Diffusion in metallic glasses and supercooled melts

Diffusion in a binary amorphous metal: Pair-correlation in Cu33Zr67

In order to clarify the role of thermal defects in diffusion in metallic glasses, we have measured the temperature and pressure dependences of diffusion of Ni, a probe for Co, in relaxed and non-relaxed amorphous Co42Zr58 alloys by means of the tracer technique in combination with secondary-ion mass spectroscopy. For the relaxed state, the activation energy and the pre-factor are Q = (1:65=0:08) eV and D 0 = 2:4+3:9 10-6m2 s-1 respectively. The pressure - 1:9 dependence yields an activation volume V act = (0:66 0:15)Omega, with Omega being the average atomic volume of the alloy. This value is similar to activation volumes in crystalline materials and is indicative of diffusion via thermally generated defects. Unlike monovacancies in crystals, these defects appear to be spread out, judging from recent isotope-effect measurements. Comparison with literature data shows that the activation volume and hence the diffusion mechanism in metallic glasses clearly depend on the structure and composition even for the very same component. The activation volume in the as-quenched state was found to be ( ) 0:93 0:25 Omega.

Diffusion in metallic glasses: Analysis from the atomic bond defect perspective

Correlation between parameters of Arrhenius-type temperature dependency for effective diffusivity governing glass crystallization

Mechanisms of bulk and surface diffusion in metallic glasses determined from molecular dynamics simulations

The enhancement of surface diffusion (DS) over the bulk (DV) in metallic glasses (MGs) is well documented and likely to strongly influence the properties of glasses grown by vapor deposition. Here, we use classical molecular dynamics (MD) simulations to identify different factors influencing the enhancement of surface diffusion in MGs. MGs have a simple atomic structure and belong to the category of moderately fragile glasses that undergo pronounced slowdown of bulk dynamics with cooling close to the glass transition temperature (Tg). We observe that DS exhibits a much more moderate slowdown compared to DV when approaching Tg, and DS/DV at Tg varies by two orders of magnitude among the MGs investigated. We demonstrate that both the surface energy and the fraction of missing bonds for surface atoms show good correlation to DS/DV, implying that the loss of nearest neighbors at the surface directly translates into higher mobility, unlike the behavior of network-bonded and hydrogen-bonded organic glasses. Fragility, a measure of the slowdown of bulk dynamics close to Tg, also correlates to DS/DV, with more fragile systems having larger surface enhancement of mobility. The deviations observed in the fragility-DS/DV relationship are shown to be correlated to the extent of segregation or depletion of the mobile element at the surface. Finally, we explore the relationship between the diffusion pre-exponential factor (D0) and the activation energy (Q) and compare it to a ln(D0)-Q correlation previously established for bulk glasses, demonstrating similar correlations from MD as in the experiments and that the surface and bulk have very similar ln(D0)-Q correlations.

Local electronic structures of Zr-based metallic glasses have been calculated using the discrete variational Xcluster molecular orbital method. The cluster models are constructed for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses on the base of local structure parameters determined by extended X-ray absorption fine structure analysis. The valence-band X-ray photoelectron spectra computed from the local electronic structures agree well with experimental spectra. Based on the feature of chemical bonds in those glasses evaluated from the electronic structures, we discuss the relationship between the chemical bond and the stability of supercooled liquid state. The strength of the Zr-Ni bond is larger than that of Zr-Cu bond, while the Cu-Cu bond strength is nearly equal to the Ni-Ni bond. Moreover, the Zr-Zr bond strength is larger in Zr-Cu glass than in Zr-Ni glass. This indicates that the formation of primary crystalline phase is more difficult in Zr-Cu glass than in Zr-Ni glass. The difficulty contributes to the relative stabilization of supercooled liquid state in Zr-Cu glass compared with that in Zr-Ni glass. It is also shown that local atomic structures of Zr-Pd glass are well described by the icosahedral atomic configuration. Recently, bulk metallic glasses have attracted much attention because of the importance in materials science and the extension of their application fields. 1) Bulk metallic glasses are characterized by a wide supercooled liquid region, � Tx which is defined as the temperature interval between glass transition temperature, Tg and crystallization temper- ature, Tx. The appearance of the wide supercooled liquid region implies the high resistance against crystallization, leading to large glass-forming ability. The stable supercooled liquid state enables the production of bulk metallic glasses. It is therefore quite important to understand the reason for the stabilization of the supercooled liquid state from the view- point of interests in materials science as well as development of new bulk metallic glasses. The three empirical rules have been proposed by Inoue 2) in order to explain the origin of the wide supercooled liquid region and good glass-forming ability in bulk metallic glasses. It is also suggested that the stability of the supercooled liquid state is strongly correlated with their local structure. 3-6) Alamgir et al. 7,8) have inves- tigated the relation between the electronic structure and the glass-forming ability in Pd-Ni-P glassy alloys, and pointed out that the existing electron models are not fully adequate to explain the high glass stability of the Pd40Ni40P20 alloy. However, little is known about the relation of the chemical bond among constituent atoms with the stability of super- cooled liquid state in bulk metallic glasses. The present study is intended to investigate the local electronic structures and the chemical bonds among constit- uent atoms in Zr-based metallic glasses, and to clarify the relation between the chemical bond and the stability of supercooled liquid state. The local electronic structures are calculated for Zr-Cu, Zr-Ni and Zr-Pd metallic glasses using the discrete variational (DV) Xcluster molecular orbital

