**1. Introduction**

### **1.1. What is metallic glass?**

The metallic glass or bulk metallic glass (BMG) is the alloy without crystals, or so-called noncrystalline alloys. In BMGs, the microstructure is so-called amorphous state, referring to the long-range disordered structures between atoms inside a material. The amorphous materials can be produced by non-crystalized cooling from melting state or vapor deposition, mechanical alloying methods, etc. To date, the amorphous materials occupy a large proportion in nature materials, from conventional oxide glass to amorphous semi-conductor, then to amorphous metals or bulk metallic glasses. The amorphous materials have been very important engineering materials to support the modern economy, as well as economic and social developments. Besides the daily-used glassy materials, in high-tech fields, amorphous materials

© 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.

have been also applied a lot to optical communication, laser technology, new solar battery, power transmission materials and so on.

The criteria [7–10] for slow crystallization kinetics, a stabilized supercooled liquid and high glass-forming ability, resulting in the formation of bulk metallic glasses, have been shown to

**1.** Multi-component alloys of increased complexity and size of crystal unit cells such that the energetic advantage of an ordered structure is reduced by increasing the configurational

**2.** Atomic radius mismatch between elements, which leads to higher packing density and smaller free volume, requiring a greater volume increase for crystallization, as well as

**3.** Negative heat of mixing between the elements, which increases the energy barrier at the solid–liquid interface and accelerates atomic diffusivity, thus slowing local atomic rearrangements and crystal nucleation rate, thereby extending the supercooled liquid

**4.** Alloy composition close to deep eutectic, which forms a liquid stable at low temperatures

The research and developments of BMGs indicate that compared with traditional crystalline

**1.** Better mechanical properties such as high yielding strength, large elastic strain limit, mainly perfect elasticity before yielding, mainly perfect plasticity after yielding, no work hardening, high fatigue resistance and high abrasive resistance. With the developments of BMGs, the ultimate strength of metallic materials is renewed again and again. The strength of Mg-based BMGs has increased from 600 to 800 MPa [11]. The strength of Cu-based BMGs is over 2000 MPa [12]. Especially for Co-Fe-Ta-B alloy, the strength is over 5000 MPa

BMGs can be 15,000% [13]. Other BMGs also show super-plasticity to varying degrees, thus according to different application, BMGs can be manufactured into micro- or nano-

**3.** Better corrosion resistance against many kinds of medium. The corrosion resistance of Fe-Cr-Mo-B-P BMGs is 10,000 times higher than conventional stainless steel and can be used

**4.** Good physical properties such as soft and hard magnetism, unique expansive quantity. For example, the saturation magnetization of Fe-based amorphous alloys can be over 1.5 T

talline alloys, better soft or hard magnetism can be obtained, which are considered as ex-

), the elongation of La-Al-Ni

Metallic Glass Matrix Composites

53

http://dx.doi.org/10.5772/intechopen.76526

[15]. When some BMGs are annealed to form nanocrys-

materials, BMGs have an advantage in usability. The main points are as follows:

include:

temperature.

entropy of the supercooled liquid phases.

that can freeze into the glassy state.

[12], which sets up a record in natural world.

level by machining deformation.

in much severed environments [14].

and coercivity is lower than 1 A/m2

cellent substitute for conventional materials.

**2.** Good processability. Near glass transition temperature (Tg

**1.2. Why study metallic glass?**

limiting the solubility of these atoms in crystalline states.

Unlike the conventional oxide glasses, the amorphous alloys or metallic glasses possess metallic bond between atoms instead of covalent bond. Thus, the characteristics related with metals are maintained, such as opacity, good toughness, etc. We can say the amorphous structure is faultless for the lack of dislocations or grain boundaries. We can also say the amorphous structure or random-arranged structure is full of defects because you can find no periodicity in it.

Even though both amorphous alloys and bulk metallic glasses are noncrystalline materials, which are obtained from rapid cooling from liquid state, hindering the crystallization kinetics [1]. The high rate of heat transfer required to prevent crystallization often limits these noncrystalline materials to thin samples or ribbon-shaped samples. These noncrystalline materials are called amorphous alloys. Recently, bulk metallic glasses with slower nucleation kinetics in undercooled liquids have been processed by conventional casting at cooling rate of 10−1–10−2 K/s [2–6]. The critical size can be larger than 1 mm rods; these "bulk" noncrystalline alloys are called bulk metallic glasses. **Figure 1** illustrates the conditions for processing both the more recently developed bulk amorphous alloys as well as traditional metallic glass alloys developed before 1990 [4]. The plot correlating critical cooling rate and maximum sample thickness as a function of the reduced glass transition temperature (Tg /Tm) shows a clear tendency for the glass-forming ability to increase with increasing Tg /Tm, as such these alloys have lower critical cooling rates and larger possible bulk cross-sectional dimensions.

**Figure 1.** Plot correlating critical cooling rate (R<sup>c</sup> ), maximum sample thickness (tmax), and reduced glass transition temperature (Tg /Tm) for bulk metallic alloys illustrating conditions for processing both the more recently developed bulk amorphous alloys as well as ordinary amorphous alloys developed before 1990 [4].

The criteria [7–10] for slow crystallization kinetics, a stabilized supercooled liquid and high glass-forming ability, resulting in the formation of bulk metallic glasses, have been shown to include:

