Engineering Materials and Their Applications
HRTEM Characterization of an Age-Hardened Mg–Ca Binary Alloy
2023 Volume 64 Issue 2 Pages 613-616
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2023 Volume 64 Issue 2 Pages 613-616
A Mg–2.8 mass% Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg2Ca phase (with C14 structure), which precipitated in the primary α-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg2Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {-1010}α columnar plane of the α-Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the α-Mg matrix for aging time of 3 h, which corresponded to the peak-aged condition. Furthermore, during the coarsening process, misfit dislocations were introduced on the planar surface of the precipitates, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.
Fig. 4 HRTEM image of the Mg–2.8 mass% Ca alloy aged at 423 K for 3 h, taken with B = [11-20]α (a). The C14–Mg2Ca precipitate was taken with B = [1-21-3]α (b).
Magnesium (Mg) alloys, as lightweight structural materials, have recently received considerable attention for potential applications in the automobile and aerospace industries where improvements in fuel efficiency and reductions in harmful emissions are desired. The Mg–Al–Ca system is considered promising for the development of cost-effective Mg alloys, because calcium (Ca) is cheaper than rare earth elements. Furthermore, the density of Ca is lower than that of Mg.1,2) Maximum utilization of thermally stable intermetallic phases is critical for enhancing the low- and high-temperature strength of Mg–Al–Ca alloys.3,4) Laves phases, such as Mg2Ca (C14, hexagonal), Al2Ca (C15, cubic), and (Mg,Al)2Ca (C36, dihexagonal), are quite promising candidates for enhancing the creep strength of such alloys.5,6)
The morphology and coarsening kinetics of the Al2Ca phase, which precipitates within the primary α-Mg phase of a Mg–Al–Ca alloy during aging treatment, have been extensively investigated.7) Coherent Al2Ca precipitates exhibit a hexagonal plate-like morphology, with a planar surface parallel to the (0001)α basal plane. Moreover, the sides of the hexagonal plate are parallel to the {11-20}α second columnar plane of the α matrix.8) Precipitation of the Mg2Ca phase has been observed in binary Mg–Ca alloys, but very little is known about the three-dimensional morphology of these precipitates.9,10) Therefore, the aim of this study is to clarify the morphological evolution of Mg2Ca precipitates and the coherence between these precipitates and the α-Mg matrix. A binary Mg–Ca alloy is investigated, with microstructural observations performed at high magnification using high-resolution transmission electron microscopy.
A binary Mg–2.8 mass% Ca alloy was investigated in this work. Using the permanent mold casting method, a sample with block dimensions of 100 × 160 × 20 mm3 was gravity cast at 1053 K under an argon atmosphere. Cuboid 10 × 8 × 8 mm3 specimens were cut out from the block and were then subjected to a 1-h homogenizing treatment at 743 K. Afterward, the specimens were subjected to an aging treatment at 423–473 K for 1.1 × 103–3.6 × 106 s (i.e., 0.3–1000 h). In addition, hardness measurements (load: 0.49 N and holding time: 10 s) were conducted using a micro Vickers hardness tester. Seven measurements were conducted on each specimen. The hardness was determined from the average of five measurement data (the maximum and minimum values were excluded).
The specimen microstructure was studied via field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Mounted specimens for FE-SEM observation were polished mechanically following standard metallographic procedures and then etched for 2 s in a solution of 2 vol% HNO3 and 98 vol% ethyl alcohol. During FE-SEM observations, secondary electron imaging was conducted at an accelerating voltage of 15 kV. Thin films for TEM and HRTEM observation were cut out from the cubic test piece and were shaped into 3 mm (diameter) × 80 µm (thickness) disk-shaped samples. The disks were further thinned thorough mechanical polishing, followed by dimple grinding and ion-milling, for perforation of each disk center. The perforated disks were examined via TEM (JEOL JEM-2010) operating at 200 kV and HRTEM (FEI Titan3 G2 60-300) operating at 300 kV.
