Effects of Ho content on microstructures and mechanical properties of Mg-Ho-Zn alloys

Effects of Ho content on microstructures and mechanical properties of Mg-Ho-Zn alloys

Materials Characterization 149 (2019) 198–205 Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.co...

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Materials Characterization 149 (2019) 198–205

Contents lists available at ScienceDirect

Materials Characterization journal homepage: www.elsevier.com/locate/matchar

Effects of Ho content on microstructures and mechanical properties of MgHo-Zn alloys

T

Jiaan Liua, , Mengli Yanga, Xiaoru Zhanga, Daqing Fangb, Chaojie Chea, Aijing Zoua ⁎

a b

Key Laboratory of Automobile Materials (Ministry of Education), College of Materials Science and Engineering, Jilin University, Changchun 130022, PR China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, PR China

ARTICLE INFO

ABSTRACT

Keywords: Mg-Ho-Zn alloy Ho element Long period stacking ordered structure Microstructure Mechanical properties

In this work, the microstructures and mechanical properties of as-cast Mg-xHo-2Zn (x = 4, 6 and 8 wt%) alloys have been studied by using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and tensile test at different temperatures. The Ho element exhibits an influence on the grain refinement of the alloys. Increasing Ho content promotes the formation of long period stacking ordered (LPSO) structure, which changes the mechanical properties of the alloys. As a consequence, the strength of the alloy rises with the increase of Ho addition at room temperature and elevated temperature. However, the elongation of the alloy increases at room temperature but slightly decreases at high temperature when the Ho addition increases. These results are explained from morphology evolution of W-phase and the presence of LPSO phase in different alloys.

1. Introduction Magnesium alloys have attracted considerable attentions due to their excellent properties, such as low density, high specific stiffness and good damping [1–3]. However, the application of magnesium alloys is still restricted due to their relative low mechanical properties at room temperature and high temperature [4]. One effective method to improve the strength and ductility of the magnesium alloys is forming a long period stacking ordered (LPSO) structure via adding RE element and Zn element into magnesium matrix [5–8]. In addition, the LPSO phase could provide good damping capacity and thermal conductivity for the magnesium alloys [9–11]. Moreover, formation of plate-shaped LPSO phase is desirable for enhancing the creep properties of the magnesium alloys [12]. Some investigations have revealed that the LPSO phase is coherent with magnesium matrix and remains a good thermal stability at elevated temperatures [13–17]. Bi et al. reported that the high mechanical properties of as-cast Mg-Dy(Zn, Cu, Ni) alloys are mainly attributed to high volume fraction of LPSO phases and small secondary dendrite arm spacing [14]. Srinivasan et al. confirmed that Mg-Gd-Zn alloys exhibit high yield strength due to the high solute contents and the presence of LPSO phase [15]. Therefore, the LPSO phase is thought to be a suitable strengthening phase to comprehensive properties of the magnesium alloys. Recent investigations demonstrate that the Mg-Ho-Zn alloys exhibit



desirable mechanical properties and good corrosion resistance and they are potentially applied in degradable implant materials. Kawamura et al. confirmed that the 18R-LPSO phase can directly form in Mg97Zn1Ho2 alloy during solidification [5]. Zhang and Jiao et al. revealed that Mg-Ho-Zn alloys possess an excellent combination of mechanical properties and corrosion resistance in a simulated body fluid as a result of the formation of profuse nano-spaced basal plane stacking faults (SFs) [18,19]. According to different Ho/Zn ratios, three kinds of ternary equilibrium phases have been identified in Mg-Ho-Zn alloys; i.e., I-phase (Mg3HoZn6), W-phase (Mg3Ho2Zn3) and LPSO phase (Mg12HoZn) [20]. Li et al. studied the microstructure and mechanical properties of Mg-ZnHo-Zr alloy with low Ho/Zn ratio. The microstructure of the alloy mainly comprises α-Mg, W-phase and I-phase. The grain size of the alloy is obviously reduced with the increase of Ho content. And the Ho element can heighten the tensile strength of the Mg-Zn-Ho-Zr alloy [21]. Lv et al. found that the HZK810 alloy owns high mechanical properties compared with the traditional Mg alloys [22]. Form the investigations mentioned above, it is confirmed that the mechanical properties of the Mg-Zn-Ho alloys containing ternary equilibrium phases are dependent on the element, phase composition and microstructure. However, only a few studies focus on the effect of Ho content on microstructure evolution and the mechanical properties of the MgHo-Zn alloys containing LPSO phase.

