Crystallization of amorphous alloys

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are also useful methods for structure determination. These methods can detect crystallization of amorphous alloy catalysts as a result of heat treatment (21, 23, 41-44) or as a result of the action of reacting gases, such as in the case of hydrogenation of carbon monoxide (53) or ammonia synthesis (22). 

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

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) factors. 

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... 

An amorphous alloy is an ideal general precursor or reaction intermediate. The amorphous state is metastable with respect to several different crystalline states but the crystalline state which is easiest to nucleate is the one which will form. Thus, the compound which crystallizes is not necessarily the most stable state. Controlling crystallization of amorphous alloys is a general route to both stable and metastable materials (10). 

All the unary and binary phases are presented in Table 2. No stable ternary compound was foimd in the system. During crystallization of amorphous alloy Fe7oCrioB2o, a tetragonal 74 Fe2Cr4Bg ternary boride was identified, which transformed to anoflier Wmcm FeeCr2B4 at further heating [2005San]. 

The equations that describe the magnetic effects and the changes of electron diffraction patterns are got in consequence with the data of X-ray investigation of amorphous alloys and the products of crystallization. 

Abstract The focus of this chapter is primarily directed towards nanocrystalline soft magnetic materials prepared by crystallization of amorphous precursors. The key elements involved in the development of this class of materials are three-fold (i) theoretical models for magnetic softness in nanostructures (ii) nanostructure-property relationships and (iii) nanostructural formation mechanisms. This chapter surveys recent research on these three areas with emphasis placed on the principles underlying alloy design in soft magnetic nanostructures. 

Two major alloy systems in the family of soft magnetic nanostructures are Fe-Si-B-Nb-Cu [1, 4-6] and Fe-M-B-(Cu) (M= Zr, Hf or Nb) [3, 7-9], commercially known as FINEMET (or VITROPERM) and NANOPERM, respectively. They are produced by primary crystallization of amorphous ribbons. Hence, the nanostructural formation upon crystallization as well as the mechanism of magnetic softening has been actively studied for these two... 

Figures 4(a) and 4(b) show the relationship between the average grain size and the coercivity in various Fe-based nanocrystalline soft magnetic alloys prepared by crystallization of amorphous precursors (For details, see Herzer [13], Yoshizawa [31], Muller and Mattem [32], Fujii et al. [33], and Suzuki et al. [34, 35]). As shown in Fig. 4(a), the coercivity Ha of the nanocrystalline Fe-Si-B-M-Cu (M = IVa to Via metal) alloys follows the predicted D6 dependence in a D range below LO ( 30 to 40 nm for this alloy system) although the plots deviate from the predicted D6 law in the range below H0 1 A/m where the effect of grain refinement on is overshadowed by magneto-elastic and annealing induced anisotropies. Hence, the experiments are better described by Hc [a2 + where a...

It is instructive to consider the free-energy hierarchy and the metastable phase equilibria when crystallization of an amorphous material is discussed. Koster and Herold [56] discussed these aspects of crystallization and showed that crystallization reactions of amorphous alloys can be classified into the following three types polymorphic, primary and eutectic crystallization reactions. Among these three types, the slowest crystal growth process is expected for primary crystallization and thus, primary crystallization is ideal for tailoring fine microstructures upon decomposition of amorphous alloys. 

By studying the Mossbauer spectra and the nuclear magnetic resonance of a series of amorphous alloys, it was established [6.44] that their LO differs slightly from that in crystals. 

The analysis of the effect of hydrostatic pressure and electron irradiation on the diffusion-controlled crystallization process of amorphous alloys of the metal-metalloid and metal-metal types in [6.48,49] leads to the following conclusions. 

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

From these results it can be concluded that any favourable (2kp= Q ) or unfavourable (2kp Q ) influence on E, even if it were of the order of AH, would have little effect on the magnitude of AE. This means that the Nagel and Tauc criterion is less suited for describing thermal stability of amorphous alloys, i.e. their resistance against crystallization. As briefly indicated above, it is mainly the transformation kinetics that governs the thermal stability of amorphous alloys. A description of the kinetic approach to thermal stability will be presented in the following section. 

Systematic investigation of the accumulation Fe and Cr in the Ni-Zr system during mechanical alloying reveals that, amount of Fe and Cr above some critical concentration push system beyond the region of amorphous phase formation and causes the crystallization of amorphous phase [109]. 

When the crystallization of liquid alloys can be prevented during cooling, glass or amorphous alloys are formed. A glass transition is a phase transition between a liquid and a glass, which... 

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