Ordering and Phase Transitions in Supercooled Metallic Liquids and Glasses

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TECHNICAL SUMMARY:Bulk metallic glasses (BMGs), which can be formed under similar processing conditions as the familiar silicate glasses, are finding an increasing number of technological applications. Why a few metallic liquids form BMGs, while the majority of metallic liquids do not, remains unclear. Previous studies by us and by others have demonstrated that metallic glasses contain medium-range and short-range structural order, with icosahedral short-range ordering (ISRO) being most common. We have demonstrated that this structural ordering originates in the high temperature liquid far above the glass transition temperature, and is often accompanied by chemical ordering. Since ISRO is incompatible with crystal periodicity, it raises the nucleation barrier for the crystal phases, thereby enhancing glass formability in some cases. Molecular dynamics studies indicate that this ordering influences the atomic motion underlying viscous behavior, which also plays a key role in glass formation. Additionally, fine-scale phase separation has been observed in many glasses; whether this occurs in supercooled liquids, where it would also impact glass formation, has not been investigated previously. Investigations of these issues require that the liquids be processed in a containerless environment, eliminating heterogeneous nucleation on container walls and increasing the amount of supercooling. This group has pioneered the use of electrostatic levitation techniques to make X-ray structural studies of deeply supercooled metallic and semiconductor liquids. Under this grant, those studies will be extended by making wide and small angle synchrotron X-ray and elastic neutron scattering studies, and correlating those with measurements of the densities, viscosities and crystallization of metallic glass forming liquids. The insights gained from this research will lead to improved control of glass formation and nanostructure development during controlled devitrification.NON-TECHNICAL SUMMARY:Silicate glasses are familiar materials, used for thousands of years to produce decorative and practical objects. Metallic glasses are much less familiar. First discovered in 1960, they typically require that the liquids from which they form be cooled at rates approaching one million degrees per second, significantly limiting their technological usefulness. A few metallic glasses are now known that can be fabricated using much slower cooling rates, comparable to those used to produce silicate glasses. These novel materials are often stronger than steel, extremely hard and resistant to corrosion. Further, they can be molded into complex shapes that cannot be obtained with conventional metals. These glasses are now used in pressure sensors, micro-geared motors, transformer cores, biomedical components and sports equipment. However, it remains unclear why a few metallic liquids will form these glasses, while most will not. The research that will be carried out under the support of this grant will address this key question. This group has pioneered the use of electrostatic levitation for containerless studies of liquid structures, and has demonstrated that metallic-glass-forming liquids develop significant topological short and medium-range order. The scattering of X-rays and neutrons from these liquids will be measured to determine the chemical ordering and to investigate possible liquid phase separation on the nanometer scale, which may accompany the structural ordering. These data will be correlated with measurements of key physical properties important for glass formation, including the liquid density, viscosity, and resistance to crystallization. In addition to their relevance to improved basic understanding, the results of these studies will lead to refined methods for metallic glass formation and for the control of nanostructure development during glass and liquid crystallization. The research will provide scientific training to both graduate and undergraduate students. Simple and practical materials science experiments and demonstrations inspired by this research, will constitute a series of workshops for secondary high-school teachers, led by the PI and his students, to help teachers convey scientific principles and the excitement of a scientific career to high school students.