Three Dimensional Characterization of Unmodified and Sr‐Modified Al‐Si Eutectics by FIB and FIB EDX Tomography

Abstract

The starting materials are high purity metals: Zr (99.6 %), Cu (99.999 %), Al (99.999 %), Nb (99.995 %), and Ni (99.995 %). Two steps are used for the preparation of master alloys. First, the binary solid-solution ingots of Zr-Nb are fabricated. Second, Zr-Nb ingots are arc melted into master alloy with the remaining elemental components, such as Ni, Cu, and Al. For each step, the ingot is melted and flipped 5 times for total about 8 min. Then, the bulk metallic glass of the nominal composition of (Zr 58 Ni 13.6 Cu 18 Al 10.4 ) 99 Nb 1 with the length, width, and thickness of 75 mm × 6 mm × 2 mm was fabricated by suction casting the alloy into copper mold under a Ti-gettered Ar atmosphere. The structure was examined by X-ray diffraction (XRD) with Cu Ka radiation and the high-resolution-transmission-electron microscopy (HRTEM). The thin film for HRTEM is obtained by twin-injection. The crystallization and melting process was given by differential thermal analysis (DTA). The crystallization kinetics of the metallic glass was characterized by continuous and isothermal heating, using DSC (Perkin-Elmer DSC-7). In the case of continuous heating, a set of DSC curves were recorded at heating rates of 30, 40, 50, and 60 K/min. For the isothermal analysis, the metallic glasses were firstly heated to a fixed temperature (723, 733, 743, 753, and 763 K) at a rate of 200 K/min., and, then, held for a certain period of time until the completion of crystallization. The DSC system was calibrated for the temperature by using zinc and indium standards, giving an accuracy of ± 0.02 K. The DSC data were processed using the laboratory-edited computer program. Fig. 4. 3D reconstruction of the eutectic Si in AlSi7Sr: a) whole studied region; b) union of three different grains (presented by different grey-scales of Si in each grain); c) three different grains showing Si fibres of different thickness.Fig. 5. 3D reconstruction of lamellar Si in unmodified AlSi7 for a,b) the whole studied region at different observation angle and c) connecting nodes.

Comparison of segregations formed in unmodified and Sr-modified Al–Si alloys studied by atom probe tomography and transmission electron microscopy

“…The unmodified plate-like eutectic Si phase presents a much coarser structure in comparison to the Srmodified eutectic. Although Figure 1(b) depicts small rounded disconnected particles in the transversal section, the three-dimensional structure consists of an interconnected corallinelike architecture [23].…”

Solidification microstructures and solid-state parallels: Recent developments, future directions

“…Focused ion beam tomography has recently been used by Lasagni et al [192] for high-resolution investigation of the 3D structure of Sr-modified and unmodified Al-Si alloys (Fig. 10), and volume-based structural parameters have been used to identify the flake/fiber transition, which is not indicated through conventional spacing measurements [193].…”

Competitive stochastic growth model for the 3D morphology of eutectic Si in Al–Si alloys

“…In Al-Si alloys, there is a strong relationship between morphology of eutectic Si and their mechanical properties. Commercially used modification of the microstructure morphology with Sr addition changes the morphology of Si from a coarse plate-like into refined fibrous structure, significantly improving mechanical properties of the alloy, particularly tensile strength and elongation [2,3,4,5,6]. The results reported by Shin et al [6] have shown that Sr-modification of Al-10.5Si-2.0Cu recycled alloy leads to an increase of the elongation by a factor of two (from 2.0 to 4.1%) and tensile strength by more than 10% (from 209 to 237 MPa), which is reflected by a higher quality index of the material.…”