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Microstructure and mechanical properties of a novel Mg–RE–Zn–Y alloy fabricated by rheo-squeeze casting
Materials and Design 94 (2016) 353–359
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Microstructure and mechanical properties of a novel Mg–RE–Zn–Y alloy fabricated by rheo-squeeze casting Xiaogang Fang, Shulin Lü, Li Zhao, Jing Wang, Longfei Liu, Shusen Wu ⁎ State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 2 December 2015 Received in revised form 13 January 2016 Accepted 14 January 2016 Available online 16 January 2016 Keywords: Mg–RE alloy Ultrasonic vibration Rheo-squeeze casting Pressure
a b s t r a c t The refinement of the microstructure and uniform distribution of RE-rich intermetallic compounds are beneficial for strengthening and toughening the rare earth (RE) containing Mg alloy castings. A novel Mg–3RE–6Zn–1.4Y alloy was designed, and fabricated by rheo-squeeze casting (RSC) in this research. The semi-solid slurry used in the process was treated by direct ultrasonic vibration (DUV) and then casted with squeeze casting. The microstructure evolution and mechanical properties of the RSC alloy with different pressures were investigated. The RE-rich intermetallic compounds were analysed by X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The results indicate that the increase of the pressure not only contributes to the refinement of α-Mg grains but also results in the increasing solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds along the grain boundary. The tensile properties increase continuously with the increase of applied pressure. The yield strength (YS), ultimate tensile strength (UTS) and elongation of RSC sample under 200 MPa pressure are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without pressure. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As the lightest structural metallic materials, magnesium alloys have a number of exceptional characteristics including low density, good castability, high specific strength and stiffness, attractive damping property and good recyclability, which brings significant application value in the industrial fields such as aerospace, automobile and telecommunications [1–3]. Magnesium cast alloys account for the major share in the actual application. Compared with aluminium cast alloys, the inferior mechanical properties and heat resistant quality of conventional magnesium cast alloys restrict their wide application. It has been concluded that addition of the rare earth (RE) elements is an effective way to greatly enhance the strength, toughness and heat resistance of the Mg alloys [4]. Addition of high content of RE brings magnesium alloys excellent properties, but it is difficult for them to be widely used because of their expensive price. Therefore, addition of cheaper alloying elements instead of part of RE becomes an important developing direction of the new RE-containing casting magnesium alloys with low cost [5]. The element Zn has a low price and strong effect of solution and ageing strengthening. Besides, Zn can react with certain RE elements to form Mg–Zn–RE phases [6–8]. Luo et al. [7] first found the stable icosahedral quasicrystal (I-phase) in an as-cast Mg–Zn–Y alloy. I phase (Mg3YZn6) is regarded as a kind of reinforcement phase with many attractive ⁎ Corresponding author. E-mail address: [email protected] (S. Wu).
http://dx.doi.org/10.1016/j.matdes.2016.01.063 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
properties, such as high strength and hardness. W phase (Mg3Zn2Y3) is another commonly ternary phase in Mg–Zn–Y alloy, and a certain fraction of it could improve the mechanical properties [8]. It has been indicated that the improvement of the properties is limited when adding one kind of rare earth element. With addition of multiple RE elements, the properties of the alloy have a significant improvement because of the interactive effects of multiple elements [5,9]. At present, studies of multiple RE magnesium alloys focus on the interactive effects of heavy rare earth elements, such as Mg–Gd–Y, Mg–Dy–Gd alloys [10,11]. However, the interactions of lower-cost light rare earth (for example, La and Ce) and heavy rare earth elements are rarely studied. Because RE elements are easy to distribute unevenly in the matrix and form coarse intermetallic compounds aggregating along grain boundaries, the properties of conventional magnesium castings are unsatisfactory. In order to solve this problem, most studies focus on the adoption of plastic forming technology to disperse the RE-rich intermetallic compounds, such as extrusion and rolling [12,13]. Rheo-squeeze casting (RSC) is a novel and applicable semisolid metal process capable of producing castings with excellent mechanical properties. Compared with traditional squeeze casting, RSC process can effectively refine primary grains and eliminate eutectic phase segregation [14].The critical step of RSC process is to prepare semisolid slurry and many successful techniques have been developed to prepare semisolid slurries of Al alloys [15,16]. In recent years, many investigations focus on preparing semisolid slurries of some common commercial Mg–Al alloys. Zhao et al. [17] developed a vibrating slope method to
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prepare semisolid slurry of AZ61 alloy and achieved grain refinement during solidification. Wu et al. [18] studied the microstructural evolution of AZ91–Ca magnesium alloy during gas bubbling, and exhibited the improvement in properties of RSC samples compared with conventional squeeze castings. However, relative studies about the effect of the RSC process on RE-containing magnesium alloys are not available. With distinct advantages in grain refinement and solute homogenization, ultrasonic vibration (USV) process is considered as a promising method of preparing semisolid slurry [19]. In previous work, a new rheo-squeeze casting (RSC) technique which combines the application of indirect ultrasonic vibration (IUV) to prepare semisolid slurry and squeeze casting was proposed and tested on aluminium alloy [20]. During the process of IUV treatment, the sonotrode was attached to the bottom of a steel cup containing the melt and worked on the slurry indirectly. The results indicated that the microstructures of the castings were fine and spherical and a considerable improvement in the mechanical properties was achieved. The investigation mainly focused on the refinement of primary grains, but the characteristics of intermetallic compounds during RSC process has not been investigated yet. In the present study, a novel Mg–3RE–6Zn–1.4Y magnesium alloy with a combination of heavy RE element (Y) and light RE elements (La and Ce) was designed and fabricated by RSC. Direct ultrasonic vibration (DUV) process was adopted to prepare the semisolid slurry of this alloy, for the reason that the process of DUV which directly introduces the sonotrode into the slurry gives higher vibration intensity and possesses better effects on the refinement and homogenization than IUV. The semisolid slurry was then casted with squeeze casting under different pressures, which varied from 0 MPa to 200 MPa. The microstructure and tensile properties of this novel magnesium alloy were studied and the effect of pressures on the castings was also discussed. 2. Experimental procedures The chemical compositions of Mg–RE–Zn–Y magnesium alloy are RE 3% (wt.%, the same in the following), Zn 6%, Y 1.4%, Zr 0.6% and Mg balance. The misch metal RE is composed of 65% Ce and 35% La. The raw materials were melted in a mild steel crucible at 750 °C under mixed atmosphere of SF6 and N2 with the volume ratio of 1:99. Fig. 1 shows the differential scanning calorimetry (DSC) curve of this alloy, which determines the liquidus temperature and the solidus temperature are 627 °C and 445 °C respectively. A DUV apparatus for preparing semisolid slurry was applied in the experiment, which was developed by our research group [21]. It mainly contains an ultrasonic generator, a piezoceramic transducer, a titanium sonotrode and a heating furnace. The ultrasonic power in this study was 1200 W and the frequency was 20 kHz. About 200 mL (about 360 g) of the melt was poured into a metallic cup in the heating furnace which was preheated to 600 °C.
