4.2 Thermoplastic forming

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

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 replace-

The flow chart for the processing of metallic glasses in order to obtain different

For the net shape fabrication of BMG, two types of casting processes (suction and die casting) have been adopted. Among the two methods the suction casting method develops a product with higher quality and lower porosity than die casting method. BMGs having low melting temperature are beneficial because this process reduces tool cost and wear, lowers energy consumption and shortens cycle time. The TTT diagram and the critical cooling rate (Tc) are highly affected by the maximum temperature prior to cooling rate [13]. The glass forming ability of the BMGs can be diminished if the maximum temperature prior to cooling does not exceed a threshold temperature (TOH). The overheating is rapid and it affects the viscosity in some type of BMGs [14]. This viscosity effect can be broadly explained by the phenomenon including the melting of an oxide phase, ordering phenomenon and a chemical decomposition process [15, 16]. During the casting processes (both die casting and suction casting) shrinkage has to be taken in to consideration. The shrinkage phenomenon is absent in the BMGs formers due to the absence of a first order phase transition during solidification. Cooling process in the casting plays a vital role. Low solidification shrinkage in BMGs develops a gap between the mold and the BMG during the cooling process [19, 20]. The heat transfer through the gap is different for the presence of atmosphere and vacuum, can become the rate

enhances the life of a BMG and increases its reliability.

ment for the recently used materials.

4. Processing of metallic glasses

products is presented in Figure 5.

4.1 Direct casting

Metallic Glasses

Figure 5.

18

Flow chart of the processing of metallic glasses.

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

Figure 6. BMG articles fabricated by direct casting method.


#### Table 3.

Advantages and disadvantages of the direct casting process.

optimum parameters for the highest formability of BMG former in its SCLR. According to the data reported in the open literature, the thermos physical properties that are detecting good formability of a BMG former are fragile liquid behavior, large poison ratio and low glass transition. Some TPF based BMG formers the glass transition temperature and the softening in the SCLR permits the use of processing temperature and pressure [18]. The capacity to plastically form metallic glasses in the zone of Tg was recognized by researchers. The enhanced formability of BMGs, which is developed by a wide range of processing methods based on the thermoplastic forming, attracts a wide range of researchers. In all the above-said processes, one thing is common that is the "feedstock". The shape and size of the feedstock are different for powder, over rods, disks, plates and films. These TPF based processing methods have been described in the below sections.

#### 4.2.1 TPF based compression and injection molding

Compression molding was an adopted form of plastic processing. In this process, the feedstock material is placed in a mold and is given temperature into SCLR and pressure must exceed the flow stress of the BMG to achieve the required strain prior to crystallization setting. Fast cooling is not required for the forming. Figure 7 represents the schematic diagram of the process equipped with examples. Under low applied pressure in the compression molding Pd-Ni-Cu-P alloy can be formed as a gear-shaped structure. It can be noticed that a dense compact part with mechanical properties close to those of bulk material and an outstanding surface finish can be obtained [11, 12]. Figure 7 indicates compression molding of BM6 former, having different feedstock shapes such as pellets, plates and rods. The needed molding pressure not only depends on the formability of the BMG at the processing temperature but also on the shape of the final product. Injection molding is also a TPF based molding process. The only difference between the injection molding and compression molding is that in injection molding, feedstock material is rendered into the mold cavity which has the benefits for the development of the commercial fabrication processes with minimized cycle time.

techniques for creating high aspect ratio structures. Due to the homogeneous and isotropic structure of BMGs in atomic scale and their superior properties over conventional materials used for miniature applications and the capability to pro-

TPF based compression molding with BMG. (A) Schematic diagram of the compression molding with BMGs. (B) Pellets used as feedstock material to compression mold Pt57.5Cu14.7Ni5.3P22.5. (Zr44Ti11Cu10Ni10Be25) formed from a flat plate into a corrugated structure. (D) Zr44Ti11Cu10Ni10Be25 formed from a flat plate to

BMGs basically formed by top-down nanofabrication. The combination of different properties like high strength at room temperature, the ability to imprint a nanometer-sized parallel print process, the non-linear softening of BMGs when reaching their glass transition and the ability to repeatedly write and erase facility on the BMG surface recommends a wide range of application. Nanoimprinting on BMG permits to directly write, such as with atomic force microscopy (AFM) tip, as

nanoimprinting can be applied with a combination of surface smoothening method and used as a rewritable high-density data storage. Several materials have been developed for the mold formation and imprint for nanoforming such as silicon,

Rolling of metallic glasses can be categorized into two processes; one is based on liquid processing and the other is on thermoplastic forming. The example of the former is the melt spinning. In the melt spinning, the liquid sample is quenched by

in a scanning probe lithography process. The capability of BMGs for direct

duce stress-free parts, these methods attracted a lot of attention.