Corresponding author. E-mail address: [email protected] (J. Liu).

https://doi.org/10.1016/j.matchar.2019.01.023 Received 2 September 2018; Received in revised form 16 January 2019; Accepted 23 January 2019 Available online 24 January 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.

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Fig. 1. OM images of microstructures of the as-cast Mg-xHo-2Zn alloys: (a) Mg-4Ho-2Zn; (b) Mg-6Ho-2Zn; (c) Mg-8Ho-2Zn.

were machined for tensile test at room temperature and elevated temperature. Before the test at 300 °C, the samples were hold for 20 min to keep a uniform temperature. Tensile tests were carried out using the MTS material testing machine (MTS-810 System Corporation). 3. Results and Discussions 3.1. Microstructures of Mg-xHo-2Zn Alloys Fig. 1 exhibits the optical images of the microstructures of as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt%) alloys. It can be seen that the microstructures of the alloys are characterized by α-Mg dendrites and eutectic phases along with grain boundaries. Fig. 1a exhibits that the Mg4Ho-2Zn alloy mainly comprises α-Mg matrix and semi-continuous eutectic phases. Fig. 1b displays that the microstructure of Mg-6Ho-2Zn alloy is similar to that of Mg-4Ho-2Zn alloy, but the eutectic phases is appeared to be continuous. Fig. 1c shows that a fine lamellar-shaped phase is formed at the grain boundary. This exhibits that the microstructure of Mg-8Ho-2Zn is different from those of other alloys. The average grain size of the Mg-4Ho-2Zn, Mg-6Ho-2Zn and Mg8Ho-2Zn alloy is 67.6 μm (Fig. 1a), 48.2 μm (Fig. 1b) and 46.9 μm (Fig. 1c), respectively. Therefore, the result demonstrates that the grain size of as-cast Mg-Ho-Zn series alloys is refined when the Ho content is increasing. As some researches reported, the mechanism of refinement on magnesium alloy can be attributed to two factors: one is lattice mismatch and the other is crystal growth restriction factor [23,24]. In this study, the Ho addition presents an obvious refinement effect on grain size of the alloys. The mechanism of grain refinement is thought that the segregated Ho elements at front of growing liquid/solid interface can restrict the grains growth [24]. This mechanism is supported by the present result because the second phases containing Ho element are along with the grain boundaries. Fig. 2 shows the XRD patterns of as-cast Mg-xHo-2Zn(x = 4, 6, 8 wt %) alloys. It can be seen that Mg-4Ho-2Zn alloy and Mg-6Ho-2Zn alloy mainly consist of α-Mg and W-phase. In contrast, Mg-8Ho-2Zn alloy exhibits additional strong peaks of LPSO phase besides the peaks of αMg and W-phase. Therefore, in this study, it is concluded that the increasing Ho content gives rise to the formation of LPSO phase. Fig. 3 shows the SEM images of as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt %) alloys. It can be seen from Fig. 3a and c that the bright eutectic structures are distributed along the grain boundaries. Fig. 3b and d show the magnified images in the region surrounded by a white frame in Fig. 3a and c, respectively. Most of the eutectic structures exhibited a feather shape in the triple junction region of the α-Mg grain. Combining with the XRD results, these bright network structures can be identified as W-phase. Except for the W-phase, neither I-phase nor LPSO phase was observed in the Mg-4Ho-2Zn alloy and the Mg-6Ho-2Zn alloy. On contrary, the Mg-8Ho-2Zn alloy displays the obviously different microstructure. It can be seen from Fig. 3e that the grey lamellar phase

Fig. 2. XRD patterns of the as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt%) alloys.