Fig. 1. The DSC curve of Mg–3RE–6Zn–1.4Y alloy.
The sonotrode was immersed into the melt about 20 mm from the liquid surface and then DUV started at 635 °C. Argon was introduced as a protective atmosphere to prevent magnesium alloy from oxidation. After the melt was treated with DUV for 1 min the melt cooled to the semi-solid temperature of about 625 °C. Some slurry with certain solid fraction was extracted by a silica tube and quenched in water promptly to analyse the effect of DUV on the microstructure. For comparison, some slurry without DUV was also extracted at the same temperature. Fig. 2 shows the schematic of direct squeeze casting process. The remaining slurry was poured into a mould preheated to 200 °C. Different squeeze pressures (0 MPa, 50 MPa, 100 MPa and 200 MPa) were applied to the slurry from the top and bottom simultaneously and held for 1 min. Cylindrical samples with 30 mm in diameter and 80 mm in length were ejected from the mould. The samples were made into standard tensile specimens with 5 mm in diameter. The room temperature tensile tests were conducted on a SHIMADZU AG-IC100 kN tester to assess the ultimate tensile strength (UTS), yield strength (YS) and elongation to failure. Specimens for metallographic observation were cut from the middle of the quenched rods and the top of the squeeze casting samples. The polished specimens were etched by a solution (4 mL HNO3 + 96 mL ethanol) and initially investigated using an optical microscope (OM, Axiovert 200MAT) for metallographic analysis. Some of them were further observed by a scanning electron microscope (SEM, Nova NanoSEM 450) equipped with an energy dispersive spectrometer (EDS). Phase analysis of the magnesium alloy was determined by the X-ray diffraction (XRD, X′ Pert PRO X). Further characterization of phases was carried out on a transmission electron microscope (TEM, Tecnai G2 F30). Thin foils for the TEM observations were prepared by twin jet electropolishing in a solution of 4 vol.% perchloric acid and 96 vol.% ethanol at −20 °C, and then low energy beam ion thinning was carried out using a Gatan precision ion polishing system (PIPS). Three pictures were randomly chosen for quantitatively analysing the average grain size and the average shape factor (SF) of the primary non-dendritic α-Mg grains. The shape factor (SF) was defined as [20]: S F ¼ 4πA=L2
ð1Þ
where A and L are the cross-sectional area and perimeter of a grain respectively.
Fig. 2. Schematic of the direct squeeze casting process: (a) squeezing; (b) ejecting. 1—movable mould; 2—semisolid slurry; 3—stationary mould; 4—ejector rod; 5—sample.
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Fig. 3. The influence of DUV on the microstructure of semisolid slurry of Mg–3RE–6Zn–1.4Y alloy: (a) without DUV; (b) with DUV.
3. Results 3.1. Microstructural characterization of semisolid slurry Fig. 3a and b shows the microstructure of Mg–3RE–6Zn–1.4Y slurry without and with DUV quenched at 625 °C, respectively. Without DUV (Fig. 3a), the α-Mg grains in the slurry shows a typical morphology of coarse dendrites with an average length of about 180 μm. After DUV with the power of 1200 W (Fig. 3b), the α-Mg phase are converted into fine and spherical grains dispersed uniformly in the matrix. The matrix is considered as a consequence of the quick solidification of the residual liquid and the solid volume fraction is about 34%. The average grain size and the average shape factor of primary Mg grains are respectively 22 μm and 0.70, which demonstrates the DUV has a distinct effect on changing the morphology and size of primary Mg grains. 3.2. Microstructure analysis of RSC samples Fig. 4 shows the morphology evolution of RSC microstructures with the increase of pressure from 0 to 200 MPa. Similar to rheo-casting AM50 alloy in reference [22], the microstructures are composed of
two different sized grains, which are respectively defined as externally solidified grains (ESGs, the larger grains) and internally solidified grains (ISGs, the smaller grains). The ESGs form primarily during slurry preparation while the ISGs correspond to grains that nucleate and grow in the mould cavity. From Fig. 4a–d, with the pressure increasing from 0 MPa to 200 MPa, the ESGs become smaller and the ISGs change gradually from coarse dendrites to fine and spherical grains. Through quantitative analysis, the changes of average grain size and average shape factor of ESGs with the increase of pressure are shown in Fig. 5. When pressure is not applied (0 MPa), the ESGs are relatively coarse and the average grain size is 70 μm. When the pressure increases to 50 MPa, the average grain size decreases obviously to 35 μm. Further increasing the pressure to 100 MPa and 200 MPa, the average grain size decreases gradually and the average grain size is 27 μm when the pressure is 200 MPa. It can be seen that the curve becomes less steep when the pressure is above 50 MPa, which means the average grain size tends gradually to be relatively stable. The average shape factor maintains basically the range of 0.63 to 0.70 and shows an increasing trend with the increase of pressure, which indicates that the grains almost retain their globular morphology previously formed in the semisolid slurry.
Fig. 4. Microstructure of RSC samples under different pressures: (a) 0 MPa, (b) 50 MPa, (c) 100 MPa, and (d) 200 MPa.
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X. Fang et al. / Materials and Design 94 (2016) 353–359 Table 1 EDS results of points in Fig. 6 and Fig. 7 (at%). Detected points
1 2 3 4
Fig. 5. Average grain size and average shape factor of ESGs in the RSC samples under different pressures.
Fig. 6a and b shows the SEM micrograph of Mg–3RE–6Zn–1.4Y alloy under pressures of 0 MPa and 200 MPa respectively. The net-like agglomeration of intermetallic compounds along the grain boundaries of α-Mg can be obviously found in Fig. 6a. With the result of EDS analyses shown in Table 1, the compositions of α-Mg (Point 1) are mainly Mg element and a small amount of solid-solution Zn element. The spectrum of Point 2 indicates that the elements of Ce, La, Y and Zn concentrate along the grain boundary and react with Mg to form RE-rich intermetallic compounds. Zr is not detected out in the EDS because of its rare amount. Under the pressure of 200 MPa (Fig. 6b), the compounds along grain boundaries are obviously refined and the agglomeration of intermetallic compounds at triple junctions of grain boundaries in Fig. 6a cannot be found, which means the pressure leads to a uniform distribution of intermetallic compounds. Fig. 7 shows the intermetallic compounds distributed continuously along the grain boundary in high magnification of Fig. 6a. The intermetallic compounds present briefly in two different kinds of brightness (Point 3 and Point 4). From Table 1, the coarse grey phase (Point 3) is an intermetallic compound of Mg–Zn–RE, and the bright phase is ternary phase of Mg–Zn–Y. The XRD patterns reveal the phase constitutions of the RSC samples under different pressures, as shown in Fig. 8a. The as-cast Mg–3RE– 6Zn–1.4Y alloy mainly consists of α-Mg matrix, α-Zr, W-Mg3Zn2Y3, IMg3Zn6Y and an unknown phase (denoted as T phase and analysed below). It also indicates that the phase constitutions are nearly the same of the RSC samples under different pressures. However, the peak of 35° disappears with pressure increasing, which is probably attributed to the increase of solid solubility.