Figure 1 shows a SEM secondary electron image of the as-cast Mg–2.8 mass% Ca alloy specimen. A mixed microstructure of primary α-Mg dendrite and α-Mg/C14–Mg2Ca eutectic lamellae (dendrite-arm spacing: ∼20 µm) was observed. According to the Mg–Ca binary phase diagram, the weight ratio of the primary α-Mg dendrite was estimated to be 90.2% based on the lever rule.11) The 1-h homogenizing treatment at 743 K had no effect on the dendrite-arm spacing. In subsequent hardness measurements and microstructural observations, we focused on the primary α-Mg dendrite within each specimen.
SEM-SEI of the as-cast Mg–2.8 mass% Ca alloy.
The age-hardening curves of the alloy at 423 K and 473 K are shown in Fig. 2. During the hardness measurements, the hardness tester indenter was positioned within the α-Mg dendrite of the alloy specimen. The hardness of the as-homogenized alloy is HV43. At 423 K, the hardness started to increase between 0.3 h and 1 h. The maximum value of HV55 was reached at 3 h. Afterward, with increasing aging time, the hardness decreases continuously to HV45 at 1000 h. At 473 K, the hardness started to increase for times <0.3 h. The maximum value of HV50 was reached at 1 h, and then the hardness decreased with increasing aging time, becoming HV46 at 30 h and 100 h.
Plots of Vickers hardness vs. aging time for the Mg–2.8 mass% Ca alloy. The data of the as-homogenizing-treated (AH) specimen is included.
TEM bright-field images of the specimens over-aged at 423 K and 473 K are shown in Fig. 3. The polygonal precipitates with a side of approximately 80 nm long are homogeneously distributed within the primary α-Mg dendrite for the over-aged at 423 K for 1000 h (Fig. 3(a)). The prolonged precipitates with a length of several hundred nanometers are observed along the (0001)α basal plane in addition to a number of polygonal precipitates for the specimen over-aged at 473 K for 100 h (Fig. 3(b)). It is evident from these microstructure observations that one kind of precipitate with a polygonal morphology is primarily detected when the alloy is subjected to aging treatment at 423 K. By contrast, two types of precipitates (polygonal and prolonged) are observed when the alloy is aged at 473 K. In this study, the peak-aged and over-aged conditions at 423 K were chosen to allow coarsening of the Mg2Ca precipitate and to facilitate HRTEM observations. Special attention was paid to the selection of the Mg2Ca precipitate with an average size in the HRTEM observation for each specimen.
TEM-BFIs of the Mg–2.8 mass% Ca alloy aged at 423 K for 1000 h (a) and at 473 K for 100 h (b), taken with B = [11-20]α.
A HRTEM image of the Mg–2.8 mass% Ca alloy peak-aged at 423 K for 3 h is shown in Fig. 4(a). The corresponding selected-area diffraction pattern (SADP), with an incident beam direction of B = [11-20]α, is also shown. As indicated by the image, the Mg2Ca phase with a plate-like morphology precipitated along the (0001)α basal plane of the α-Mg matrix. The length and thickness of the precipitate are 14 nm and 2.1 nm, respectively, corresponding to four stacks of the (0001)α basal plane comprising the matrix. A semicircular strain contrast is visible on the lower side of the precipitate, with the strain contrast at a distance of 6.4 nm away. A typical coffee bean contrast that can be observed around the precipitate is indicative of the coherent precipitation of the plate-like C14–Mg2Ca phase with respect to the α-Mg matrix.
HRTEM image of the Mg–2.8 mass% Ca alloy aged at 423 K for 3 h, taken with B = [11-20]α (a). The C14–Mg2Ca precipitate was taken with B = [1-21-3]α (b).
A HRTEM image of the alloy taken with B = [1-21-3]α is shown in Fig. 4(b); the corresponding SADP is shown. The plate-like Mg2Ca precipitate exhibited hexagonal morphology, where the sides of the precipitate hexagon are parallel to the {-1010}α columnar plane of the α-Mg matrix. The above HRTEM observations revealed that the Mg2Ca phase precipitated during the aging treatment of the Mg–2.8 mass% Ca alloy has a hexagonal plate-like morphology. The planar surface of the precipitate is parallel to the (0001)α basal plane and the sides of the hexagonal plate are parallel to the {-1010}α columnar plane of the matrix.
