Stabilization of metallic supercooled liquid and bulk amorphous alloys☆
Section snippets
History and alloy components of bulk amorphous alloys
Since the first synthesis of an amorphous phase in the Au–Si system by a rapid solidification technique in 1960 [1], a great number of amorphous alloys has been produced for the last three decades. It is well known that Fe-, Co- and Ni-based amorphous alloys found before 1990 require high cooling rates above 105 K/s for glass formation and the resulting sample thickness is limited to less than about 50 μm [2]. As exceptional examples, one can observe that Pd–Ni–P and Pt–Ni–P amorphous alloys have
Structure of bulk amorphous alloys and reasons for high glass-forming ability
Firstly, the reason for the stabilization of the supercooled liquid for the alloys belonging to the groups (i)–(iv) is discussed. All the alloy systems in these groups are based on the following three empirical rules 17, 18, 19, 20, 21: (1) multicomponent systems consisting of more than three elements; (2) significant difference in atomic size ratios above about 12% among the three main constituent elements; and (3) negative heats of mixing among the three main constituent elements. We want to
Production of bulk amorphous alloys
By choosing the above-described multicomponent alloy systems, we can produce bulk amorphous alloys by using two kinds of production techniques of solidification and consolidation 17, 18, 19, 20, 21. As a solidification technique, one can list water-quenching, copper-mold casting, high-pressure die casting, arc melting, unidirectional melting, suction casting and squeeze casting. Bulk amorphous alloys are also produced by hot pressing and warm extrusion of atomized amorphous powders in the
Mechanical properties
In addition to the importance of basic science, it is important in applications as engineering materials to clarify the mechanical properties of bulk amorphous alloys. Figure 8 shows the relationship between the tensile fracture strength (σf) and Young's modulus (E) for the cast bulk amorphous Zr–Ti–Al–Ni–Cu alloys in sheet and cylinder forms with thicknesses (or diameters) of 1–5 mm [21]. The bulk amorphous alloys have high σf of 840–2100 MPa combined with E of 47–102 GPa which depend on alloy
Corrosion resistance
When bulk amorphous alloys for their good static and dynamic mechanical properties are used as structural materials, it is essential for the bulk amorphous alloys to have good corrosion resistance in various kinds of corrosive solutions. There have been no data published on the corrosion resistance of Zr-based bulk amorphous alloys in any kinds of corrosive solution. We examined the corrosion resistance of melt-spun amorphous alloys in Zr–TM–Al–Ni–Cu (TM=Ti,Cr,Nb,Ta) systems in HCl and NaCl
High strain-rate superplasticity
As described above, the bulk amorphous alloys have a wide supercooled liquid region of more than 60 K before crystallization. Figure 14 shows the change in viscosity with reduced temperature (Tr=T/Tm) for the Pd40Cu30Ni10P20 amorphous alloy [49]. The data of SiO2 glass are also shown for comparison. The viscosity of the Pd-based amorphous alloy has much larger temperature dependence as compared with SiO2 glass, indicating that the Pd-based amorphous alloy belongs to the fragile type glass which
Magnetic properties
Based on the three empirical rules for achievement of high GFA, a new bulk amorphous alloy with ferromagnetism at room temperature has been developed. As described in Section 1, soft ferromagnetic bulk amorphous alloys have been synthesized in multicomponent Fe–(Al,Ga)–(P,C,B,Si) [10], Co–Cr–(Al,Ga)–(P,B,C) [57], Fe–(Co,Ni)–(Zr,Nb,Ta)–B [11] and Co–Fe–(Zr,Nb,Ta)–B [58] systems. This section describes the relationship of soft magnetic properties of new Fe- and Co-based amorphous alloys with high
Formation and structures
The single-stage crystallization mode typical for bulk amorphous alloys also implies that the amorphous phase containing homogeneously nanocrystalline particles is not formed. It has previously been pointed out that the mixed structure consisting of nanoscale crystalline particles embedded in an amorphous phase is formed in the satisfaction of the four following criteria [69]:
- 1.
multistage crystallization process;
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existence of homogeneous nucleation sites in an amorphous phase;
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suppression of growth
Formation and mechanical properties of bulk-clustered amorphous alloys
It was shown in the previous section that the most important point in obtaining high Vf for the mixed-phase alloys is attributed to the good ductility of the remaining amorphous phase. The above-described nanostructured amorphous alloys were obtained by annealing-induced partial crystallization in the supercooled liquid region, followed by water-quenching. In addition to the annealing treatment, as another route to producing the similar nanostructured amorphous structure, one can observe a
Applications and conclusions
Table 8 summarizes fields of application in which the bulk amorphous alloys have expected uses. As particularly important application fields, one can list machinery/structural materials, magnetic materials, acoustic materials, somatologic materials, optical machinery materials, sporting goods materials and electrode materials. Finally, it is pleasing to introduce a successful example of a real application of bulk amorphous alloys as sporting goods materials. As exemplified in Fig. 39, the
Acknowledgements
This work is partly supported by the Inoue Superliquid Glass Project of Exploratory Research for Advanced Technology, Japan Science and Technology Corporation (JST).
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