Element (at.%) Mg
Zn
Y
La
Ce
Total
99.35 69.47 77.45 73.17
0.65 20.94 15.51 17.45
– 5.17 0.25 8.01
– 1.89 3.81 0.51
– 2.53 2.98 0.86
100 100 100 100
The phase transition temperatures in Fig. 1 are in accordance with the transition temperatures of W phase (Mg3Zn2Y3) and I phase (Mg3Zn6Y) reported by Xu et al. [23]. The results further confirm the formation of cubic W phase and icosahedral quasicrystal I phase. The eutectic reaction happens at about 534 °C to form the W phase (Eq. (2)) and then the remaining liquid reacts with the W phase to form I phase (Eq. (3)). L
534
→
LþW
C
α‐Mg þ W
445
→
C
I
ð2Þ ð3Þ
However, the intensity of diffraction peaks corresponding to the W phase and I phase (shown in Fig. 8) is relatively low, indicating the low fraction of the W phase and I phase. The XRD patterns also prove that the unknown T phase accounts for the major fraction of intermetallic compounds. To have a further investigation of the unknown phase, a bright-field TEM image of the RSC sample under pressure of 0 MPa is shown in Fig. 9a. It can be seen that a variety of phases are hybrid to form a ribbon surrounding α-Mg grains. An EDS analysis (Fig. 9b) of the particle (region A in Fig. 9a) reveals prominent Mg peaks, significant intensity of the Zn peak, and low intensity of the La and Ce peak, which means the unknown phase is a kind of Mg–Zn–La–Ce quaternary phase. The selected area electron diffraction (SAED) pattern of the quaternary phase in Fig. 9a is given in Fig. 9c. It is in accordance with Ce(Mg1 − xZnx)11, which has a C-centred orthorhombic structure with the lattice parameters of around a = 1.019 nm, b = 1.119 nm, and c = 1.009 nm. The presence of T phase (Ce(Mg1 − xZnx)11) was first found in equilibrium Mg–Zn–Ce alloys by Huang et al. [24]. Jeong et al. [25] also observed Ce(Mg1 − xZnx)11 in extruded Mg–Zn–Ce–Zr alloys, and claimed that the agglomeration of Ce(Mg1 − xZnx)11 provides sites for fracture initiation during tensile testing. Since both the light RE elements La and Ce belong to the lanthanide series and their atomic number are adjacent, they have extremely similar chemical properties. As to this novel alloy, it is considered that La atoms substitute the positions of some Ce atoms of Ce(Mg1 − xZnx)11 to form a new quaternary phase
Fig. 6. SEM micrographs of RSC samples under different pressures: (a) 0 MPa; (b) 200 MPa.
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Fig. 7. SEM micrographs in high magnification of Fig. 6a.
(La,Ce)(Mg1 − xZnx)11, which is also denoted as T phase. Besides, it can be inferred that the phase transition temperature of T phase is 579 °C (see in Fig. 1). 3.3. Mechanical properties The tensile properties of as-cast RSC samples under different pressures are shown in Fig. 10. An increase in pressure brings a certain improvement in the YS and a significant improvement in the UTS and the elongation. The YS, UTS and elongation of RSC sample under the pressure of 200 MPa are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without applied pressure. Besides, when the pressure is beyond 50 MPa, the improvements tend to be slight gradually. 4. Discussion An understanding of the formation and growth of both primary phase and intermetallic compounds during the solidification is necessary, as the mechanical properties strongly depend on the size, amount, shape and distribution of them. The RSC process consists mainly of two stages of solidification. The solidification during the preparation of semisolid slurry is the first stage, and the solidification of the slurry being squeezed in the mould cavity is the second stage.
Fig. 8. (a) XRD patterns for the RSC samples under different pressures; (b) partial enlargement of α-Mg peak.
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As the first step of the RSC process, the preparation of semisolid slurry is of vital importance. Fig. 3 indicated that good semisolid slurry with fine and spherical primary α-Mg grains can be obtained by DUV process. During the first stage of solidification, the combined effect of both ultrasonic cavitation and acoustic streaming is regarded as the mechanism that explains the refinement and spheroidization of primary Mg grains [19,26]. The sonotrode was immersed directly into the melt and ultrasound propagated in the magnesium alloy melt during the process of DUV. Liquid molecules bear the effect of a cyclic alternating sound field. The liquid is under tensile stress during the negative cycle of ultrasound. A large number of cavitation bubbles will form and grow in the melt if the negative pressure reaches a certain value. The expansion of the cavitation bubbles absorbs heat and decreases the ambient temperature. As a consequence, undercooling occurs in the surrounding melt and a large number of nuclei form. During the subsequent positive cycle, the cavitation bubbles are subjected to compressive stress and collapse violently. Consequently, instantaneous high temperature and pressure occur, which further refines the newly formed nuclei. The acoustic pressure gradient formed in the propagation of ultrasound leads to the acoustic streaming effect. The intensive streaming produces a pronounced effect on transporting the cavitation bubbles to the bulk of the melt, which leads to the homogenization of temperature and solute. As a consequence, the growth mode of primary Mg grains changes from dendritic to non-dendritic. Besides, the homogenization of alloying elements contributes to the uniform distribution of intermetallic compounds which form during the subsequent solidification. The subsequent growth of ESGs and the formations of ISGs and intermetallic compounds proceed during the second stage of solidification. For the RSC process, the applied pressure plays an important role in determining the characteristics of microstructures. It is considered that the application of pressure will lead to an increase in the freezing temperature [27]. The theoretical value of the rise (ΔTf) can be calculated according to the Clausius–Clapeyron equation: ΔT f T f ðV l −V s Þ ¼ ΔH f ΔP
ð4Þ
where Tf is the equilibrium freezing temperature, Vl and Vs are respectively the specific volumes of the liquid and solid, and ΔHf is the latent heat of fusion. Thus, for an alloy having a change in volume from liquid to solid, the application of pressure during solidification will increase the freezing temperature of the alloy. For pure magnesium, the increase being calculated is 0.069 K MPa−1, which is close to pure aluminium (0.068 K MPa−1) in reference [28]. For the process of RSC, because the temperature of the slurry is below the liquidus temperature, a high degree of undercooling caused by applying pressure effectively stimulates the nucleation. For this reason, the refinement of α-Mg is significant, especially for ISGs which nucleate and grow during the second stage. On the other hand, the pressure could reinforce the heat transfer across the interface between the mould and the slurry and generate a high cooling rate. According to Lee et al. [29], the air gap between the material and the mould is easily formed during conventional die casting due to the thermal contraction. For RSC process, the sustaining applied pressure is effective in eliminating the air gap. The higher of the pressure, the more thoroughly the air gap is eliminated. With the pressure increasing, the elimination of the air gap is more thorough. In other words, the increase of the pressure makes the slurry and the mould contact more closely, thereby leading to a greater heat transfer and a higher cooling rate. During the second stage, a high cooling rate makes the morphology of spherical grains in the slurry retained completely and helps α-Mg grains get further refined. Thus, the decrease in the average grain size of ESGs and the morphological change of ISGs in Fig.4 are mainly due to the differences in ΔTf and cooling rates induced by varied pressures. To quantitatively characterize the effect of pressure on the intermetallic compounds, the volume fraction of intermetallic compounds (fi) is
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Fig. 9. (a) A bright field TEM graph of the second phase along the boundary; (b) the corresponding EDS result of region A arrowed in (a); (c) [0 0 1] zone axis, the SAED pattern obtained from the Mg–Zn–RE phase.
