Tohoku University, Institute of Materials Science

The Tohoku University Institute of Materials Science occupies facilities at the old campus in the central part of the city of Sendai, which are separate from those of the College of Engineering. The institute is housed in a multistory facility of recent construction. The WTEC team visited Prof. Inoue, who was cited as the second most-quoted worker in the field of materials science by a recent study reported in Science magazine. The same study ranked Tohoku University as the sixth most-quoted institution in materials science (after Oak Ridge National Laboratory). Prof. Inoue's major efforts are devoted to the study of amorphous metallic materials (Inoue 1995; Inoue n.d.; Inoue et al. n.d.).

Prof. Inoue has a staff of about 30 research associates. His efforts are supported by Japanese industry and the Japanese government (primarily the Ministry of Education, Culture and Science as well as other ministries). The ratio of government to private funding has been in the range of 4:1 to 3:1. The level of industrial funding has been fairly constant and depends on the number of industrial workers on Prof. Inoue's staff at any time. He has a budget of ¥30 million to support operating and overhead costs for his operations, which is equivalent to a cost of ¥1 million for supporting each associate. These costs do not include the salaries of his associates.

In the several research laboratories under Prof. Inoue's direction, there was a complete range of processing, testing, and analytical equipment to support his studies on amorphous metallic materials. Professor Inoue presented a comprehensive review of the history of the efforts of his group on amorphous and nanocrystalline metallic materials. In addition to interest in these materials in the amorphous state, there is also interest in them as precursors of nanocrystalline materials. The new materials are being developed for their unique properties including the following:

Over the period Prof. Inoue and his colleagues have progressed in understanding the proper compositions to the point that bulk amorphous materials can be produced using conventional casting procedures. Whereas, conventional glassy materials are produced at cooling rates of from 102 to 100 °K/sec, the earlier amorphous metal compositions required cooling rates of from 106 to 104 °K/sec. The cooling rates for some of the newer compositions are now similar to those of conventional glassy materials. The goal has been the development of compositions which simultaneously exhibit:

  1. A large difference between the crystallization temperature (Tx) and glass forming temperatures (Tg), DTx=(Tx-Tg)
  2. A high reduced glass forming temperature (Tg/Tm)
  3. Low cooling rates

To achieve these conditions, Prof. Inoue's group has developed three rules for metallic glass forming:

  1. Use multicomponent systems containing three or more elements
  2. There should be significantly different atomic size ratios, >12%
  3. The mixtures should have large negative heats of mixing

Glass formation is encouraged when several phases are formed during normal crystallization as crystallization is suppressed by the long-range rearrangements required for the formation of the several phases. A high liquid-solid surface energy becomes a barrier to crystallization. Prof. Inoue indicated that both nucleation and growth of the crystalline phases should be difficult for good amorphous metal systems.

Figure 3.2 presents experimental results for numerous alloy systems showing variations in critical cooling rates versus reduced glass forming temperatures. Figure 3.3 presents results for the same systems showing cooling rate versus DTx. Figure 3.4 shows atomic radii of the constituent atoms for new Mg-, Ti-, and Al-based amorphous alloys. Figure 3.5 presents a summary of the reasons for the achievement of glass forming tendencies.

Fig. 3.2. Relation between the minimum critical cooling rate for glass formation (Rc) and reduced glass transition temperature.

Through their many studies, Prof. Inoue's group has demonstrated solidification of amorphous metal materials using the following procedures:
  1. Water quenching a melt in a quartz tube
  2. Casting in a copper mold
  3. High pressure die casting
  4. Arc melting in a water-cooled crucible
  5. Unidirectional solidification
  6. Suction casting

Consolidation of powdered amorphous metals has been demonstrated by both hot pressing and warm extrusion.

Fig. 3.3. Relation between Rc and the temperature interval of the supercooled liquid region DTx.

Fig. 3.4. Atomic radii of the constituent elements for new ternary amorphous alloys.

Fig. 3.5. Summary of the reasons for the achievement of large glass forming ability for ternary alloy systems.

The use of a wedge-shaped copper mold has been most helpful in determining critical cooling rates for amorphous structure formation as well as for predicting interface velocities versus cooling rates for unidirectional solidification. By water quenching a melt in a quartz glass tube, amorphous structures in a Zr-Al-Ni-Cu alloy have been produced with a diameter of 16 mm. Using a unidirectional zone melting method, samples of a Zr-Al-Ni-Cu-Pd alloy have been produced with a thickness of 10 mm, a width of 12 mm, and a length of 300 mm.

Mechanical studies of amorphous metal systems have shown them to exhibit Newtonian viscous behavior (stress proportional to strain rate) with no indications of work hardening. Tensile elongations up to 15,000% have been demonstrated when such materials are deformed in the supercooled liquid region. Nanocrystalline structures can be formed when amorphous metals are heated to temperatures at which crystallization can occur. The extremely fine structures so produced demonstrate quasi-viscous superplastic behavior (stress proportional to the square root of strain rate). With such ultra-fine structures superplastic behavior can be obtained at strain rates as high as 1 reciprocal second.

For the various amorphous metal compositions that are under investigation, potential applications include the following: electro-processes, small mechanical parts, magnetic applications, and biomedical applications based on high corrosion resistance, particularly with no grain boundaries.

For example, studies of high strength aluminum alloys are being carried out for YKK (zipper manufacturer). Amorphous structures have been produced by melt spinning in numerous systems including the following:

     -  Al-La                         -   Al-Ca
     -  Al-early transition metal     -   Al-late transition metal
     -  Al-Ca-Mg                      -   Al-Ca-Zn
     -  Al-Ca-late transition metal   -   Al-B-transition metal
     -  Al-Si-transition metal        -   Al-Ge-transition metal
     -  Al-Misch metal-Ni

Recent research has shown that very high strengths can be obtained by production of Al-based materials with a mixed amorphous +fcc Al structure. Such structures can be produced by controlled cooling conditions in which the equilibrium intermetallic phase formation is avoided, but a metastable +fcc Al phase can be nucleated. Similar studies with an Fe-Nd-B alloy have demonstrated the formation of a mixed triplex nanostructure consisting of +bcc Fe, tetragonal Fe14Nd2B, and remaining amorphous phases. The triplex nanostructure is a magnetically hard material while the totally amorphous structure has soft magnetic behavior.

National Industrial Research Institute of Nagoya

The WTEC team did not visit the National Industrial Research Institute of Nagoya. The institute is jointly sponsored by the Agency of Industrial Science and Technology and MITI. We learned that this center will be responsible for the super metals program on intermetallics and amorphous metals. During the trip we learned that the center is also involved in a program of reactive synthesis and melting of intermetallics through chemical reaction and subsequent pouring with vacuum assist.

Recent publications by members of the center indicate extensive ongoing efforts in the following areas:

Published: March 1997; WTEC Hyper-Librarian