**4. Potential applications**

**Figure 4.** Using the rapid uniform heating approach with heating rates in the order of 10<sup>6</sup> K/s, the undercooled liquid is accessible at any temperature above the glass transition, through the melting point and beyond, where the liquid enters

the lifetime of the mold. Furthermore, the processing is accomplished in the laminar flow regime; therefore, higher quality and reliable parts could be obtained by comparison with the current mold-casting technique [8, 42]. However, the viscosity of the supercooled liquid MGs is much higher than that of the plastics melt, which poses a challenge for mass

In order to improve the thermoplastic formability of supercooled liquid MGs, micro-backextrusion was proposed by Wu et al. [14], and a three-dimensional cup-shaped object with wall thickness of 0.05 mm was successfully fabricated. To reduce the contact area between MGs and mold materials, rolling was developed by Schroers et al. [43] who not only hot-rolled high-quality MG sheets but also replicated micro-patterns with featured size of 300 nm. The micro-replication of MGs through hot-rolling is actually similar to hotembossing process, wherein the high viscosity and interfacial effect are main reasons limit the processability. Subsequently, Schroers et al. [44, 45] developed blow molding (see **Figure 3**), which allows blowing hollow products by using gas pressure to inflate the thermoplastic MGs enclosed in the mold. The low-forming pressure and high-dimensional accuracy indicates that this net-shaping technology could bring economic and environ-

Recently, an ultra-fast MGs' hot-processing technique was probed by Johnson et al. [47], as illustrated in **Figure 4**. When rapidly and uniformly heating a metallic glass at rates of 10<sup>6</sup>

to temperatures spanning the undercooled liquid region, rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. Owing to the millisecond time window, this method is able to "beat" the intervening crystallization and successfully process even marginal glassforming alloys with very limited stability against crystallization that are not processable by

K/s

the equilibrium state.

production.

14 Metallic Glasses - Properties and Processing

mental benefits.

The above thermoplastic forming techniques endow MGs with superiority in net-shaping precise and versatile structures comprising of macro-/micro-/nano-sized features. Through nanoimprinting, Schroers et al. [5, 8, 9] fabricated metallic glass nanowires with very high aspect ratios (>200); these nanorods not only exhibit enhanced thermal stability [4] but also display superb durability combined with high electrocatalytic activity toward methanol, ethanol oxidation and CO, exhibiting great potential in energy conversion/storage and sensors fields [52]. The superb durability and high-surface area of these MG nano-structures motivate the generation of first functional proton exchange membrane micro fuel cells (MFCs). Such novel MFCs have been identified as a promising alternative power sources for portable electronics [53].

In addition to the potential applications in energy sector, the micro-/nano-gratings hotimprinted on MGs surfaces also exhibit excellent spectroscopic performance [54, 55]. For example, Chu et al. [54] fabricated nano-scale gratings, and Ma et al. [55] hot-embossed micro-scale gratings with fine periodicity on Pd-based MGs surfaces, both surface exhibit beautiful optical properties such as rainbow-like spectrum when shone by fluorescent lamp light, as shown in **Figure 6**. Inoue et al. [56] pointed out that these nano-imprinted MG surfaces exhibit potential applications as anti-reflection materials, electrode materials, hologram technology, next generation ultra-high density of information data storage material and cell culture medium for bio-chips.

By integrating macro-, micro- and nano-scale features in a sequential order, Kumar et al. [13] hot-embossed hierarchical structures and displayed potential applications in optical devices, electrochemical activity and cellular response. Through micro-imprinting, some micro-lens arrays [57], micro-channel geometries [58] have been fabricated, showing potential applications in aspheric lens and fuel cell interconnect plates, respectively. Furthermore, the thermoplastic formed MG components have been used as a master mold (see **Figure 7**) to imprint polymers (such as PMMA) [10, 24, 59, 60], and an integrated PMMA micro-channel part was fabricated, implying that MG is a robust, attractive and viable mold material for thermoplastic imprinting of polymer devices [10]. Bardt et al. [23] thermoplastic formed some complex 3-D micro-topologies and envisaged potential application as high-Q micro-resonators, microwave waveguides, microsurgical tools and devices, connectors for higher frequency operations, micro-scale motors and transmission components, microfluidic arrays, and free-form reflective micro-optics.

