Continuous chemical redistribution following amorphous-to-crystalline structural ordering in a Zr-Cu-Al bulk metallic glass

doi:10.1016/j.jmst.2021.05.062

Abstract

Abstract:

Bulk metallic glasses (BMGs) are thermodynamically metastable. As such, crystallization occurs when a BMG is thermally annealed at a temperature above the glass transition temperature. While extensive studies have been performed on the crystallization kinetics of BMGs, most of them have focused on the amorphous-to-crystalline structural ordering, and little attention has been paid to chemical distribution and its relationship with the structural ordering during the crystallization process. In this paper, a new approach, with simultaneous differential scanning calorimetry (DSC) and small angle neutron scattering (SANS) measurements, was applied to study in situ the crystallization of a Zr_45.5Cu_45.5Al₉ BMG upon isothermal annealing at a temperature in the supercooled liquid region. Quantitative analysis of the DSC and SANS data showed that the structural evolution during isothermal annealing could be classified into three stages: (I) incubation; (II) amorphous-to-crystalline structural ordering; (III) continuous chemical redistribution. This finding was validated by composition analysis with atom probe tomography (APT), which further identified a transition region formed by expelling Al into the matrix. The transition region, with a composition of (Cu,Al)₅₀Zr₅₀, served as an intermediate step facilitating the formation of a thermodynamically stable crystalline phase with a composition of (Cu,Al)₁₀Zr₇.

Key words: Bulk metallic glass, Crystallization, In situ, Continuous chemical redistribution, Transition region

Xuelian Wu, Si Lan, Xiyang Li, Ming Yang, Zhenduo Wu, Xiaoya Wei, Haiyan He, Muhammad Naeem, Jie Zhou, Zhaoping Lu, Elliot Paul Gilbert, Dong Ma, Xun-Li Wang. Continuous chemical redistribution following amorphous-to-crystalline structural ordering in a Zr-Cu-Al bulk metallic glass[J]. J. Mater. Sci. Technol., 2022, 101: 285-293.

Figures/Tables 9

Fig. 1. DSC scan of Zr45.5Cu45.5Al9 BMG conducted at a heating rate of 10 K/min. The onset temperatures of glass transition (Tg) and crystallization (Tx) are indicated by arrows.

Fig. 2. The structure factor S(Q) of as-cast Zr45.5Cu45.5Al9 BMG measured by high energy synchrotron X-ray diffraction.

Fig. 3. Summarized data for DSC (a), and SANS (b) and the mean particle radius (open blue squares) deduced from fitting of the SANS data. The DSC trace is shown by a red line, the integration of DSC trace is shown by a blue line, and the integration of SANS is shown by open red triangles. The dashed lines indicate the borders for the three-stage structural evolution of Zr45.5Cu45.5Al9 BMG upon isothermal annealing in the supercooled liquid region (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4. Representative scattering profiles in a log-log (a), Porod (b), and Kratky representations (c) for Zr45.5Cu45.5Al9 BMG during isothermal annealing. The green lines are drawn as a guide to the eye (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 5. (a) Experimental SANS scattering profile (symbols) collected at 80 min and associated fit (solid line) using a structure model consisting of a log-normal size distribution of spherical particles and a flat background. Temporal evolution of mean particle radius Ravg (b), and the polydispersity σ (c) obtained by model fitting.

Fig. 6. (a) TEM bright field image for the annealed Zr45.5Cu45.5Al9 BMG, and (b) the corresponding particle size distribution. The size distribution obtained by model fitting of SANS data collected at 80 min, represented as a solid line, is superimposed for comparison.

Fig. 7. Rietveld refinement of the XRD pattern for the annealed Zr45.5Cu45.5Al9 BMG. The experimental data points are indicated by black open circles, and the calculated pattern by the red solid line. The green vertical bars are the positions of the Bragg peaks for the Cu10Zr7 crystalline phase (space group Cmca). The difference plot between the experimental data and the modelling results is shown at the bottom. The inset presents the structure model of the orthogonal Cu10Zr7 crystalline phase (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 8. APT atom maps of Zr45.5Cu45.5Al9 BMG after isothermal annealing at 730 K for (a) 40 min and (b) 80 min, revealing the presence of the Cu-rich region, the transition region, and the remaining amorphous matrix.

Table 1 Representative compositions (at.%) of different regions in Zr45.5Cu45.5Al9 BMG after isothermal annealing at 730 K for 40 and 80 min, respectively as determined by APT. The scattering length densities (SLDs) of different regions are included for discussion.

Sample information		Zr	Cu	Al	SLD (× 10^-6 Å^-2)
t = 40 min	Cu-rich region	42.9 ± 0.4	52.8 ± 0.5	4.3 ± 0.2	4.314
	Transition region	49.0 ± 1.4	44.1 ± 1.5	6.8 ± 1.2	4.036
	Matrix	48.6 ± 0.3	39.2 ± 0.3	12.2 ± 0.3	3.862
t = 80 min	Cu-rich region	43.1 ± 0.3	51.5 ± 0.3	5.4 ± 0.1	4.268
	Transition region	50.1 ± 1.7	42.1 ± 1.7	7.8 ± 1.1	3.973
	Matrix	47.2 ± 0.5	35.2 ± 0.6	17.6 ± 0.4	3.717
	Nominal	45.5	45.5	9.0

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