**5.2 Net-shape (micro-)forming in supercooled liquid region**

Although amorphous alloys have exhibited unique properties compared the conventional polycrystalline materials, the metastable nature of amorphous phase has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. In general, amorphous alloys are super-strong with compressive yield strengths of approximately 2 GPa and even up to 5 GPa for some exotic bulk glass-forming alloys, as has already shown in Fig. 1. However, amorphous alloys suffer from a strong tendency toward shear localization upon yielding, which results in macroscopically brittle failure at ambient temperatures. Therefore, processing of amorphous alloys at ambient temperatures is extremely hard. When an amorphous solid is continuously heated in the supercooled liquid region the viscosity decreases dramatically as the alloy relaxes into the metastable equilibrium state of the supercooled liquid and the large viscous flowability is obtained (Bakke et al., 1995; Volkert

Crystallization Behavior and Control of Amorphous Alloys 205

of the gear agree with the inner size of the die within a scattering of ±1% (Nishiyama & Inoue, 1999). The utilization of viscous flow of supercooled liquid is useful for secondary working of the bulk amorphous alloys which can produce a final product with near-net shape. In addition, in the supercooled liquid region, successful joining of the Pd40Ni40P20 bulk amorphous components has been achieved by the friction-welding method utilizing

Recently, Schroers and his co-workers (Kumar et al., 2009; Schroers et al., 2007; Schroers, 2008; 2010; Schroers et al., 2011) have used a developed novel thermoplastic forming (TPF) based processing to fabricate complex amorphous components. The process of TPF is also known as hot forming, hot pressing, super plastic forming, viscous flow working, and viscous flow forming. TPF takes advantage of the drastic softening of amorphous alloys upon heating above glass transition temperature and its thermal stability of supercooled liquid, which is quantified by the width of the supercooled liquid region. During TPF, the amorphous

Fig. 11. Through TPF-based blow molding Blow molding with bulk metallic glasses (BMGs) permits hollow, thin, seamless shapes, which can include undercuts. These shapes were previously unachievable with any other metal processing method (A–C). The surface can be patterned, e.g., to reveal a hologram (D), joints can be created such as threads (F,H), and a second material can be joined to the BMG (E) in the same processing step than the blow molding. Reprinted from (Schroers, 2010), with permission from John Wiley and Sons.

the viscous flowability of the supercooled liquid (Kawamura & Ohno, 2001).

& Spaepen, 1989). Larger values of Δ*Tx* (*Tx* – *T*g) indicate higher metastability of the liquid with respect to crystallization. The considerable softening of an amorphous alloy (viscous flowability) in its supercooled liquid region can be used for net-shape micro-forming of bulk amorphous alloy components and creation of bulk amorphous alloy samples via powder processing of amorphous powder precursors (see S*ection 5.3*). In order to maintain their unique properties, processing of an amorphous alloy requires special attention. The main challenge is to avoid crystallization during the processing of amorphous alloy. By utilizing the low viscosity and large viscous flowability, bulk amorphous alloys could be deformed (Inoue & Takeuchi, 2002; Nishiyama & Inoue, 1999) to various complicated shapes in the maintenance of good mechanical properties. For example, from a bulk amorphous Pd40Cu30Ni10P20 alloy rod in 6 mm diameter, the die-forging into a three-stage die with pitch circle diameters of 4, 5 and 6 mm and a module of 0.3 was made for 120 s at 610 K under a compressive stress of 10 kPa and a three-stage gear was formed. The shape and dimension

Fig. 10. Summary of the microstructure and mechanical properties of aluminum based alloys produced from amorphous alloy precursors. Reprinted from (Inoue & Kimura, 2000), with permission from Elsevier.

& Spaepen, 1989). Larger values of Δ*Tx* (*Tx* – *T*g) indicate higher metastability of the liquid with respect to crystallization. The considerable softening of an amorphous alloy (viscous flowability) in its supercooled liquid region can be used for net-shape micro-forming of bulk amorphous alloy components and creation of bulk amorphous alloy samples via powder processing of amorphous powder precursors (see S*ection 5.3*). In order to maintain their unique properties, processing of an amorphous alloy requires special attention. The main challenge is to avoid crystallization during the processing of amorphous alloy. By utilizing the low viscosity and large viscous flowability, bulk amorphous alloys could be deformed (Inoue & Takeuchi, 2002; Nishiyama & Inoue, 1999) to various complicated shapes in the maintenance of good mechanical properties. For example, from a bulk amorphous Pd40Cu30Ni10P20 alloy rod in 6 mm diameter, the die-forging into a three-stage die with pitch circle diameters of 4, 5 and 6 mm and a module of 0.3 was made for 120 s at 610 K under a compressive stress of 10 kPa and a three-stage gear was formed. The shape and dimension

Fig. 10. Summary of the microstructure and mechanical properties of aluminum based alloys produced from amorphous alloy precursors. Reprinted from (Inoue & Kimura, 2000),

with permission from Elsevier.

of the gear agree with the inner size of the die within a scattering of ±1% (Nishiyama & Inoue, 1999). The utilization of viscous flow of supercooled liquid is useful for secondary working of the bulk amorphous alloys which can produce a final product with near-net shape. In addition, in the supercooled liquid region, successful joining of the Pd40Ni40P20 bulk amorphous components has been achieved by the friction-welding method utilizing the viscous flowability of the supercooled liquid (Kawamura & Ohno, 2001).