calculated by image analysis. As shown in Fig. 6a and b, with the pressure increasing from 0 MPa to 200 MPa, the fi decreases from 15.1% to 8.5%. The result is in accordance with the investigation of Jie et al. [30]. They reported that applying high pressure during solidification of metals can increase the solid solubility of some elements. As shown in Fig. 8b, it indicates that a little migration of α-Mg diffraction peak appears under different pressures in the XRD patterns. This can be attributed to the increase of solid solubility in the matrix with the increase of pressure, which may lead to the change of lattice constant. Without applied pressure, strong agglomerations of intermetallic compounds are easy to form along grain boundaries (see Fig. 6a). In the situation of applying pressure, the grain boundaries are refined and the strong agglomerations of intermetallic compounds disappear. This is attributed to the refinement of α-Mg grains and the increased solid solubility.
Besides, the high cooling rate induced by pressure contributes to block the formation of agglomerations. Porosity is easy to form in the conventional castings of Mg–3RE– 6Zn–1.4Y alloy due to the broad crystallization temperature interval as shown in Fig. 1. Masoumi et al. [31] found that the increase of applied pressure from 3 MPa to 90 MPa led to the increase in density and the decrease in porosity of conventional squeeze casting AX51 alloys. Therefore, with the pressure varied from 0 MPa to 50 MPa, the significant improvement in the YS, UTS and elongation (Fig. 10) could be mainly attributed to the materials densification. In a situation where the porosity is almost completely eliminated, the grain size becomes the dominant factor in influencing the properties. The yield strength and grain size are inversely related, as demonstrated by the Hall–Petch equation: −1=2
σ ¼ σ 0 þ kd
ð5Þ
where d is the grain size, σ0 and k are the experimentally derived constants. As Mg has a hexagonal close-packed (hcp) crystal structure, the constant k can reach a high value (280–320 MPa μm− 1/2) [32]. Hence, the grain refinement leads to an increase in the strength. In addition to grain refinement, the increased solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds also contribute to the improvement of the tensile properties. Therefore, the applied pressure during the RSC process results in the materials densification, the refinement of grains, the increase of solid solubility and the elimination of intermetallic compounds agglomeration, which contribute to the improvement of mechanical properties. 5. Conclusions
Fig. 10. Tensile properties of the as-cast RSC samples under different pressures.
1) Good semisolid slurry of Mg–3RE–6Zn–1.4Y alloy with fine and spherical primary α-Mg grains can be obtained with the application
X. Fang et al. / Materials and Design 94 (2016) 353–359
of direct ultrasonic vibration, and the average grain size and average shape factor are 22 μm and 0.70 respectively. 2) There are two different sized grains (ESGs and ISGs) in the microstructure of RSC samples. The increase of pressure not only contributes to the refinement and spheroidization of α-Mg grains but also results in the increased solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds along the grain boundary. The phase constitutions of this novel alloy are mainly αMg, α-Zr, W phase (Mg3Zn2Y3), icosahedral quasicrystal I phase (Mg3Zn6Y) and T phase ((La,Ce)(Mg1 − xZnx)11). 3) With the increase of pressure from 0 MPa to 200 MPa, the tensile properties of the RSC samples increase continuously. The YS, UTS and elongation of RSC sample under the pressure of 200 MPa are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without applied pressure. Acknowledgements This work was funded by Project 51275183 supported by National Natural Science Foundation of China. The authors would also like to express their appreciation to the Analytical and Testing Centre, HUST. References [1] D. Eliezer, E. Aghion, F.H. Froes, Magnesium science, technology and applications, Adv. Perform. Mater. 5 (1998) 201–212. [2] F.A. Mirza, D.L. Chen, D.J. Li, X.Q. Zeng, Low cycle fatigue of an extruded Mg–3Nd– 0.2Zn–0.5Zr magnesium alloy, Mater. Des. 64 (2014) 63–73. [3] M. Easton, A. Beer, M. Barnett, C. Davies, G. Dunlop, Y. Durandet, S. Blacket, T. Hilditch, P. Beggs, Magnesium alloy applications in automotive structures, JOM 60 (2008) 57–62. [4] S. Tekumalla, S. Seetharaman, A. Almajid, M. Gupta, Mechanical properties of magnesium–rare earth alloy systems: a review, Metals 5 (2015) 1–39. [5] J. Wang, P. Song, S. Huang, F. Pan, High-strength and good-ductility Mg–RE–Zn–Mn magnesium alloy with long-period stacking ordered phase, Mater. Lett. 93 (2013) 415–418. [6] Z. Zhang, X. Liu, Z. Wang, Q. Le, W. Hu, L. Bao, J. Cui, Effects of phase composition and content on the microstructures and mechanical properties of high strength Mg–Y– Zn–Zr alloys, Mater. Des. 88 (2015) 915–923. [7] Z. Luo, S. Zhang, Y. Tang, D. Zhao, Quasicrystals in as-cast Mg–Zn–RE alloys, Scr. Metall. Mater. 28 (1993) 1513–1518. [8] D. Xu, W. Tang, L. Liu, Y. Xu, E. Han, Effect of W-phase on the mechanical properties of as-cast Mg–Zn–Y–Zr alloys, J. Alloys Compd. 461 (2008) 248–252. [9] J. Gröbner, A. Kozlov, R. Schmid-Fetzer, M.A. Easton, S. Zhu, M.A. Gibson, J.F. Nie, Thermodynamic analysis of as-cast and heat-treated microstructures of Mg–Ce– Nd alloys, Acta Mater. 59 (2011) 613–622. [10] F.A. Mirza, D.L. Chen, D.J. Li, X.Q. Zeng, Effect of rare earth elements on deformation behavior of an extruded Mg–10Gd–3Y–0.5Zr alloy during compression, Mater. Des. 46 (2013) 411–418. [11] D.H. Li, J. Dong, X.Q. Zeng, Characterization of precipitate phases in a Mg–Dy–Gd–Nd alloy, J. Alloys Compd. 439 (2007) 254–257.
