Three-Dimensional Morphology of C15–Al2Ca Precipitates in a Mg–Al–Ca Alloy
2019 Volume 60 Issue 9 Pages 2048-2052
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2019 Volume 60 Issue 9 Pages 2048-2052
The three-dimensional morphology and thickness of an Al2Ca Laves phase with a C15 crystal structure, which precipitated within the primary α-Mg grain of a Mg–5Al–1.5Ca alloy that had been over-aged at 523 K for 100 h, were investigated using high-resolution transmission electron microscopy. The C15–Al2Ca precipitate exhibits a hexagonal plate-like morphology, with a planar surface parallel to the (0001)α basal plane and the sides of the hexagonal plate parallel to the $\{ 11\bar{2}0\} _{\alpha }$ second columnar plane of the α matrix. A typical coffee bean contrast was clearly visible around the precipitate, which is indicative of the coherent precipitation of the C15–Al2Ca phase with respect to the α-Mg matrix. The thickness of the Al2Ca precipitate, which corresponds to six layers of the (111)C15 plane composed of Ca atoms, was evaluated as approximately 1.5 nm.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 193–197.
HRTEM image of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with $\mathbf{B} = [11\bar{2}0]_{\alpha }$.
Magnesium alloys are gaining interest as lightweight structural materials for use in the automotive and aerospace industries, where they are instrumental in increasing fuel efficiency and minimizing carbon dioxide emissions.1,2) Due to its low density and low cost, calcium is a promising substitute for rare-earth elements for improving the mechanical strength of Mg–Al alloys, and the resulting Mg–Al–Ca alloys usually exhibit excellent non-flammability. As a result of the recent efforts aimed at developing high-strength magnesium alloys for automotive and aerospace applications,3–5) cost-effective Mg–Al–Ca alloys have been successfully prepared with superior mechanical strength.6–9) The microstructures of die-cast Mg–Al–Ca alloys generally exhibit a primary α-Mg solid-solution phase with a dendritic morphology and a eutectic structure in the interdendritic regions.10,11) When Mg–Al–Ca alloys are subjected to aging treatments in the temperature range of 448–623 K, a fine Al2Ca Laves phase with a C15 crystal structure precipitates along the (0001)α basal plane of the primary α-Mg grain, and the low- and high-temperature strength of the material is increased via the precipitation strengthening of the fine C15–Al2Ca Laves phase.12,13)
A typical transmission electron microscopy (TEM) image of the primary α-Mg grain of the candidate Mg–Al–Ca alloy (Mg–5 mass% Al–1.5 mass% Ca alloy) subjected to an aging treatment under the peak-aging condition at 523 K for 10 h is shown in Fig. 1.13) A fine C15–Al2Ca Laves phase with a length of approximately 20 nm can be observed to have uniformly precipitated along the (0001)α basal plane of the primary α-Mg phase, and a strain contrast is visible that is associated with the individual precipitate. Since the thickness of the fine Al2Ca precipitate is extremely thin, the precipitate thickness is difficult to identify using conventional TEM. Basic microstructural information such as morphology and thickness of the Al2Ca precipitate is required to quantitatively evaluate the precipitation strengthening derived from the Al2Ca precipitate in the Mg–Al–Ca alloy. This study aims to clarify the three-dimensional morphology and thickness of a fine C15–Al2Ca Laves phase, which precipitates during the aging treatment of a Mg–Al–Ca alloy, through microstructure observations under high magnification using high-resolution transmission electron microscopy (HRTEM).
TEM BFI of the Mg–5Al–1.5Ca alloy aged at 523 K for 10 h, taken with $\mathbf{B} = [11\bar{2}0]_{\alpha }$, g = 0002α.13)
A Mg–5Al–1.5Ca (mass%) alloy was used in this study, the composition of which is shown in Table 1. The alloy was produced using a cold chamber die-cast machine in a 1 vol% SF6–99 vol% CO2 atmosphere at casting and die temperatures of 993 and 473 K, respectively. Samples were obtained in the form of plates with a length of 50 mm, a width of 70 mm, and a thickness of 3 mm. The aging treatment was carried out at 523 K for 3.6 × 105 s (100 h). Note that in this study, the over-aging condition was chosen to allow the C15–Al2Ca precipitate to fully coarsen and make the HRTEM observations as easy as possible.13) Thin films cut from the aged sample were shaped into disk-like samples with a diameter of 3 mm and a thickness of 120 µm by mechanical polishing. These samples were electrolytically polished using a standard twin-jet polisher and a solution of methanol and perchloric acid (9:1). The polishing conditions were set to 243 K and 25 V, resulting in a polishing current of approximately 0.1 A. The perforated foils were examined using a Cs-corrected scanning transmission electron microscope FEI Titan3 G2 60-300 operating at 300 kV with various incident vectors of the electron beam.
