Investigation on the Thermal Stability of Bulk Glass Forming Alloy Pd-Cu-Ni-P
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Investigation on the Thermal Stability of Bulk Glass Forming Alloy Pd-Cu-Ni-P I.-R. Lu, G. P. Görler and R. Willnecker Institute of Space Simulation, DLR, Linder Hoehe, D-51147 Cologne, Germany H. J. Fecht Faculty of Engineering, Materials Division, Ulm University Albert Einstein Allee 47, D-89081 Ulm, Germany
ABSTRACT The metallic glass former Pd-Ni-P is well known for its pronounced stability against crystallization. Samples of this alloy vitrify completely at low cooling rates down to 0.1 K/s. The addition of copper to this alloy system reduces further the crystallization kinetics significantly. Investigations on critical cooling rates were performed on Pd-Cu-Ni-P alloy by means of isothermal nucleation experiments. The results indicate a critical cooling rate in the order of 5⋅10-3 K/s, which is the lowest one presently known for metallic glass-formers. The high stability against crystallization during cooling allows for simultaneous measurements of its thermodynamic properties within the entire temperature range from the regime of the liquid to the glassy state. Heat capacity measurements were carried out by differential heat-flow calorimetry and the coefficient of thermal expansion was determined by applying sessile drop technique. The results can be described within the free-volume model. INTRODUCTION The Pd-based metallic alloys Pd-Cu-Si and Pd-Ni-P are well known for their glass forming ability [1,2]. Glassy samples of Pd-Ni-P can be prepared at cooling rates of about 0.1 K/s [3]. Especially, alloys composed of Pd-Cu-Ni-P can be vitrified even more easily and have the largest interval between the glass transition and the crystallization temperature in comparison to other metallic alloys [4,5]. Therefore, the properties of the undercooled liquid and the vitreous state for these metallic alloys can be investigated during continuous cooling without interference from crystallization. In previous works, the thermodynamic glass temperatures, Tg*, defined in the same way as the limiting fictive temperatures, were derived from calorimetric and first volumetric measurements for samples vitrified at constant cooling rates. Consistent Tg* values of glasses were determined independently from the differential calorimetry and thermal expansion measurements [6]. The change of enthalpy and volume in the glass transition region could be described within the model of free volume and attributed to structural relaxation processes. In the present work, investigations on the accessibility to the undercooled liquid regime on long time scales as well as on the dependence of heating rate are performed in order to further study the thermal stability of the liquid samples. The results allow us to perform additional measurements of the thermal expansion coefficient in an extended temperature range in the liquid, glassy and crystalline state of the alloys.
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EXPERIMENTAL DETAILS Pd-Cu-Ni-P alloys composed of Pd40Cu30Ni10P20, Pd43Cu27Ni10P20 and Pd44Cu30Ni6P20 (at.%) were prepared by induction melting of the pure elements under
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