Three Dimensional (3D) Microstructural Characterization and Quantitative Analysis of Solidified Microstructures in Magnesium-Based Alloys
Magnesium alloys have the attractive combination of lightweight and strength. An understanding of solidification microstructures in these materials is important. An accurate means of quantifying microstructure in 3D is extremely important. In this study, we have used serial polishing and synchrotron-based x-ray tomography technique as a means of 3D characterization of the solidified microstructures of magnesium-based alloys. These models were also used to conduct quantitative analysis in 3D. The phase fraction and morphologies of intermetallics and α-Mg matrix phase were obtained. The phase fractions of β-Mg17Al12 and Al–Mn intermetallics are consistent with measurements in the literature and calculations based on the Scheil–Gulliver solidification model. Our 3D reconstructions also show that the dendrite morphology has sixfold symmetry. The results of 3D microstructural characterization and analysis will enable a comprehensive understanding of solidification variables, microstructure, and properties.
KeywordsSolidification Magnesium alloys Three-dimensional characterization Synchrotron-based x-ray tomography Serial sectioning
Magnesium alloys are of interest as structural materials due to their lightweight and high strength-to-weight ratio . These alloys are also beginning to find use as biocompatible implant materials [2, 3, 4, 5]. Dendritic microstructures are observed in a wide range of solidification processes, and play a vital role in determining the properties of the material [6, 7, 8]. A limited number of studies have focused on the evolution of solidification patterns and preferred crystallographic orientations [9, 10, 11]. To date, a few studies on dendritic microstructures in magnesium alloys have been carried out [12, 13, 14, 15, 16]. In order to develop accurate anisotropic models for the solid–liquid (S/L) interface in hexagonal close-packed alloys, quantitative experimental characterization in three dimensions needs to be carried out. A significant fraction of commercial magnesium alloys is based on hypoeutectic Mg–Al alloys with Al concentration in the range of 3–9 wt.%. These alloys exhibit a mixture of primary α-Mg dendrites surrounded by a eutectic network eutectic with Mg–Al intermetallic precipitates. Thus, understanding and quantifying the solidified microstructural features of Mg-rich alloys in binary Mg–Al binary and other multi-component systems are crucial. Furthermore, a fundamental understanding of the microstructures of materials in three dimensions (3D) is necessary to accurately model the evolution and formation of their microstructures.
The objective of the present study is to investigate the morphologies and phase fraction of microstructures of Mg–9Al alloy processed by directional solidification and a commercial AZ91D alloy made by high-pressure die casting (HPDC). A combination of metallographic serial sectioning and synchrotron-based x-ray phase-contrast tomography techniques were used to visualize and quantify the volume fraction of microstructures in 3D.
Materials and Experimental Procedure
HPDC process parameters used for producing AZ91D die castings
In order to carry out serial sectioning experiments, the Mg–9 wt.% Al samples were mounted and polished using an automatic polisher to obtain a reproducible and controlled removal rate. The serial sectioning procedure was as follows. A square region of interest was selected from the sample by indenting four equally spaced fiducial marks using a Vickers pyramidal indenter. In general, selecting the region of interest is a very subjective step in the 3D reconstruction process. In this study, the size of the microstructural region of interest was taken as approximately 500 × 400 μm, and a depth of approximately 150 μm. Fiducial marks were placed on the sample to define the region of interest, to measure the material loss during serial sectioning, and to align the indentations during reconstruction. This method of quantifying the material removal has been proven to be quite effective because the cross sections of the indentations are nearly square, making it relatively simple to measure the length of the diagonals [18, 19]. The approximate depth (h) of the indentation was determined by the following equation: h = D/2 tan(θ/2), where D is the average of the indentation diagonals (D1 and D2) on a 2D projection, and θ is the angle between the two diagonals.
Polishing routine used in the serial sectioning process
Al2O3 grit size, μm
Synchrotron-Based X-Ray Computed Tomography
X-ray microtomography measurements were carried out at the Advanced Photon Source (APS) at Argonne National Laboratory (beamline 2-BM) which offers near-video-rate acquisition of tomographic data at micrometer spatial resolution . An x-ray energy of ~30 keV provided the combination of high penetration ability and excellent phase contrast for 2 mm × 2 mm × 2 mm volume of Mg–9 wt.% Al as-solidified and AZ91D specimens. A CdWO4 scintillator screen was used to convert the x-rays to visible light, and acquired with a 2048 × 2048 pixel CoolSnap K4 CCD camera. Typical exposure times ranging between 80 and 200 ms per projection were used. A resolution of 1.4 μm/voxel was obtained. A projection was acquired every 1/8°. Including the readout time and disk input/output, the tomography was completed in about 20 min. The 2D projections were reconstructed in 3D using a filtered-back-projection algorithm.
After x-ray tomography, all the images were segmented to black and white images. To reconstruct the 3D solid for visualization, the sections were aligned and stacked in Matlab, and then the images were segmented using conventional image analysis software (ImageJ, Bethesda, MD). Separate gray scale values were assigned to each phase, e.g., α-Mg dendrite matrix phase, β-Mg17Al12 eutectic, Al–Mn intermetallics, and the porosity. The 3D microstructures were digitally reconstructed using image reconstruction software Mimics (Materialise, Ann Arbor, MI), which was also use for quantitative analysis of the 3D volumetric data.
Results and Discussion
Volume fractions of phase in Mg–9Al and AZ91D alloys, along with comparison with literature data
In conclusion, we successfully used the combined method of serial-sectioning and synchrotron-based x-ray computed tomography to characterize and quantify microstructures of Mg–9Al and AZ91D in three-dimensions. The phase fraction and morphologies of intermetallics and α-Mg matrix phase were obtained. The phase fractions of β-Mg17Al12 and Al–Mn intermetallics are consistent with measurements in the literature and calculations based on the Scheil–Gulliver solidification model. Our 3D reconstructions also show that the dendrite morphology has sixfold symmetry. More generally, these quantitative results of phase fraction and shapes should be used in validating phase-field modeling and as an input in microstructure-based finite element analysis to better understand the structure–property relationships in these materials.
MYW and TJ gratefully acknowledge the financial support for this study by the National Science and Technology Major Project of China, under Grant No. 2011ZX04014-052; the National Science Foundation of China, under Grant No. 51175292; and the Doctoral Fund of Ministry of Education of China, under Grant No. 20090002110031. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. MYW also acknowledges the financial support from the China Scholarship Council during his stay at Arizona State University.
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