Recently, Schroers and his co-workers (Kumar et al., 2009; Schroers et al., 2007; Schroers, 2008; 2010; Schroers et al., 2011) have used a developed novel thermoplastic forming (TPF) based processing to fabricate complex amorphous components. The process of TPF is also known as hot forming, hot pressing, super plastic forming, viscous flow working, and viscous flow forming. TPF takes advantage of the drastic softening of amorphous alloys upon heating above glass transition temperature and its thermal stability of supercooled liquid, which is quantified by the width of the supercooled liquid region. During TPF, the amorphous

Fig. 11. Through TPF-based blow molding Blow molding with bulk metallic glasses (BMGs) permits hollow, thin, seamless shapes, which can include undercuts. These shapes were previously unachievable with any other metal processing method (A–C). The surface can be patterned, e.g., to reveal a hologram (D), joints can be created such as threads (F,H), and a second material can be joined to the BMG (E) in the same processing step than the blow molding. Reprinted from (Schroers, 2010), with permission from John Wiley and Sons.

Crystallization Behavior and Control of Amorphous Alloys 207

with wide supercooled liquid region has been achieved by warm extrusion, spark plasma sintering and equal channel angular pressing (ECAP) (Choi et al., 2007; Ishihará et al., 2002; Itoi et al., 2001; Karaman et al., 2004; Kawamura et al., 1997; Kim et al., 2004; Kim et al., 2009; Lee et al., 2003; Mear et al., 2009; Robertson et al., 2003; Senkov et al., 2004; Senkov et al., 2005; Sordelet et al., 2002; Zhang et al., 2006b; Zhang, et al., 2007a). The consolidated samples show almost the same thermal properties, mechanical properties, and/or soft magnetic properties as those of the BMGs prepared by direct melt casting from molted

Fig. 12. (a) A representative DSC curve to determine the holding time (*τ*) up to the initial crystallization, and (b) TTT diagram for the onset of crystallization of the amorphous Ti50Cu18Ni22Al4Sn6 powders heated to set temperatures at heating rate of 0.33 or 0.67 K s-1. The data of the onset temperature of crystallization (*T*x) and the glass transition temperature (*T*g) at the heating rate of 0.67 K s-1 are also shown. Reprinted from (Zhang, et al., 2006b),

with permission from Elsevier.

solid is reheated into the supercooled liquid region, where it relaxes into a supercooled and metastable liquid before it eventually crystallizes. For a variety of BMG formers, a large processing window exists, which permits access to temperatures in this region on a practical experimental time scale in order to avoid crystallization. In general, at low temperatures a long processing time is available accompanied by a high viscosity. In contrast, at high temperatures, the viscosity is significantly reduced but, at the same time, the processing time is shortened. Currently, for a wide range of alloys, viscosities of 106 Pa s and lower can be accessed in the supercooled liquid region on a practical time scale (Schroers, 2010). For the highest formability of the BMG former in supercooled liquid region, optimum processing such as low viscosity and long processing time are required. The formability is a material property that reflects the maximum strain a metastable material can undergo before crystallization under given geometry and processing parameters.

As a novel technique with integration of shaping, joining, and finishing into one processing step, TPF-based blow molding allows one to net shape complex geometries in an economical and precise manner, including shapes, which can not be produced with any other metal processing method. In particular when pre-shaped parisons are used, BMGs can be blow molded into shapes that were previously not achievable with any metal processing method. Examples of such shapes are given in Fig. 11. They include hollow seamless shapes, which can comprise of complex undercuts, and very large thin sections. Due to the low forming pressure, together with the ability to replicate smallest features, as shown in Fig. 11D, the dimensional accuracy that can be achieved with this process is even superior over other TPF-based processes. In addition, this method is capable to combine the three processing steps typically required for metal processing—shaping, joining, and finishing—into one step (Schroers, 2010). For example potential joints such as threads, as shown in Fig. 11E–H can be formed in the BMG during the expansion process. Surface finishes that can be achieved with blow molding of BMGs include mirror finish. The superior properties of BMGs relative to plastics and typical structural metals, combined with the ease, economy, and precision of blow molding, have the potential to impact society in a manner similar to the development of synthetic plastics and their associated processing methods.

