Amorphous and Nanocrystalline Materials

This chapter aims to review our recent research results on new bulk amorphous alloys. The main topics are the following: (1) the finding of new amorphous alloys with high glass-forming ability in a number of alloy systems; (2) the mechanism for achieving high glass-forming ability; (3) the fundamental properties of the new amorphous alloys; (4) successful examples of producing bulk amorphous alloys by different techniques of water quenching, metallic mold casting, arc melting and unidirectional zone melting, etc.; (5) the high tensile strength, low Young’s modulus, and high impact fracture energy of nonferrous metal-based bulk amorphous alloys; (6) the soft magnetic properties of Fe- and Co-based bulk amorphous alloys; (7) hard magnetic properties of Nd- and Pr-based bulk amorphous alloys; (8) the viscous flow and microformability of bulk amorphous alloys in a supercooled liquid region, and (9) future aspects of applications. These new results enable eliminating of the limitation of sample shape which has prevented the development of amorphous alloys as engineering materials. They are expected to give rise to a new era of amorphous alloys.

We investigated the stress relaxation behavior and diffusion of Zr-Al-Ni-Cu metallic glasses having a wide supercooled liquid region. The stress relaxation was more pronounced after yielding and its relaxation rate increased with temperature. The stress relaxation was associated with the stress overshoot that is a transient stress-strain phenomenon. The stress overshoot appeared again after the stress relaxation in the course of stretch, and increased with an increase in stress relaxation fraction. The glassy structure appears to change into a state with higher atomic mobility by yielding, and to return its initial structure by the stress relaxation. The temperature dependence of the diffusivity in the supercooled liquid phase above the glass transition temperature was significantly different from that in the amorphous phase. The activation energy for diffusion in the supercooled liquid phase was much larger than that in amorphous phase below the glass transition temperature.

The thermal stability in the supercooled liquid region was examined for Pd₈₂Si₁₈, Pd₇₆Cu₆Si₁₈, Pd₄₀Ni₁₀Cu₃₀P₂₀ and Zr₆oAl₁₅Ni₂₅ glasses by means of mainly in-situ electrical resistance measurement carried out under various atmospheres, such as Ar, He, H₂ and vacuum. A clear variation was found out in the slope of electrical resistance curve after glass transition for all glasses. The glass transition and crystallization temperatures corresponded to those obtained by differential scanning calorimetry. No crystallites were detected in a Zr₆₀Al₁₅Ni₂₅, Pd₇₆Cu₆Si₁₈ and Pd₄₀Ni₁₀Cu₃₀P₂₀ glasses heated to the supercooled liquid region at least within X-ray diffraction and transmission electron microscopy. Some anomalous behaviors of electrical resistance were observed around room temperature for Pd-Si based glasses and in the super-cooled liquid region for a Pd₄₀Ni₁₀Cu₃₀P₂₀ glass, possibly resulting from hydrogen absorption and desorption. On the other hand, the behavior of electrical resistance for a Zr₆₀Al₁₅Ni₂₅ glass was strongly dependent on the surface state of the sample, containing the oxidation. The change in electrical resistance after glass transition was also examined in detail and explained by Faber-Zimann theory for Pd₇₆Cu₆Si₁₈ and Pd₄₀Ni₁₀Cu₃₀P₂₀ glasses.

There are a lot of possible techniques that can be used to prepare amorphous materials. This Chapter is denoted to the three widely used methods of preparation of amorphous and metastable materials namely: ball milling, electrode-position and sputtering. The properties of materials obtained by these methods are also to be discussed in the present Chapter. Among the materials studied are: amorphous and nanocrystalline Co-Ti, Ni-W alloys containing from 5 to 30 at.% W, Ti-B, Ti-Si, Ti-C, Ti-Al and Ni-Mo alloys. The structure and phase transformations in the above-mentioned alloys have been studied by means of X-ray diffraction, small angle X-ray scattering, high resolution transmission electron microscopy and differential thermal analysis.

The present chapter describes Si,Ge-Al-TM (TM-transition metals) amorphous alloys produced by rapid solidification. Although Al-Si-Fe and Al-Ge-Cr alloys have similar composition ranges for the formation of an amorphous phase in the Al-rich area and the largest possible among the Al-Si,Ge-TM systems metalloid concentration in the amorphous phase achieved, influence of transition metals on the amorphous phase formation between Ge- and Si-based (Si,Ge-Al-TM) alloys is significantly different. Composition range of an amorphous single phase in the Si-Al-Fe system was extended up to 50–60 at % Si by the alloying with Ni, Cr and Zr transition metals while the addition of different transition metals to Ge-Al-Cr alloys with 55–60 % Ge causes appearance of crystallinity. Moreover, microstructure of the Si-based alloys produced by rapid solidification changes from homogeneous amorphous to heterogeneous — (amorphous+crystalline Ge solid solution) by the addition of Ge. The Ge particle size increases with increasing Ge content from 5–7 nm at 7 at % Ge to 30–40 nm at 40 at % Ge. The amount of Si dissolved in Ge decreases with increasing Ge concentration. At the same time replacement of Ge by Si for Ge-Al-Cr-Si causes the precipitation of Ge particles from the amorphous matrix. In contrast to Si-Al-TM-Ge alloys where homogeneous distribution of c-Ge particles in an amorphous matrix was observed, the distribution of the Ge particles in the Ge-Al-Cr-Si alloy is inhomogeneous. This phenomenon as well as the properties of the obtained materials are also discussed in the present chapter.

CO₂ emissions, which induce global warming, increase with economic growth. It is impossible to demand that CO₂ emission should be reduced by suppressing economic activity. Global CO₂ recycling can solve this problem. The global CO₂ recycling involves three geographical regions: The electricity is generated by solar cells on deserts. At coasts close to the deserts, the electricity generated on the deserts is used for H₂ production by seawater electrolysis and H₂ is used for CH4 production by the reaction with CO₂. CH₄ is liquefied and transported to energy consuming districts where, after the CH₄ is used as a fuel, CO₂ is recovered, liquefied and transported to the coasts close to the deserts. Since 90% of city gases in Japan are liquefied natural gas (LNG) consisting mostly of CH₄, CH₄ produced in the global CO₂ recycling can be used immediately as city gas. A CO₂ recycling plant for substantiation of our idea to solve global warming and energy problems has been built on the roof of our institute (IMR) in 1996. Key materials necessary for the global CO₂ recycling are the anode and cathode for seawater electrolysis and the catalyst for CO₂ conversion. All of them have been tailored by us. Since the quantities of CO₂ to be converted far exceed an industrial level, the system must be very simple, the rate of conversion must be very fast and precious metals must not be required for the system. All these requirements are satisfied in the global CO₂ recycling. When the global CO₂ recycling is conducted on a large scale, the energies and costs required to form liquefied CH₄ in the global CO₂ recycling are almost the same as those for production of LNG from currently operating natural gas sources. The project for the field experiment on global CO₂ recycling using pilot plants in Egypt was planned in cooperation with Egyptian scientists, engineers and industries.

In this chapter, the formation of micro and nano-sized martensite particles and their application to materials strengthening are described. The size of martensite can be controlled by the control of dislocation density and temperature. It is implied that the nucleation site of martensite is the intersection of two partial dislocations, however, further studies are necessary. Fatigue strength increased especially in the high cycle regime including fatigue limit due to the existence of nano-sized martensite particles.