Site:               Tohoku University
                    Institute for Material Research
                    Katahiro 2-1-1, Aoba-ku
                    Sendai 980-77, Japan

Date Visited:       January 9, 1996

WTEC Attendees:     H.W. Hayden (report author), T.S. Piwonka

Host:               Dr. Akihisa Inoue, Professor
                      Tel: (81) 022-2152110; Fax: (81) 022-2152111

BACKGROUND

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

Professor 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 the direct costs, e.g. equipment, associated with the research of each associate. These costs do not include the salaries of his associates.

In the several research laboratories under Professor Inoue's direction, there is a complete range of processing, testing, and analytical equipment to support his studies on amorphous metallic materials.

RESEARCH AND DEVELOPMENT

Professor Inoue presented a comprehensive review of the history of the efforts of his group on amorphous and nano-crystalline metallic materials. Amorphous metallic materials are of great interest for their unique properties including the following:

Over the period this group has progressed in understanding of 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 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 in which there can be a large difference between the crystallization and glass forming temperatures at low cooling rates.

As a result of these studies, Professor Inoue's group has developed three rules for metallic glass forming:

  1. Use multi-component 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 rearrangments required for the formation of the several phases. A high liquid-solid surface energy becomes a barrier to crystallization. Professor Inoue indicated that both nucleation and growth of the crystalline phases should be difficult for good amorphous metal systems.

Through many studies, Professor 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. Uni-directional solidification
  6. Suction casting

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

The use of a wedge-shaped copper mold has been most useful for the determination of critical cooling rates for amorphous structure formation as well as for the prediction of interface velocities versus cooling rates for unidirectional solidification.

Mechanical studies on amorphous metal systems have shown them to exhibit Newtonian viscous behavior (stress being proportional to strain-rate) with no indication of work hardening. Nano-crystalline structures can be formed when amorphous metals are heated to temperatures at which crystallization can occur. The extremely fine structures so produced demonstrate high-temperature quasi-viscous superplastic behavior (stress being 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 which are under investigation, potential applications include bio-medical applications based on high corrosion resistance, particularly with no grain boundaries; electro-processes; small mechanical parts; and magnetic applications.

As an example, studies of high strength aluminum alloys are being carried out for YKK (a zipper manufacturer). Amorphous structures have been produced by melt spinning in numerous systems including 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, and 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 metal stable +fcc Al phase can be nucleated. Similar studies with an Fe-Misch metal-B alloy have shown that the formation of a mixed amorphous +fcc Fe structure leads to a hard magnetic material while the totally amorphous structure has soft magnetic behavior.

REFERENCES

Inoue, Akihisa. 1995. "High Strength Bulk Amorphous Alloys with Low Critical Cooling Rates (Overview)," Materials Transactions, JIM, Vol. 36, No. 7, July, pp. 866-875.

Inoue, Akihisa. 1995. "Recent Progress of Zr-Based Amorphous Alloys," submitted for publication November 30.

Inoue, A., A. Takeuchi, A. Makino, and T. Masumoto. 1995. "Soft and Hard Magnetic Properties of Nanocrystalline Fe-M-B (M=Zr, Nd) Base Alloys Containing Intergranular Amorphous Phase," submitted for publication November 30.


Published: March 1997; WTEC Hyper-Librarian