The HRTEM image of the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h is shown in Fig. 2, together with its selected-area diffraction pattern (SADP), with an incident beam direction of $\mathbf{B} = [11\bar{2}0]_{\alpha }$. The C15–Al2Ca phase with a plate-like morphology can be observed to have precipitated along the (0001)α basal plane of the α-Mg matrix, with a length of 24 nm and a thickness of 1–2 nm, corresponding to 5–7 atomic layers. A semicircular strain contrast is visible on both the upper and lower sides of the precipitate, with the strain contrast at a distance of 12 nm away. A typical coffee bean contrast that can be observed around the precipitate is indicative of the coherent precipitation of the plate-like C15–Al2Ca phase with respect to the α-Mg matrix. The α-Mg matrix phase has a hexagonal hcp (A3) crystal structure, whereas the crystal structure of the Al2Ca precipitate is a cubic C15 structure. The precipitation of the plate-like Al2Ca phase was induced through a nucleation and growth mechanism rather than spinodal decomposition, since the crystal system of the precipitate phase differs from that of the matrix phase.
HRTEM image (a) and corresponding SADP (b) of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with $\mathbf{B} = [11\bar{2}0]_{\alpha }$. A schematic of the SADP, together with the indices of the major spots, is shown in (c).
The HRTEM image of the alloy aged at 523 K for 100 h taken with $\mathbf{B} = [01\bar{1}1]_{\alpha }$, which is perpendicular to the pyramidal plane of the α-Mg matrix, is shown in Fig. 3, together with its SADP. The plate-like Al2Ca precipitate exhibits a polygonal morphology rather than a circular or elliptical one, where the sides of plate-like Al2Ca precipitate polygons are parallel to the $\{ 11\bar{2}0\} _{\alpha }$ plane of the α-Mg matrix. The above HRTEM observations reveal that the Al2Ca phase precipitated during the aging treatment for the Mg–5Al–1.5Ca alloy has a hexagonal plate-like morphology, where the planar surface is parallel to the (0001)α basal plane and the sides of the hexagonal plate are parallel to the $\{ 11\bar{2}0\} _{\alpha }$ second columnar plane of the α-Mg matrix.
HRTEM image (a) and corresponding SADP (b) of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with $\mathbf{B} = [01\bar{1}1]_{\alpha }$. A schematic of the SADP, together with the indices of the major spots, is shown in (c).
In order to closely evaluate the thickness of the hexagonal plate-like C15–Al2Ca phase precipitated during the aging treatment for the Mg–5Al–1.5Ca alloy, the edge of the Al2Ca precipitate shown in Fig. 2(a) was investigated. The HRTEM image obtained by focusing on the right-hand edge of the plate-like Al2Ca precipitate in Fig. 2(a) is shown in Fig. 4(a), which reveals that white spots are regularly arranged in most areas and the AB stacking can be clearly identified in the α-Mg matrix with an hcp crystal structure. By contrast, the regular arrangement of the white spots is obscured in the region surrounded by the square in Fig. 4(a).
Enlarged HRTEM image of the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with $\mathbf{B} = [11\bar{2}0]_{\alpha }$ (a). The edge portion of the C15–Al2Ca precipitate, surrounded by the square in (a), is further magnified in (b). The inverse fast-Fourier transformation (IFFT) image of (b) is shown in (c).
The enlarged HRTEM image of the region surrounded by the square in Fig. 4(a) is shown in Fig. 4(b). The crystal lattice planes of the upper five layers are apparently continuous from the left to the right sides of the figure, whereas the crystal planes are obscured in the left-center field of view. To clarify the HRTEM image of Fig. 4(b), the area other than the high-intensity spots derived from the periodic structures of the α-Mg matrix in the fast-Fourier transformation (FFT) image of Fig. 4(b) was masked and inversely transformed, and the resulting inverse fast-Fourier transformation (IFFT) image is shown in Fig. 4(c). The interplanar spacing of the (0001)α plane is constant in the α-Mg matrix, as seen in the right-hand field of the figure. By contrast, the intensity of the spot itself decreases and the wide and narrow interplanar spacings indicated by the black and white arrowheads appear alternatingly in the left-center field of view in Fig. 4(c).
