**1. Introduction**

184 Advances in Crystallization Processes

[45] P. J. Flory, J. Chem. Phys. 17, 223(1949).

Since the discovery in 1960 by Duwez (Klement et al., 1960), considerable effort has been devoted to form amorphous (or glassy) alloys either by rapid solidification techniques or by solid-state amorphization techniques (Inoue, 2000; Johnson, 1999; Suryanarayana & Inoue, 2011; Wang et al., 2004). However, the geometry of the amorphous samples has long time been limited in the form of ribbons or wires. The first "bulk" amorphous alloys, arbitrarily defined as the amorphous alloys with a dimension no less than 1 mm in all directions, was discovered by Chen and Turnbull (Chen & Turnbull, 1969) in ternary Pd-Cu-Si alloys. These ternary bulk glass-forming alloys have a critical cooling rate of about 102 K s-1 and can be obtained in amorphous state with a thickness up to 1 mm and more. Since then, especially after the presence of new bulk metallic glasses (BMGs) in La55Al25Ni20 (Inoue et al., 1989) and Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (Peker & Johnson, 1993), multicomponent BMGs, which could be prepared by direct casting from molten liquid at low cooling rates, have been drawing increasing attention in the scientific community. A great deal of effort has been devoted to developing and characterizing BMGs with a section thickness or diameter of a few millimetres to a few centimetres (Suryanarayana & Inoue, 2011). A large variety of multicomponent BMGs in a number of alloy systems, such as Pd-, Zr-, Mg-, Ln-, Ti-, Fe-, and Ni-based BMGs, have been developed via direct casting method with low cooling rates of the order of 1 – 102 K s-1 (Inoue, 2000; Johnson, 1999; Suryanarayana & Inoue, 2011; Wang, et al., 2004). In this method, the alloy compositions were carefully designed to have large glassforming ability (GFA) so that "bulk" amorphous alloys can be formed at a low cooling rate to frustrate crystallization from molted liquid state. A number of parameters/indicators have been proposed to evaluate the GFA of multicomponent alloy systems to search for BMGs with larger dimensions (Suryanarayana & Inoue, 2011). So far, the "record" size of the BMGs is 72 mm diameter for a Pd40Cu30Ni10P20 bulk metallic glass (Inoue et al., 1997). The discovery of amorphous alloys has attracted widespread research interests because of their technological promise for practical applications and scientific importance in understanding glass formation and glass phenomena.

Arising from their disordered atomic structure and unique glass-to-supercooled liquid transition, amorphous alloys represent a new class of structural and functional materials with excellent properties (Eckert et al., 2007; Inoue, 2000; Johnson, 1999; Suryanarayana & Inoue, 2011; Wang, 2009; Xu et al., 2010), e.g. high strength about 2–3 times of their

Crystallization Behavior and Control of Amorphous Alloys 187

considered metastable. By considering the local potential wells between which atoms must make diffusional jumps, even states which are thermodynamically unstable may be regarded as kinetically metastable. Such kinetic metastability can exist only where thermal activation of atomic jumps is required. Regardless of the processing route used for the formation of amorphous state, the amorphous alloys are in thermodynamically metastable state and are susceptible to transform into more stable states under appropriate circumstances. Crystallization is such a transformation during which an amorphous phase devitrified into one or more metastable or stable crystalline phases. The driving force for the crystallization is the Gibbs free energy difference between the amorphous and the crystalline states. Crystallization could happen when an amorphous solid is subject to heat treatment (Calin et al., 2007; Suryanarayana & Inoue, 2011; Zhang et al., 2002; Zhang et al., 2003; Zhang & Xu, 2004; Zhang et al., 2005a; 2005b; Zhang et al., 2006a; Zhang et al., 2007a; Zhang et al., 2007b), mechanical deformation (Fornell et al., 2010; Lohwongwatana et al., 2006; Setyawan et al., 2010), pressure (Jiang et al., 2000; Jiang et al., 2002; Jiang et al., 2003b; Yang et al., 2006; Ye & Lu, 1999), and/or irradiations (Azam et al., 1979). Amongst these processing techniques, conventionally thermal annealing is the most commonly used in the investigation on crystallization of amorphous alloys. The dimensions and morphologies of the crystallization products strongly depend on the transformation mechanism, which is closely related to the chemical composition of the amorphous phase and to the thermodynamic properties of the corresponding crystalline phase. The crystallization products could include crystalline solids (solid solution, intermetallics, and/or compounds) (Foley et al., 1997; Kelton et al., 2003; Lu, 1996; Sahu et al., 2010; Zhang, et al., 2002; Zhang, et al., 2003) or quasicrystalline (Murty et al., 2000). As the crystallization process upon annealing of an amorphous phase is much slower than during solidification of liquids, it is relatively easier to fundamentally investigate crystallization in amorphous phases than in liquids on the processes of nucleation and growth, in particular of nucleation kinetics difficult to study quantitatively in the liquid state. The study of crystallization behaviors on amorphous alloys is of primary importance not only to characterize the thermal stability of amorphous alloys against crystallization but also to investigate the fundamental aspect of the processes of

