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Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering
Journal Pre-proof Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering Yuanhang Luo, Yujuan Wu, Qingchen Deng, Yu Zhang, Juan Chen, Liming Peng PII:
S0925-8388(19)34651-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153405
Reference:
JALCOM 153405
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2019 Revised Date:
10 December 2019
Accepted Date: 13 December 2019
Please cite this article as: Y. Luo, Y. Wu, Q. Deng, Y. Zhang, J. Chen, L. Peng, Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153405. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement Yuanhang Luo1, Yujuan Wu1,2,3*, Qingchen Deng1,Yu Zhang4,Juan Chen1,2,3, Liming Peng1,2,3 1
National Engineering Research Center of Light Alloy Net Forming and Key State
Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China; 2
Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd., Shanghai, 201615, China; 3 4
Shanghai Innovation Institute for Materials, Shanghai, 200444, China;
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
* Corresponding authors: Yujuan Wu: [email protected]
As the authors of this paper, we ensure that the work is entirely original, any raw data exiting in the article can be provided. Besides, the work never published in other journals. All authors' individual contributions: 1. Yuanhang Luo: Prepared samples for all experiment, analyzed experimental results and write the manuscript. 2. Yujuan Wu: Designed experiment plan, organized discussion and revised the article. 3. Qingchen Deng：Completed BSE-SEM micrographs of the powders experiment. 4. Yu Zhang: Completed the TEM experiment. 5. Juan Chen: Completed the XRD experiment. 6. Liming Peng: Completed the DSC experiment.
Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering Yuanhang Luo1, Yujuan Wu1,2,3∗, Qingchen Deng1,Yu Zhang4,Juan Chen1,2,3,Liming Peng1,2,3 1
National Engineering Research Center of Light Alloy Net Forming and Key State Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China;
2
Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd., Shanghai, 201615, China; 3
4
Shanghai Innovation Institute for Materials, Shanghai, 200444, China;
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
Abstract: Two Mg-Gd-Zn-Zr alloys (GZ100K and GZ122K) with Gd/Zn atomic ratios of 24 and 3 were prepared by spark plasma sintering (SPS) and the effect of sintering temperature on microstructures and mechanical properties of these two alloys was systematically investigated. With increasing sintering temperature, the volume fraction of β-(Mg,Zn)3Gd phase decreases accompanied with the grain size increasing. Besides, the normal grain growth turns into abnormal grain growth in the as-sintered GZ100K when the sintering temperature increases to 440°C. A 14H long-period stacking ordered (LPSO) phase forms in the as-sintered GZ122K at 440-480°C, which can delay grain coarsening. The GZ122K alloy sintered at 400°C achieves ultimate compressive stress (UCS) of 410 MPa and yield stress (CYS) of 222 MPa. While, the GZ100K alloy sintered at 400°C reaches the highest UCS of 386 MPa and CYS of 210 MPa. Compared with the as-cast counterparts, the increment in strength of as-sintered alloys is mainly due to grain boundary strengthening arising from the fine grains produced by SPS. Keywords： Mg-Gd-Zn-Zr alloy, Spark plasma sintering, Long-period stacking ordered phase, Mechanical properties
∗
Corresponding author at : National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong
University, Shanghai 200240, China Tel: +86 21 54742627; fax: +86 21 34202794; E-mail address: [email protected] (Yujuan Wu)
1. Introduction In recent years, increasing applications of magnesium (Mg) alloys have been witnessed in aerospace and automobile industries. Such enlarged use of Mg alloys is mainly due to their low density and excellent specific strength. Gadolinium (Gd) has a large solubility in Mg alloys and causes strong age hardening effect during low-temperature ageing [1, 2]. The age-hardening response of Mg-Gd alloys can be further enhanced by a small amount of Zn addition [3, 4]. This is mainly attributed to the fact that Zn can introduce the formation of long-period stacking ordered (LPSO) phase/structure. In Mg-Gd-Zn-Zr alloys, the Gd/Zn atomic ratio, solidification condition, heat treatment parameters and deformation process are key factors of controlling the formation, distribution and transformation of LPSO phase/structure [5-10]. An as-cast Mg96.5Gd2.5Zn1 alloy consists of α-Mg matrix, intragranular 14H-LPSO structure and intergranular β-(Mg,Zn)3Gd eutectic secondary phase, while, 14H-LPSO phase in grain boundaries forms in the Mg96.5Gd2.5Zn1 after solution treatment [11]. The strengths of Mg alloys exhibit strong dependence on grain size owing to the limited slip systems and large Taylor factors [12]. Rapid solidification process is an effective way to obtain fine grains [13]. Kawamura et al. [14] prepared a Mg97Y2Zn1 alloy via rapid solidification, showing tensile yield stress above 600 MPa. Such super-high strength mainly arises from the nanocrystalline and dispersive LPSO structure. Powder metallurgy is a feasible method to reduce macro-segregation and form fine grains. Moreover, spark plasma sintering (SPS) is a solid-state sintering technique, which combines the discharge heating, plasma activation and compaction into sintering [15-17]. Short sintering time accompanied by high heating rate can retard grain growth during densification. At present, most studies of SPS focus on the fabrication process of Mg-Al alloys [18-20]. In addition, some attention has been paid to the control of precipitation in Mg alloy to improve the mechanical properties during the SPS process [20-23]. In this regard, the microstructure evolution in sintering process of Mg-Gd-Zn-Zr alloys (GZK) is complex, which can be divided to two types according to formation of different phase: (1) with the formation of 14H-LPSO phase or (2) without 14H-LPSO phase during the SPS process. In this paper, two Mg-Gd-Zn-Zr alloys (GZ100K and GZ122K) with two different Gd/Zn ratios were prepared by SPS. The impacts of the sintering temperature on the
microstructures and the mechanical properties were analyzed. At last, two sintering strategies of GZK alloys were extracted from two different experimental phenomena within two alloys. 2. Experimental procedures Two kinds of gas atomized Mg-Gd-Zn-Zr powders were used as the starting materials for the SPS. The Mg-Gd-Zn-Zr powders are spherical with a mean diameter of 42 µm. The chemical compositions of the powders were measured by an inductively coupled plasma atomic emission spectroscopy analyzer (ICP-AES, Perkin-Elmer, Plasma 400) and tabulated in Table 1. The as-atomized powders were consolidated into cylindrical samples with a diameter of 28 mm and a height of 30 mm using the SPS system (HPD 25/4-SD). Five sintering temperatures (520, 480, 440, 400, 360 °C) were designed ranging from the single-phase solid solution region to the two-phase region. To acquire a uniform temperature field, slower heating rates ranging from 20 to 50 °C/min were used in the heating-up stage. A holding time of 10 min and a pressure of 50 MPa was applied in the soaking stage. As a reference, parallel samples of GZ100K and GZ122K alloys prepared by conventional gravity casting and solution treatment were also characterized and tested. X-ray diffraction with a Cu target (XRD, Rigaku D/max 2550 V) was performed with a scanning speed of 1 °/min and a scanning range from 20 ° to 60 °. Differential scanning calorimetry (DSC, a Netzsch STA449 F3 machine) tests were performed via increasing the testing temperature from room temperature to 600 °C at a heat rate of 5 °C/min. Densities of sintered GZ100K and GZ122K alloys were measured by the Sartorius densimeter. After mechanical grounding and polishing, the specimens were etched using an acetic-picral solution for microstructure observations using a scanning electron microscope (SEM, FEI Nova Nano SEM 230) equipped with a backscatter electron (BSE) detector and an energy dispersive X-ray spectrometer (EDS)). The volume fractions of secondary phases were quantitatively measured by using Image pro software based on the SEM micrographs. The grain sizes of the as-sintered samples were measured via the linear intercept method according to ASTM E112-12. Transmission electron microscopy (TEM) experiments were carried out on the FEI Tecnai G2 at 200 kV. Specimens for compression tests were electrical discharge machined to 10 mm in height, 5 mm in diameter. Compressive tests
were performed on the MTS material test machine with a constant loading speed of 0.5 mm/min (a strain rate of ~0.001 s−1). Compressive properties such as the true compressive yield strength (CYS), true ultimate compressive strength (UCS) and true strain at UCS were averaged from at least three parallel tests. 3. Results 3.1 Microstructure of the as-atomized powder Cross-section microstructures of GZ100K and GZ122K powders are shown in Fig. 1. It can be seen that both GZ100K and GZ122K powders consist of α-Mg matrix and eutectic β-phase [(Mg,Zn)3Gd]. Such character is further confirmed via XRD patterns in Fig. 2a. Besides, a weak diffraction peak at ∼43°corresponding to MgO can be noticed. The volume fraction of β-phase in GZ122K powder was ∼ 6.0%, while that of the β-phase in GZ100K was near zero. The average grain sizes of GZ100K and GZ122K powders were 2.8 µm and 2.4 µm, respectively. Lamellar LPSO structures were not detected in both powders, indicating that the rapid solidification rate in the process of gas atomization can retard the formation of LPSO structure. Fig. 2b shows the DSC curves of GZ100K and GZ122K powders. An endothermic peak at 510 °C was found in GZ122K, indicating the dissolution of β-phase. While, no peaks was observed in GZ100K, further verifying the absence (or extremely low volume) of β-phase in GZ100K. 3.2 Microstructure of the as-sintered GZK alloys prepared by SPS The densities of as-sintered GZ100K and GZ122K alloys under different temperatures are given in Fig. 3. The measured densities of GZ100K-360°C and GZ122K-360°C are below 99.0% of the theoretical densities, while the other samples are thought to achieve almost full relative density except GZ122K-520°C. As for GZ122K-520°C, some liquid was squeezed out from the mould under the applied pressure during the sintering processing and the actual composition of as-sintered GZ122K-520°C is Mg-1.61Gd-0.56Zn-0.13Zr (at.%), which deviates from the nominal composition. According to the DSC curve, Gd-rich β-phase may turn into liquid at 520°C. A small portion of β-phase with higher density than α-Mg was lost, which is believed to lead to the low density. The microstructures of GZ100K powder and GZ100K alloy sintered at different temperatures (360,440,520°C) are shown in Fig. 4. As the sintering temperature is elevated, the amount of β-phase at grain boundaries is reduced. The β-phase almost dissolves into the
α-Mg matrix when the sintering temperature is above 440°C. The grain size increases with the increasing sintering temperature. When the sintering temperature is below 440°C, the grain growth rate is slow. When the sintering temperature is above 440°C, a bimodal grain size distribution with coarse grains (Dave. ≈ 25µm) and fine grains (Dave. ≈ 5µm) forms. Such bimodal structure is believed to be associated with the abnormal grain growth, which is different from the behavior in the sintering temperature below 440°C. The microstructures of GZ122K powder and GZ122K alloy sintered at different temperatures (360,440,520°C) are shown in Fig. 5. The volume fraction of β-phase decreases as the sintering temperature increases. The particles at grain boundaries are β-phase rather than lamellar 14H-LPSO structure at 360°C (Fig. 5b). The β-phase with a reticular shape turns into a blocky shape, and a dispersed lamellar phase appears at grain boundaries when the sintering temperature increases to 440°C (Fig. 5c). Such microstructure of GZ122K is distinguishably different from those of GZ100K at the same sintering temperature. The formation of lamellar 14H-LPSO phase occurs in the range of 440°C to 480°C and the 14H-LPSO phase vanishes at sintering temperature of 520°C. Moreover, the grain size and the density of LPSO structure in the grain interiors increase with the increasing sintering temperature from 360°C to 520°C. At sintering temperature of 520°C, the average grain size of GZ122K is 9.1 µm, which is close to that of GZ100K-520°C. In contrast, the continuous oxide layer of GZ122K-520°C along the grain boundaries was broken (Fig. 5d), while the layer keeps consecutive in the GZ100K-520°C (Fig. 4d). Fig. 6 shows TEM bright-field (BF) images and select area electron diffractions (SAEDs) of as-sintered GZ122K-440°C. GZ122K-440°C consists of β-phase, lamellar phase both at grain boundaries and within the matrix. The β-phase has an FCC structure with the lattice constant of a = 0.74 nm, as verified by the select area electron diffraction (SAED) inset in Fig. 6a. Both of the lamellar phases at grain boundaries (Fig. 6b) and within the matrix(Fig. 6c) are 14H-LPSO structure with the lattice constant of a=0.34 nm and c=3.58 nm, same to those in the solution-treated Mg96.5Gd2.5Zn1 alloy prepared by conventionally ingot metallurgy(I/M) reported by Wu [11]. Therefore, it is verified that the as-sintered GZ122K-440°C alloy contains β-phase and 14H-LPSO phase at grain boundaries and lamellae with 14H-LPSO structure within the matrix.
