Papers by Author: Nobuyuki Nishiyama

Abstract: The kinetics of structural relaxation in fragile glass former, Pd46Cu35.5P18.5 BMG, and strong glass former, Zr50Cu40Al10 BMG, was investigated by volume relaxation. The former exhibited a relaxation phenomenon that is well understood by the local topological instability model, while the latter showed monotonous relaxation behavior over a wide range down to Tg-60 K. The discrepancy may be closely related to the difference in the fragility of both glasses.

Abstract: Nanoscale structural change in (Fe0.5Co0.5)72B20Si4Nb4 bulk glassy alloy on annealing has been investigated using transmission electron microscopy. On annealing at temperatures above 773K, electron diffraction intensity analysis showed a clear structure change for a Cr23C6-type local atomic ordering. The local structure formation of Cr23C6-type nanophase was confirmed by nanobeam electron diffraction. A development process of dense precipitates of the Cr23C6-type nanophase was further studied by high-resolution electron microscopy. It was found that the formation of the highly-dense nanoprecipitates provides an increase in Vickers hardness.

Abstract: Viscosity of a Zr55Cu30Al10Ni5 supercooled liquid alloy having bulk metallic glass forming ability has been measured by using a penetration viscometer with a cylindrical probe under high speed heating conditions at rates between 20°C/min and 400°C/min in the temperature range from the glass transition temperatures (Tg) up to above the crystallization temperatures (Tx). The viscosity of the alloy decreased with increasing the heating rate and tended to saturate at the heating rate of 200°C/min and above. Corresponding measurements of the differential thermal calorimetry (DSC) under the high-speed heating conditions have been also done.

Abstract: The free volume relaxation just under the glass transition region was investigated by the high-resolution density measurement using the bulk metallic glasses with the compositions of Pd40Ni40P20 and Zr55Cu30Ni5Al10. The relaxation process was well described by a stretched exponential function with Kohlrausch exponent values less than unity. The reduced free volumes in an as-quenched state were estimated as 0.0117 and 0.0353 for Pd40Ni40P20 glass and Zr55Cu30Ni5Al10 glass, respectively. The specific heat curves Cp(T) for Pd42.5Cu30Ni7.5P20 alloy were obtained for the supercooled liquid, the equilibrium liquid and the crystallized alloy. The isenthalpic Kauzmann temperature TKH and isentropic Kauzmann temperature TKS were estimated as 471 K and 522 K, respectively, from the specific heat data.