nucleation and growth, which are of relevance for the understanding glass formation.

solid solution α and a compound β, are included) at a chosen annealing temperature.

**2.1 Polymorphous crystallization** 

Three types of crystallization reactions that may occur during devitrification can be classified, depending on their chemical compositions (Köster & Herold, 1981; Lu, 1996): *polymorphous*, *eutectic* and *primary* crystallization. Fig. 2 shows a hypothetical free energy diagram to illustrate the crystallization reactions during crystallization. This schematic is essentially a representation of the variation of free energy with the chemical compositions of the amorphous phase and various crystalline phases (in this case, two crystalline phases, a

In *polymorphous* crystallization, an amorphous solid crystallizes into a single crystalline phase with different structure but with same chemical composition as the amorphous phase. This reaction can only occur in concentration ranges near to those of stable compounds (*C1* in Fig. 2) or pure elements (*C2*) and needs only single jumps of atoms across the crystallization front. The polymorphous crystallization reaction (reaction (1) or (2)) may produce a single compound phase (β) or a supersaturated solid solution phase (α), as shown

crystalline counterparts, large elastic limit about 2% which is very near to some polymer materials, including extreme strength at low temperature and high flexibility at high temperature, high corrosion resistance, high wear resistance, superior chemical and physical properties, etc. These properties, which can be rarely found in crystalline materials, are attractive for the practical applications as a new class of structural and functional materials. Fig. 1 summarizes the relationship between fracture strength and Young's modulus for typical engineering materials in amorphous and crystalline states. There is a clear tendency for fracture strength to increase with increasing Young's modulus, but the slope of the linear relation corresponding to elastic elongation is significantly different between the bulk amorphous and crystalline alloys and the elastic elongation of the amorphous alloys is ~3 times larger than those for the crystalline alloys. The amorphous alloys also exhibit high strength which is ~3 times higher than those for crystalline alloys, when the comparison is made at the same Young's modulus level. Currently, amoprhous alloys have a variety of uses for sports and luxury goods, microelectromechanical systems (MEMS), biomedicine and nanotechnology.

Fig. 1. Relation between strength and Young's modulus for bulk alloys in amorphous and crystalline states. Reprinted from (Inoue et al., 2004b), with permission from Elsevier.