3.3 Room-temperature mechanical properties of as-sintered GZK alloys Fig. 7 gives the evolution of Vickers hardness of as-sintered GZ100K and GZ122K alloys under different sintering temperatures. Generally, the hardness values of GZ100K and GZ122K alloys increase at first, reaching their maximum values of 83 HV and 95 HV, respectively, then their hardness values gradually decrease. In the range of 360-400°C, the relative density becomes higher owing to the elevated sintering temperature, which results in the hardness increasing. In the range of 400-520°C, microstructure evolution is the dominant factor on the mechanical properties instead of the effect of relative density. Figs. 8-9 show the compressive properties of as-sintered GZ100K and GZ122K alloys. Compared with the as-cast GZK-T4 alloys, the as-sintered GZK alloys by SPS process exhibit higher compressive strength but poorer ductility. The evolution of CYS and UCS versus temperature shown in Figs. 8a and 9a is similar to that of hardness shown in Fig. 7. The strain at UCS shown in Figs. 8b and 9b gradually increases with sintering temperature. To be specific, the CYS of both as-sintered GZ100K and GZ122K alloys always decreases with increasing temperature. The UCS of GZ100K and GZ122K exhibits a growing trend in the range of 360-400°C, relating to the relative higher density, and decreases in the range of 400-520°C due to the decreased hardness. At 400°C, the as-sintered GZ100K and GZ122K alloys achieve UCS of 386 MPa and CYS of 210 MPa, UCS of 410 MPa and CYS of 222 MPa, respectively. 4. Discussion 4.1 The impact of sintering temperature on microstructure Statistical analyses were done to describe the volume fraction of β-phase and average grain size to understand the impact of sintering temperature on microstructures of as-sintered GZ100K and GZ122K alloys. Before sintering, little β-phase exists in the initial GZ100K powder because of the rapid solidification rate of gas atomization. After sintering, the volume fraction of β-phase gets the maximum of 5.36% at 360°C, then constantly decreases until 440°C, and finally keeps stable above 440°C (Fig. 10a). This indicates that β-phase precipitates below 440 °C and then dissolves into the α-Mg matrix above 440 °C. Thus, it can be deduced that re-precipitation of β-phase is included in the cooling stage of SPS process if the sintering temperature is over 440°C. Compared with the initial grain size of as-atomized powder, the growth of grain size can be divided into three stages as
illustrated in Fig. 10b. In the first stage (360-400°C), dispersed secondary eutectic β-phase [(Mg,Zn)3Gd] continuously distributes at grain boundaries. These β-phase can greatly hinder the grain boundary migration, so the grain shows a low growth rate. In the second stage (400-480°C), partial intergranular β-phase dissolves into the α-Mg matrix, the normal grain growth transfers into the abnormal grain growth. Some grains grow rapidly while others keep fine size, which leads to the bimodal grain size distribution. The bimodal structure often occurs in the recrystallization of extruded alloys[24, 25]. The appearance of abnormal grain growth depends on temperature, pinning force, stability of particles and mobility of grain boundary [26]. This phenomenon was also reported in the SPS of Mg-10Al alloy [27]. Gas atomized powder is metastable. Besides, fine grains have strong driving force to grow. When some surface contact areas between powder particles are firstly formed, the current density will be highly localized at these areas during SPS. As a result, these local areas are heated up quicker due to Joule effect, leading to heterogeneous temperature distribution and intimately to the local grain growth [28, 29]. In the third stage (480-520°C), the complete dissolution of β-phases at grain boundaries results in great decrease of the pinning force, facilitating the abnormal grain growth. When the sintering temperature comes to 520°C, most grains are coarse and surrounded by an oxide layer (Fig. 4d). Grain growth is restricted by the oxide layer; hence the average grain size keeps relatively stable. Regarding as-sintered GZ122K alloy, the volume fraction of β-phase declines as the sintering temperature increases (Fig. 11a-b). Compared with the initial volume fraction of β-phase in the as-atomized powder, the β-phase has a higher volume fraction when it was sintered at 360°C. Thus, precipitation of the β-phase happened during sintering at 360°C. Dissolution of β-phase happened during sintering at 400-520°C. Between 440°C and 480°C, the 14H-LPSO phase forms (Fig. 5c). The formation temperature window of the 14H-LPSO phase is close to that of the as-cast Mg96.5Gd2.5Zn1 alloy prepared by conventional I/M process[11]. The volume fraction of 14H-LPSO phase stays at ∼2.2%. Compared with the initial grain size of as-atomized powder, the grain growth behavior can be also divided into three stages. In the first stage (360-400°C), the β-phase distributes along grain boundaries, which impedes the grain growth. In the second stage (440-480°C), the 14H-LPSO phase forms which can hinder the grain growth. Grain growth is an interface migration by thermal
activation and the formation of LPSO structure is a thermal activated diffusional-displacive course [30]. The activation energy for grain growth of Mg-3Gd-1Zn is 101 kJ/mol [25] and the activation energy of LPSO phase formation is 125 kJ/mol [31]. Thus, it is speculated that the formation of LPSO phase is comparable to the grain growth which can delay grain growth in return [32]. It’s also reported that 14H-LPSO phases in the dynamic recrystallization have the delaying effect on the grain boundaries migration [33]. When the temperature is 520°C, the grain grows rapidly during sintering. According to the DSC result (Fig. 2b), β-phase turns to liquid at 520°C. It is speculated that some liquid β-phase was squeezed out from the mould under the pressure during the process, leading to the rapid grain growth without the restriction of β-phase. 4.2 The impact of sintering temperature on mechanical properties As mentioned in section 3.3, the CYS values of both as-sintered GZ100K and GZ122K alloys always decrease with increasing temperature. The UCS values of GZ100K and GZ122K exhibit a growing trend in the range of 360-400°C, relating to the higher relative density, and decrease in the range of 400-520°C due to the microstructure evolution. As the sintering temperature increases, grain growth and dissolution of intergranular β-phase occur in both GZ100K and GZ122K alloys. In addition, the 14H-LPSO phase forms in GZ122K alloy in the range of 440-480°C. In these conditions of GZ122K, the grain growth slows down and the formation of 14H-LPSO phase prevents the decline of CYS and UCS. Therefore, the curve of CYS and UCS in this temperature region keeps stable (Fig. 9a). Subsequently, CYS and UCS of GZ122K-520°C are lower than that of solution-treated GZ122K-T4 prepared by conventional I/M, because the β-phase turns to liquid when the eutectic temperature is reached and liquid β-phase was squeezed out from the mould. With the loss of Gd and Zn, the amount of β-phase and the solubility of Gd and Zn decreases, meanwhile the grain size increases. In such circumstance, the microstructure with coarse grains and less β-phase causes lower strength. Comparing the maximum of compressive strength of as-sintered GZK with solution-treated GZ100K and GZ122K alloys prepared by conventional I/M, the increment of CYS is 50-60 MPa, which can be deduced from Hall-Petch equation taking the difference of grain size into equation. The increment of UCS is about 90MPa for GZ100K and 30MPa for GZ122K, respectively. The improvement of strength is taken as the contribution of fine
