**2.3. Other kind of ex-situ BMGMCs**

Besides fiber and particle reinforcement, recently, some new kind of secondary phases have been introduced to BMG matrix, such as pores and porous particles [43–46].

Wada et al. introduced pores into Pd-based BMG matrix [44]. The master alloy melts are subjected to four distinct hydrogenation treatments at 853 K in tubes of fused silica: (i) 12 h at 1 MPa, then oil quench, (ii) the same, then water quench; (iii) 3 h at 4 MPa, then oil quench, and (iv) the same, then water quench to form porous rods. The porosities of BMG rods are calculated to be 1.7–3.7% from their density. The pore size is observed to be 20–30 μm, as shown in **Figure 6a**. The stability of the fine uniform pore distribution during casting follows from the high viscosity of the melt. Compressive stress-strain curves are strongly affected by porosity, as shown in **Figure 6b**. The porous alloy with the highest porosity of 3.7% shows the plastic strain over 18%, greatly enhanced compared to the pore-free BMG. But during tension, no plasticity can be observed. The shear band pattern is affected by the pores acting as stress concentrators. The pores are comparable in radius with the notch roots giving enhanced toughness, and are expected to induce extensive shear banding.

**Figure 6.** (a) Pore distributed BMG rods, water quenched from a melt held at 853 K for 3 h under 4 MPa hydrogen; (b) tensile and compressive stress-strain curves of pore reinforced BMGMCs with different porosities.

without interfacial reaction. **Figure 5b** shows stress-strain curves for monolithic BMG and Nb-containing composite with 4 and 8% Nb particles. The BMG fails just after the elastic limit of 2%, but the composites yield at about 900 MPa and exhibit significantly plastic strain as well as work hardening and softening. The overall engineering plastic strain is determined to be about 12.1% for 8% Nb-containing composite. Ductile Nb particles serve as obstacles to impede shear bands propagation. When encountering an Nb particle, the shear band has to be either blocked or bypass around the particle due to the strong bonding between the particles and the matrix. Furthermore, if the sear deformation travels into the ductile Nb particles, the particle can dramatically plastic deform by dislocation mode to absorb the shear strain and prevent the

**Figure 5.** (a) SEM image of the as-cast BMGMC containing 10% Ta; (b) compressive stress-strain curves of the BMGMCs containing 0–20% Ta; (c) SEM image of the composite containing 8% Nb (inset shows the XRD pattern); (d) compressive

stress-strain curves for single phase BMG and Nb-containing composite (4 and 8%).

catastrophic failure from taking place by the free propagation of unstable shear bands.

been introduced to BMG matrix, such as pores and porous particles [43–46].

toughness, and are expected to induce extensive shear banding.

Besides fiber and particle reinforcement, recently, some new kind of secondary phases have

Wada et al. introduced pores into Pd-based BMG matrix [44]. The master alloy melts are subjected to four distinct hydrogenation treatments at 853 K in tubes of fused silica: (i) 12 h at 1 MPa, then oil quench, (ii) the same, then water quench; (iii) 3 h at 4 MPa, then oil quench, and (iv) the same, then water quench to form porous rods. The porosities of BMG rods are calculated to be 1.7–3.7% from their density. The pore size is observed to be 20–30 μm, as shown in **Figure 6a**. The stability of the fine uniform pore distribution during casting follows from the high viscosity of the melt. Compressive stress-strain curves are strongly affected by porosity, as shown in **Figure 6b**. The porous alloy with the highest porosity of 3.7% shows the plastic strain over 18%, greatly enhanced compared to the pore-free BMG. But during tension, no plasticity can be observed. The shear band pattern is affected by the pores acting as stress concentrators. The pores are comparable in radius with the notch roots giving enhanced

**2.3. Other kind of ex-situ BMGMCs**

58 Metallic Glasses - Properties and Processing

Jang et al. have introduced porous Mo particles into Mg-Cu-Gd-Ag BMG matrix [45]. During master alloy melting, high purity porous Mo particles with a spherical shape and a size of 20–70 μm are added in the matrix alloy. The volume fraction of porous Mo particles ranges from 10 to 25%. For the introduced porous particles, the overall microstructure of the composite is separated into larger-scale compartment, ~50 μm between Mo particles, and the fine-scale compartment, 1–5 μm within one porous Mo particle, as shown in **Figure 7a** and **b**. The composites containing 20–25 vol% porous Mo particles exhibit superior mechanical performance with ultimate compression stress up to 0.95 and 1.1 GPa and plastic strain up to 10%, as shown in **Figure 7c**. Unlike solid particles, the crack propagation in the present composite is arrested by the porous Mo particles. For the porous nature of the reinforcing phase, the numbers of particles are calculated to be 1.4 times higher than those with solid ones, i.e., the mean interspacing of the porous Mo particles is less than that of the solid Mo ones. This should favor to the confining the shear-banding behavior, thus enable more halting propagation of the shear bands. The porous Mo particles separates and restricts the highly localized shear banding into many isolated small regions, and can confine lots of microsized compartments of the matrix within porous particles, which results in the formation of multiple shear bands within or around the porous particles, promoting the deformation to distribute more uniformly across the sample.