A molecular dynamics simulation of the effects of excess free volume on the diffusion in metallic glasses

Nuclear-reaction methods to measure boron diffusion in metallic glasses

Point Defects and Atomic Diffusion in Metallic Glasses

Amorphous metal alloys (AMА), obtained by the method of ultra-fast cooling from the melt, have a non-crystalline structure, which determines their special physicochemical properties. Alloys based on Co are characterized by high magnetic permeability, which makes it possible to use them in the production of transformers and other devices. In addition, amorphous metal alloys based on Co are promising for use in biomedical technologies because they are highly biocompatible with human tissues and organs. Since amorphous metal alloys are unique materials, there is a need to study their corrosion resistance in aggressive environments, in particular, potassium hydroxide. In order to increase the catalytic activity of amorphous alloys for the release of hydrogen from alkaline solutions, alloys are modified, in particular, by heat treatment. In order to preserve the integrity of the alloy, but possibly increase the catalytic activity, samples of AMА Со72,0Fе5,0Sі11,0В12,0 were preheated to temperatures of 473 and 673 K at a speed of 10 K/s. The corrosion resistance of the strip amorphous alloy Со72,0Fе5,0Sі11,0В12,0 in a 1.0 M aqueous solution of KOH was investigated by the method of cyclic voltammetry in the potentiodynamic mode. During contact with an alkaline solution, the corrosion potential shifts to the cathodic side, and the corrosion currents increase, which indicates a decrease in the durability of the sample. Preheating of AMC samples to 673 K helps to increase their corrosion durability. Such dependencies are determined by the change in morphology, distribution of elements, and elemental composition of modified samples of AMC Со72,0Fе5,0Sі11,0В12,0, which determines their electrochemical and catalytic activity. Therefore, such temperature modification can be used for additional protection of the surface from destruction in aggressive environments, as well as for intensification of hydrogen release processes from alkaline solutions. Keywords: amorphous metal alloys, cobalt, corrosion resistance, heat treatment.

Previously published results have shown that viscosity greatly influences on the deformation behavior of the bulk amorphous alloy in supercooled liquid region during microforming process. And viscosity is proved to be a component of the evaluation index which indicating microformability. Based on the fluid flow theory and assumptions, bulk amorphous alloy can be regarded as the viscous materials with a certain viscosity. It is helpful to understand how the viscosity plays an important role in viscous materials with various viscosities by numerical simulation on the process. Analysis is carried out by linear state equation in FEM with other three materials, water, lubricant oil and polymer melt, whose viscosities are different obviously. The depths of the materials flow into the U‐shaped groove during the microimprinting process are compared in this paper. The result shows that the deformation is quite different when surface tension effect is not considered in the case. With the lowest viscosity, water can reach the bottom of micro groove in a very short time. Lubricant oil and polymer melt slower than it. Moreover bulk amorphous alloys in supercooled liquid state just flow into the groove slightly. Among the alloys of different systems including Pd‐, Mg‐ and Zr‐based alloy, Pd‐based alloy ranks largest in the depth. Mg‐based alloy is the second. And Zr‐based alloy is the third. Further more the rank order of the viscosities of the alloys is Pd‐, Mg‐ and Zr‐based. It agrees well with the results of calculation. Therefore viscosity plays an important role in the microforming of the bulk amorphous alloy in the supercooled liquid state.

Effect of reversible structural relaxation on diffusion in bulk metallic glasses

Formation of Zr-based bulk metallic glass with large amount of yttrium addition

Microformability of new amorphous alloys in the supercooled liquid state and microforming techniques for the materials are shown. In the supercooled liquid state, the materials reveal perfect Newtonian viscous flow characteristics and furthermore exhibit an excellent property of microformability on a submicron scale. Microforming techniques are shown as follows; microextrusion with micro-dies made of photochemically machinable glass and made of laser-micromachined polyimide, submicron imprinting with silicon die fabricated by EB lithography and etching, microforging of microgear of 10 /spl mu/m in module with microdie fabricated by UV-LIGA process. As a result, the materials are expected to become some of the most useful materials for fabricating MEMS.