grains. The much finer grains are believed to derive from the rapid solidification during gas atomization (the preparation of powders) and the SPS process, which effectively refine grain and retard grain growth. The strain at UCS of the as-sintered GZK alloy is inferior to solution-treated GZK alloy prepared by conventional I/M, except the GZ122K-520°C. This is related to the oxide layer hindering the diffusion in the sintering process ,which becomes the main reason for limited elongation during the tensile test [34]. SPS is a solid-state sintering technique, so the oxide layer remains at the powder outer surface during SPS. As illustrated in Fig. 12, the continuous oxide layer has a complicated structure with the width of about 200 nm. As for GZ122K-520°C, the sintering temperature reaches the eutectic point so that liquid β-phase appears and breaks through the oxide layer on the compression of SPS. So the strain (24.3%) at UCS of GZ122K-520°C is near that of GZ122K-T4 alloy (24.2%). This paper provides two sintering strategies of Mg-Gd-Zn-Zr alloys for SPS process, depending on whether formation of 14H-LPSO phase is induced or not. The introduction of 14H-LPSO phase depends on adjustment of Gd/Zn atomic ratio. For one thing, the GZ100K alloy with higher Gd/Zn atomic ratio of 24 does not form the 14H-LPSO phase in the process of SPS process. A higher solubility of Gd and Zn atoms can be achieved by the adjustment of sintering temperature under the premise of fine grains. It is possible to generate more precipitates by aging treatment which makes greater contribution to high strength. For another, the GZ122K alloy with the Gd/Zn atomic ratio of 3 can produce the 14H-LPSO at the appropriate sintering temperature. The phase formation can impede the grain growth and bring a strengthening effect [35]. 5. Conclusions The microstructures and mechanical properties of GZ100K and GZ122K alloys prepared by SPS process under different sintering temperatures were investigated. The impacts of sintering temperature on microstructures and mechanical properties were analyzed. The conclusions are as follows: 1. For the GZ100K and GZ122K alloys, the grain grows and β-phase [(Mg,Zn)3Gd] dissolves into the matrix with increasing sintering temperature from 400 to 520°C. Therefore, an appropriate sintering temperature ranges from 400 to 480°C. 2. The as-sintered GZ122K alloy involves the formation of 14H-LPSO phase in the
sintering temperature of 440-480°C. The formation of 14H-LPSO phase can delay the grain growth. 3. Both GZ122K and GZ100K alloys achieved their maximum strength sintered at 400°C. Specifically, the as-sintered GZ122K reaches UCS of 410 MPa and CYS of 222 MPa; GZ100K alloys obtains UCS of 386 MPa and CYS of 210 MPa.
Acknowledgement This work is funded by the National Key Research and Development Program of China (No. 2016YFB0701201), the National Natural Science Foundation (Nos. 51771113, 51971130, 51605288) and the United Fund of National Department of Education and Equipment Development (No.6141A02033213).
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Table captions Table 1 Compositions of GZK powders by ICP (at.%)
Figure captions Fig.1 Cross-section BSE-SEM micrographs of the powders: (a) GZ100K; (b) GZ122K Fig.2 XRD patterns (a) and DSC curves (b) of the GZ100K and GZ122K powders Fig.3 The densities of GZ100K and GZ122K alloys under different sintering temperatures Fig.4 BSE-SEM micrographs of the GZ100K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C Fig.5 BSE-SEM micrographs of the GZ122K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C Fig.6 TEM bright-field images and corresponding selected area electron diffraction(SAED) of as-sintered GZ122K-440°C: (a) β-phase (B//[0001]α); (b) 14H-LPSO phase in grain boundaries (B//[2-1-10]α);(c) 14H-LPSO structure within matrix (B//[2-1-10]α) Fig.7 Vickers hardness of as-sintered GZK alloys with sintering temperatures Fig.8 Compressive properties of as-sintered GZ100K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS Fig.9 Compressive properties of as-sintered GZ122K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS Fig.10 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ100K under different sintering temperatures Fig.11 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ122K under different sintering temperatures Fig.12 Bright-field TEM micrograph, corresponding SAED patterns (a) and EDS (b) of the oxide layer
Table 1 Compositions of GZK powders by ICP (at.%) Alloy
Gd
Zn
Zr
Mg
Gd/Zn atomic ratio
GZ100K
1.64
0.07
0.10
Bal.
23.4
GZ122K
2.09
0.82
0.14
Bal.
2.5
Fig. 1 Cross-section BSE-SEM micrographs of the powders: (a) GZ100K; (b) GZ122K
Fig. 2 XRD patterns (a) and DSC curves (b) of the GZ100K and GZ122K powders
Fig. 3 The densities of GZ100K and GZ122K alloys under different sintering temperatures
Fig. 4 BSE-SEM micrographs of the GZ100K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C
Fig. 5 BSE-SEM micrographs of the GZ122K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C
Fig. 6 TEM bright-field images and corresponding selected area electron diffraction(SAED) of as-sintered GZ122K-440°C: (a) β-phase (B//[0001]α); (b) 14H-LPSO phase in grain boundaries (B//[2-1-10]α);(c) 14H-LPSO structure within matrix (B//[2-1-10]α)
Fig. 7 Vickers hardness of as-sintered GZK alloys with sintering temperatures
Fig. 8 Compressive properties of as-sintered GZ100K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS
Fig. 9 Compressive properties of as-sintered GZ122K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS
Fig. 10 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ100K under different sintering temperatures
Fig. 11 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ122K under different sintering temperatures
Fig. 12 Bright-field TEM micrograph, corresponding SAED patterns (a) and EDS (b) of the oxide layer
Highlights 1. Two Mg-Gd-Zn-Zr alloys with different Gd/Zn ratios were first fabricated by SPS. 2. The formation of 14H-LPSO phase in GZ122K can delay the grain growth. 3. GZ122K and GZ100K alloys achieve maximum compressive strength sintered at 400°C.