Therefore, in this study, the microstructures and mechanical properties of the Mg-xHo-2Zn (x = 4, 6 and 8 wt%) alloys were systematically investigated to get insight to the effects of Ho content on morphology of W-phase, formation of LPSO phase and tensile properties of the alloys at room temperature and elevated temperature. 2. Experimental Procedures Experimental Mg-xHo-2Zn (x = 4, 6 and 8 wt%) alloys were prepared from commercial pure Mg (99.95 wt%), pure Zn (99.95 wt%) and Mg-25Ho (wt%) master alloy. The pure magnesium (99.95 wt%) was melted in an electric resistant furnace under the protection of 1.5 vol% SF6 ± CO2 mixed gas. And then the pure Zn (99.95 wt%) and the Mg25Ho master alloy were added at 720 °C. Afterwards, the melts were stirred at about 700 °C. Finally, the ingots were obtained after the solidification of the melts by semi-continuous casting. The samples were polished and then were etched. The grain sizes of the alloys were characterized by optical microscope (OM, Scope Axio ZEISS). The microstructures of the alloys were observed by the scanning electron microscope (SEM, EVO-18 ZEISS and VEGA3 TESCAN) and the transmission electron microscope (TEM, JEM-2100F). The element compositions of the microstructure were analyzed by X-ray energy dispersive spectrometer (EDS, INCA Oxford). The TEM observation was employed using a thin foil selected from the corresponding alloys and the foils were pretreated using argon ion thinning technique. The phase compositions of the alloys were analyzed by X-ray diffraction (XRD, X'Pert3 Powder) using Cu Kα radiation with a scanning angle from 20° to 80°. The samples with gage dimensions of 15 mm × 3.5 mm × 2 mm

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Fig. 3. SEM images of the as-cast Mg-xHo-2Zn alloys: (a), (b) Mg-4Ho-2Zn; (c), (d) Mg-6Ho-2Zn; (e), (f) Mg-8Ho-2Zn.

and bright W-phase with distinguished interface are located at the grain boundaries in the Mg-8Ho-2Zn alloy. The grey lamellar phase could be identified as LPSO phase because the XRD results indicated the presence of the LPSO phase besides W-phase. It is evident from the magnified image (shown in Fig. 3f) that the W-phases in the Mg-8Ho-2Zn alloy are separated by LPSO phase, and therefore W-phases are refined due to the presence of the LPSO phase, whereas the W-phases in the Mg-4Ho-2Zn

alloy and Mg-6Ho-2Zn alloy are almost in the form of coarse structure. Fig. 4a and b display the TEM images and the corresponding selected area diffraction (SAED) patterns in Mg-4Ho-2Zn alloy and Mg6Ho-2Zn alloy, respectively. It can be seen from the corresponding SAED patterns that the dark phase was further identified to be W-phase with a face-centered cubic (fcc) crystal structure, and the W-phases in these alloys exhibit similar coarse morphology. Fig. 4c shows the TEM

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Fig. 4. TEM and corresponding SAED images of the main second phases in the ascast Mg-xHo-2Zn alloys: (a) network Wphase and the corresponding SAED image in Mg-4Ho-2Zn alloy; (b) network W-phase and the corresponding SAED image in Mg6Ho-2Zn alloy; (c) W-phase(region A) and LPSO phase(region B) and the corresponding SAED image in Mg-8Ho-2Zn alloy.