#### **5.3 Bulk amorphous alloy consolidated from amorphous powder precursor**

Synthesis of three-dimensional bulk amorphous materials has been an attractive object for several decades, not only for its significance in basic studies of the intrinsic properties of bulk amorphous materials (instead of the form of powders, fibers, or ribbons), but also for technological applications of these advanced materials with many novel properties. In principle, there are two approaches to obtain bulk amorphous samples. The first one is direct casting of alloy melts into bulk form in amorphous state (Suryanarayana & Inoue, 2011). An alternative approach that can potentially lead to bulk amorphous alloys is to exploit the viscous flow resulting from the significant decrease of the viscosity in supercooled liquid region. This is an especially attractive route to bulk amorphous alloys, especially to obtain bulk samples for the alloy systems with insufficient or limited glassforming ability. A number of amorphous alloys with a sizable supercooled liquid region have been reported (Inoue, 2000; Johnson, 1999). This opens up the possibility of preparing truly bulk samples through powder consolidation in supercooled liquid region. In the Zr-, Cu-, Fe- and Ni-based alloy systems, some successful consolidation of amorphous powders

solid is reheated into the supercooled liquid region, where it relaxes into a supercooled and metastable liquid before it eventually crystallizes. For a variety of BMG formers, a large processing window exists, which permits access to temperatures in this region on a practical experimental time scale in order to avoid crystallization. In general, at low temperatures a long processing time is available accompanied by a high viscosity. In contrast, at high temperatures, the viscosity is significantly reduced but, at the same time, the processing time is shortened. Currently, for a wide range of alloys, viscosities of 106 Pa s and lower can be accessed in the supercooled liquid region on a practical time scale (Schroers, 2010). For the highest formability of the BMG former in supercooled liquid region, optimum processing such as low viscosity and long processing time are required. The formability is a material property that reflects the maximum strain a metastable material can undergo before

As a novel technique with integration of shaping, joining, and finishing into one processing step, TPF-based blow molding allows one to net shape complex geometries in an economical and precise manner, including shapes, which can not be produced with any other metal processing method. In particular when pre-shaped parisons are used, BMGs can be blow molded into shapes that were previously not achievable with any metal processing method. Examples of such shapes are given in Fig. 11. They include hollow seamless shapes, which can comprise of complex undercuts, and very large thin sections. Due to the low forming pressure, together with the ability to replicate smallest features, as shown in Fig. 11D, the dimensional accuracy that can be achieved with this process is even superior over other TPF-based processes. In addition, this method is capable to combine the three processing steps typically required for metal processing—shaping, joining, and finishing—into one step (Schroers, 2010). For example potential joints such as threads, as shown in Fig. 11E–H can be formed in the BMG during the expansion process. Surface finishes that can be achieved with blow molding of BMGs include mirror finish. The superior properties of BMGs relative to plastics and typical structural metals, combined with the ease, economy, and precision of blow molding, have the potential to impact society in a manner similar to the development

crystallization under given geometry and processing parameters.

of synthetic plastics and their associated processing methods.

**5.3 Bulk amorphous alloy consolidated from amorphous powder precursor** 

Synthesis of three-dimensional bulk amorphous materials has been an attractive object for several decades, not only for its significance in basic studies of the intrinsic properties of bulk amorphous materials (instead of the form of powders, fibers, or ribbons), but also for technological applications of these advanced materials with many novel properties. In principle, there are two approaches to obtain bulk amorphous samples. The first one is direct casting of alloy melts into bulk form in amorphous state (Suryanarayana & Inoue, 2011). An alternative approach that can potentially lead to bulk amorphous alloys is to exploit the viscous flow resulting from the significant decrease of the viscosity in supercooled liquid region. This is an especially attractive route to bulk amorphous alloys, especially to obtain bulk samples for the alloy systems with insufficient or limited glassforming ability. A number of amorphous alloys with a sizable supercooled liquid region have been reported (Inoue, 2000; Johnson, 1999). This opens up the possibility of preparing truly bulk samples through powder consolidation in supercooled liquid region. In the Zr-, Cu-, Fe- and Ni-based alloy systems, some successful consolidation of amorphous powders with wide supercooled liquid region has been achieved by warm extrusion, spark plasma sintering and equal channel angular pressing (ECAP) (Choi et al., 2007; Ishihará et al., 2002; Itoi et al., 2001; Karaman et al., 2004; Kawamura et al., 1997; Kim et al., 2004; Kim et al., 2009; Lee et al., 2003; Mear et al., 2009; Robertson et al., 2003; Senkov et al., 2004; Senkov et al., 2005; Sordelet et al., 2002; Zhang et al., 2006b; Zhang, et al., 2007a). The consolidated samples show almost the same thermal properties, mechanical properties, and/or soft magnetic properties as those of the BMGs prepared by direct melt casting from molted

Fig. 12. (a) A representative DSC curve to determine the holding time (*τ*) up to the initial crystallization, and (b) TTT diagram for the onset of crystallization of the amorphous Ti50Cu18Ni22Al4Sn6 powders heated to set temperatures at heating rate of 0.33 or 0.67 K s-1. The data of the onset temperature of crystallization (*T*x) and the glass transition temperature (*T*g) at the heating rate of 0.67 K s-1 are also shown. Reprinted from (Zhang, et al., 2006b), with permission from Elsevier.