Declaration of Interest Statement
On behalf of all the authors, I declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Sincerely yours,
Yujuan Wu
National Engineering Research Center of Light Alloy Net Forming Shanghai Jiao Tong University 800 Dongchuan Road Shanghai 200240, PR China. [email protected]
S0925-8388(19)34651-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153405
Reference:
JALCOM 153405
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2019 Revised Date:
10 December 2019
Accepted Date: 13 December 2019
Please cite this article as: Y. Luo, Y. Wu, Q. Deng, Y. Zhang, J. Chen, L. Peng, Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153405. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contribution Statement Yuanhang Luo1, Yujuan Wu1,2,3*, Qingchen Deng1,Yu Zhang4,Juan Chen1,2,3, Liming Peng1,2,3 1
National Engineering Research Center of Light Alloy Net Forming and Key State
Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China; 2
Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd., Shanghai, 201615, China; 3 4
Shanghai Innovation Institute for Materials, Shanghai, 200444, China;
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
* Corresponding authors: Yujuan Wu: [email protected]
As the authors of this paper, we ensure that the work is entirely original, any raw data exiting in the article can be provided. Besides, the work never published in other journals. All authors' individual contributions: 1. Yuanhang Luo: Prepared samples for all experiment, analyzed experimental results and write the manuscript. 2. Yujuan Wu: Designed experiment plan, organized discussion and revised the article. 3. Qingchen Deng：Completed BSE-SEM micrographs of the powders experiment. 4. Yu Zhang: Completed the TEM experiment. 5. Juan Chen: Completed the XRD experiment. 6. Liming Peng: Completed the DSC experiment.
Microstructures and mechanical properties of Mg-Gd-Zn-Zr alloys prepared by spark plasma sintering Yuanhang Luo1, Yujuan Wu1,2,3∗, Qingchen Deng1,Yu Zhang4,Juan Chen1,2,3,Liming Peng1,2,3 1
National Engineering Research Center of Light Alloy Net Forming and Key State Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China;
2
Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd., Shanghai, 201615, China; 3
4
Shanghai Innovation Institute for Materials, Shanghai, 200444, China;
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
Abstract: Two Mg-Gd-Zn-Zr alloys (GZ100K and GZ122K) with Gd/Zn atomic ratios of 24 and 3 were prepared by spark plasma sintering (SPS) and the effect of sintering temperature on microstructures and mechanical properties of these two alloys was systematically investigated. With increasing sintering temperature, the volume fraction of β-(Mg,Zn)3Gd phase decreases accompanied with the grain size increasing. Besides, the normal grain growth turns into abnormal grain growth in the as-sintered GZ100K when the sintering temperature increases to 440°C. A 14H long-period stacking ordered (LPSO) phase forms in the as-sintered GZ122K at 440-480°C, which can delay grain coarsening. The GZ122K alloy sintered at 400°C achieves ultimate compressive stress (UCS) of 410 MPa and yield stress (CYS) of 222 MPa. While, the GZ100K alloy sintered at 400°C reaches the highest UCS of 386 MPa and CYS of 210 MPa. Compared with the as-cast counterparts, the increment in strength of as-sintered alloys is mainly due to grain boundary strengthening arising from the fine grains produced by SPS. Keywords： Mg-Gd-Zn-Zr alloy, Spark plasma sintering, Long-period stacking ordered phase, Mechanical properties
∗
Corresponding author at : National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong
University, Shanghai 200240, China Tel: +86 21 54742627; fax: +86 21 34202794; E-mail address: [email protected] (Yujuan Wu)
1. Introduction In recent years, increasing applications of magnesium (Mg) alloys have been witnessed in aerospace and automobile industries. Such enlarged use of Mg alloys is mainly due to their low density and excellent specific strength. Gadolinium (Gd) has a large solubility in Mg alloys and causes strong age hardening effect during low-temperature ageing [1, 2]. The age-hardening response of Mg-Gd alloys can be further enhanced by a small amount of Zn addition [3, 4]. This is mainly attributed to the fact that Zn can introduce the formation of long-period stacking ordered (LPSO) phase/structure. In Mg-Gd-Zn-Zr alloys, the Gd/Zn atomic ratio, solidification condition, heat treatment parameters and deformation process are key factors of controlling the formation, distribution and transformation of LPSO phase/structure [5-10]. An as-cast Mg96.5Gd2.5Zn1 alloy consists of α-Mg matrix, intragranular 14H-LPSO structure and intergranular β-(Mg,Zn)3Gd eutectic secondary phase, while, 14H-LPSO phase in grain boundaries forms in the Mg96.5Gd2.5Zn1 after solution treatment [11]. The strengths of Mg alloys exhibit strong dependence on grain size owing to the limited slip systems and large Taylor factors [12]. Rapid solidification process is an effective way to obtain fine grains [13]. Kawamura et al. [14] prepared a Mg97Y2Zn1 alloy via rapid solidification, showing tensile yield stress above 600 MPa. Such super-high strength mainly arises from the nanocrystalline and dispersive LPSO structure. Powder metallurgy is a feasible method to reduce macro-segregation and form fine grains. Moreover, spark plasma sintering (SPS) is a solid-state sintering technique, which combines the discharge heating, plasma activation and compaction into sintering [15-17]. Short sintering time accompanied by high heating rate can retard grain growth during densification. At present, most studies of SPS focus on the fabrication process of Mg-Al alloys [18-20]. In addition, some attention has been paid to the control of precipitation in Mg alloy to improve the mechanical properties during the SPS process [20-23]. In this regard, the microstructure evolution in sintering process of Mg-Gd-Zn-Zr alloys (GZK) is complex, which can be divided to two types according to formation of different phase: (1) with the formation of 14H-LPSO phase or (2) without 14H-LPSO phase during the SPS process. In this paper, two Mg-Gd-Zn-Zr alloys (GZ100K and GZ122K) with two different Gd/Zn ratios were prepared by SPS. The impacts of the sintering temperature on the
