Effects of dopants on the glass forming ability in Al-based metallic alloy

Yang Sun, Feng Zhang, Lin Yang, Huajing Song, Mikhail I. Mendelev, Cai-Zhuang Wang, and Kai-Ming Ho

Phys. Rev. Materials 3, 023404 – Published 15 February 2019

Abstract

The effect of dopants on the metallic glass forming ability is usually considered based on the analysis of changes in the liquid structure or thermodynamics. What is missing in such considerations is an analysis of how a dopant changes the properties of the crystal phases which can form instead of the glass. In order to illuminate this aspect we performed molecular dynamics simulations to study the effects of Mg and Sm dopants on the crystal nucleation in Al. The simulation data were found to be consistent with the experimental observations that addition of Mg to Al does not lead to vitrification but addition of only 8% Sm does. The significant effect of Sm doping was related to the intolerance of Al to this dopant. This leads to increase in the solid-liquid interfacial free energy, and therefore, to increase in the nucleation barrier and to a dramatic decrease in the nucleation rate. The intolerance mechanism also significantly affects the growth kinetics.

Received 16 May 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.3.023404

Physics Subject Headings (PhySH)

Growth Nucleation

Liquid-solid interfaces Metallic glasses

Molecular dynamics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yang Sun¹, Feng Zhang¹, Lin Yang^1,2, Huajing Song¹, Mikhail I. Mendelev^1,*, Cai-Zhuang Wang^1,2, and Kai-Ming Ho^1,2,†

¹Ames Laboratory, US Department of Energy, Ames, Iowa 50011, USA
²Department of Physics, Iowa State University, Ames, Iowa 50011, USA

^*mendelev@ameslab.gov
^†kmh@ameslab.gov

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Vol. 3, Iss. 2 — February 2019

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Figure 1
The comparison of liquid structures of the Al-Sm and Al-Mg alloys. (a) The total pair correlation functions. (b) The icosahedra and FCC SRO of Al-centered clusters in the Al-Sm and Al-Mg alloys with the dopant concentrations of 5 and 10 at. % at 560 K obtained by the cluster alignment. The score cutoff to identify the SRO was set to 0.16 [28]. (c) The population of the top two Voronoi polyhedra $〈 0, 0, 12, 0 〉 and 〈 0, 1, 10, 2 〉$ in the Al-Sm and Al-Mg alloys. Both two Voronoi polyhedra are associated with the icosahedral ordering [2].
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Figure 2
The comparison of bulk liquid kinetics of Al-Sm and Al-Mg alloys. (a) Normalized stress autocorrelation functions of $A l_{90} M g_{10}$ (upper) and $A l_{90} S m_{10}$ (lower) melts at various temperatures. The colors from red to blue indicate the decreasing temperatures. The inset shows the computed shear viscosity using the Green-Kubo relation and the temperature dependences are fitted using the VFT equation. The fitting functions are $η = 0.0038 exp (\frac{841.0}{T - 444.2})$ and $η = 0.0042 exp (\frac{1060.6}{T - 586.9})$ for the $A l_{90} M g_{10}$ and $A l_{90} S m_{10}$ alloys, respectively. The error bar was obtained by averaging over all the autocorrelation functions of three off-diagonal components of the stress tensor. (b) The viscosity as function of $T_{0}$ -scaled temperature for the Al-Mg and Al-Sm alloys. Note that we follow the common practice by setting the diverging temperature $T_{0}$ equal to the glass transition temperature since the two are close for many metallic liquids [30, 36, 37].
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Figure 3
FCC nucleation in the Al-Mg and Al-Sm alloys obtained from the PEM at moderate undercooling $T = 700 K$ . (a) The critical nucleus size, (b) the nucleation barrier, and (c) the nucleation rate as functions of the doping concentrations. All the quantities are scaled by the corresponding values obtained for the pure Al (denoted by subscript “0”).
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Figure 4
The main quantities controlling the nucleation barrier. (a) The chemical potential difference and (b) the SLI free energy as functions of the doping concentrations at $T = 700 K$ . The quantities are scaled by the corresponding values obtained for the pure Al (denoted by subscript “0”).
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Figure 5
The SLI profiles for AlSm and AlMg. (a) The snapshots at the SLI for the Al-Mg (upper panel) and Al-Sm (lower panel) alloys. Two fcc layers along the in-plane direction (001) are shown for clarity. The grey atoms are liquid-like Al atoms. The blue are the solid-like Al atoms while light blue indicates the initial bulk fcc phase. The red atoms are Sm and green are Mg. For all the Al-Mg, as well as $A l_{99} S m_{1}$ and $A l_{97} S m_{3}$ alloys, the snapshots were taken at the moment when the as-grown solid phase reaches the similar population (∼25 000 atoms). For the $A l_{95} S m_{5}$ and $A l_{93} S m_{7}$ alloys, the snapshots were taken after significate longer-time simulation. (b) The interfacial area as function of doping concentration. The quantities are scaled with the interfacial area in the pure Al system.
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Figure 6
Trapping Sm into the fcc phase growing from the liquid $A l_{97} S m_{3}$ alloy. From left to right, it shows a time serial of the Sm atom incorporating into the fcc-Al lattice.
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