame in tennis rackets, various shapes of optical mirrors, casing in cellular phones, casing in electromagnetic instruments, connecting part of optical fibers, shot penning balls, electromagnetic shielding plates, soft magnetic choke coils, soft magnetic high frequency power coils, high torque geared motor parts, high corrosion resistance coating plates, vessels for lead-free soldering, colliori type liquid flow meter, spring, inprinting plate, high frequency type antenna material, biomedical instruments such as endoscope parts etc. [12]. Metallic glasses are very strong compared to other conventional materials and that makes it a very good candidate for military applications like armor (Bulletproof vest) piercing bullets, anti-tank projectiles etc. Those metallic glasses, which are stronger than titanium, are also tried for aerospace application. Recently researchers from NASA and different research centers and organizations of China, Britain and Japan are doing several tests to get such ultimate material. This type of materials can give relatively double the performance compared to that of a titanium product in the space application. The major problem lies in the BMGs are the very quick aging of these materials. They become fragile after exposed to external physical stress conditions. The non-uniformity or not enough randomness inside the glass structure leads to the quick aging of these BMGs. So reliability cannot be achieved for the space applications. To remove such weakness of these materials, Wei-Hua Wang from China did experiments like the operation of a blacksmith. Cryogenic treatment of melted metal was done by using liquid nitrogen and maintained to room temperature after solidification. Again the material was melted and the process was repeated for several cycles. The purpose of

solvent-atom sharing schemes [11].

atoms centered around Pt solute atoms [10].

Metallic Glasses: A Revolution in Material Science DOI: http://dx.doi.org/10.5772/intechopen.90165

Figure 4.

17

Table 2.

Composition and properties of different types of BMGs (composition of base metal is greater than 50 at. %) [8].

## 3. Structure, properties and applications

Structure of material defines its property. BMGs do not exhibit a long-range order structure, as they solidify from liquid without reaching the crystalline ground state. However, short to medium-range structural order does develop to a considerable extent under the given kinetic constraints. This happens because the atoms strive to find comfortable configurations to lower their energy. The structure of the bulk metallic liquids was first observed by Bernal [9] and it was described as dense random packing. Structural features of metallic glasses are discussed by Michael et al. where the concept of efficient filling of space is supported [10]. The rationalization of the good glass forming compositions can be possible by the analysis of dense packing. An example of simple binary metallic glass is shown in Figure 4 [10]. Metallic Glasses: A Revolution in Material Science DOI: http://dx.doi.org/10.5772/intechopen.90165

#### Figure 4.

Model of a simple binary metallic glass: Interpenetrating quasi-equivalent clusters sharing faces, edges, or vertices in the atomic packing configuration of a Zr-Pt metallic glass. The blue balls represent the solvent Zr atoms centered around Pt solute atoms [10].

These structural motifs arise from the strong tendency to form as many bonds as possible between unlike species because of the large negative heat of mixing which is usual in good glass formers. The size of the cluster and its type depend on the relative size of the solvent and the solute. The replacement of Pt solute in Figure 4 by much smaller Be reduce the number of Zr neighbors which can be accommodated around the solute, and the solute concentration in the alloy would be correspondingly much higher. The medium-range order and dense packing in threedimensional space can be possible by the overlapping of the cluster via various solvent-atom sharing schemes [11].

The adaptability of the metallic glass in the real world applications is spread in various fields, such as striking face plate in golf clubs, frame in tennis rackets, various shapes of optical mirrors, casing in cellular phones, casing in electromagnetic instruments, connecting part of optical fibers, shot penning balls, electromagnetic shielding plates, soft magnetic choke coils, soft magnetic high frequency power coils, high torque geared motor parts, high corrosion resistance coating plates, vessels for lead-free soldering, colliori type liquid flow meter, spring, inprinting plate, high frequency type antenna material, biomedical instruments such as endoscope parts etc. [12]. Metallic glasses are very strong compared to other conventional materials and that makes it a very good candidate for military applications like armor (Bulletproof vest) piercing bullets, anti-tank projectiles etc. Those metallic glasses, which are stronger than titanium, are also tried for aerospace application. Recently researchers from NASA and different research centers and organizations of China, Britain and Japan are doing several tests to get such ultimate material. This type of materials can give relatively double the performance compared to that of a titanium product in the space application. The major problem lies in the BMGs are the very quick aging of these materials. They become fragile after exposed to external physical stress conditions. The non-uniformity or not enough randomness inside the glass structure leads to the quick aging of these BMGs. So reliability cannot be achieved for the space applications. To remove such weakness of these materials, Wei-Hua Wang from China did experiments like the operation of a blacksmith. Cryogenic treatment of melted metal was done by using liquid nitrogen and maintained to room temperature after solidification. Again the material was melted and the process was repeated for several cycles. The purpose of