Guo et al. have used an original method, so-called top-down process, to fabricate porous NiTi shape memory alloy (SMA) powders [46]. In this process, the multiphase precursor powder of Ni-Ti-Gd is firstly produced with B2-NiTi and Ni-Gd phase, then by leaching in nitric acid solution to remove Ni-Gd phase and leaving pore in the powder, as shown in **Figure 8a**. The size and interspacing between NiTi within one porous particle are as small as 200 nm. The porous NiTi powders are subsequently added to Mg-Cu-Gd-Ag glass former liquid to fabricate the BMGMC, as shown in **Figure 8b**, with volume fraction ranged from 5 to 20%. The porous particles are homogeneously distributed in the glassy matrix. The composites containing 20 vol% porous NiTi particles exhibits the best mechanical properties, including a true plastic strain of up to 10.6% and a fracture stress of up to 1173 MPa, as shown in **Figure 8c**. Similar to porous Mo, many microcracks are confined in the inter-particle regions and should result from local shear banding within the amorphous matrix. Furthermore, compared with solid particles, the porous particles can generate

structures are characterized by primary dendrite axes with lengths of 50–150 μm and radius of about 1.5–2 μm. Regular patterns of secondary dendrite arms with spacing of 6–7 μm can be observed from SEM images. This composite shows about 5% plastic strain under three point bending. **Figure 9b** shows that the shear bands propagate preferentially through many successive dendrite arms, occasionally initiate or terminate within the arms, and clearly propagate as localized bands through the β-phase arms. This composite also shows good plasticity during compression, as shown in **Figure 9c**. It yields at 1.3 GPa when the β phase yields and deforms, and shear band patterns develop, as the glassy matrix is locally loaded beyond its critical shear stress. The plastic strain is over 6%. The composite even shows about 5% plastic strain during tension. Clear necking and deformation can be observed. The dendritic microstructure of the β phase acts to seed the initiation of organized shear band patterns, confines the propagation of individual shear bands to domains having a spatial scale of the order of the primary dendritic axes length, and lead to shear band spacing which is related to the dendrite arm spacing.

Metallic Glass Matrix Composites

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

Another important in-situ secondary phase in Zr-based BMGMCs is refractory metal phase. Fan et al. have introduced in-situ Ta-rich precipitates in Zr-Cu-Al-Ni-Ta BMGMCs [48]. For the high melting temperature of Ta, there are two steps arc-melting during master alloy preparation. Firstly, Zr-Ta ingot is fabricated which forms solid solutions of Zr-Ta, and remaining elements are subsequently mixed with Zr-Ta ingots. The microstructure of the as-cast composite samples consists of both glassy matrix and Ta-rich particles with an average size of 10–30 μm, as shown in **Figure 10a**. The particles are oblong in shape and do not appear to possess a dendritic structure, they distribute homogeneously among the matrix and the volume faction is about 4%. This composite yields at 1.7 GPa and exhibits apparent work hardening and significant plastic strain, as shown in **Figure 10b**. The elastic incompatibility between the particles and the matrix introduces stress concentrations which may promote shear band initiation. The particles may also impede shear band propagation. Guo et al. have applied dealloying in metallic melt method to further optimize the microstructure and mechanical properties of Ta-rich phase reinforced BMGMCs [49]. The dealloying in metallic melt phenomenon occurs when immerging Zr-Ta solid solution precursor in Cu-Al-Ni melt. For the negative enthalpy of mixing between Zr and Cu-Al-Ni and positive enthalpy of mixing between Ta and Cu-Al-Ni, Zr is gradually selectively leached from precursor ingot to

**Figure 9.** (a) SEM image of in-situ β-Zr reinforced BMGMCs (inset: XRD patterns); (b) shear band patterns array from

compressive failure region of bend test sample; (c) compressive stress strain curve for the composite.

**Figure 7.** (a) SEM observation of the porous Mo particles in the BMG matrix, with the inserted XRD pattern; (b) an enlarged image of a single porous Mo particle; (c) representative room-temperature compressive engineering stressstrain curves for the BMGMCs.

**Figure 8.** (a) Porous NiTi powder by top-down process; (b) SEM images of BMGMCs containing 20 vol% porous particles; (c) compressive true stress-strain curves for monolithic base BMG and BMGMCs with various volume fraction of porous NiTi addition.

more interfaces, which makes the yield strength follows the load-bearing mode even with low volume fraction of particles. This composite also shows very obvious work hardening behavior, which is considered to originate from the stress-induced martensitic transformation of B2-NiTi phase, which is very attractive and different from those conventional metal or ceramic particles.