#### **2. Crystallization mechanisms**

In general, the best practice way to describe a microstructure is in terms of its thermodynamic state before configurationally freezing set in (Turnbull, 1981). In this way, an amorphous alloy in configurationally frozen state as an undercooled liquid would be

crystalline counterparts, large elastic limit about 2% which is very near to some polymer materials, including extreme strength at low temperature and high flexibility at high temperature, high corrosion resistance, high wear resistance, superior chemical and physical properties, etc. These properties, which can be rarely found in crystalline materials, are attractive for the practical applications as a new class of structural and functional materials. Fig. 1 summarizes the relationship between fracture strength and Young's modulus for typical engineering materials in amorphous and crystalline states. There is a clear tendency for fracture strength to increase with increasing Young's modulus, but the slope of the linear relation corresponding to elastic elongation is significantly different between the bulk amorphous and crystalline alloys and the elastic elongation of the amorphous alloys is ~3 times larger than those for the crystalline alloys. The amorphous alloys also exhibit high strength which is ~3 times higher than those for crystalline alloys, when the comparison is made at the same Young's modulus level. Currently, amoprhous alloys have a variety of uses for sports and luxury goods, microelectromechanical systems (MEMS), biomedicine

Fig. 1. Relation between strength and Young's modulus for bulk alloys in amorphous and crystalline states. Reprinted from (Inoue et al., 2004b), with permission from Elsevier.

In general, the best practice way to describe a microstructure is in terms of its thermodynamic state before configurationally freezing set in (Turnbull, 1981). In this way, an amorphous alloy in configurationally frozen state as an undercooled liquid would be

and nanotechnology.

**2. Crystallization mechanisms** 

considered metastable. By considering the local potential wells between which atoms must make diffusional jumps, even states which are thermodynamically unstable may be regarded as kinetically metastable. Such kinetic metastability can exist only where thermal activation of atomic jumps is required. Regardless of the processing route used for the formation of amorphous state, the amorphous alloys are in thermodynamically metastable state and are susceptible to transform into more stable states under appropriate circumstances. Crystallization is such a transformation during which an amorphous phase devitrified into one or more metastable or stable crystalline phases. The driving force for the crystallization is the Gibbs free energy difference between the amorphous and the crystalline states. Crystallization could happen when an amorphous solid is subject to heat treatment (Calin et al., 2007; Suryanarayana & Inoue, 2011; Zhang et al., 2002; Zhang et al., 2003; Zhang & Xu, 2004; Zhang et al., 2005a; 2005b; Zhang et al., 2006a; Zhang et al., 2007a; Zhang et al., 2007b), mechanical deformation (Fornell et al., 2010; Lohwongwatana et al., 2006; Setyawan et al., 2010), pressure (Jiang et al., 2000; Jiang et al., 2002; Jiang et al., 2003b; Yang et al., 2006; Ye & Lu, 1999), and/or irradiations (Azam et al., 1979). Amongst these processing techniques, conventionally thermal annealing is the most commonly used in the investigation on crystallization of amorphous alloys. The dimensions and morphologies of the crystallization products strongly depend on the transformation mechanism, which is closely related to the chemical composition of the amorphous phase and to the thermodynamic properties of the corresponding crystalline phase. The crystallization products could include crystalline solids (solid solution, intermetallics, and/or compounds) (Foley et al., 1997; Kelton et al., 2003; Lu, 1996; Sahu et al., 2010; Zhang, et al., 2002; Zhang, et al., 2003) or quasicrystalline (Murty et al., 2000). As the crystallization process upon annealing of an amorphous phase is much slower than during solidification of liquids, it is relatively easier to fundamentally investigate crystallization in amorphous phases than in liquids on the processes of nucleation and growth, in particular of nucleation kinetics difficult to study quantitatively in the liquid state. The study of crystallization behaviors on amorphous alloys is of primary importance not only to characterize the thermal stability of amorphous alloys against crystallization but also to investigate the fundamental aspect of the processes of nucleation and growth, which are of relevance for the understanding glass formation.

Three types of crystallization reactions that may occur during devitrification can be classified, depending on their chemical compositions (Köster & Herold, 1981; Lu, 1996): *polymorphous*, *eutectic* and *primary* crystallization. Fig. 2 shows a hypothetical free energy diagram to illustrate the crystallization reactions during crystallization. This schematic is essentially a representation of the variation of free energy with the chemical compositions of the amorphous phase and various crystalline phases (in this case, two crystalline phases, a solid solution α and a compound β, are included) at a chosen annealing temperature.