microstructures and the mechanical properties were analyzed. At last, two sintering strategies of GZK alloys were extracted from two different experimental phenomena within two alloys. 2. Experimental procedures Two kinds of gas atomized Mg-Gd-Zn-Zr powders were used as the starting materials for the SPS. The Mg-Gd-Zn-Zr powders are spherical with a mean diameter of 42 µm. The chemical compositions of the powders were measured by an inductively coupled plasma atomic emission spectroscopy analyzer (ICP-AES, Perkin-Elmer, Plasma 400) and tabulated in Table 1. The as-atomized powders were consolidated into cylindrical samples with a diameter of 28 mm and a height of 30 mm using the SPS system (HPD 25/4-SD). Five sintering temperatures (520, 480, 440, 400, 360 °C) were designed ranging from the single-phase solid solution region to the two-phase region. To acquire a uniform temperature field, slower heating rates ranging from 20 to 50 °C/min were used in the heating-up stage. A holding time of 10 min and a pressure of 50 MPa was applied in the soaking stage. As a reference, parallel samples of GZ100K and GZ122K alloys prepared by conventional gravity casting and solution treatment were also characterized and tested. X-ray diffraction with a Cu target (XRD, Rigaku D/max 2550 V) was performed with a scanning speed of 1 °/min and a scanning range from 20 ° to 60 °. Differential scanning calorimetry (DSC, a Netzsch STA449 F3 machine) tests were performed via increasing the testing temperature from room temperature to 600 °C at a heat rate of 5 °C/min. Densities of sintered GZ100K and GZ122K alloys were measured by the Sartorius densimeter. After mechanical grounding and polishing, the specimens were etched using an acetic-picral solution for microstructure observations using a scanning electron microscope (SEM, FEI Nova Nano SEM 230) equipped with a backscatter electron (BSE) detector and an energy dispersive X-ray spectrometer (EDS)). The volume fractions of secondary phases were quantitatively measured by using Image pro software based on the SEM micrographs. The grain sizes of the as-sintered samples were measured via the linear intercept method according to ASTM E112-12. Transmission electron microscopy (TEM) experiments were carried out on the FEI Tecnai G2 at 200 kV. Specimens for compression tests were electrical discharge machined to 10 mm in height, 5 mm in diameter. Compressive tests
were performed on the MTS material test machine with a constant loading speed of 0.5 mm/min (a strain rate of ~0.001 s−1). Compressive properties such as the true compressive yield strength (CYS), true ultimate compressive strength (UCS) and true strain at UCS were averaged from at least three parallel tests. 3. Results 3.1 Microstructure of the as-atomized powder Cross-section microstructures of GZ100K and GZ122K powders are shown in Fig. 1. It can be seen that both GZ100K and GZ122K powders consist of α-Mg matrix and eutectic β-phase [(Mg,Zn)3Gd]. Such character is further confirmed via XRD patterns in Fig. 2a. Besides, a weak diffraction peak at ∼43°corresponding to MgO can be noticed. The volume fraction of β-phase in GZ122K powder was ∼ 6.0%, while that of the β-phase in GZ100K was near zero. The average grain sizes of GZ100K and GZ122K powders were 2.8 µm and 2.4 µm, respectively. Lamellar LPSO structures were not detected in both powders, indicating that the rapid solidification rate in the process of gas atomization can retard the formation of LPSO structure. Fig. 2b shows the DSC curves of GZ100K and GZ122K powders. An endothermic peak at 510 °C was found in GZ122K, indicating the dissolution of β-phase. While, no peaks was observed in GZ100K, further verifying the absence (or extremely low volume) of β-phase in GZ100K. 3.2 Microstructure of the as-sintered GZK alloys prepared by SPS The densities of as-sintered GZ100K and GZ122K alloys under different temperatures are given in Fig. 3. The measured densities of GZ100K-360°C and GZ122K-360°C are below 99.0% of the theoretical densities, while the other samples are thought to achieve almost full relative density except GZ122K-520°C. As for GZ122K-520°C, some liquid was squeezed out from the mould under the applied pressure during the sintering processing and the actual composition of as-sintered GZ122K-520°C is Mg-1.61Gd-0.56Zn-0.13Zr (at.%), which deviates from the nominal composition. According to the DSC curve, Gd-rich β-phase may turn into liquid at 520°C. A small portion of β-phase with higher density than α-Mg was lost, which is believed to lead to the low density. The microstructures of GZ100K powder and GZ100K alloy sintered at different temperatures (360,440,520°C) are shown in Fig. 4. As the sintering temperature is elevated, the amount of β-phase at grain boundaries is reduced. The β-phase almost dissolves into the
α-Mg matrix when the sintering temperature is above 440°C. The grain size increases with the increasing sintering temperature. When the sintering temperature is below 440°C, the grain growth rate is slow. When the sintering temperature is above 440°C, a bimodal grain size distribution with coarse grains (Dave. ≈ 25µm) and fine grains (Dave. ≈ 5µm) forms. Such bimodal structure is believed to be associated with the abnormal grain growth, which is different from the behavior in the sintering temperature below 440°C. The microstructures of GZ122K powder and GZ122K alloy sintered at different temperatures (360,440,520°C) are shown in Fig. 5. The volume fraction of β-phase decreases as the sintering temperature increases. The particles at grain boundaries are β-phase rather than lamellar 14H-LPSO structure at 360°C (Fig. 5b). The β-phase with a reticular shape turns into a blocky shape, and a dispersed lamellar phase appears at grain boundaries when the sintering temperature increases to 440°C (Fig. 5c). Such microstructure of GZ122K is distinguishably different from those of GZ100K at the same sintering temperature. The formation of lamellar 14H-LPSO phase occurs in the range of 440°C to 480°C and the 14H-LPSO phase vanishes at sintering temperature of 520°C. Moreover, the grain size and the density of LPSO structure in the grain interiors increase with the increasing sintering temperature from 360°C to 520°C. At sintering temperature of 520°C, the average grain size of GZ122K is 9.1 µm, which is close to that of GZ100K-520°C. In contrast, the continuous oxide layer of GZ122K-520°C along the grain boundaries was broken (Fig. 5d), while the layer keeps consecutive in the GZ100K-520°C (Fig. 4d). Fig. 6 shows TEM bright-field (BF) images and select area electron diffractions (SAEDs) of as-sintered GZ122K-440°C. GZ122K-440°C consists of β-phase, lamellar phase both at grain boundaries and within the matrix. The β-phase has an FCC structure with the lattice constant of a = 0.74 nm, as verified by the select area electron diffraction (SAED) inset in Fig. 6a. Both of the lamellar phases at grain boundaries (Fig. 6b) and within the matrix(Fig. 6c) are 14H-LPSO structure with the lattice constant of a=0.34 nm and c=3.58 nm, same to those in the solution-treated Mg96.5Gd2.5Zn1 alloy prepared by conventionally ingot metallurgy(I/M) reported by Wu [11]. Therefore, it is verified that the as-sintered GZ122K-440°C alloy contains β-phase and 14H-LPSO phase at grain boundaries and lamellae with 14H-LPSO structure within the matrix.
