Structural and dynamical properties of glass-forming Ni66.7B33.3 melts

Abstract

One central goal of understanding transport phenomena in liquid matter is to establish relations between dynamical and structural properties on atomic scale. One possibility to link directly the structure and the atomic dynamics of densely packed melts is provided by the mode coupling theory (MCT) of glass formation [1], which makes predictions of dynamic quantities using partial structure factors of the liquid as an input. Detailed investigations of the short-range order and the atomic dynamics have recently been reported for melts of Zr-Ni [2,3] and Hf-Ni [4]. For these alloy systems MCT is able to reproduce the experimentally determined temperature dependence of the diffusion coefficients as well as the coupling behavior of the self-diffusion coefficients of the constituents reasonably well. Here, we report on investigations of the structure-dynamics relationship in melts of the metallic glass forming alloy Ni66.7B33.3, which exhibits a similar packing fraction, as well as almost identical Ni self-diffusion coefficients and melt viscosities as Zr64Ni36 [5]. Ni66.7B33.3 also shows a similar deviation from the Stokes-Einstein relation as observed in Zr64Ni36 melts, indicating the collective nature of the mass transport in the melt [5]. However, Ni-B exhibits also distinct differences when compared with the Zr-Ni system: boron is a non-metallic constituent with a considerably smaller atomic size than Ni. This raises the question whether these differences influence also the structural and dynamical features of the system and how the transport coefficients are affected by the atomic size ratio and chemical short-range order. We have studied the short-range order in Ni66.7B33.3 melts using a combination of neutron diffraction and isotopic substitution as well as X-ray diffraction. By utilizing additionally the containerless processing technique of electrostatic levitation, we were able to obtain partial structure factors of high precision. These are indicative of a pronounced chemical short-range order with a preference for the formation of Ni-B nearest neighbor pairs. Using the measured partial structure factors as an input, MCT predicts dynamic quantities like e.g. self- and interdiffusion coefficients, which are in good agreement with experimental results for liquid Ni66.7B33.3. The calculated B self-diffusivity is twice as large as the Ni self-diffusivity indicating decoupled atomic dynamics. With the knowledge of the mobility of both constituents, we find that interdiffusion in Ni66.7B33.3 is well described by Darken's equation. In contrast, for Zr64Ni36 it was found that the self-diffusion coefficients of Zr and Ni are almost equal [3], and the Onsager coefficient is considerably lower than predicted by Darken's equation [2]. Both can be attributed to the strong affinity between Zr and Ni [2,3]. We emphasize here again that the Ni self-diffusion coefficient and the melt viscosity are very similar for both melts and both systems show a chemical short-range order with a preference for the formation of heterogeneous nearest neighbor pairs [5]. However, concerning the coupling of the self-diffusion coefficients and the validity of Darken's equation, for Ni66.7B33.3 the situation appears to be different. This might be a consequence of the different atomic size ratios or bonding characteristics in Ni-B and Zr-Ni. [1] W. Götze (2008) Complex Dynamics of Glass-Forming Liquids: A Mode-Coupling Theory, Oxford University Press, New York; [2] T. Voigtmann et al., EPL 82, (2008) 66001; [3] B. Nowak et al., Phys. Rev. Mater. 1, (2017) 025603; [4] B. Nowak et al., PRB B 96 (2017) 054201; [5] S. Nell et al., PRB 103 (2021) 064206.