**5.1 Nanocrystalline alloy created from amorphous precursor via partial crystallization**

The subject of preparation of nanostructured alloys by nanocrystallization of amorphous solid precursors has been reviewed by Lu (Lu, 1996) and by McHenry et al. (McHenry et al., 1999). The formation of nanocrystalline structures during crystallization of amorphous alloys is of a great interest from both the fundamental and the technical point of view. Fundamental studies of the mechanisms of crystal nucleation and growth as well as kinetics of transformation will to a certain degree aid in tailoring the structure for excellent physical (e.g. magnetic properties) and mechanical properties of nanostructured materials attractive for practical applications. In general, this method has extensively applied for those amorphous alloys where ductile solid solution phase(s) or functional phase(s) is formed through primary crystallization. Amorphous alloys of appropriate chemical compositions, crystallized at temperatures above their primary crystallization temperature but below the secondary crystallization temperature, can yield nanocrystalline grains dispersed in an amorphous matrix. Three important groups of nanocrystalline materials produced by primary crytallization from amorphous alloy precursors can be distinguished: constructional Al-based alloys (Kim et al., 1990; Latuch et al., 1997; Zhong et al., 1997), magnetically soft (Lachowicz & Slawskawaniewska, 1994; Makino et al., 1997; Suzuki et al., 1990; Suzuki et al., 1993; Willard et al., 1998) and magnetically hard (Inoue et al., 1995b; Manaf et al., 1993; Takeuchi et al., 1997; Withanawasam et al., 1994) Fe-based alloys. Examples of the alloys compositions and main aspects of their structure are presented in Table 3. There are two basic parameters characterizing structure of these materials: crystallite diameter, *D*, and volume fraction, *V*cr, of nanocrystals. The optimum amount of nanocrystalline phase differs from each group. In the case of magnetically hard nanocrystalline materials, full (Manaf, et al., 1993; Takeuchi, et al., 1997) or almost full (Inoue, et al., 1995b) crystallization is required. For constructional and magnetically soft nanocrystalline materials the optimum mechanical and magnetic properties, respectively, are obtained after partial crystallization of their amorphous precursors (Inoue et al., 1988), which means that they are dual-phase materials composed of nanocrystals and an amorphous matrix. To preserve ductility in Al-based nanocrystalline alloys, *V*cr should not exceed 20% in ternary Al–Y–Ni (Inoue, et al., 1988) and 40% in quaternary Al–Y–Ni–Cu (Latuch, et al., 1997) alloys. Mechanical properties of these materials can be explained and predicted using mixture model based on the volume fractions of amorphous matrix and nanocrystals, proposed by Kim et al. (Kim et al., 1999).

Inoue and Kimura (Inoue & Kimura, 2000) have summarized the microstructure and mechanical properties of aluminum based alloys produced by controlling the crystallization of amorphous alloy precursors, as shown in Fig. 10. A high mechanical strength exceeding

in isothermal annealing and those obtained by isochronal annealing as revealed by Kissinger analysis for the Ti-based amorphous alloy with and without TiC particles. The activation energy of crystallization determined from the Kissinger analysis and the Arrhenius equationfor both powders show that the composite has slightly lower activation energy. The addition of 10 vol.% TiC particles into the Ti-based amorphous alloy may slightly affect the crystallization kinetics of the amorphous phase and the TiC particles may

**5.1 Nanocrystalline alloy created from amorphous precursor via partial crystallization**  The subject of preparation of nanostructured alloys by nanocrystallization of amorphous solid precursors has been reviewed by Lu (Lu, 1996) and by McHenry et al. (McHenry et al., 1999). The formation of nanocrystalline structures during crystallization of amorphous alloys is of a great interest from both the fundamental and the technical point of view. Fundamental studies of the mechanisms of crystal nucleation and growth as well as kinetics of transformation will to a certain degree aid in tailoring the structure for excellent physical (e.g. magnetic properties) and mechanical properties of nanostructured materials attractive for practical applications. In general, this method has extensively applied for those amorphous alloys where ductile solid solution phase(s) or functional phase(s) is formed through primary crystallization. Amorphous alloys of appropriate chemical compositions, crystallized at temperatures above their primary crystallization temperature but below the secondary crystallization temperature, can yield nanocrystalline grains dispersed in an amorphous matrix. Three important groups of nanocrystalline materials produced by primary crytallization from amorphous alloy precursors can be distinguished: constructional Al-based alloys (Kim et al., 1990; Latuch et al., 1997; Zhong et al., 1997), magnetically soft (Lachowicz & Slawskawaniewska, 1994; Makino et al., 1997; Suzuki et al., 1990; Suzuki et al., 1993; Willard et al., 1998) and magnetically hard (Inoue et al., 1995b; Manaf et al., 1993; Takeuchi et al., 1997; Withanawasam et al., 1994) Fe-based alloys. Examples of the alloys compositions and main aspects of their structure are presented in Table 3. There are two basic parameters characterizing structure of these materials: crystallite diameter, *D*, and volume fraction, *V*cr, of nanocrystals. The optimum amount of nanocrystalline phase differs from each group. In the case of magnetically hard nanocrystalline materials, full (Manaf, et al., 1993; Takeuchi, et al., 1997) or almost full (Inoue, et al., 1995b) crystallization is required. For constructional and magnetically soft nanocrystalline materials the optimum mechanical and magnetic properties, respectively, are obtained after partial crystallization of their amorphous precursors (Inoue et al., 1988), which means that they are dual-phase materials composed of nanocrystals and an amorphous matrix. To preserve ductility in Al-based nanocrystalline alloys, *V*cr should not exceed 20% in ternary Al–Y–Ni (Inoue, et al., 1988) and 40% in quaternary Al–Y–Ni–Cu (Latuch, et al., 1997) alloys. Mechanical properties of these materials can be explained and predicted using mixture model based on the volume fractions of amorphous matrix and

Inoue and Kimura (Inoue & Kimura, 2000) have summarized the microstructure and mechanical properties of aluminum based alloys produced by controlling the crystallization of amorphous alloy precursors, as shown in Fig. 10. A high mechanical strength exceeding

act as potential heterogeneous nucleation sites.

**5. Crystallization control for applications** 

nanocrystals, proposed by Kim et al. (Kim et al., 1999).

1000 MPa is achieved by the formation of an amorphous phase. The bulk nanocrystalline alloys, which contain a mixed structure of intermetallic compounds embedded fcc-Al matrix by the crystallization of Al-based amorphous phase, exhibit high mechanical strength of 700–1000 MPa and have been commercialized as a commercial name of GIGAS. By controlling the crystallization of Al-based amorphous alloys, the tensile strength of the Albased amorphous alloys increases to 1560 MPa by the homogeneous precipitation of nanoscale fcc-Al particles into an amorphous phase, which is higher than the strength of 1260 MPa by the formation of an amorphous single phase.


Table 3. General characteristics of the three main groups of nanocrystalline materials produced by devitrification of amorphous alloys (*V*cr – volume fraction of crystalline phase, *D* – diameter of nanocrystals, *λ*s – saturation magnetostriction constant, <*K*> – averaged magnetocrystalline anisotropy, *σ*f – fracture strength). Reprinted from (Kulik, 2001), with permission from Elsevier.