## **2.1 Polymorphous crystallization**

In *polymorphous* crystallization, an amorphous solid crystallizes into a single crystalline phase with different structure but with same chemical composition as the amorphous phase. This reaction can only occur in concentration ranges near to those of stable compounds (*C1* in Fig. 2) or pure elements (*C2*) and needs only single jumps of atoms across the crystallization front. The polymorphous crystallization reaction (reaction (1) or (2)) may produce a single compound phase (β) or a supersaturated solid solution phase (α), as shown

Crystallization Behavior and Control of Amorphous Alloys 189

or an intermetallic compound) embedded in an amorphous matrix (amorphous phase (C4) = α + amorphous phase' (C3)). During this reaction, a concentration gradient occurs at the interface between the precipitate and the matrix until the reaction reaches the metastable equilibrium. The residual amorphous phase (with the new concentration C3) crystallizes, in the second step, into crystalline phases through the mechanism of either the *eutectic* or *polymorphous* crystallization. The crystallization mechanisms of most of Al-based, e.g. Al88Ni4Y8 (Jiang et al., 1997), and Fe-based amorphous alloys, e.g. Fe73.5Si13.5B9Nb3Cu1 (Finemet) (Hono et al., 1992), are typically primary crystallization (Foley, et al., 1997; Kelton, et al., 2003). The control of primary crystallization behaviors could lead to nanocrytalline– amorphous composites with special mechanical or functional properties (see *Section 5.1*).

The mechanisms and products of crystallization of amorphous alloys are influenced by both inherent (e.g. composition, oxygen) and extraneous (e.g. preparation method, pressure, etc)

During the searching for strong glass-forming alloys, the effect of composition on the crystallization behavior has been extensively studied in a variety of amorphous alloys, despite of the preparation methods (Suryanarayana & Inoue, 2011). Two examples are listed in this section to show how the chemical compositions of amorphous alloys influence the

Zhang et al. (Zhang, et al., 2002) has investigated the addition of Al on the glass formation and crystalliztion in the ball-milled amorphous Ti50(Cu0.45Ni0.55)44-xAlxSi4B2 (x=0, 4, 8 and 12) alloys. Al additions were introduced to simultaneously replace part of the Cu and Ni in Ti50Cu20Ni24Si4B2 (Zhang & Xu, 2002) to further reduce the density of the resulting alloys and improve the thermal stability of the supercooled liquid. The Ti-based amorphous alloy powders prepared through this solid-state process exhibit a well-defined glass transition and a supercooled liquid region. Al addition has changed the crystallization mechanims and crystallizaiton products of the amoprhous Ti50Cu20Ni24Si4B2 alloy. Fig. 3 (a) displays the differential scanning calorimetry (DSC) scans for the as-milled samples with different Al contents. In all cases, an endothermic signal associated with the glass transition is evident. As see from Fig. 3 (a), the onset of glass transition temperature (*T*g) is apparently insensitive to the change in the overall alloy composition. With increasing Al substitution, the exothermic reaction due to crystallization occurs at higher temperatures and the single-step crystallization event changes to a two-step process. X-ray diffraction (XRD) has been used to identify the structural changes associated with the exothermal events at several different temperatures, as marked by dots in the DSC traces in Fig. 3 (b). For x = 0, the XRD pattern at 777 K crystallization peak and after the crystallization event (810 K) showed that the amorphous phase transformed into the cubic NiTi phase and an unknown phase. The same products were found for x = 4 after crystallization, as shown in the XRD pattern at 820 K. Such a transition can be regarded as a eutectic crystallization, by which the amorphous phase simultaneously transforms into more than two phases in one step (as stated in *Section 2.2*). For x = 8 and x = 12, on the other hand, crystallization is completed in two steps. Fig. 3

**3. Influences on crystallization of amorphous alloys** 

crystallization mechanism and crystallization products.