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Microstructure and mechanical properties of a novel Mg–RE–Zn–Y alloy fabricated by rheo-squeeze casting Xiaogang Fang, Shulin Lü, Li Zhao, Jing Wang, Longfei Liu, Shusen Wu ⁎ State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 2 December 2015 Received in revised form 13 January 2016 Accepted 14 January 2016 Available online 16 January 2016 Keywords: Mg–RE alloy Ultrasonic vibration Rheo-squeeze casting Pressure
a b s t r a c t The refinement of the microstructure and uniform distribution of RE-rich intermetallic compounds are beneficial for strengthening and toughening the rare earth (RE) containing Mg alloy castings. A novel Mg–3RE–6Zn–1.4Y alloy was designed, and fabricated by rheo-squeeze casting (RSC) in this research. The semi-solid slurry used in the process was treated by direct ultrasonic vibration (DUV) and then casted with squeeze casting. The microstructure evolution and mechanical properties of the RSC alloy with different pressures were investigated. The RE-rich intermetallic compounds were analysed by X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The results indicate that the increase of the pressure not only contributes to the refinement of α-Mg grains but also results in the increasing solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds along the grain boundary. The tensile properties increase continuously with the increase of applied pressure. The yield strength (YS), ultimate tensile strength (UTS) and elongation of RSC sample under 200 MPa pressure are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without pressure. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As the lightest structural metallic materials, magnesium alloys have a number of exceptional characteristics including low density, good castability, high specific strength and stiffness, attractive damping property and good recyclability, which brings significant application value in the industrial fields such as aerospace, automobile and telecommunications [1–3]. Magnesium cast alloys account for the major share in the actual application. Compared with aluminium cast alloys, the inferior mechanical properties and heat resistant quality of conventional magnesium cast alloys restrict their wide application. It has been concluded that addition of the rare earth (RE) elements is an effective way to greatly enhance the strength, toughness and heat resistance of the Mg alloys [4]. Addition of high content of RE brings magnesium alloys excellent properties, but it is difficult for them to be widely used because of their expensive price. Therefore, addition of cheaper alloying elements instead of part of RE becomes an important developing direction of the new RE-containing casting magnesium alloys with low cost [5]. The element Zn has a low price and strong effect of solution and ageing strengthening. Besides, Zn can react with certain RE elements to form Mg–Zn–RE phases [6–8]. Luo et al. [7] first found the stable icosahedral quasicrystal (I-phase) in an as-cast Mg–Zn–Y alloy. I phase (Mg3YZn6) is regarded as a kind of reinforcement phase with many attractive ⁎ Corresponding author. E-mail address: [email protected] (S. Wu).
http://dx.doi.org/10.1016/j.matdes.2016.01.063 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
properties, such as high strength and hardness. W phase (Mg3Zn2Y3) is another commonly ternary phase in Mg–Zn–Y alloy, and a certain fraction of it could improve the mechanical properties [8]. It has been indicated that the improvement of the properties is limited when adding one kind of rare earth element. With addition of multiple RE elements, the properties of the alloy have a significant improvement because of the interactive effects of multiple elements [5,9]. At present, studies of multiple RE magnesium alloys focus on the interactive effects of heavy rare earth elements, such as Mg–Gd–Y, Mg–Dy–Gd alloys [10,11]. However, the interactions of lower-cost light rare earth (for example, La and Ce) and heavy rare earth elements are rarely studied. Because RE elements are easy to distribute unevenly in the matrix and form coarse intermetallic compounds aggregating along grain boundaries, the properties of conventional magnesium castings are unsatisfactory. In order to solve this problem, most studies focus on the adoption of plastic forming technology to disperse the RE-rich intermetallic compounds, such as extrusion and rolling [12,13]. Rheo-squeeze casting (RSC) is a novel and applicable semisolid metal process capable of producing castings with excellent mechanical properties. Compared with traditional squeeze casting, RSC process can effectively refine primary grains and eliminate eutectic phase segregation [14].The critical step of RSC process is to prepare semisolid slurry and many successful techniques have been developed to prepare semisolid slurries of Al alloys [15,16]. In recent years, many investigations focus on preparing semisolid slurries of some common commercial Mg–Al alloys. Zhao et al. [17] developed a vibrating slope method to
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prepare semisolid slurry of AZ61 alloy and achieved grain refinement during solidification. Wu et al. [18] studied the microstructural evolution of AZ91–Ca magnesium alloy during gas bubbling, and exhibited the improvement in properties of RSC samples compared with conventional squeeze castings. However, relative studies about the effect of the RSC process on RE-containing magnesium alloys are not available. With distinct advantages in grain refinement and solute homogenization, ultrasonic vibration (USV) process is considered as a promising method of preparing semisolid slurry [19]. In previous work, a new rheo-squeeze casting (RSC) technique which combines the application of indirect ultrasonic vibration (IUV) to prepare semisolid slurry and squeeze casting was proposed and tested on aluminium alloy [20]. During the process of IUV treatment, the sonotrode was attached to the bottom of a steel cup containing the melt and worked on the slurry indirectly. The results indicated that the microstructures of the castings were fine and spherical and a considerable improvement in the mechanical properties was achieved. The investigation mainly focused on the refinement of primary grains, but the characteristics of intermetallic compounds during RSC process has not been investigated yet. In the present study, a novel Mg–3RE–6Zn–1.4Y magnesium alloy with a combination of heavy RE element (Y) and light RE elements (La and Ce) was designed and fabricated by RSC. Direct ultrasonic vibration (DUV) process was adopted to prepare the semisolid slurry of this alloy, for the reason that the process of DUV which directly introduces the sonotrode into the slurry gives higher vibration intensity and possesses better effects on the refinement and homogenization than IUV. The semisolid slurry was then casted with squeeze casting under different pressures, which varied from 0 MPa to 200 MPa. The microstructure and tensile properties of this novel magnesium alloy were studied and the effect of pressures on the castings was also discussed. 2. Experimental procedures The chemical compositions of Mg–RE–Zn–Y magnesium alloy are RE 3% (wt.%, the same in the following), Zn 6%, Y 1.4%, Zr 0.6% and Mg balance. The misch metal RE is composed of 65% Ce and 35% La. The raw materials were melted in a mild steel crucible at 750 °C under mixed atmosphere of SF6 and N2 with the volume ratio of 1:99. Fig. 1 shows the differential scanning calorimetry (DSC) curve of this alloy, which determines the liquidus temperature and the solidus temperature are 627 °C and 445 °C respectively. A DUV apparatus for preparing semisolid slurry was applied in the experiment, which was developed by our research group [21]. It mainly contains an ultrasonic generator, a piezoceramic transducer, a titanium sonotrode and a heating furnace. The ultrasonic power in this study was 1200 W and the frequency was 20 kHz. About 200 mL (about 360 g) of the melt was poured into a metallic cup in the heating furnace which was preheated to 600 °C.
Fig. 1. The DSC curve of Mg–3RE–6Zn–1.4Y alloy.