3.3 Room-temperature mechanical properties of as-sintered GZK alloys Fig. 7 gives the evolution of Vickers hardness of as-sintered GZ100K and GZ122K alloys under different sintering temperatures. Generally, the hardness values of GZ100K and GZ122K alloys increase at first, reaching their maximum values of 83 HV and 95 HV, respectively, then their hardness values gradually decrease. In the range of 360-400°C, the relative density becomes higher owing to the elevated sintering temperature, which results in the hardness increasing. In the range of 400-520°C, microstructure evolution is the dominant factor on the mechanical properties instead of the effect of relative density. Figs. 8-9 show the compressive properties of as-sintered GZ100K and GZ122K alloys. Compared with the as-cast GZK-T4 alloys, the as-sintered GZK alloys by SPS process exhibit higher compressive strength but poorer ductility. The evolution of CYS and UCS versus temperature shown in Figs. 8a and 9a is similar to that of hardness shown in Fig. 7. The strain at UCS shown in Figs. 8b and 9b gradually increases with sintering temperature. To be specific, the CYS of both as-sintered GZ100K and GZ122K alloys always decreases with increasing temperature. The UCS of GZ100K and GZ122K exhibits a growing trend in the range of 360-400°C, relating to the relative higher density, and decreases in the range of 400-520°C due to the decreased hardness. At 400°C, the as-sintered GZ100K and GZ122K alloys achieve UCS of 386 MPa and CYS of 210 MPa, UCS of 410 MPa and CYS of 222 MPa, respectively. 4. Discussion 4.1 The impact of sintering temperature on microstructure Statistical analyses were done to describe the volume fraction of β-phase and average grain size to understand the impact of sintering temperature on microstructures of as-sintered GZ100K and GZ122K alloys. Before sintering, little β-phase exists in the initial GZ100K powder because of the rapid solidification rate of gas atomization. After sintering, the volume fraction of β-phase gets the maximum of 5.36% at 360°C, then constantly decreases until 440°C, and finally keeps stable above 440°C (Fig. 10a). This indicates that β-phase precipitates below 440 °C and then dissolves into the α-Mg matrix above 440 °C. Thus, it can be deduced that re-precipitation of β-phase is included in the cooling stage of SPS process if the sintering temperature is over 440°C. Compared with the initial grain size of as-atomized powder, the growth of grain size can be divided into three stages as
illustrated in Fig. 10b. In the first stage (360-400°C), dispersed secondary eutectic β-phase [(Mg,Zn)3Gd] continuously distributes at grain boundaries. These β-phase can greatly hinder the grain boundary migration, so the grain shows a low growth rate. In the second stage (400-480°C), partial intergranular β-phase dissolves into the α-Mg matrix, the normal grain growth transfers into the abnormal grain growth. Some grains grow rapidly while others keep fine size, which leads to the bimodal grain size distribution. The bimodal structure often occurs in the recrystallization of extruded alloys[24, 25]. The appearance of abnormal grain growth depends on temperature, pinning force, stability of particles and mobility of grain boundary [26]. This phenomenon was also reported in the SPS of Mg-10Al alloy [27]. Gas atomized powder is metastable. Besides, fine grains have strong driving force to grow. When some surface contact areas between powder particles are firstly formed, the current density will be highly localized at these areas during SPS. As a result, these local areas are heated up quicker due to Joule effect, leading to heterogeneous temperature distribution and intimately to the local grain growth [28, 29]. In the third stage (480-520°C), the complete dissolution of β-phases at grain boundaries results in great decrease of the pinning force, facilitating the abnormal grain growth. When the sintering temperature comes to 520°C, most grains are coarse and surrounded by an oxide layer (Fig. 4d). Grain growth is restricted by the oxide layer; hence the average grain size keeps relatively stable. Regarding as-sintered GZ122K alloy, the volume fraction of β-phase declines as the sintering temperature increases (Fig. 11a-b). Compared with the initial volume fraction of β-phase in the as-atomized powder, the β-phase has a higher volume fraction when it was sintered at 360°C. Thus, precipitation of the β-phase happened during sintering at 360°C. Dissolution of β-phase happened during sintering at 400-520°C. Between 440°C and 480°C, the 14H-LPSO phase forms (Fig. 5c). The formation temperature window of the 14H-LPSO phase is close to that of the as-cast Mg96.5Gd2.5Zn1 alloy prepared by conventional I/M process[11]. The volume fraction of 14H-LPSO phase stays at ∼2.2%. Compared with the initial grain size of as-atomized powder, the grain growth behavior can be also divided into three stages. In the first stage (360-400°C), the β-phase distributes along grain boundaries, which impedes the grain growth. In the second stage (440-480°C), the 14H-LPSO phase forms which can hinder the grain growth. Grain growth is an interface migration by thermal
activation and the formation of LPSO structure is a thermal activated diffusional-displacive course [30]. The activation energy for grain growth of Mg-3Gd-1Zn is 101 kJ/mol [25] and the activation energy of LPSO phase formation is 125 kJ/mol [31]. Thus, it is speculated that the formation of LPSO phase is comparable to the grain growth which can delay grain growth in return [32]. It’s also reported that 14H-LPSO phases in the dynamic recrystallization have the delaying effect on the grain boundaries migration [33]. When the temperature is 520°C, the grain grows rapidly during sintering. According to the DSC result (Fig. 2b), β-phase turns to liquid at 520°C. It is speculated that some liquid β-phase was squeezed out from the mould under the pressure during the process, leading to the rapid grain growth without the restriction of β-phase. 4.2 The impact of sintering temperature on mechanical properties As mentioned in section 3.3, the CYS values of both as-sintered GZ100K and GZ122K alloys always decrease with increasing temperature. The UCS values of GZ100K and GZ122K exhibit a growing trend in the range of 360-400°C, relating to the higher relative density, and decrease in the range of 400-520°C due to the microstructure evolution. As the sintering temperature increases, grain growth and dissolution of intergranular β-phase occur in both GZ100K and GZ122K alloys. In addition, the 14H-LPSO phase forms in GZ122K alloy in the range of 440-480°C. In these conditions of GZ122K, the grain growth slows down and the formation of 14H-LPSO phase prevents the decline of CYS and UCS. Therefore, the curve of CYS and UCS in this temperature region keeps stable (Fig. 9a). Subsequently, CYS and UCS of GZ122K-520°C are lower than that of solution-treated GZ122K-T4 prepared by conventional I/M, because the β-phase turns to liquid when the eutectic temperature is reached and liquid β-phase was squeezed out from the mould. With the loss of Gd and Zn, the amount of β-phase and the solubility of Gd and Zn decreases, meanwhile the grain size increases. In such circumstance, the microstructure with coarse grains and less β-phase causes lower strength. Comparing the maximum of compressive strength of as-sintered GZK with solution-treated GZ100K and GZ122K alloys prepared by conventional I/M, the increment of CYS is 50-60 MPa, which can be deduced from Hall-Petch equation taking the difference of grain size into equation. The increment of UCS is about 90MPa for GZ100K and 30MPa for GZ122K, respectively. The improvement of strength is taken as the contribution of fine