Fig. 5. Schematic diagram of solidification process of the as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt.%) alloys. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

image and the corresponding selected area diffraction (SAED) patterns in Mg-8Ho-2Zn alloy. W-phase and LPSO phase can be identified in the Mg-8Ho-2Zn alloy. The TEM image displays that LPSO phase has a fine lamellar morphology. The SAED pattern also exhibits there are five additional reflection-spots at the positions of n/6(0002)α spots (n is an interval), indicating that this LPSO phase can be characterized by 18R structure. Therefore, it can be inferred that the orientation relationships between 18R-LPSO and α-Mg are [010]18R//[11−20]α-Mg and

(001)18R//(0001)α-Mg, which has been confirmed in the previous studies [25–29]. In addition, The TEM image also confirms that the Wphase in the Mg-8Ho-2Zn alloy is refined due to the presence of 18RLPSO phase. This microstructure evolution is thought to derive from the different solidification processes of the alloys with different RE contents [24,30]. The schematic diagram of solidification process is shown in the Fig. 5. The L → α-Mg occurs firstly, leading to the enrichment of Ho 201

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content. Generally, the mechanical properties of the alloys are varied with the compositions, grain size and morphologies of phases. The strength of Mg-6Ho-2Zn is slightly higher than that of Mg-4Ho-2Zn. This result may be attributed to the comprehensive effects from two competitive aspects: on one hand, the strengthening by grain size reduction is beneficial to both strength and ductility. The solubility of Ho element in Mg is considerable. When the Ho content is increased, the solid solution of Ho into the Mg matrix is relatively increased, resulting in the further solid solution strengthening. On the other hand, the W-phase which has been confirmed a face-centered cubic (fcc) crystal structure forms a poor bonding interface with Mg matrix which has a hexagonal closed packed (hcp) crystal structure [26,31–33]. The W-phases in these alloys are coarse, and therefore they play a negative role on ductility. As for Mg-8Ho-2Zn alloy, Wphase and LPSO phase are paralleled to each other along the grain boundaries. It is reported that the LPSO phase is characterized by high hardness and high modulus, and it can enhance the strength by inhibiting dislocation movement without the expense of ductility [10,34]. Also, the refined W-phase in Mg-8Ho-2Zn alloy contributes to the strength of the alloys [24,31]. Fig. 7 shows the mechanical properties of as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt%) alloys at 300 °C. The ultimate strengths of the as-cast Mg-4Ho-2Zn, Mg-6Ho-2Zn and Mg-8Ho-2Zn alloys are 82.3 MPa, 111.6 MPa and 124.6 MPa, respectively. The yield strength of the three alloys are 44.2 MPa, 51.7 MPa and 70.8 MPa respectively when the Ho content is increased from 4 wt% to 8 wt%. Besides, the elongations of three corresponding alloys are 13.1%, 13.0%, and 11.9%, respectively. Among three alloys, the Mg-8Ho-2Zn alloy shows the highest strength but the lowest elongation at 300 °C. The LPSO phase is thermal stable, and it can maintain the alloy a relative high strength at high temperature [35,36]. Besides, non-basal plane dislocations are actuated when the testing temperature is increasing. Therefore, dislocation jog is also favor for the strength by blocking dislocation motion [37]. Under the high temperature condition, the grain boundary sliding (GBS) of the Mg alloy makes a substantial contribution to the plastic deformation. The LPSO phase also can enhance the strength by inhibiting the GBS [38].

Fig. 6. The mechanical properties of the as-cast Mg-xHo-2Zn alloys (x = 4, 6, 8 wt%) at room temperature.

Fig. 7. The mechanical properties of the as-cast Mg-xHo-2Zn alloys (x = 4, 6, 8 wt%) at 300 °C.

