**1.4. How to overcome the problem?**

For the better physical, chemical, mechanical properties and precision shaping abilities of BMGs than conventional materials, BMGs have shown important application value in aerospace device, precision machine, information technology and so on. The researches of BMGs

Because of the metallic bonding in amorphous alloys, strain can be accommodated at the atomic level through changes in neighborhood; atomic bonds can be broken and reformed at the atomic scale. However, unlike crystalline metals and alloys, metallic glasses do not exhibit long-range translational symmetry. Thus, the deformation mechanisms such as dislocations, which allow changes in atomic neighborhood at low energies or stresses, do not exist in metallic glasses. The local rearrangement of atoms in metallic glasses is a relatively highenergy or high-stress process. The exact nature of local atomic motion in deforming metallic glasses is not fully understood, although there is general consensus that the fundamental unit must be a local rearrangement of atoms accommodating the shear strain. An example of such a local rearrangement is depicted in the two-dimensional schematic of **Figure 2a**, originally proposed by Argon and Kuo [16] on the basis of an atomic-analog bubble-raft model, called a "shear transformation zone" (STZ) [17–20]. The STZ is essentially a local cluster of atoms that undergoes an inelastic shear distortion from one relatively low energy configuration to a second such configuration, crossing an activated configuration of higher energy and volume. The STZs are common to deformation of all amorphous metals, although details of the structure, size and energy scales of STZs may vary from one glass to the other. In a metallic glass body experiencing uniform stress, the STZ that is activated first is selected from among many potential sites on the basis of energetics, which vary with the local atomic arrangements [21–23]. The continued propagation of the applied shear strain occurs when one STZ creates a localized distortion of the surrounding material, which triggers the formation of large planar bands of STZs along the maximum shear stress plane, or so-called "shear bands", as shown in **Figure 2b**. For most BMGs, the deformation occurs in homogeneous through plastic strains

**Figure 2.** Schematic illustrating of (a) "shear transformation zone" in which strain accommodation occurs through localized cluster of atoms undergoing intense distortion and (b) shear band formation along maximum shear stress

plane.

have attracted a lot of attentions from physical, chemical and material scientists.

**1.3. Room-temperature brittleness of metallic glass**

54 Metallic Glasses - Properties and Processing

To date, there are several ways that have been developed to improve the room-temperature plasticity of BMGs, including intruding a secondary phase to develop a composite microstructure, surface coating, composition adjustment to induce intrinsic heterogeneity, severe plastic deformation such as shot peening and high pressure torsion [24–27]. Although introducing a secondary phase to make a composite structure seems to be most primal methods, it is reported that composite structure is one of the most efficient method and very easy to realize. Furthermore, there are many combinations of amorphous matrix and reinforcing phases, giving an infinite possibility to improve the mechanical properties of BMGs.

For now, there are mainly two ways to introduce the secondary phases, ex-situ direct adding and in-situ precipitation. For ex-situ method, the various combinations of secondary particle or fiber and the amorphous matrix makes the fabrication process easier to design. But the interface bonding between the secondary phase and the matrix is not strong because of the formation of surface oxide layers, which degrades the mechanical properties of them. For in-situ method, even though the interface bonding is stronger than those ex-situ composites because the secondary phases are intrinsically formed in the melt during cooling, but the fabrication process is very difficult to design. Furthermore, for ex-situ method, the size and volume fraction of secondary phases can be easily controlled by using various sized particles or fibers with various amounts when adding. However, for in-situ method, it is difficult to optimize the microstructures because the optimization process is related with the composition adjustment. Hereafter, we will introduce the researches on both ex-situ and in-situ BMGMCs, including their composite structure and mechanical properties.