Crystallization Behavior and Control of Amorphous Alloys 209

importance in understanding glass formation and glass phenomena. Due to the nature of metastability, amorphous phase tends to crystalize to more stable crystalline state through *polymorphous*, *eutectic* and/or *primary* crystallization mechanisms. The crystallization mechanisms and crystallization products are influenced by both inherent (e.g. chemical composition of amorphous phase, oxygen) and extraneous (e.g. preparation method, pressure, etc.) factors. The study of kinetic behavior associated with a structural change in amorphous alloys above glass transition temperature could provide opportunities for structure control by innovative design and processing strategies. By controlling the crystallization of amorphous alloys, bulk nanocrystalline alloys and/or nanocrystallineamorphous composites with excellent properties could be achieved from amorphous alloys precursors. By utilizing the viscous flowability of amorphous alloys in supercooled liquid region, net-shaped microforming could be realized for bulk amorphous alloys and bulk amorphous components with "true" bulk size might be produced from amorphous powder

The author is grateful to J. Xu, E. Ma, J. Eckert, H.B. Lu and M. Calin for their stimulating discussions. Financial support provided by Research Services of The University of Western Australia (through UWA Research Development Award Scheme) is gratefully

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acknowledged.

**8. References** 

**7. Acknowledgments** 

0003-6951

ISSN 0001-6160

liquid. Among the aforementioned consolidation methods, it has recently been shown that ECAP is a particular effective and novel approach used for the consolidation of amorphous powders. ECAP is a method for subjecting a volume fraction of materials to severe shear deformation by forcing them around a mold corner (Karaman, et al., 2004; Robertson, et al., 2003; Zhang, et al., 2006b). The advantages of ECAP have allowed to fabricate bulk materials with large cross-sections.

In order to utilize the viscous flow of amorphous phase, the crystallization of an amorphous alloys must be well controlled. Therefore, the temperature–time– transformation (TTT) diagram should be determined for the selected amorphous powders by measuring the onset time of the exothermic reaction due to crystallization on the DSC curves during isothermal annealing (e.g. see Fig. 12 (a)), where the sample was heated to the selected annealing temperature(s) in the supercooled liquid region, and the time that the sample began to crystallize (the onset of an exothermic reaction) was recorded. Fig. 12 (b) shows an example of the TTT diagram for the amorphous Ti50Cu18Ni22Al4Sn6 powders, which provides a window for processing in supercooled liquid state. The temperature and the time before crystallization (or the time to remain in the fully amorphous state at a certain temperature) exhibits approximately a linear relationship. At a given heating rate, the lower the temperature is, the longer the time is for the supercooled liquid to remain stable without crystallization. For the same temperature, the time window is longer at a faster heating rate. Therefore, for the ECAP processing at a given length of the can, it is necessary to select a suitable extrusion temperature (*T*e) and extrusion rate (*v*e). Two extrusion temperatures (700 and 705 K) near the calorimetric glass transition temperature (*T*g) were used in when with extrusion rate of 0.40 mm s-1 (Zhang, et al., 2006b). By using ECAP with these processing parameters, bulk nanocrystal-amorphous composites with a relative density about 97% have been achieved from the amorphous Ti50Cu18Ni22Al4Sn6 powders. Full densification was not reached, mainly owing to that the powders experienced insufficient shear deformation and that partial crystallization occurred during ECAP processing (Zhang, et al., 2006b).

Karaman et al (Karaman, et al., 2004) has optimized the ECAP process to consolidate the gas-atomized Vitreloy 106a (Zr58.5Nb2.8Cu15.6Ni12.8Al10.3) powder in supercooled liquid region at different strain rates and temperatures. The microstructure of all consolidates shows significant particle deformation. The increase in aspect ratio of particles due to shear strain is correlated with the extrusion temperature. Extrusions processed close to glass transition temepraure showed significant porosity. There is an increase in the consolidate hardness, depending on the extrusion temperature. Compression experiments on the consolidated V106a shows that good consolidate samples have strength levels of 1500 – 1700 MPa, which are comparable to that of cast V106 (Zr57Nb5Al10Cu15.4Ni12.6). In spite of some nanocrystallization and short-range order formation upon processing, most of the fracture surfaces of the consolidates show shear banding and well-developed vein patterns, typical fracture characteristics of amorphous alloys with good ductility.
