**3. Thermoplastic forming techniques**

Thermoplastic forming map clarifies the relationship between flow features and formability and provides the selection of processing parameters. However, the Newtonian flow usually locates at regions with high processing temperature and low strain rate, which would induce the crystallization of amorphous alloys. In addition, the interfacial effect between amorphous alloys and mold materials becomes prominent during micro- and nano-scale forming, which seriously hinders the forming of metallic glasses [5, 9]. In order to improve the formability of supercooled MGs, various forming techniques have been developed.

By comparison with the hot-embossing technique as mentioned earlier, injection molding [42] as a net-shaping method for MGs exhibits superiorities in development of commercial manufacturing processes with minimized production cycle and high-volume production. Wherein the feedstock melt is gathered and forced into the part forming mold cavity at high pressure and velocity. As a potential forming process for MG parts, the injection molding is conducted at temperatures much lower than direct casting, which can improve

**Figure 3.** These shapes were previously unachievable with any other metal processing method that can be fabricated by blow molding [46].

**Figure 2.** Thermoplastic forming map that reveals the relationship between the formability and the flow characteristics

thermoplastic forming map (see **Figure 2**), which reveals an inherent relationship between the thermoplastic formability and the flow characteristics, namely, Newtonian flow facilitates the forming capability, while the thermoplastic forming in a non-Newtonian flow regime tends to be difficult. Li et al. believe that this scenario is caused by the spatio-temporally homoge-

neous/inhomogeneous flow of MGs in Newtonian/non-Newtonian flow regime.

**Figure 1.** Angell plots of conventional MGs and high entropy MG [39].

12 Metallic Glasses - Properties and Processing

[34].

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 production.

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 environmental benefits.

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> K/s 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

conventional heating. Take advantage of unique rheological property along with the classic Lorentz force concept, electromagnetic coupling of electric current and a magnetic field was then thermoplastically shape a metallic glass without conventional heating sources or

**Figure 5.** The displacement-temperature (time) curves of Zr35Ti30Be26.75Cu8.25 MG after vibrational tension under various loading frequencies ranging from 0.05 to 10 Hz (temperature rises from 23 to 365°C with scanning rate of 5°C/min−1) [50].

Thermoplastic Forming of Metallic Glasses http://dx.doi.org/10.5772/intechopen.78016 15

Based on improvements of formability made in the traditional metal formed by employing ultrasonic vibration [49], and considering that the viscosity is closely related to the dynamic relaxation of the alloy system, namely the shortening of the relaxation time, reduced viscosity is caused. Li et al. [50] introduced vibrational loading in thermoplastic forming of MGs; the intriguing finding was that the formability of supercooled liquid MGs is facilitated by vibrational loading (**Figure 5**). This technique exhibits potential applications in micro-/nano-scale forming of MGs. By increasing loading frequency to about 20 KHz, Ma et al. [51] used high frequency ultrasonic beating method to fabricate micro- to macro-scale structures, avoiding

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

applied mechanical forces [48].

crystallization and oxidation of MGs.

**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 equilibrium state.

**Figure 5.** The displacement-temperature (time) curves of Zr35Ti30Be26.75Cu8.25 MG after vibrational tension under various loading frequencies ranging from 0.05 to 10 Hz (temperature rises from 23 to 365°C with scanning rate of 5°C/min−1) [50].

conventional heating. Take advantage of unique rheological property along with the classic Lorentz force concept, electromagnetic coupling of electric current and a magnetic field was then thermoplastically shape a metallic glass without conventional heating sources or applied mechanical forces [48].

Based on improvements of formability made in the traditional metal formed by employing ultrasonic vibration [49], and considering that the viscosity is closely related to the dynamic relaxation of the alloy system, namely the shortening of the relaxation time, reduced viscosity is caused. Li et al. [50] introduced vibrational loading in thermoplastic forming of MGs; the intriguing finding was that the formability of supercooled liquid MGs is facilitated by vibrational loading (**Figure 5**). This technique exhibits potential applications in micro-/nano-scale forming of MGs. By increasing loading frequency to about 20 KHz, Ma et al. [51] used high frequency ultrasonic beating method to fabricate micro- to macro-scale structures, avoiding crystallization and oxidation of MGs.