**3.1 Effect of chemical composition** 

factors.

in Fig. 2. The crystallization mechanisms of Fe33Zr67 (Spassov & Koster, 1993), Ni33Zr67 (Lu et al., 1996), Co33Zr67 (Nicolaus et al., 1992) and Zr50Co50 (Köster & Meinhardt, 1994) amorphous alloys are the typical polymorphous crystallization.

Fig. 2. Hypothetical free energy diagram to illustrate the crystallization of amorphous alloys. Reprinted from (Lu, 1996), with permission from Elsevier.

#### **2.2 Eutectic crystallization**

In case of *eutectic* crystallization, amorphous phase crystallizes into two crystalline phases simultaneously (e.g. reaction (3) in Fig. 2, α *+* β), during which two phases grow in a coupled fashion. This is similar to the eutectic crystallization of liquids. The reaction has the largest driving force and the overall composition of the two phases remains the same as that of the amorphous matrix. The eutectic crystallization can occur within a concentration range around the equilibrium eutectic composition rather than a specific eutectic composition as observed in conventional crystallization. A possible reason might be that the crystalline material contains a large amount of interface that may have higher energetic configurations and thus allows a relatively wide composition range (Lu, 1996). For example, e.g. in the Ni-P binary system eutectic crystallization occurs within 18.2–20.0 at.% P (i.e. amorphous Ni + Ni3P), where the equilibrium eutectic composition is 19.0 at.% (Dong et al., 1994).

#### **2.3 Primary crystallization**

In *primary* crystallization, amorphous phase crystallizes into a phase with different composition (C4 in Fig. 2) in the first step (this can be either a supersaturated solid solution

in Fig. 2. The crystallization mechanisms of Fe33Zr67 (Spassov & Koster, 1993), Ni33Zr67 (Lu et al., 1996), Co33Zr67 (Nicolaus et al., 1992) and Zr50Co50 (Köster & Meinhardt, 1994)

Fig. 2. Hypothetical free energy diagram to illustrate the crystallization of amorphous alloys.

In case of *eutectic* crystallization, amorphous phase crystallizes into two crystalline phases simultaneously (e.g. reaction (3) in Fig. 2, α *+* β), during which two phases grow in a coupled fashion. This is similar to the eutectic crystallization of liquids. The reaction has the largest driving force and the overall composition of the two phases remains the same as that of the amorphous matrix. The eutectic crystallization can occur within a concentration range around the equilibrium eutectic composition rather than a specific eutectic composition as observed in conventional crystallization. A possible reason might be that the crystalline material contains a large amount of interface that may have higher energetic configurations and thus allows a relatively wide composition range (Lu, 1996). For example, e.g. in the Ni-P binary system eutectic crystallization occurs within 18.2–20.0 at.% P (i.e. amorphous Ni +

In *primary* crystallization, amorphous phase crystallizes into a phase with different composition (C4 in Fig. 2) in the first step (this can be either a supersaturated solid solution

Ni3P), where the equilibrium eutectic composition is 19.0 at.% (Dong et al., 1994).

amorphous alloys are the typical polymorphous crystallization.

Reprinted from (Lu, 1996), with permission from Elsevier.

**2.2 Eutectic crystallization** 

**2.3 Primary crystallization** 

or an intermetallic compound) embedded in an amorphous matrix (amorphous phase (C4) = α + amorphous phase' (C3)). During this reaction, a concentration gradient occurs at the interface between the precipitate and the matrix until the reaction reaches the metastable equilibrium. The residual amorphous phase (with the new concentration C3) crystallizes, in the second step, into crystalline phases through the mechanism of either the *eutectic* or *polymorphous* crystallization. The crystallization mechanisms of most of Al-based, e.g. Al88Ni4Y8 (Jiang et al., 1997), and Fe-based amorphous alloys, e.g. Fe73.5Si13.5B9Nb3Cu1 (Finemet) (Hono et al., 1992), are typically primary crystallization (Foley, et al., 1997; Kelton, et al., 2003). The control of primary crystallization behaviors could lead to nanocrytalline– amorphous composites with special mechanical or functional properties (see *Section 5.1*).