3.3. Fracture Morphologies Fig. 8 exhibits the fracture morphologies of the as-cast Mg-xHo2Zn (x = 4, 6, 8 wt%) alloys at room temperature. It can be seen from Fig. 8a and d that both dimples and cleavage steps can be observed in the fracture surface of Mg-4Ho-2Zn alloy, revealing that the mixed fracture is the primary mechanism [38]. The fracture surface of Mg6Ho-2Zn alloy exhibits some deep dimples and tear ridges, as shown in Fig. 8b and e. As for Mg-8Ho-2Zn alloy (shown in Fig. 8c and f), the fracture surface consists of a large number of shallow dimples and a small range of cleavage steps, revealing that ductile fracture is the main mechanism. These fracture morphologies are line in the experimental results on the mechanical properties of the alloys at room temperature. Fig. 9 shows the fracture morphologies of the as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt%) alloys at 300 °C. As shown in Fig. 9a and d, Mg-4Ho2Zn displays some small dimples, indicating a ductility fracture happen. It can be seen from the Fig. 9b and e that some regions remain the small and deep dimples but the other regions exhibit shallow and large dimples in the fractures surface of Mg-6Ho-2Zn. Fig. 9c and f show the fracture morphologies of Mg-8Ho-2Zn. It is clear that there are a lot of tear ridges and wide cleavage steps. It is indicated that there is a

and Zn atoms in the remaining liquid. And then, there are different solidification processes depending on the Ho content of the alloys. As for the Mg-4Ho-2Zn and Mg-6Ho-2Zn alloys, W phase could be separated out from the remaining liquid. With respect to the Mg-8Ho-2Zn alloy, the W phase and the LPSO phase could form due to the high Ho addition [24]. 3.2. Mechanical Properties at Room Temperature and Elevated Temperature Fig. 6 shows mechanical properties of as-cast Mg-xHo-2Zn (x = 4, 6, 8 wt%) alloys at room temperature. The ultimate tensile strengths of the as-cast Mg-4Ho-2Zn, Mg-6Ho-2Zn, Mg-8Ho-2Zn alloys are 149.8 MPa, 155.1 MPa, and 168.9 MPa, respectively. The yield strength of the three alloys are 61.9 MPa, 71.4 MPa and 82.7 MPa respectively when the Ho content is increased from 4 wt% to 8 wt%. In addition, the elongations of the three alloys are 10.4%, 11.8%, and 12.5%, respectively. Therefore, it can be concluded that the strength and elongation of the alloy are promoted with the increase of Ho

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Fig. 8. SEM morphologies of tensile fractures of the alloys at room temperature: (a), (d) Mg-4Ho-2Zn; (b), (e) Mg-6Ho-2Zn; (c), (f) Mg-8Ho-2Zn.

debonding between the second phases and the α-Mg grains, and this result is similar to the previous study [39].

the second phase along the grain boundary. Increasing content of Ho element gives rise to the formation of 18R-LPSO phase, and therefore the W-phase is also refined during the solidification process. (3) The mechanical properties of the alloys can be influenced by various Ho additions. With the increase of Ho content, the strength and elongation of the alloy are increased at room temperature. When temperature is 300 °C, with the Ho addition varying from 4 wt% to 8 wt%, the strength of the alloy is increased but the elongation is slightly reduced.

4. Conclusions This study focuses on the effects of Ho content on the microstructure evolution and mechanical properties of the Mg-xHo-2Zn (x = 4, 6 and 8 wt%) alloys. The conclusions were drawn as follows: (1) The grain size and phase composition of the as-cast alloy vary with the Ho content. The grain size of the alloy is reduced with the increasing Ho content. When the Ho content is 4 or 6 wt%, the alloy is composed of α-Mg and W-phase. While when the Ho content is 8 wt %, the alloy comprises α-Mg, W-phase and LPSO-phase. (2) Different Ho additions of the alloys can change the morphology of

This study demonstrates that the addition of the Ho element is an effective method to tailor the microstructure and mechanical properties of the Mg alloy at room temperature and elevated temperature.

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Fig. 9. SEM morphologies of tensile fractures of the alloys at 300 °C: (a), (d) Mg-4Ho-2Zn; (b), (e) Mg-6Ho-2Zn; (c), (f) Mg-8Ho-2Zn.

Acknowledgements

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