**1. Introduction**

Unlike crystalline metals where dislocations or grain boundaries carry the plastic deformation, metallic glasses (MGs) usually deform inhomogeneous plastic deformation at ambient temperature caused by high localization of shear stress, resulting in fail catastrophe with zero tensile plasticity [1], which severely constraints their structural applications in macro-scale. This challenge tends to be mediated by reducing the sample size or feature below a critical length scale (<1 mm), wherein large tensile-plasticity and enhanced strength could be observed [2, 3],

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

exhibiting size-dependent deformation behavior. Furthermore, MGs also illustrate sizedependent crystallization kinetics at nano-scale, such as the crystallization temperature rapidly increases with reduction in the diameter of nanorods, disclosing the enhanced thermal stability [4]. Consequently, the potential applications of MGs in micro- and nano-fields such as micro and nano-electro mechanical systems (MEMS/NEMS) have attracted enduring attentions [5]. However, the poor manufacturing ability origins from the high strength and ambient-temperature brittleness has been the Achilles' heel to structural applications of MGs [6, 7]. In the past decade, efforts have been devoted to fabricate MGs components with precise and versatile geometries, though the main techniques mainly focus on mold casting [8], thermoplastic forming [5, 8–26] and additive manufacturing [27–29]. By comparing with mold casting and additive manufacturing, the superiorities of thermoplastic forming is worth noting, for example, (1) the existence of supercooled liquid regime (SCLR) between the glass-transition temperature (Tg) and the crystallization temperature (Tx) allows thermoplastic forming (TPF) of MGs under low-forming strength [6], which breaks through the limitations of poor processability of MGs at ambient temperature; (2) net-shaping of precise and versatile geometries with minimum size of atom-scale could be realized, that were previously unachievable with any conventional crystalline metals; (3) the absence of phase transition of MGs during solidification endows them small solidification shrinkage (1/20 of typical casting alloys) [30], which is beneficial to the net-shaping with high precision and (4)as mentioned earlier, MGs maintain more excellent mechanical properties than crystalline metals.

carried out by Saotome et al. [35, 36], who regarded that the micro-nano formability of super-

the supercooled liquid MGs exhibit superior formability on micro-nanometer scales. It is easy to find that the alloys used in thermoplastic forming are generally with wide temperature

indicates that the MGs have opportunities to obtain low viscosity, long forming time,

important indicators of the formability [31, 37], similar to the normalized parameter (*S*) [30] that should reflect better the formability of a MG, particularly when comparing different MG

*S* = Δ *Tx* /(*Tl* − *Tg*) (2)

was proposed to measure the formability of supercooled liquid MG [37]. As the Angell plots of conventional MGs and high entropy MG as shown in **Figure 1**, wherein the temperature dependent viscosity among alloys exhibits various steepness index, that is, fragility parameter (*m*). A large steepness index corresponds to fragile liquid behavior, such as Pt57.5Cu14.7Ni5.3P22.5 MG shows the largest value of fragility, exhibits fragile liquids with the best micro-formability, is ideal candidate for near-net shape processing with fine printability. While a small index corresponds to strong liquid and exhibits poor formability, such as the thermoplastic forming of TiZrHfNiCuBe, high entropy MG becomes arduous with reducing mold size to tens micrometer, owing to the strong supercooled TiZrHfNiCuBe high entropy MG with small value of *m* [39]. Similar results are also observed by Schroers [31], who proposed a simple and precise standard to characterize the formability of BMGs, and the maximum diameter (*d*) of

 , Δ *Tx*

formability of MGs focus on amorphous alloys with various compositions. As for MG with certain composition, it is well understood that low viscosity is crucial to improve the thermoplastic formability of supercooled liquid MG. The viscosity of MGs not only depends on the temperature but is also sensitive to the strain rate. For example, with increasing strain rates under a certain temperature in the supercooled liquid region, there is a remarkable decrease in the viscosity, accompanied by the transitioning from Newtonian to non-Newtonian behavior [40]. In this case, thermoplastic forming becomes increasingly difficult, rather than enhancement [41]. In order to probe the physical origin of this phenomenon, Li et al. [34] established a

is the melting temperature. While for MGs with different compositions, fragility

), expressed as,

11

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

/*Ag* (1)

is the area of the V-groove. They found that

, to reduce the risk of crystallization. Large

, *m* and *d*) in evaluating the thermoplastic

is also regarded as one of the

(3)

cooled liquid MGs could be quantified by the percentage of flowed area (*Rf*

*Rf* = *Af*

ranges of supercooled liquid region, <sup>Δ</sup> *Tx* <sup>=</sup> *Tx* <sup>−</sup> *Tg*

is the flowed area into the V-groove, *Ag*

and enhance the thermoplastic formability. Accordingly, Δ *Tx*

*<sup>m</sup>* <sup>=</sup> <sup>∂</sup> log*η*/∂(*Tg* /*T*)|*<sup>T</sup>*=*Tg*

the hot-formed disc was taken as a measure of the MG's formability.

It is essential that all these parameters (such as *Rf*

where, *Af*

alloy families,

in which *Tl*

parameter (*m*) [38],

Δ *Tx*

In investigating the thermoplastic micro-forming of MGs, formability, namely the filling ability of supercooled liquid MGs in the mold, has been proposed to the MGs processability in the supercooled liquid region [31]. For MGs with various alloy compositions, previous literatures have reported that the thermoplastic formability was related to fragility of the supercooled liquid MGs and the width of supercooled liquid region. While for an MG with certain composition, the low viscosity and the long processing time are always appreciated [8, 32], in which the viscosity of supercooled liquid MGs is determined by processing parameters such as temperature, stress and strain rate [33]. The forming parameters actually affect the materials flow characteristics (i.e. Newtonian and non-Newtonian flow) [34]; therefore, the fundamental understanding the correlation between materials flow characteristics and thermoplastic formability is attractive with great significance. To improve the thermoplastic formability of supercooled liquid MGs, various forming techniques have been developed; these novel methods could also hot-process MGs components with macro size. It is worth noting that the potential applications of these thermoplastic formed parts especially the microcomponents/patterns have been probed, which would broaden the real application of MGs. On the basis of the above descriptions, this chapter reviews the related aspects and provides in-depth understanding of the fundamental issues.