The hot-embossed surface micro-components can be used in MEMS, biochips, such as microspring, micro-gear, micro-motor, micro-fan, micro-honeycomb structure, micro-gyroscope and micro-accelerometer structure and micro-turbines; some beautiful surface features such as micro-bats and micro-poetry of Tang Dynasty "Yellow Crane Tower" have also been fabricated by Li et al. [6] through thermoplastic forming. Similar to micro-/nano-scale hot-imprinting, the TPF-based blow molding has also been used to fabricate ultra-smooth and symmetric 3-D metallic glass resonators, which demonstrates precision over 5 orders of magnitude without the use of cleanroom facilities or traditional microfabrication techniques, displaying potential applications in future MEMS vibratory devices, such as accelerometers and gyroscopes, with

**Figure 7.** SEM micrographs of the silicon master (a) with single (c) and continuous bends structure (e); the corresponding

Thermoplastic Forming of Metallic Glasses http://dx.doi.org/10.5772/intechopen.78016 17

The thermoplastic micro-forming technique also exhibits great potential in fabrication superhydrophobic surfaces with long lifespan in service, as demonstrated by Li et al. [62, 63]. Who found that without any modification or post-treatment, superhydrophobic surfaces with good stability could be fabricated by hot-embossing honeycomb patterns on Pd40Cu30Ni10P<sup>20</sup> MG [62]. By constructing micro-/nano-hierarchical structures on Zr35Ti30Be26.75Cu8.25 MG surface, Li et al. [63] not only fabricated superhydrophobic MG surface with water contact angle over 150°, but also found that these surfaces exhibit strong adhesion with water droplets. The combined properties of both superhydrophobicity and strong adhesion toward liquid exhibit promising applications as dry adhesives and transport of liquid micro-droplets, as well as desirable mechanical and corrosion resistance showing potential applications in modern industries [64]. Furthermore, Li et al. revealed that MGs surfaces with hot-embossed textures exhibit low friction coefficient especially under dry contact (see **Figure 8**), which indicates that

reduced energy dissipation mechanisms, increased performance and low costs [61].

hot-embossed metallic glass micro-channel structure (b, d, and f) [10].

the lifetime of the textured surfaces could be optimized by minimizing friction [65].

**Figure 6.** (a) Photographs of polished BMG plate (left) and BMG grating (right) when fluorescent lamp light shines upon them (b) photographs of Si die (left) and BMG grating (right) under the shine of fluorescent lamp light [55].

#### Thermoplastic Forming of Metallic Glasses http://dx.doi.org/10.5772/intechopen.78016 17

CO, exhibiting great potential in energy conversion/storage and sensors fields [52]. The superb durability and high-surface area of these MG nano-structures motivate the generation of first functional proton exchange membrane micro fuel cells (MFCs). Such novel MFCs have been

In addition to the potential applications in energy sector, the micro-/nano-gratings hotimprinted on MGs surfaces also exhibit excellent spectroscopic performance [54, 55]. For example, Chu et al. [54] fabricated nano-scale gratings, and Ma et al. [55] hot-embossed micro-scale gratings with fine periodicity on Pd-based MGs surfaces, both surface exhibit beautiful optical properties such as rainbow-like spectrum when shone by fluorescent lamp light, as shown in **Figure 6**. Inoue et al. [56] pointed out that these nano-imprinted MG surfaces exhibit potential applications as anti-reflection materials, electrode materials, hologram technology, next generation ultra-high density of information data storage material and cell

By integrating macro-, micro- and nano-scale features in a sequential order, Kumar et al. [13] hot-embossed hierarchical structures and displayed potential applications in optical devices, electrochemical activity and cellular response. Through micro-imprinting, some micro-lens arrays [57], micro-channel geometries [58] have been fabricated, showing potential applications in aspheric lens and fuel cell interconnect plates, respectively. Furthermore, the thermoplastic formed MG components have been used as a master mold (see **Figure 7**) to imprint polymers (such as PMMA) [10, 24, 59, 60], and an integrated PMMA micro-channel part was fabricated, implying that MG is a robust, attractive and viable mold material for thermoplastic imprinting of polymer devices [10]. Bardt et al. [23] thermoplastic formed some complex 3-D micro-topologies and envisaged potential application as high-Q micro-resonators, microwave waveguides, microsurgical tools and devices, connectors for higher frequency operations, micro-scale motors and transmission components, microfluidic arrays, and free-form reflec-

The hot-embossed surface micro-components can be used in MEMS, biochips, such as microspring, micro-gear, micro-motor, micro-fan, micro-honeycomb structure, micro-gyroscope and micro-accelerometer structure and micro-turbines; some beautiful surface features such as micro-bats and micro-poetry of Tang Dynasty "Yellow Crane Tower" have also been fabricated

**Figure 6.** (a) Photographs of polished BMG plate (left) and BMG grating (right) when fluorescent lamp light shines upon

them (b) photographs of Si die (left) and BMG grating (right) under the shine of fluorescent lamp light [55].

identified as a promising alternative power sources for portable electronics [53].

culture medium for bio-chips.