Diffraction pattern analysis by TEM revealed that the following orientation relationship is satisfied between the fine C15–Al2Ca precipitate and α-Mg matrix for the Mg–Al–Ca alloy: (111)C15 // (0001)α and $[01\bar{1}]_{\text{C15}} \parallel[0\bar{1}10]_{\alpha }$, where the crystal planes with the greatest planar density of both phases are parallel to each other.12) A schematic illustration of the unit cell of the Al2Ca phase with a C15 crystal structure is shown in Fig. 5(a).14) The Ca atoms are located at the 8 corners of the unit cell, the 6 centers of the faces, and the 4 positions within the unit cell, which are identical to the atomic positions in a diamond cubic crystal structure. By contrast, four Al atoms constitute a regular tetrahedron, and the four regular tetrahedrons composed of Al atoms are staggered with the four Ca atoms positioned within the unit cell.15)
A reduced-sphere unit cell (a) and $[\bar{1}10]$ projection (b) of the C15–Al2Ca.
In order to clearly identify the stacking of the (111)C15 plane with the greatest planar density for the C15 crystal structure, which is parallel to the (0001)α plane of the α-Mg matrix, the $[\bar{1}10]_{\text{C15}}$ projection of the Al2Ca phase is shown in Fig. 5(b). The stacking of the (111)C15 plane consists of single layers A, B, and C, composed of Al atoms, and triple layers αcβ, βaγ, and γbα, composed of a layer (a, b, or c) of small Al atoms sandwiched between two layers of large Ca atoms (α, β, or γ). The (111)C15 planes composed of Al atoms are equidistant, whereas the wide and narrow interplanar spacings of the (111)C15 planes composed of Ca atoms appear alternatingly.
In the C15–Al2Ca precipitate, the sum of the wide and narrow spacings of the (111)C15 plane composed of Ca atoms corresponds to one third of the diagonal length of the C15 unit cell. Since the lattice constant of the Al2Ca phase is 0.8005 nm,16) the sum of the wide and narrow spacings of the (111)C15 plane composed of Ca atoms was quantitatively evaluated as 0.4622 nm. This value is close to 0.5230 nm, which is twice as large as the (0001)α plane spacing for the α-Mg matrix.17) It was inferred that the C15–Al2Ca phase precipitated coherently in the α-Mg matrix, where the two layers of the (111)C15 plane composed of Ca atoms correspond to the two layers of the (0001)α plane of the α-Mg matrix. Furthermore, the repetition of the wide and narrow interplanar spacings in the IFFT image in Fig. 4(c) is presumed to reflect the spacing of the (111)C15 plane composed of Ca atoms in the C15–Al2Ca precipitate.
From the IFFT image shown in Fig. 4(c), the thickness of the plate-like C15–Al2Ca phase was identified to correspond to six layers of the (111)C15 plane composed of Ca atoms and was quantitatively evaluated as approximately 1.5 nm. It can be seen from Fig. 4(c) that the six layers of the (111)C15 plane composed of Ca atoms in the C15–Al2Ca precipitate correspond respectively to the six layers of the (0001)α plane in the α-Mg matrix. The coherency between the C15–Al2Ca precipitate and the α-Mg matrix is also evident on the sides of the hexagonal plate-like Al2Ca precipitate. In the future, the coarsening kinetics of the C15–Al2Ca precipitate during the aging treatment on the basis of the aforementioned results obtained in this study need to be quantitatively clarified.
A Mg–5Al–1.5Ca alloy was subjected to an aging treatment under the over-aging condition at 523 K for 100 h. The three-dimensional morphology and thickness of the resultant C15–Al2Ca Laves phase precipitated in the primary α-Mg grain were investigated using HRTEM and the following results were obtained.
The alloy samples used in this study were manufactured and provided by Mitsubishi Aluminum Co. The authors would like to thank Mr. Kenji Ohkubo and Mr. Ryo Ota of Hokkaido University for their kind assistance with the microstructure observation using electron microscopy. One of the authors (Y. Terada) greatly appreciates the support from the Tanikawa Fund Promotion of Thermal Technology and the Light Metal Educational Foundation. A part of this work was conducted at Hokkaido University, supported by the Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