The sonotrode was immersed into the melt about 20 mm from the liquid surface and then DUV started at 635 °C. Argon was introduced as a protective atmosphere to prevent magnesium alloy from oxidation. After the melt was treated with DUV for 1 min the melt cooled to the semi-solid temperature of about 625 °C. Some slurry with certain solid fraction was extracted by a silica tube and quenched in water promptly to analyse the effect of DUV on the microstructure. For comparison, some slurry without DUV was also extracted at the same temperature. Fig. 2 shows the schematic of direct squeeze casting process. The remaining slurry was poured into a mould preheated to 200 °C. Different squeeze pressures (0 MPa, 50 MPa, 100 MPa and 200 MPa) were applied to the slurry from the top and bottom simultaneously and held for 1 min. Cylindrical samples with 30 mm in diameter and 80 mm in length were ejected from the mould. The samples were made into standard tensile specimens with 5 mm in diameter. The room temperature tensile tests were conducted on a SHIMADZU AG-IC100 kN tester to assess the ultimate tensile strength (UTS), yield strength (YS) and elongation to failure. Specimens for metallographic observation were cut from the middle of the quenched rods and the top of the squeeze casting samples. The polished specimens were etched by a solution (4 mL HNO3 + 96 mL ethanol) and initially investigated using an optical microscope (OM, Axiovert 200MAT) for metallographic analysis. Some of them were further observed by a scanning electron microscope (SEM, Nova NanoSEM 450) equipped with an energy dispersive spectrometer (EDS). Phase analysis of the magnesium alloy was determined by the X-ray diffraction (XRD, X′ Pert PRO X). Further characterization of phases was carried out on a transmission electron microscope (TEM, Tecnai G2 F30). Thin foils for the TEM observations were prepared by twin jet electropolishing in a solution of 4 vol.% perchloric acid and 96 vol.% ethanol at −20 °C, and then low energy beam ion thinning was carried out using a Gatan precision ion polishing system (PIPS). Three pictures were randomly chosen for quantitatively analysing the average grain size and the average shape factor (SF) of the primary non-dendritic α-Mg grains. The shape factor (SF) was defined as [20]: S F ¼ 4πA=L2
ð1Þ
where A and L are the cross-sectional area and perimeter of a grain respectively.
Fig. 2. Schematic of the direct squeeze casting process: (a) squeezing; (b) ejecting. 1—movable mould; 2—semisolid slurry; 3—stationary mould; 4—ejector rod; 5—sample.
X. Fang et al. / Materials and Design 94 (2016) 353–359
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Fig. 3. The influence of DUV on the microstructure of semisolid slurry of Mg–3RE–6Zn–1.4Y alloy: (a) without DUV; (b) with DUV.
3. Results 3.1. Microstructural characterization of semisolid slurry Fig. 3a and b shows the microstructure of Mg–3RE–6Zn–1.4Y slurry without and with DUV quenched at 625 °C, respectively. Without DUV (Fig. 3a), the α-Mg grains in the slurry shows a typical morphology of coarse dendrites with an average length of about 180 μm. After DUV with the power of 1200 W (Fig. 3b), the α-Mg phase are converted into fine and spherical grains dispersed uniformly in the matrix. The matrix is considered as a consequence of the quick solidification of the residual liquid and the solid volume fraction is about 34%. The average grain size and the average shape factor of primary Mg grains are respectively 22 μm and 0.70, which demonstrates the DUV has a distinct effect on changing the morphology and size of primary Mg grains. 3.2. Microstructure analysis of RSC samples Fig. 4 shows the morphology evolution of RSC microstructures with the increase of pressure from 0 to 200 MPa. Similar to rheo-casting AM50 alloy in reference [22], the microstructures are composed of
two different sized grains, which are respectively defined as externally solidified grains (ESGs, the larger grains) and internally solidified grains (ISGs, the smaller grains). The ESGs form primarily during slurry preparation while the ISGs correspond to grains that nucleate and grow in the mould cavity. From Fig. 4a–d, with the pressure increasing from 0 MPa to 200 MPa, the ESGs become smaller and the ISGs change gradually from coarse dendrites to fine and spherical grains. Through quantitative analysis, the changes of average grain size and average shape factor of ESGs with the increase of pressure are shown in Fig. 5. When pressure is not applied (0 MPa), the ESGs are relatively coarse and the average grain size is 70 μm. When the pressure increases to 50 MPa, the average grain size decreases obviously to 35 μm. Further increasing the pressure to 100 MPa and 200 MPa, the average grain size decreases gradually and the average grain size is 27 μm when the pressure is 200 MPa. It can be seen that the curve becomes less steep when the pressure is above 50 MPa, which means the average grain size tends gradually to be relatively stable. The average shape factor maintains basically the range of 0.63 to 0.70 and shows an increasing trend with the increase of pressure, which indicates that the grains almost retain their globular morphology previously formed in the semisolid slurry.
Fig. 4. Microstructure of RSC samples under different pressures: (a) 0 MPa, (b) 50 MPa, (c) 100 MPa, and (d) 200 MPa.
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X. Fang et al. / Materials and Design 94 (2016) 353–359 Table 1 EDS results of points in Fig. 6 and Fig. 7 (at%). Detected points
1 2 3 4
Fig. 5. Average grain size and average shape factor of ESGs in the RSC samples under different pressures.
Fig. 6a and b shows the SEM micrograph of Mg–3RE–6Zn–1.4Y alloy under pressures of 0 MPa and 200 MPa respectively. The net-like agglomeration of intermetallic compounds along the grain boundaries of α-Mg can be obviously found in Fig. 6a. With the result of EDS analyses shown in Table 1, the compositions of α-Mg (Point 1) are mainly Mg element and a small amount of solid-solution Zn element. The spectrum of Point 2 indicates that the elements of Ce, La, Y and Zn concentrate along the grain boundary and react with Mg to form RE-rich intermetallic compounds. Zr is not detected out in the EDS because of its rare amount. Under the pressure of 200 MPa (Fig. 6b), the compounds along grain boundaries are obviously refined and the agglomeration of intermetallic compounds at triple junctions of grain boundaries in Fig. 6a cannot be found, which means the pressure leads to a uniform distribution of intermetallic compounds. Fig. 7 shows the intermetallic compounds distributed continuously along the grain boundary in high magnification of Fig. 6a. The intermetallic compounds present briefly in two different kinds of brightness (Point 3 and Point 4). From Table 1, the coarse grey phase (Point 3) is an intermetallic compound of Mg–Zn–RE, and the bright phase is ternary phase of Mg–Zn–Y. The XRD patterns reveal the phase constitutions of the RSC samples under different pressures, as shown in Fig. 8a. The as-cast Mg–3RE– 6Zn–1.4Y alloy mainly consists of α-Mg matrix, α-Zr, W-Mg3Zn2Y3, IMg3Zn6Y and an unknown phase (denoted as T phase and analysed below). It also indicates that the phase constitutions are nearly the same of the RSC samples under different pressures. However, the peak of 35° disappears with pressure increasing, which is probably attributed to the increase of solid solubility.