A HRTEM image of the alloy over-aged at 423 K for 10 h is shown in Fig. 5 (incident beam direction of B = [11-20]α). The rectangular Mg2Ca phase with a flat planar surface precipitates on the (0001)α basal plane of the α-Mg matrix. The length and thickness of the Mg2Ca precipitate were determined to be 37 nm and 8 nm, respectively. In addition, the aspect ratio of the precipitate was obtained by dividing the length of the precipitate by its thickness. The ratio of the alloy over-aged for 10 h (4.6) was considerably smaller than that of the specimen peak-aged for 3 h (6.7). The length and thickness of the precipitate increased substantially to 76 nm and 60 nm, respectively, for the specimen aged at 423 K for 100 h, as shown in Fig. 6. Although the planar surface of the precipitate is located along the (0001)α basal plane of the α-Mg matrix, the linearity of the interface between the precipitate and the matrix decreased. The aspect ratio of the precipitate decreased further to 1.3 for the specimen over-aged for 100 h.
HRTEM image, taken with B = [11-20]α, of the Mg–2.8 mass% Ca alloy aged at 423 K for 10 h.
STEM-BFI, taken with B = [11-20]α, of the Mg–2.8 mass% Ca alloy aged at 423 K for 100 h.
In order to closely evaluate the coherency of the polygonal Mg2Ca phase precipitated in the alloy over-aged at 423 K for 100 h, the planar surface of the Mg2Ca precipitate shown in Fig. 6 was investigated. A high-magnification HRTEM image obtained by focusing on the planar surface of the polygonal precipitate is shown in Fig. 7(a) (the corresponding SADP is also shown). As indicated in the figure, the planar surface is parallel to the (0001)α basal plane of the α-Mg matrix in only some cases. The fast-Fourier transformation image of the area enclosed in the square shown in Fig. 7(a) was masked and inversely transformed (the resulting inverse fast-Fourier transformation image is shown in Fig. 7(b)). The results revealed that the coherence between the Mg2Ca precipitate and the α-Mg matrix is unsustainable and misfit dislocations are introduced on the planar surface of the precipitate. Consequently, the coherent interface evolves into a semi-coherent interface. It is noted that the two types of crystallographic orientation relationships between the rectangular Mg2Ca and the α-Mg matrix has been reported for the age-hardened Mg–Ca–Zn ternary alloy.12) The assumption was that the higher value of misfit perpendicular to the c-axis (compared with that parallel to this axis) enhanced the misfit strain on the planar surface of the Mg2Ca precipitate during the coarsening process. This resulted in the introduction of misfit dislocations on the planar surface and the transition of the coherent α/C14 interface to a semi-coherent interface. In the future, the crystallographic orientation relationship between the Mg2Ca precipitate and the α-Mg matrix should be clarified for the binary Mg–Ca alloys.
HRTEM image, taken with B = [10-10]α, of the Mg–2.8 mass% Ca alloy aged at 423 K for 100 h (a). The inverse fast-Fourier transformation (IFFT) image of the surrounded area in (a) is shown in (b).
A homogenized Mg–2.8 mass% Ca binary alloy was subjected to an aging treatment at 423–473 K to clarify the age-hardening response associated with precipitation of the Mg2Ca (C14) phase. The three-dimensional morphology and coherency of the Mg2Ca phase precipitated in the primary α-Mg dendrite were investigated in the peak-aged and over-aged conditions at 423 K through microstructural observation performed via HRTEM. The Mg2Ca precipitate has a hexagonal plate-like morphology, characterized by a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {-1010}α columnar plane of the α-Mg matrix. The coherency of the Mg2Ca precipitate with the α-Mg matrix was retained for aging time of 3 h, which corresponded to the peak-aged condition. Misfit dislocations were generated on the planar surface of the precipitate during the coarsening process, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.
This work was supported by JSPS KAKENHI Grant Number JP22K04735, Japan. One of the authors (Y. Terada) greatly appreciates the support of 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.