grains. The much finer grains are believed to derive from the rapid solidification during gas atomization (the preparation of powders) and the SPS process, which effectively refine grain and retard grain growth. The strain at UCS of the as-sintered GZK alloy is inferior to solution-treated GZK alloy prepared by conventional I/M, except the GZ122K-520°C. This is related to the oxide layer hindering the diffusion in the sintering process ,which becomes the main reason for limited elongation during the tensile test [34]. SPS is a solid-state sintering technique, so the oxide layer remains at the powder outer surface during SPS. As illustrated in Fig. 12, the continuous oxide layer has a complicated structure with the width of about 200 nm. As for GZ122K-520°C, the sintering temperature reaches the eutectic point so that liquid β-phase appears and breaks through the oxide layer on the compression of SPS. So the strain (24.3%) at UCS of GZ122K-520°C is near that of GZ122K-T4 alloy (24.2%). This paper provides two sintering strategies of Mg-Gd-Zn-Zr alloys for SPS process, depending on whether formation of 14H-LPSO phase is induced or not. The introduction of 14H-LPSO phase depends on adjustment of Gd/Zn atomic ratio. For one thing, the GZ100K alloy with higher Gd/Zn atomic ratio of 24 does not form the 14H-LPSO phase in the process of SPS process. A higher solubility of Gd and Zn atoms can be achieved by the adjustment of sintering temperature under the premise of fine grains. It is possible to generate more precipitates by aging treatment which makes greater contribution to high strength. For another, the GZ122K alloy with the Gd/Zn atomic ratio of 3 can produce the 14H-LPSO at the appropriate sintering temperature. The phase formation can impede the grain growth and bring a strengthening effect [35]. 5. Conclusions The microstructures and mechanical properties of GZ100K and GZ122K alloys prepared by SPS process under different sintering temperatures were investigated. The impacts of sintering temperature on microstructures and mechanical properties were analyzed. The conclusions are as follows: 1. For the GZ100K and GZ122K alloys, the grain grows and β-phase [(Mg,Zn)3Gd] dissolves into the matrix with increasing sintering temperature from 400 to 520°C. Therefore, an appropriate sintering temperature ranges from 400 to 480°C. 2. The as-sintered GZ122K alloy involves the formation of 14H-LPSO phase in the
sintering temperature of 440-480°C. The formation of 14H-LPSO phase can delay the grain growth. 3. Both GZ122K and GZ100K alloys achieved their maximum strength sintered at 400°C. Specifically, the as-sintered GZ122K reaches UCS of 410 MPa and CYS of 222 MPa; GZ100K alloys obtains UCS of 386 MPa and CYS of 210 MPa.
Acknowledgement This work is funded by the National Key Research and Development Program of China (No. 2016YFB0701201), the National Natural Science Foundation (Nos. 51771113, 51971130, 51605288) and the United Fund of National Department of Education and Equipment Development (No.6141A02033213).
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Table captions Table 1 Compositions of GZK powders by ICP (at.%)
Figure captions Fig.1 Cross-section BSE-SEM micrographs of the powders: (a) GZ100K; (b) GZ122K Fig.2 XRD patterns (a) and DSC curves (b) of the GZ100K and GZ122K powders Fig.3 The densities of GZ100K and GZ122K alloys under different sintering temperatures Fig.4 BSE-SEM micrographs of the GZ100K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C Fig.5 BSE-SEM micrographs of the GZ122K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C Fig.6 TEM bright-field images and corresponding selected area electron diffraction(SAED) of as-sintered GZ122K-440°C: (a) β-phase (B//[0001]α); (b) 14H-LPSO phase in grain boundaries (B//[2-1-10]α);(c) 14H-LPSO structure within matrix (B//[2-1-10]α) Fig.7 Vickers hardness of as-sintered GZK alloys with sintering temperatures Fig.8 Compressive properties of as-sintered GZ100K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS Fig.9 Compressive properties of as-sintered GZ122K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS Fig.10 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ100K under different sintering temperatures Fig.11 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ122K under different sintering temperatures Fig.12 Bright-field TEM micrograph, corresponding SAED patterns (a) and EDS (b) of the oxide layer
Table 1 Compositions of GZK powders by ICP (at.%) Alloy
Gd
Zn
Zr
Mg
Gd/Zn atomic ratio
GZ100K
1.64
0.07
0.10
Bal.
23.4
GZ122K
2.09
0.82
0.14
Bal.
2.5
Fig. 1 Cross-section BSE-SEM micrographs of the powders: (a) GZ100K; (b) GZ122K
Fig. 2 XRD patterns (a) and DSC curves (b) of the GZ100K and GZ122K powders
Fig. 3 The densities of GZ100K and GZ122K alloys under different sintering temperatures
Fig. 4 BSE-SEM micrographs of the GZ100K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C
Fig. 5 BSE-SEM micrographs of the GZ122K alloys: (a) powder and as-sintered samples (b)360°C ;(c)440°C ;(d)520°C
Fig. 6 TEM bright-field images and corresponding selected area electron diffraction(SAED) of as-sintered GZ122K-440°C: (a) β-phase (B//[0001]α); (b) 14H-LPSO phase in grain boundaries (B//[2-1-10]α);(c) 14H-LPSO structure within matrix (B//[2-1-10]α)
Fig. 7 Vickers hardness of as-sintered GZK alloys with sintering temperatures
Fig. 8 Compressive properties of as-sintered GZ100K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS
Fig. 9 Compressive properties of as-sintered GZ122K alloys with sintering temperatures: (a) CYS and UCS;(b) true strain at UCS
Fig. 10 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ100K under different sintering temperatures
Fig. 11 Volume fractions of β-phase (a) and average grain sizes (b) of as-sintered GZ122K under different sintering temperatures
Fig. 12 Bright-field TEM micrograph, corresponding SAED patterns (a) and EDS (b) of the oxide layer
Highlights 1. Two Mg-Gd-Zn-Zr alloys with different Gd/Zn ratios were first fabricated by SPS. 2. The formation of 14H-LPSO phase in GZ122K can delay the grain growth. 3. GZ122K and GZ100K alloys achieve maximum compressive strength sintered at 400°C.
Declaration of Interest Statement
On behalf of all the authors, I declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Sincerely yours,
Yujuan Wu
National Engineering Research Center of Light Alloy Net Forming Shanghai Jiao Tong University 800 Dongchuan Road Shanghai 200240, PR China. [email protected]