Element (at.%) Mg
Zn
Y
La
Ce
Total
99.35 69.47 77.45 73.17
0.65 20.94 15.51 17.45
– 5.17 0.25 8.01
– 1.89 3.81 0.51
– 2.53 2.98 0.86
100 100 100 100
The phase transition temperatures in Fig. 1 are in accordance with the transition temperatures of W phase (Mg3Zn2Y3) and I phase (Mg3Zn6Y) reported by Xu et al. [23]. The results further confirm the formation of cubic W phase and icosahedral quasicrystal I phase. The eutectic reaction happens at about 534 °C to form the W phase (Eq. (2)) and then the remaining liquid reacts with the W phase to form I phase (Eq. (3)). L
534
→
LþW
C
α‐Mg þ W
445
→
C
I
ð2Þ ð3Þ
However, the intensity of diffraction peaks corresponding to the W phase and I phase (shown in Fig. 8) is relatively low, indicating the low fraction of the W phase and I phase. The XRD patterns also prove that the unknown T phase accounts for the major fraction of intermetallic compounds. To have a further investigation of the unknown phase, a bright-field TEM image of the RSC sample under pressure of 0 MPa is shown in Fig. 9a. It can be seen that a variety of phases are hybrid to form a ribbon surrounding α-Mg grains. An EDS analysis (Fig. 9b) of the particle (region A in Fig. 9a) reveals prominent Mg peaks, significant intensity of the Zn peak, and low intensity of the La and Ce peak, which means the unknown phase is a kind of Mg–Zn–La–Ce quaternary phase. The selected area electron diffraction (SAED) pattern of the quaternary phase in Fig. 9a is given in Fig. 9c. It is in accordance with Ce(Mg1 − xZnx)11, which has a C-centred orthorhombic structure with the lattice parameters of around a = 1.019 nm, b = 1.119 nm, and c = 1.009 nm. The presence of T phase (Ce(Mg1 − xZnx)11) was first found in equilibrium Mg–Zn–Ce alloys by Huang et al. [24]. Jeong et al. [25] also observed Ce(Mg1 − xZnx)11 in extruded Mg–Zn–Ce–Zr alloys, and claimed that the agglomeration of Ce(Mg1 − xZnx)11 provides sites for fracture initiation during tensile testing. Since both the light RE elements La and Ce belong to the lanthanide series and their atomic number are adjacent, they have extremely similar chemical properties. As to this novel alloy, it is considered that La atoms substitute the positions of some Ce atoms of Ce(Mg1 − xZnx)11 to form a new quaternary phase
Fig. 6. SEM micrographs of RSC samples under different pressures: (a) 0 MPa; (b) 200 MPa.
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Fig. 7. SEM micrographs in high magnification of Fig. 6a.
(La,Ce)(Mg1 − xZnx)11, which is also denoted as T phase. Besides, it can be inferred that the phase transition temperature of T phase is 579 °C (see in Fig. 1). 3.3. Mechanical properties The tensile properties of as-cast RSC samples under different pressures are shown in Fig. 10. An increase in pressure brings a certain improvement in the YS and a significant improvement in the UTS and the elongation. The YS, UTS and elongation of RSC sample under the pressure of 200 MPa are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without applied pressure. Besides, when the pressure is beyond 50 MPa, the improvements tend to be slight gradually. 4. Discussion An understanding of the formation and growth of both primary phase and intermetallic compounds during the solidification is necessary, as the mechanical properties strongly depend on the size, amount, shape and distribution of them. The RSC process consists mainly of two stages of solidification. The solidification during the preparation of semisolid slurry is the first stage, and the solidification of the slurry being squeezed in the mould cavity is the second stage.
Fig. 8. (a) XRD patterns for the RSC samples under different pressures; (b) partial enlargement of α-Mg peak.
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As the first step of the RSC process, the preparation of semisolid slurry is of vital importance. Fig. 3 indicated that good semisolid slurry with fine and spherical primary α-Mg grains can be obtained by DUV process. During the first stage of solidification, the combined effect of both ultrasonic cavitation and acoustic streaming is regarded as the mechanism that explains the refinement and spheroidization of primary Mg grains [19,26]. The sonotrode was immersed directly into the melt and ultrasound propagated in the magnesium alloy melt during the process of DUV. Liquid molecules bear the effect of a cyclic alternating sound field. The liquid is under tensile stress during the negative cycle of ultrasound. A large number of cavitation bubbles will form and grow in the melt if the negative pressure reaches a certain value. The expansion of the cavitation bubbles absorbs heat and decreases the ambient temperature. As a consequence, undercooling occurs in the surrounding melt and a large number of nuclei form. During the subsequent positive cycle, the cavitation bubbles are subjected to compressive stress and collapse violently. Consequently, instantaneous high temperature and pressure occur, which further refines the newly formed nuclei. The acoustic pressure gradient formed in the propagation of ultrasound leads to the acoustic streaming effect. The intensive streaming produces a pronounced effect on transporting the cavitation bubbles to the bulk of the melt, which leads to the homogenization of temperature and solute. As a consequence, the growth mode of primary Mg grains changes from dendritic to non-dendritic. Besides, the homogenization of alloying elements contributes to the uniform distribution of intermetallic compounds which form during the subsequent solidification. The subsequent growth of ESGs and the formations of ISGs and intermetallic compounds proceed during the second stage of solidification. For the RSC process, the applied pressure plays an important role in determining the characteristics of microstructures. It is considered that the application of pressure will lead to an increase in the freezing temperature [27]. The theoretical value of the rise (ΔTf) can be calculated according to the Clausius–Clapeyron equation: ΔT f T f ðV l −V s Þ ¼ ΔH f ΔP
ð4Þ
where Tf is the equilibrium freezing temperature, Vl and Vs are respectively the specific volumes of the liquid and solid, and ΔHf is the latent heat of fusion. Thus, for an alloy having a change in volume from liquid to solid, the application of pressure during solidification will increase the freezing temperature of the alloy. For pure magnesium, the increase being calculated is 0.069 K MPa−1, which is close to pure aluminium (0.068 K MPa−1) in reference [28]. For the process of RSC, because the temperature of the slurry is below the liquidus temperature, a high degree of undercooling caused by applying pressure effectively stimulates the nucleation. For this reason, the refinement of α-Mg is significant, especially for ISGs which nucleate and grow during the second stage. On the other hand, the pressure could reinforce the heat transfer across the interface between the mould and the slurry and generate a high cooling rate. According to Lee et al. [29], the air gap between the material and the mould is easily formed during conventional die casting due to the thermal contraction. For RSC process, the sustaining applied pressure is effective in eliminating the air gap. The higher of the pressure, the more thoroughly the air gap is eliminated. With the pressure increasing, the elimination of the air gap is more thorough. In other words, the increase of the pressure makes the slurry and the mould contact more closely, thereby leading to a greater heat transfer and a higher cooling rate. During the second stage, a high cooling rate makes the morphology of spherical grains in the slurry retained completely and helps α-Mg grains get further refined. Thus, the decrease in the average grain size of ESGs and the morphological change of ISGs in Fig.4 are mainly due to the differences in ΔTf and cooling rates induced by varied pressures. To quantitatively characterize the effect of pressure on the intermetallic compounds, the volume fraction of intermetallic compounds (fi) is
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Fig. 9. (a) A bright field TEM graph of the second phase along the boundary; (b) the corresponding EDS result of region A arrowed in (a); (c) [0 0 1] zone axis, the SAED pattern obtained from the Mg–Zn–RE phase.
