Results and discussion

Optical micrographs of AZX912 as-cast alloy and the ingots STed at 420, 510, and 525 °C are shown in Figs. 1(a), 1(c), 1(e), and 1(g). The as-cast ingot contained coarse Mg₁₇Al₁₂ phase particles and Al₂Ca compound with a large network morphology. After ST at 420 °C, Mg₁₇Al₁₂ phase disappeared due to dissolution into α-Mg matrix, whereas no apparent change was detected for Al₂Ca compound. However, with increasing ST temperature, Al₂Ca compound changed from a network-like morphology to a spheroidized shape. In addition, for the ingot STed at the highest temperature of 525 °C, some Mg₁₇Al₁₂ phase particles appeared again around Al₂Ca particles. For AZX912 alloy, the solidus temperature and the liquidus temperature are 522 °C and 595 °C, respectively [Reference Suzuki, Saito, Huang, Nakatsugawa and Chino24]. In the present work, the highest ST temperature (525 °C) is slightly higher than the solidus temperature (522 °C), and thus, it is possible to induce partial melting. Therefore, the appearance of Mg₁₇Al₁₂ phase particles might be the result from the partial melting during ST for the ingot STed at 525 °C.

Figure 1:

Optical micrographs of AZX912 (a, c, e, g) cast and (b, d, f, h) extruded (ED: horizontal direction) alloys (a, b) without ST and with ST at (c, d) 420 °C, (e, f) 510 °C, and (g, h) 525 °C. (i) Optical micrograph of etched surface and (j) (0002) pole figure of AZX912 extruded alloy with ST at 480 °C.

The optical micrographs of the corresponding extruded alloys are shown in Figs. 1(b), 1(d), 1(f), and 1(h). For the extruded alloys without ST and with ST at 420 °C, coarse Al₂Ca particles were broken during extrusion. In contrast, Al₂Ca particles became unbroken with increasing ST temperature. In addition, Mg₁₇Al₁₂ phase particles were significantly elongated along the extrusion direction (ED) as indicated by the arrows for the extruded alloy without ST [see Fig. 1(b)]. The elongated Mg₁₇Al₁₂ phase was also observed in AZ91 Mg alloy extruded at 270 °C [Reference Li, Dong, Zeng, Lu and Ding25]. Even though Mg₁₇Al₁₂ phase is extremely brittle at room temperature, a high deformability accompanied by a significant softening can be obtained at elevated temperatures (∼2/3 melting point), originating from the onset of dislocation climb [Reference Ragani, Donnadieu, Tassin and Blandin18, Reference Mathur, Maier-Kiener and Korte-Kerzel19]. It is suggested that this transition of deformation mechanism is responsible for the formation of significantly elongated Mg₁₇Al₁₂ phase particles without fracture during extrusion at 280 °C (0.75 melting point) in the present work.

The influences of the ST temperature on grain size and texture after extrusion appear to be weak. The grain structure and (0002) pole figure of the extruded alloy with ST at 480 °C are shown as the examples in Figs. 1(i) and 1(j). All extruded alloys exhibited approximately the same grain sizes of 2–3 μm. The refined microstructure can be attributed to dynamic recrystallization during extrusion. In addition, all extruded alloys exhibited a typical extrusion texture of round bar. For all extruded alloys, basal planes are roughly parallel to the ED, and there is a large spread of (0002) orientation along the transverse direction (i.e., the radial direction of the extruded bar). The extruded alloys exhibited the similar weak textures with the low basal texture intensities of 2.6–3.6.

