Time-Temperature Superposition of Structural Relaxation in a Viscous Metallic Liquid

Abstract

Bulk metallic glass-forming Pd 40 Ni 10 Cu 30 P 20 has been investigated in its equilibrium liquid by quasielastic neutron scattering. The quasielastic signal exhibits a structural relaxation as known from nonmetallic viscous liquids. Even well above the melting point, the structural relaxation is nonexponential and obeys a universal time-temperature superposition. From the mean relaxation times average diffusivities have been determined, resulting in values on a 10 210 m 2 s 21 scale, 3 orders of magnitude slower than in simple metallic liquids. PACS numbers: 61.25.Mv, 61.20.Lc, 64.70.Pf During the past few years, new multicomponent Zrbased alloys have been found which show a high stability against crystallization in their undercooled liquid state when cooled below their melting point as well as when heated above the calorimetric glass transition temperature [1,2]. Cooling rates as low as 1 K͞s allow for the casting of bulk metallic glass of up to 50 mm in the smallest dimension. The stability with respect to crystallization comes along with a viscosity of several Pa s at their liquidus temperatures [3,4], which is some 3 orders of magnitude larger than in simple metallic liquids. Hence, one expects atomic motion in the liquid state to differ from the microscopic dynamics in simple metals [5].Recently, we investigated the fast dynamics in liquid Zr 46.8 Ti 8.2 Cu 7.5 Ni 10 Be 27.5 ͑V 4͒ by inelastic neutron scattering [6]. Our results compare well to the behavior of certain molecular and ionic glass-forming liquids [7], and to predictions by the mode-coupling theory (MCT) of the liquid-to-glass transition [8]. Within mode-coupling theory, a fast relaxation process on a meV scale-the fast b relaxation, which can be visualized as a rattling of the atoms in the transient cages formed by their neighborsprepares structural a relaxation responsible for viscous flow. It turned out, that, indeed, there is a fast b relaxation in liquid Zr 46.8 Ti 8.2 Cu 7.5 Ni 10 Be 27.5 and that it is in full accord with mode-coupling theory. The dynamics in an equilibrium metallic liquid towards longer times as well as its behavior relative to the structural a relaxation known from nonmetallic glass-forming systems, remains an open question.In this Letter we report on the study of structural relaxation in liquid Pd 40 Ni 10 Cu 30 P 20 over a broad temperature range. This alloy is the best metallic glass former known so far. A critical cooling rate of only 0.1 K͞s is sufficient to avoid crystallization and to obtain bulk metallic glass castings of up to 72 mm in diameter [9]. Pd 40 Ni 10 Cu 30 P 20 combines a deep eutectic composition with a liquidus temperature T liq at only 865 K (V 4: 1050 K) with a high glass transition temperature T g Ӎ 578 K (V 4: Ӎ 626 K). Therefore, in contrast to simple metallic liquids and most alloys, relaxational dynamics in liquid Pd 40 Ni 10 Cu 30 P 20 is sufficiently slow to study structural relaxation in an equilibrium metallic liquid with quasielastic neutron scattering.Common features of structural...

Crossover of Microscopic Dynamics in Metallic Supercooled Liquid Observed by NMR

“…(4) with T c 660 10 K assuming the shift is proportional to hu 2 i over the entire temperature range. The T c value agrees within the range of uncertainties with diffusion results [10,27]. This by no means implies that the change is abrupt at 660 K. The change might be gradual below 700 K and the effective T c might be as high as 700 K. The detail is obscured by the lack of K data in that temperature range due to fast crystallization.…”

Atomic transport in dense multicomponent metallic liquids

“…In an experiment on viscous Pd 40 Ni 10 Cu 30 P 20 (Pd40) the focus was on slow dynamics [23]: The long-time decay of correlations exhibits the common features of glass forming liquids, i.e. structural relaxation obeys a stretching in time and a universal time-temperature superposition.…”

The inquiry of liquids and glass transition by heat capacity

“…Bulk metallic glasses 53 are much more stable up to temperatures well above T g , and allow accurate measurement of the properties above T g without intervening crystallization. [54][55][56][57] Even that, only the special bulk metallic glass-forming alloy, Pd 40 Ni 10 Cu 30 P 20 , can make it possible to measure the heat capacity of liquid metal in whole undercooled liquid region. 58 Figure 4 shows the specific heat (C P ) of the Pd 40 Ni 10 Cu 30 P 20 metallic glass former in the temperature region covering the solid, undercooled liquid and normal liquid region.…”