4.2.3 Nano forming

create an embossing mold.

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

Figure 7.

quartz, and alumina.

4.2.4 Rolling

21

#### 4.2.2 Miniature fabrication

The development in technologies like micro-electromechanical systems (MEMS), electronics devices, and medical devices have created a rising demand for miniature products and parts. The miniature formation is done by different processes like German method LIGA (lithography, electroplating and molding), UV-LIGA etc. Due to the drawbacks of the LIGA i.e. cost, UV-LIGA is developed giving the similar products. Both processes can be used as the surface patterning

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

#### Figure 7.

optimum parameters for the highest formability of BMG former in its SCLR. According to the data reported in the open literature, the thermos physical properties that are detecting good formability of a BMG former are fragile liquid behavior, large poison ratio and low glass transition. Some TPF based BMG formers the glass transition temperature and the softening in the SCLR permits the use of processing temperature and pressure [18]. The capacity to plastically form metallic glasses in the zone of Tg was recognized by researchers. The enhanced formability of BMGs, which is developed by a wide range of processing methods based on the thermoplastic forming, attracts a wide range of researchers. In all the above-said processes, one thing is common that is the "feedstock". The shape and size of the feedstock are different for powder, over rods, disks, plates and films. These TPF based processing

• Low melting temperature • Cooling and forming are coupled • Low shrinkage • Processing environment can influence

crystallization kinetics

• BMGs contaminate during processing

Compression molding was an adopted form of plastic processing. In this process, the feedstock material is placed in a mold and is given temperature into SCLR and pressure must exceed the flow stress of the BMG to achieve the required strain prior to crystallization setting. Fast cooling is not required for the forming. Figure 7 represents the schematic diagram of the process equipped with examples. Under low applied pressure in the compression molding Pd-Ni-Cu-P alloy can be formed as a gear-shaped structure. It can be noticed that a dense compact part with mechanical properties close to those of bulk material and an outstanding surface finish can be obtained [11, 12]. Figure 7 indicates compression molding of BM6 former, having different feedstock shapes such as pellets, plates and rods. The needed molding pressure not only depends on the formability of the BMG at the processing temperature but also on the shape of the final product. Injection molding is also a TPF based molding process. The only difference between the injection molding and compression molding is that in injection molding, feedstock material is rendered into the mold cavity which has the benefits for the development of the

methods have been described in the below sections.

Advantages Disadvantages

• One step process • High viscosity • Homogeneous microstructure • Internal stresses

• Mechanical properties are already matured in

Advantages and disadvantages of the direct casting process.

the as-cast state

Metallic Glasses

Table 3.

commercial fabrication processes with minimized cycle time.

The development in technologies like micro-electromechanical systems (MEMS), electronics devices, and medical devices have created a rising demand for miniature products and parts. The miniature formation is done by different processes like German method LIGA (lithography, electroplating and molding), UV-LIGA etc. Due to the drawbacks of the LIGA i.e. cost, UV-LIGA is developed giving

the similar products. Both processes can be used as the surface patterning

4.2.2 Miniature fabrication

20

4.2.1 TPF based compression and injection molding

TPF based compression molding with BMG. (A) Schematic diagram of the compression molding with BMGs. (B) Pellets used as feedstock material to compression mold Pt57.5Cu14.7Ni5.3P22.5. (Zr44Ti11Cu10Ni10Be25) formed from a flat plate into a corrugated structure. (D) Zr44Ti11Cu10Ni10Be25 formed from a flat plate to create an embossing mold.

techniques for creating high aspect ratio structures. Due to the homogeneous and isotropic structure of BMGs in atomic scale and their superior properties over conventional materials used for miniature applications and the capability to produce stress-free parts, these methods attracted a lot of attention.

#### 4.2.3 Nano forming

BMGs basically formed by top-down nanofabrication. The combination of different properties like high strength at room temperature, the ability to imprint a nanometer-sized parallel print process, the non-linear softening of BMGs when reaching their glass transition and the ability to repeatedly write and erase facility on the BMG surface recommends a wide range of application. Nanoimprinting on BMG permits to directly write, such as with atomic force microscopy (AFM) tip, as in a scanning probe lithography process. The capability of BMGs for direct nanoimprinting can be applied with a combination of surface smoothening method and used as a rewritable high-density data storage. Several materials have been developed for the mold formation and imprint for nanoforming such as silicon, quartz, and alumina.

#### 4.2.4 Rolling

Rolling of metallic glasses can be categorized into two processes; one is based on liquid processing and the other is on thermoplastic forming. The example of the former is the melt spinning. In the melt spinning, the liquid sample is quenched by


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