Figure 2 shows the X-ray diffraction (XRD) patterns of the extruded alloys without ST and with ST at the lowest and highest temperatures of 420 °C and 525 °C, respectively. For all extruded alloys, the XRD patterns exactly show the existence of the α-Mg, Mg₁₇Al₁₂, and Al₂Ca (C15 Laves crystal structure) phases. Existence of the C15 Al₂Ca Laves phase is consistent with the result that this stable phase forms in the case of Ca/Al ratio smaller than 0.4 for Mg–Al–Ca alloy system [Reference Zhang, Deng, Nie, Xu, Su and Lian26, Reference Zubair, Sandlöbes, Wollenweber, Kusche, Hildebrandt, Broeckmann and Korte-Kerzel27]. The extruded alloy with ST at 420 °C exhibited the pronounced peaks of the Mg₁₇Al₁₂ phase, even though this alloy before extrusion did not contain the Mg₁₇Al₁₂ phase due to dissolution into the α-Mg matrix [see Fig. 1(c)]. This indicates the dynamic precipitation of the Mg₁₇Al₁₂ phase during extrusion. Even after ST at a high temperature of 525 °C, the Al₂Ca compound with the C15 Laves crystal structure is still stable, which can be attributed to its high melting point of 1079 °C [Reference Huang, Chino, Yuasa, Ueda, Inoue, Kido and Matsumoto6, Reference Koltygin, Bazhenov, Belova and Nikitina21, Reference Hort, Huang and Kainer22]. In addition, it is interesting to note that the peaks of the Al₂Ca compound tend to slightly shift to the left (i.e., decreased diffraction angle) for the extruded alloy with the ST at 525 °C. For example, regarding the diffraction peak (511), 2θ deceased from 60.2° to 59.8° for the extruded alloy with ST at 525 °C compared with that with ST at 420 °C. This implies the expansion of lattice of the Al₂Ca compound for the alloy subjected to high-temperature ST.

Figure 2:

XRD patterns of AZX912 extruded alloys (a) without ST, with ST at (b) 420 °C and (c) 525 °C.

Figure 3 shows the changes in Vickers hardness, size, and area ratio of Al₂Ca particles in cast and extruded alloys as a function of the ST temperature. As shown in Fig. 3(a), with increasing ST temperature, the measured Vickers harness increased about twice for both cast and extruded alloys. The average Vickers harnesses remarkably increased from 300 to 665 and from 236 to 516 for cast and extruded alloys, respectively. As shown in Figs. 3(c) and 3(d), the pictures after hardness measurements for the cast alloys without ST and with ST at 525 °C display a much smaller indentation size for the latter.

Figure 3:

Changes in (a) Vickers hardness (The error bars indicate the minimum and maximum values) and (b) size and area ratio of Al₂Ca particles in cast and extruded alloys as a function of the ST temperature. The pictures after hardness measurements for AZX912 cast alloy (c) without ST and (d) with ST at 525 °C.

As shown in Fig. 3(b), the cast alloys exhibited approximately the same size (∼3 μm) of Al₂Ca particles. However, for the alloy without ST, the size of Al₂Ca particles considerably decreased from 3.0 to 1.1 μm after extrusion, resulting from fracture during extrusion. The difference in the size of Al₂Ca particles between the cast alloys and the extruded alloys gradually reduced and became almost the same when increasing the ST temperature up to ∼500 °C, indicating the restricted fracture of Al₂Ca particles. This can be attributed to the hardening of Al₂Ca particles induced by high-temperature ST. Furthermore, it can be expected that stress concentration on Al₂Ca particles caused by plastic flow during extrusion reduces due to the change from network morphology to spheroidized shape. The reduction of stress concentration would be beneficial in preventing Al₂Ca particles from fracture. In addition, the area ratio of Al₂Ca particles concurrently decreased from ∼6 to ∼4% with increasing ST temperature.

Figure 4 shows the transmission electron microscope (TEM) images of coarse Al₂Ca particles and nanoscale Mg₁₇Al₁₂ particles in the extruded alloys with ST at 420 °C and 510 °C. Because Mg₁₇Al₁₂ phase was dissolved into the matrix during ST [see Figs. 1(c) and 1(e)], the presence of nanoscale Mg₁₇Al₁₂ phase particles with the sizes of 50–300 nm can be attributed to dynamic precipitation during extrusion.

Figure 4:

TEM images showing (a, b) coarse Al₂Ca particles and (c, d) nanoscale Mg₁₇Al₁₂ precipitates in AZX912 extruded alloys with ST at (a, c) 420 °C and (b, d) 510 °C.

