Strong Liquid Behavior of Zr-Ti-Cu-Ni-Be Bulk Metallic Glass Forming Alloys
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ABSTRACT The viscosities of the Zr 46 .75 Ti 8 .25 Cu 7 .5 Ni10Be 2 7 .5 and the Zr 4 1 .2 Ti 13 .8Cu 12 .5 Ni10 Be 22 .5 bulk metallic glass forming liquids was determined from the melting point down to the glass transition in the entire temperature range of the supercooled liquid. The temperature dependence of the viscosity in the supercooled liquid obeys the Vogel-Fulcher-Tammann (VFT) relation. The fragility index D is about 20 for both alloys and the ratio between glass transition temperature and VFT temperature is found to be 1.5. A comparison with other glass forming systems shows that these bulk metallic glass formers are strong liquids comparable to sodium silicate glass. Furthermore, they are the strongest among metallic glass forming liquids. This behavior is a main contributing factor to the glass forming ability since it implicates a higher viscosity from the melting point down to the glass transition compared to other metallic liquids. Thus, the kinetics in the supercooled liquid is sluggish and yields a low critical cooling rate for glass formation. The relaxation behavior in the glass transition region of the alloys is consistent with their strong glassy nature as reflected by a stretching exponent that is close to 0.8. The microscopic origin of the strong liquid behavior of bulk metallic glass formers is discussed. INTRODUCTION Glass formation is observed in a large variety of materials such as silicates, polymers and ionically bonded systems. In metallic systems glass formation could be observed up to recently only after rapidly quenching the melt with rates of the order of 104-106 K/s 1-2. This resulted in thin ribbons or sheets with thicknesses of typically 20-50 l.tm. For a long time the only exception was the Ni-Pd-P alloy system where bulk material with thicknesses up to 1cm could be produced by fluxing the surface to avoid heterogeneous nucleation of crystals 3-4 . Recently the judicious choice of the compositions in multicomponent alloy systems made bulk metallic glass (BMG) formation much more common. Examples are La-Al-Ni 5, Zr-Al-Ni-Cu 6 and Zr-Ti-Cu-Ni-Be 7 of which the latter one is by far the best bulk metallic glass forming alloy system with critical cooling rates as low as 1K/s 8 Bulk metallic glasses have promising properties like high yield strength and a high elastic strain limit together with fatigue and corrosion resistance 9-10. These features make them interesting engineering materials e.g. as high strength, light weight materials, as springs, for bonding, or as composites with ceramics or intermetallic compounds. BMG forming liquids exhibit very good thermal stability with respect to crystallization in their supercooled state when cooled below the equilibrium melting point as well as when heated above their glass temperature. This high resistance with respect to crystallization allows for the first time to measure thermophysical properties of these supercooled metallic liquids in a broad time and temperature range. Results were, e.g., obtained on the specific heat capacityl
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