Metallic Glass

“Metallic Glass” opens a new field in materials science Development of new light-weight, high-strength materials

Prof. Inoue developed "metallic glass" having excellent mechanical properties, e.g., high tensile strength and large elastic strains, and he is currently a leader of worldwide researchers in materials science. It has been generally known that, when solid lacks a systematic atomic arrangement, that is, when it is in an amorphous state, its strength and corrosion resistance are enhanced. People had believed, to make an amorphous alloy, the rapid cooling of the molten alloys is required and thus it is quite difficult to obtain amorphous alloys in bulk. Contrary to this widely conceived belief, Prof. Inoue succeeded, for the first time in the world, in developing metallic glass which makes it possible to prepare bulk amorphous alloys without rapid cooling. The papers published by him and his colleagues about their discoveries have been highly evaluated by other researchers. Indeed, the number of their citations is ranked at the highest level, and Prof. Inoue has been awarded many prizes for his many discoveries including the Japan Academy Award in 2002. In 1982, Prof. Inoue became a research fellow at AT&T Bell Laboratories (currently Lucent Technologies, Bell Laboratories), and there, met Dr. Chen who had discovered a glass-transition phenomenon in metals. This led Prof. Inoue to develop an interest in the structural relaxation phenomena observed among non-equilibrium materials. From then on, his study on the structural relaxation and glass transition of non-equilibrium materials advanced steadfastly. During the course of his study, he came to conceive the belief, "if it were possible to reveal the principle governing the formation of a glassy metal which exhibits a glass-transition phenomenon and supercooled liquid state, it would be possible to produce bulk amorphous alloys." When a liquid material is cooled very rapidly, it does not crystallize even when it is cooled below its freezing point, and maintains its liquid state, which is called a supercooled state. When Prof. Inoue began to study metallic supercooled liquids, he decided to reveal the principle underlying the phenomena, and studied to obtain reliable thermodynamic data related to the phenomena. In 1987, he found an alloy having a wide temperature range in which a supercooled state is maintained. This discovery stimulated his interest in developing bulk amorphous alloys. In 1988, Prof. Inoue found a Zr-based alloy which maintains a supercooled state down to a temperature equal to 60% of the freezing point even when cooled at a rate as slow as 10 K/sec, and then solidifies as glass. This alloy exhibits markedly different mechanical properties depending on its microscopic structure: the crystallized alloy is broken to pieces when hit with a hammer, but the glassy alloy is quite resistant to the same impact. This glassy alloy was a "bulk metallic glass" that Prof. Inoue had sought. This new alloy was found to have excellent mechanical properties. It exhibited ideal superplasticity. It had a tensile strength three times as high as that of the crystalline alloys that had the same Young's modulus. It also had an elastic elongation at least five times as high as that of conventional crystalline alloys. The elastic energy the glassy alloy could store just before it reached a yield point was twenty times or more as high as that of conventional crystalline alloys. Prof. Inoue reported his discoveries at some meetings in Japan. At that time, his papers did not attract much attention from the audience. This was probably because people confounded the metallic glass he had discovered with an amorphous metal. However, the situation changed dramatically when the results were made public to scientists around the world. In 1993, five years after Prof. Inoue's publication of Zr-based metallic glasses, a group of researchers in the USA who had secretly traced his research, published their discovery of a metallic glass obtained from a beryllium-based alloy system, which in turn suddenly ignited the interest of researchers in metallic glasses. By that time, Prof. Inoue had discovered several hundreds of kinds of metallic glasses, and in 1994 he deduced, from the observations accumulated during the course of his study, empirical rules determining the glass-forming ability of an alloy which are now called "Inoue's three empirical rules." Based on these rules, Prof. Inoue further continued his search for new metallic glasses, and added new alloys to a list of metallic glasses he had prepared. Establishment of these empirical rules is based on his enthusiasm towards finding a fundamental concept applicable to all materials having a glass-forming ability and thus profitable to all materials scientists interested in metallic glasses around the world, rather than being based on a simple desire to devise a method for finding a new resource of metallic glasses. Metallic glasses having a thickness ranging from 1 to 100 mm have been fabricated by employing various casting processes appropriate to the alloy systems. Indeed, the face plate of a golf club made of a metallic glass has been put to practical use. Currently, Prof. Inoue's interest has shifted to nanostructured bulk alloys with high strength and toughness, and his studies in this field also lead materials scientists around the world. He carries out research on strengthening of materials by crystallizing metallic glasses partially. That is, bulk metallic glasses are partially crystallized by adding a small amount of elements that do not satisfy the Inoue's empirical rules into conventional metallic glass systems. The partially crystallized metallic glasses have nanoscale crystals with a diameter of 1 nm or more in their glassy matrix. This nanostructural feature is responsible for the improved tensile strength and toughness of these new alloys. Prof. Inoue predicts confidently, "Maybe in ten years the metallic glasses we have developed will be used as a basic material for nanotechnology because of their excellent viscosity, fluidity and workability. This is because there are no metallic materials that are more readily amenable to fine processing than these metallic glasses."