Results of the compositions of matrix and secondary-phase particles in the extruded alloys with ST at 420 and 510 °C, investigated by TEM-EDS analyses, are given in Table I. Regarding Al₂Ca particles, the alloy with ST at 510 °C exhibited a great decrease from 21.7 ± 1.8 to 5.3 ± 0.2 at.% in the concentration of Mg atoms dissolving into Al₂Ca compound compared with the alloy with ST at 420 °C. The concentrations of Al and Ca correspondingly increased. The Al concentration slightly increased from 58.5 ± 1.3 to 65.2 ± 0.7 at.%, whereas the Ca concentration largely increased from 19.4 ± 0.6 to 29.4 ± 0.5 at.%. This resulted in a decrease in the ratio of Al/Ca from 3.0 to 2.2, which is close to the stoichiometric proportion of 2:1. As abovementioned, the XRD results indicate the expansion of lattice of Al₂Ca compound for the alloy subjected to high-temperature ST (see Fig. 2). The atomic radius is 0.160 nm for Mg, 0.143 nm for Al, and 0.197 nm for Ca. The expansion of lattice may be attributed to large increase in the concentration of Ca, which has the largest atomic radius.

TABLE I:

Results of average compositions of α-Mg matrix, Al₂Ca, and Mg₁₇Al₁₂ particles in AZX912 extruded alloys with ST at 420 and 510 °C and investigated by TEM-EDS analyses (in at.%).

The reason for hardening of Al₂Ca particles during high-temperature ST is still unclear. It is known that the hardness of intermetallic compound is sensitive to composition change and vacancy concentration [Reference Fukuchi and Watanabe28, Reference Chang, Pike, Liu, Bilbrey and Stone29, Reference Chen, Allen and Livingston30, Reference Chen, Chu, Kotula and Thoma31]. For example, it has been observed that the hardness of Co₂Hf C15 Laves single phase, which has the same type of crystal structure as that of Al₂Ca [Reference Liu, Wang, Wang, Ma and Zhao32], increased with decreasing the Co concentration and reached the maximum value around the stoichiometric Co₂Hf composition [Reference Chen, Chu, Kotula and Thoma31]. In addition, it has been reported that the hardness of FeAl intermetallic compound increased as a function of the square root of vacancy concentration [Reference Chang, Pike, Liu, Bilbrey and Stone29]. It is well known that a higher heating temperature may lead to a higher equilibrium vacancy concentration [Reference Fredriksson and Akerlind33]. It can be expected that more vacancies are frozen by water quenching after high-temperature ST. It is suggested that the hardening phenomenon may be related to the composition change and also the increased vacancy concentration. Further research will be necessary for elucidating the mechanism of the hardening of Al₂Ca compound.

On the other hand, the content of Ca in matrix was very low (only 0.02 ± 0.01 at.%). This is related to its low solid solubility limit in Mg. The compositions of Mg₁₇Al₁₂ precipitates are almost the same between the alloys with ST at 420 and 510 °C. Hence, high-temperature ST does not result in the solid solution of Ca atoms into matrix and also has no influence on the composition of the precipitated Mg₁₇Al₁₂ phase. In addition, no fine Ca-containing precipitate was detected. Therefore, it can be considered that Ca atoms mostly exist in coarse Al₂Ca particles forming during solidification. Considering the total amount of Ca atoms in Al₂Ca particles does not change, the increase in the Ca concentration in Al₂Ca compound from 19.4 ± 0.6 to 29.4 ± 0.5 at.% implies that the amount of Al₂Ca particles should be decreased by the same ratio (i.e., ∼1/3) for the extruded alloy with ST at 510 °C. This is roughly consistent with the reduction ratio (∼1/3) with a decrease from ∼6 to ∼4% in the area ratio of Al₂Ca particles induced by high-temperature ST [see Fig. 3(b)]. According to the large decreases in the Mg concentration and also the ratio of Al/Ca, it is suggested that the abovementioned decrease in the area ratio of Al₂Ca particles caused by high-temperature ST may be mainly due to the solid solution of excess Mg and Al atoms from Al₂Ca compound into matrix.