calculated by image analysis. As shown in Fig. 6a and b, with the pressure increasing from 0 MPa to 200 MPa, the fi decreases from 15.1% to 8.5%. The result is in accordance with the investigation of Jie et al. [30]. They reported that applying high pressure during solidification of metals can increase the solid solubility of some elements. As shown in Fig. 8b, it indicates that a little migration of α-Mg diffraction peak appears under different pressures in the XRD patterns. This can be attributed to the increase of solid solubility in the matrix with the increase of pressure, which may lead to the change of lattice constant. Without applied pressure, strong agglomerations of intermetallic compounds are easy to form along grain boundaries (see Fig. 6a). In the situation of applying pressure, the grain boundaries are refined and the strong agglomerations of intermetallic compounds disappear. This is attributed to the refinement of α-Mg grains and the increased solid solubility.
Besides, the high cooling rate induced by pressure contributes to block the formation of agglomerations. Porosity is easy to form in the conventional castings of Mg–3RE– 6Zn–1.4Y alloy due to the broad crystallization temperature interval as shown in Fig. 1. Masoumi et al. [31] found that the increase of applied pressure from 3 MPa to 90 MPa led to the increase in density and the decrease in porosity of conventional squeeze casting AX51 alloys. Therefore, with the pressure varied from 0 MPa to 50 MPa, the significant improvement in the YS, UTS and elongation (Fig. 10) could be mainly attributed to the materials densification. In a situation where the porosity is almost completely eliminated, the grain size becomes the dominant factor in influencing the properties. The yield strength and grain size are inversely related, as demonstrated by the Hall–Petch equation: −1=2
σ ¼ σ 0 þ kd
ð5Þ
where d is the grain size, σ0 and k are the experimentally derived constants. As Mg has a hexagonal close-packed (hcp) crystal structure, the constant k can reach a high value (280–320 MPa μm− 1/2) [32]. Hence, the grain refinement leads to an increase in the strength. In addition to grain refinement, the increased solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds also contribute to the improvement of the tensile properties. Therefore, the applied pressure during the RSC process results in the materials densification, the refinement of grains, the increase of solid solubility and the elimination of intermetallic compounds agglomeration, which contribute to the improvement of mechanical properties. 5. Conclusions
Fig. 10. Tensile properties of the as-cast RSC samples under different pressures.
1) Good semisolid slurry of Mg–3RE–6Zn–1.4Y alloy with fine and spherical primary α-Mg grains can be obtained with the application
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of direct ultrasonic vibration, and the average grain size and average shape factor are 22 μm and 0.70 respectively. 2) There are two different sized grains (ESGs and ISGs) in the microstructure of RSC samples. The increase of pressure not only contributes to the refinement and spheroidization of α-Mg grains but also results in the increased solid solubility in the matrix and uniform distribution of RE-rich intermetallic compounds along the grain boundary. The phase constitutions of this novel alloy are mainly αMg, α-Zr, W phase (Mg3Zn2Y3), icosahedral quasicrystal I phase (Mg3Zn6Y) and T phase ((La,Ce)(Mg1 − xZnx)11). 3) With the increase of pressure from 0 MPa to 200 MPa, the tensile properties of the RSC samples increase continuously. The YS, UTS and elongation of RSC sample under the pressure of 200 MPa are 110 MPa, 180 MPa and 8.6%, which are improved by 17.0%, 19.2% and 81.7% respectively than the corresponding values of the samples without applied pressure. Acknowledgements This work was funded by Project 51275183 supported by National Natural Science Foundation of China. The authors would also like to express their appreciation to the Analytical and Testing Centre, HUST. References [1] D. Eliezer, E. Aghion, F.H. Froes, Magnesium science, technology and applications, Adv. Perform. Mater. 5 (1998) 201–212. [2] F.A. Mirza, D.L. Chen, D.J. Li, X.Q. Zeng, Low cycle fatigue of an extruded Mg–3Nd– 0.2Zn–0.5Zr magnesium alloy, Mater. Des. 64 (2014) 63–73. [3] M. Easton, A. Beer, M. Barnett, C. Davies, G. Dunlop, Y. Durandet, S. Blacket, T. Hilditch, P. Beggs, Magnesium alloy applications in automotive structures, JOM 60 (2008) 57–62. [4] S. Tekumalla, S. Seetharaman, A. Almajid, M. Gupta, Mechanical properties of magnesium–rare earth alloy systems: a review, Metals 5 (2015) 1–39. [5] J. Wang, P. Song, S. Huang, F. Pan, High-strength and good-ductility Mg–RE–Zn–Mn magnesium alloy with long-period stacking ordered phase, Mater. Lett. 93 (2013) 415–418. [6] Z. Zhang, X. Liu, Z. Wang, Q. Le, W. Hu, L. Bao, J. Cui, Effects of phase composition and content on the microstructures and mechanical properties of high strength Mg–Y– Zn–Zr alloys, Mater. Des. 88 (2015) 915–923. [7] Z. Luo, S. Zhang, Y. Tang, D. Zhao, Quasicrystals in as-cast Mg–Zn–RE alloys, Scr. Metall. Mater. 28 (1993) 1513–1518. [8] D. Xu, W. Tang, L. Liu, Y. Xu, E. Han, Effect of W-phase on the mechanical properties of as-cast Mg–Zn–Y–Zr alloys, J. Alloys Compd. 461 (2008) 248–252. [9] J. Gröbner, A. Kozlov, R. Schmid-Fetzer, M.A. Easton, S. Zhu, M.A. Gibson, J.F. Nie, Thermodynamic analysis of as-cast and heat-treated microstructures of Mg–Ce– Nd alloys, Acta Mater. 59 (2011) 613–622. [10] F.A. Mirza, D.L. Chen, D.J. Li, X.Q. Zeng, Effect of rare earth elements on deformation behavior of an extruded Mg–10Gd–3Y–0.5Zr alloy during compression, Mater. Des. 46 (2013) 411–418. [11] D.H. Li, J. Dong, X.Q. Zeng, Characterization of precipitate phases in a Mg–Dy–Gd–Nd alloy, J. Alloys Compd. 439 (2007) 254–257.
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