16 Metallic Glasses - Properties and Processing

tive micro-optics.

**Figure 7.** SEM micrographs of the silicon master (a) with single (c) and continuous bends structure (e); the corresponding hot-embossed metallic glass micro-channel structure (b, d, and f) [10].

by Li et al. [6] through thermoplastic forming. Similar to micro-/nano-scale hot-imprinting, the TPF-based blow molding has also been used to fabricate ultra-smooth and symmetric 3-D metallic glass resonators, which demonstrates precision over 5 orders of magnitude without the use of cleanroom facilities or traditional microfabrication techniques, displaying potential applications in future MEMS vibratory devices, such as accelerometers and gyroscopes, with reduced energy dissipation mechanisms, increased performance and low costs [61].

The thermoplastic micro-forming technique also exhibits great potential in fabrication superhydrophobic surfaces with long lifespan in service, as demonstrated by Li et al. [62, 63]. Who found that without any modification or post-treatment, superhydrophobic surfaces with good stability could be fabricated by hot-embossing honeycomb patterns on Pd40Cu30Ni10P<sup>20</sup> MG [62]. By constructing micro-/nano-hierarchical structures on Zr35Ti30Be26.75Cu8.25 MG surface, Li et al. [63] not only fabricated superhydrophobic MG surface with water contact angle over 150°, but also found that these surfaces exhibit strong adhesion with water droplets. The combined properties of both superhydrophobicity and strong adhesion toward liquid exhibit promising applications as dry adhesives and transport of liquid micro-droplets, as well as desirable mechanical and corrosion resistance showing potential applications in modern industries [64]. Furthermore, Li et al. revealed that MGs surfaces with hot-embossed textures exhibit low friction coefficient especially under dry contact (see **Figure 8**), which indicates that the lifetime of the textured surfaces could be optimized by minimizing friction [65].

**Conflict of interest**

**Author details**

Wuhan, PR China

\* and Jiang Ma<sup>2</sup>

10.1016/j.msea.2017.03.062

DOI: 10.1016/j.jallcom.2009.12.174

DOI: 10.1002/adma.200902776

Ning Li1

PR China

**References**

nmat2622

ncomms9157

The authors declare that they have no competing financial interests.

1 State Key Laboratory for Materials Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology,

Thermoplastic Forming of Metallic Glasses http://dx.doi.org/10.5772/intechopen.78016 19

2 Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, College of Mechatronics and Control Engineering, Shenzhen University, Shenzhen,

[1] Li N, Liu Z, Wang XY, Zhang M. Vibration-accelerated activation of flow units in a Pd-based bulk metallic glass. Materials Science and Engineering A. 2017;**692**:62-66. DOI:

[2] Guo H, Yan PF, Wang YB, Tan J, Zhang ZF, Sui ML, Ma E. Tensile ductility and necking

[3] Jang D, Greer JR. Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses. Nature Materials. 2010;**9**:215-219. DOI: 10.1038/

[4] Sohn S, Jung Y, Xie Y, Osuji C, Schroers J, Cha JJ. Nanoscale size effects in crystallization of metallic glass nanorods. Nature Communications. 2015;**6**:8167. DOI: 10.1038/

[5] Kumar G, Desai A, Schroers J. Bulk metallic glass: The smaller the better. Advanced

[6] Li N, Chen W, Liu L. Thermoplastic micro-forming of bulk metallic glasses: Areview.

[7] Li N, Chen Q, Liu L. Size dependent plasticity of a Zr-based bulk metallic glass during room temperature compression. Journal of Alloys and Compounds. 2010;**493**:142-147.

[8] Schroers J. Processing of bulk metallic glass. Advanced Materials. 2010;**22**:1566-1597.

Materials. 2011;**23**:461-476. DOI: 10.1002/adma.201002148

Journal of Metals. 2016;**68**:1246-1261. DOI: 10.1007/s11837-016-1844-y

of metallic glass. Nature Materials. 2007;**6**:735-739. DOI: 10.1038/nmat1984

\*Address all correspondence to: hslining@mail.hust.edu.cn

**Figure 8.** The experimental and theoretically calculated coefficients of friction with the contact area fraction [65].