The scanning electron microscope (SEM) images of AZX912 extruded alloys without ST and with ST at 510 °C are shown in Fig. 5. For the extruded alloy without ST [Fig. 5(a)], some nanoscale Mg₁₇Al₁₂ precipitates with a spherical shape exist besides the significantly elongated Mg₁₇Al₁₂ phase particles. It is obvious that nanoscale Mg₁₇Al₁₂ precipitates in the extruded alloy with ST at 510 °C [Fig. 5(b)] are much more than those in the extruded alloy without ST. For the extruded alloy without ST, the reduced amount of nanoscale Mg₁₇Al₁₂ precipitates can be attributed to the existence of coarse Mg₁₇Al₁₂ phase particles in the as-cast alloy. This reduces the content of Al atoms dissolving into matrix and, thus, decreases the amount of nanoscale Mg₁₇Al₁₂ precipitates induced by dynamic precipitation during subsequent extrusion.

Figure 5:

SEM images of AZX912 extruded alloys (a) without ST and (b) with ST at 510 °C.

The nominal stress–strain curves of the extruded alloys with different ST temperatures are shown in Fig. 6(a). It is obvious that tensile ductility remarkably improves with increasing ST temperature. The changes in ultimate tensile strength (UTS), 0.2% offset yield strength (YS), and fracture elongation (FE), as a function of the ST temperature, are shown in Fig. 6(b). As also summarized in Table II, the FE significantly enhanced from 11.2 to 19.2% with increasing the ST temperature. High mechanical strength (∼360 MPa for UTS) was maintained with an improvement in ductility when increasing the ST temperature up to 510 °C. The high mechanical strength can be mainly attributed to the refined grain structure and the large amount of nanoscale Mg₁₇Al₁₂ precipitates (see Figs. 1(i), 4, and 5). However, the UTS slightly decreased to 344 MPa for the alloy with ST at 525 °C, even though it exhibited the highest FE of 19.2%. The decrease in mechanical strength may be related to the formation of Mg₁₇Al₁₂ phase particles in the STed ingot [see Fig. 1(g)], which may reduce the content of Al atoms dissolving into the matrix and, thus, decrease the amount of dynamically precipitated nanoscale Mg₁₇Al₁₂ particles during extrusion. Regardless of relatively low amount of dynamically precipitated nanoscale Mg₁₇Al₁₂ particles as shown in Fig. 5(a), the alloy without ST exhibited a high UTS of 373 MPa. The high mechanical strength may be related to the unbroken Mg₁₇Al₁₂ phase particles with the significantly elongated shape deformed by extrusion [see Figs. 1(b) and 5(a)], which are likely to act as sources for inducing fiber strengthening effect, as has been observed in carbon nanotube–reinforced Mg matrix composites [Reference Li, Wang, Liu, Wu, Shi, Ding, Hu and Zheng34]. In this case, the load can be transferred from matrix to fiber by a shear stress developing along the interface of matrix and fiber [Reference Li, Wang, Liu, Wu, Shi, Ding, Hu and Zheng34]. However, the FE was only 11.2% for the alloy without ST. It has been reported that stress concentration and subsequent fracture occurred in massive Mg₁₇Al₁₂ particles, and thus, resulted in early fracture during tensile deformation for AZ91 alloy hot compressed from an as-cast state [Reference Xu, Kamado and Honma35]. Therefore, the lowest FE (11.2%) of the alloy without ST in this work may be attributed to the fracture of brittle coarse elongated Mg₁₇Al₁₂ particles under high stress during tensile deformation. Consequently, the alloy with ST at 510 °C exhibited a superior balance between mechanical strength and tensile ductility with a high UTS of 367 MPa and a good FE of 16.8%.

Figure 6:

(a) Nominal stress–strain curves of AZX912 extruded alloys with different ST temperatures. (b) Changes in UTS, YS, and FE as a function of the ST temperature.

TABLE II:

Results of the tensile tests of AZX912 extruded alloys with different ST temperatures.

