2015 Fiscal Year Research-status Report
Probing the atomic structure and physical properties of super-cooled liquid alloys during vitrification
| Project/Area Number |
15K18201
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| Research Institution | Tohoku University |
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Principal Investigator |
G コンスタンティノス 東北大学, 原子分子材料科学高等研究機構, 助教 (30526794)
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| Project Period (FY) |
2015-04-01 – 2017-03-31
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| Keywords | アモルファス / 金属ガラス / 準結晶 |
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| Outline of Annual Research Achievements |
The project aims in tracing the structural pathway of liquid metallic alloys to vitrification and contribute to the understanding of the long standing puzzle of glass formation and glass transition. Upon cooling from the melt, a remarkable increase in viscosity occurs below the liquidus temperature T(liq) resulting in a dynamic arrest at the glass transition temperature Tg, below which the amorphous structure “freezes”. The structural evolution between T(liq)and Tg has been so far hindered due to crystallization upon cooling during in-situ structural experiments. In the framework of this project, we were able to overcome this limitation and achieve in-situ vitrification of various metallic alloys in a synchrotron beam. This achievement allowed the acquisition of structural data in the whole range from the liquid through the super-cooled liquid and the glassy state offering a unique possibility to uncover the structural evolution during vitrification. This outstanding achievement was rendered possible by combining the aerodynamic levitation technique and synchrotron radiation X-ray Diffraction using an ultra-fast acquisition 2-D detector. The results indicate that three different structural regimes exist in the pathway of metallic liquids to vitrification, pointing to a structural transition (unknown until today) below the melting temperature. The undercooled liquid enters into a different structural configuration below the liquidus temperature T(liq) and undergoes fast structural rearrangements, thus enhancing strongly the short (SRO) and medium (MRO) range atomic order.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
A variety of glass forming alloys were rapidly solidified in a synchrotron beam using containeless aerodynamic levitation. The structural evolution during solidification was observed by high energy synchrotron radiation X-ray diffraction from well above the liquidus down to temperatures close to room temperature. High cooling rates, up to about 80 K/s, were achieved allowing the in-situ vitrification of several of the studied alloys. In particular, the structural information during vitrification was obtained for ternary and multicomponent Zr-based and Ti-based alloys, whereas the atomic structure of highly undercooled liquids was observed for binary Zr- and Cu- based alloys. The investigation has taken place primarily at Tohoku University, whereas experiments including synchrotron measurements were performed at the ESRF and INP Grenoble, France. The evolution of the atomic structure is analyzed using the structure factor, pair distribution function and simulations. The rate of structural rearrangements is enhanced in the super-cooled liquid region between T(liq) and Tg with icosahedral local symmetry becoming predominant as Tg is approached, leading to the suppression of crystal nucleation and growth. The scenario of a possible fragile to strong transition in the super-cooled liquid is under investigation. The structural evolution below the melting temperature deviates from that of the liquid, pointing to a liquid-liquid transition. The structural rearrangements in the supercooled liquid are correlated with rapid increase in viscosity of the metallic liquids upon cooling.
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| Strategy for Future Research Activity |
Research activities will primarily focus on the analysis of the structural evolution of the glass forming liquid vitrified in-situ, as well as to the comparison of crystal formation during in-situ containerless solidification of glass forming and non-forming metallic melts. The evolution of the structure factor, S(Q) and the pair distribution function,G(r) with the temperature will be used to probe the structure of the supercooled liquids and investigate how the atomic chemical and topological short (SRO) and medium -range-order (MRO) develop from the liquid to the glassy state. The results from a variety of glass forming liquids will be critically compared aiming to a qualitative and quantitative analysis of the structural evolution. The density of metallic liquids will be analyzed as a function of temperature during cooling by accurate volume measurements in the levitating liquids using a high-resolution high- speed CCD camera. The evolution of heat expansion coefficient during cooling will be estimated from the volume changes. The verification of a second order structural transition between the liquid and the supercooled liquid phase, below the melting temperature Tm (implied by the results so far) is expected to lead to groundbreaking improvements to our understanding of glass formation in metals. The itineraries of atoms and atomic configurations during vitrification will shed light on the glass formation and allow a better understanding of the structural origins of the sluggish long-range order kinetics that suppress nucleation and growth of crystalline phases.
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| Causes of Carryover |
A part of the budget previewed for the first year has not been spent and is planned to be used during the second year of the project. The main reason for this was a delay related to the provision of a proper high- speed camera equipped with appropriate filters for the acquisition of high-resolution images of metallic melts at high temperatures, aiming to study the volume evolution during cooling. However the limitations in defining the proper equipment have now been overcome and the purchase of the proper camera is previewed for the near future. This delay has not significantly affected the research activities, since according to the research plan of the project, the investigation of volume evolution of metallic melts during cooling has been planned for the second year.
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| Expenditure Plan for Carryover Budget |
The remaining budget for the second year (the amount previewed and the amount requested to be transferred to FY 2016) is essential for the successful completion of the research project. A part of the budget (~900,000Yen) is planned to be used for equipment such as a high-speed, high-resolution camera and a viscometer. Another part (~300,000 Yen) of the budget will be dedicated to laboratory consumables. A third part of the budget (~600,000 Yen) will be used to cover travel expenses for measurements at the Synchrotron Radiation Facility (ESRF) in Grenoble (France) and the participation to International Conferences (invited presentations). In addition, a part of the budget (~300,000 yen) will be used for publication expenses, books and technical software.
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