3. Structure, properties and applications

Pt–Cu–Co–P (Cal Tech)

Pt–Pd–Cu–P

Structure of material defines its property. BMGs do not exhibit a long-range order structure, as they solidify from liquid without reaching the crystalline ground state. However, short to medium-range structural order does develop to a considerable extent under the given kinetic constraints. This happens because the atoms strive to find comfortable configurations to lower their energy. The structure of the bulk metallic liquids was first observed by Bernal [9] and it was described as dense random packing. Structural features of metallic glasses are discussed by Michael et al. where the concept of efficient filling of space is supported [10]. The rationalization of the good glass forming compositions can be possible by the analysis of dense packing. An example of simple binary metallic glass is shown in Figure 4 [10].

Composition and properties of different types of BMGs (composition of base metal is greater than 50 at. %) [8].

Base metal Metal–Metalloids Metal–Metal

Fe–Nd–Al

Co–Nd–Al Co–Sm–Al

Ni–Nb–Ti Ni–Nb–Zr Ni–Nb–Hf Ni–Nb–Zr–Ti Ni–Nb–Zr–Ti–M (M = Fe, Co, Cu) Ni–Nb–Hf–Ti Ni–Nb–Hf–Ti–M Ni–Nb–Sn (Cal Tech)

Cu–Zr–Ti Cu–Hf–Ti Cu–Zr–Ti–Ni Cu–Hf–Ti–Ni Cu–Zr–Ti–Y Cu–Hf–Ti–Y Cu–Zr–Ti–Be Cu–Hf–Ti–Be Cu–Zr–Al Cu–Hf–Al Cu–Zr–Al–M Cu–Hf–Al–M (M = Ni, Co, Pd, Ag) Cu–Zr–Ga Cu–Hf–Ga Cu–Zr–Ga–M Cu–Hf–Ga–M Cu–Zr–Al–Y (Cal Tech)

Fe–Ga–(Nb,Cr,Mo)–(P,C,B)

Fe–Ga–(P,C,B,Si)

Co–(Zr,Hf,Nb,Ta)–B

Ni–(Ta,Cr,Mo)–(P,B) Ni–Zr–Ti–Sn–Si (Yonsei University)

(Los Alamos) Fe–(Cr,Mo)–(B,C) Fe–(Zr,Hf,Nb,Ta)–B Fe–(B,Si)–Nb

Fe-based Fe–(Al,Ga)–(P,C,B,Si)

Metallic Glasses

Co-based Co–Ga–(Cr,Mo)–(P,C,B)

Ni-based Ni–(NbCr,Mo)–(P,B)

Cu-based Cu–Pd–P

Pd-based Pt–Cu–P

Table 2.

16

Co–Ln–B

Ni–Pd–P

Cu–Ni–Pd–P

choosing the repeated cryo-treatment is to reduce the instability of the material by increasing the randomness inside the material. This type of processing technique enhances the life of a BMG and increases its reliability.

limiting factor and leads to affect the cooling rate. In the process of direct casting of BMG formers, fast cooling and forming have to be done simultaneously due to the crystallization mechanism and the crystallization kinetics [7, 8]. Direct casting needs care during the mold filling and at the same time to avoid crystallization during solidification. This process is very difficult while making an intricate part which has attractive properties. Figure 6 represents some application of BMGs fabricated by direct casting. The advantages and disadvantages of the direct casting

Thermoplastic forming (TPF) is the alternative process to direct casting for developing BMGs. This process has different nomenclature such as hot forming, hot pressing, superplastic forming, viscous flow working and viscous flow forming. The preferable condition for TPF is the drastic softening of the BMG former upon heating above Tg and its thermal stability. The measure of the ability of BMG formers to adopt an amorphous structure by heating above its glass transition temperature is known as thermal stability and it can be quantified by the width of the supercooled liquid region (SCLR). For Zr44Ti11Cu10Ni10Be25 a very long processing time is available at low temperature by a high viscosity [17]. Furthermore, the viscosity is significantly reduced at a high temperature which leads to the reduction in processing time. Low viscosity and the long processing time are the

BMG process are given in Table 3.

Metallic Glasses: A Revolution in Material Science DOI: http://dx.doi.org/10.5772/intechopen.90165

4.2 Thermoplastic forming

Figure 6.

19

BMG articles fabricated by direct casting method.

Firstly China and secondly the United States are the major producers of BMGs. Currently, most applications are focused on electric based products like transformer core. Because of the good conductivity properties of BMGs, it dominates in that sector. Other applications like high-temperature applications, aerospace applications and military applications have a long way to go for becoming a better replacement for the recently used materials.