The SEM images of the polished surfaces of the fractured tensile specimens are shown in Fig. 7. In the SEM images, intermetallic compound particles exhibit a light gray white color, whereas cracks and voids exhibit a dark color. For the extruded alloy with ST at 420 °C shown in Fig. 7(a), the cracking of brittle Al₂Ca particles occurred predominately perpendicular to the tensile direction, as frequently observed in Al₂Ca-containing Mg alloys after tensile deformation [Reference Saito, Suzuki, Fukuda, Ito, Noda, Gonda and Chino11]. No distinct void formation originating from particle–matrix decohesion can be observed. This indicates that Al₂Ca particles cannot provide sufficient strength for restricting cracking before the occurrence of particle–matrix decohesion. In the case of particle cracking, further deformation may result in the rapid widening of initial cracks and a junction between initial cracks in the second-phase particles [Reference Babout, Maire and Fougeres9, Reference Pineau, Benzerga and Pardoen10]. The coalescent cracks may act as fracture sites and cause a significant deterioration of ductility [Reference Babout, Maire and Fougeres9, Reference Pineau, Benzerga and Pardoen10, Reference Saito, Suzuki, Fukuda, Ito, Noda, Gonda and Chino11]. In contrast, particle cracking was remarkably restricted for the extruded alloy with ST at 510 °C [see Fig. 7(b)]. As indicated by the arrows in Fig. 7(b), with respect to the horizontal tensile direction, small voids tended to form at the left and/or right sides of Al₂Ca particles, which may be attributed to particle–matrix decohesion. The change from particle cracking to particle–matrix decohesion may occur when the second-phase particles are much harder than matrix [Reference Babout, Maire and Fougeres9, Reference Pineau, Benzerga and Pardoen10]. The restriction of particle cracking may be attributed to the hardening of Al₂Ca particles and the relaxation of stress concentration on Al₂Ca particles due to the spheroidized shape. Compared with particle cracking, void formation due to particle–matrix decohesion may occur at a later stage of tensile deformation under a higher stress state. This should weaken the detrimental impact of brittle Al₂Ca particles and in turn leads to an enhanced ductility. Furthermore, the decrease in the area ratio of Al₂Ca particles as shown in Fig. 3(b), may also contribute to the improvement of ductility as a result of the reduction of possible fracture sites.

Figure 7:

SEM images of the polished surfaces of the fractured tensile specimens for AZX912 extruded alloys with ST at (a) 420 °C and (b) 510 °C.

The comparison of mechanical properties between the extruded alloys STed at different temperatures in this work and the reported flame-retardant Mg alloys with high Al and Ca contents processed by extrusion, rolling, or drawing, is illustrated in Fig. 8. As guided by the red line, for the conventionally processed alloys in the literature [Reference Huang, Chino, Yuasa, Ueda, Inoue, Kido and Matsumoto6, Reference Saito, Suzuki, Fukuda, Ito, Noda, Gonda and Chino11, Reference Watanabe, Yamaguchi, Takigawa and Higashi36, Reference Masaki, Ochi, Kakiuchi, Kurata, Hirasawa, Matsumura, Takigawa and Higashi37, Reference Hristov and Yoshida38, Reference Li, Qiao, Xu, Kamado, Zheng and Luo39], the UTS and the FE roughly obey the trade-off strength–ductility relationship. In this case, a high UTS of ∼400 MPa results in a low FE of ∼3%, although a relatively good FE of ∼13% tends to be accompanied by a low UTS of ∼300 MPa. In contrast, the extruded alloys subjected to high-temperature ST in this work largely deviates from the red line and shifts to the high ductility region with almost no sacrifice of strength. The extruded alloys with ST at 510 °C exhibited a combination of a UTS of 367 MPa and a FE of 16.8%. In addition, the extruded alloy with ST at 525 °C exhibited a combination of a UTS of 344 MPa and a FE of 19.2%. These mechanical properties are superior to those conventionally processed flame-retardant Mg alloys in the literature. This work reveals that mechanical properties can be significantly improved by using high-temperature ST to tailor the morphology and properties of Al₂Ca compound, which commonly exists in flame-retardant Ca-containing AZX and AMX series Mg alloys.

Figure 8:

UTS and FE of AZX912 extruded alloys obtained in the present work and other conventionally processed flame-retardant Mg alloys with high Al and Ca contents.