Microstructure and mechanical properties of an Mg-4.0Sm-1.0Ca alloy during thermomechanical treatment

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016, P. 1134

Microstructure and mechanical properties of an Mg-4.0Sm-1.0Ca alloy during thermomechanical treatment LUO Xiaoping (罗小萍), FANG Daqing (房大庆)*, LI Qiushu (李秋书), CHAI Yuesheng (柴跃生) (College of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China) Received 29 March 2016; revised 23 July 2016

Abstract: The microstructure and mechanical properties of an as-cast Mg-4.0Sm-1.0Ca alloy were investigated during thermomechanical treatments consisting of hot extrusion, rolling, and aging at 473 K. Mg41Sm5 phases containing Ca and needle-like Mg2Ca phases formed in the Mg matrix, and the average grain size and elongation were 4.2 μm and 27%, respectively, after hot extrusion, which implied an increase in ductility. In addition, after the rolling, the grain size was further refined, and the tensile strength increased to 293 MPa. A new precipitate Mg3Sm was found in the peak-aged Mg-4.0Sm-1.0Ca alloy and this alloy displayed the best mechanical properties, with a peak hardness of 83 HV and ultimate tensile strength of 313 MPa; these properties were attributed to grain refinement strengthening, solid solution strengthening, work hardening, and precipitation strengthening. Keywords: Mg-Sm-Ca alloy; thermomechanical treatment; microstructure; mechanical properties; rare earths

Increasing demands to reduce the weight of automobiles and efforts to reduce the emission of greenhouse gases have accelerated the development of lightweight metallic materials. Many studies on Mg alloys for automotive applications have focused on weight reduction while maintaining their mechanical properties[1–4]. However, the mechanical properties of most of the commonly used Mg alloys cannot match the requirements of many applications because of the intrinsic characterization of the hcp structure. With the aim of improving their mechanical properties, intensive research efforts have been focused on Mg alloys. Grain refinement is one of the key processes in the development of both high strength and ductility. The currently available grain-refinement techniques for Mg alloys can be divided into two basic approaches. One involves thermomechanical treatment of solid Mg alloys, which leads to plastic deformation and dynamic recrystallization, hence, small equiaxed grains can be obtained[5–8]. The other approach involves inoculation of efficient grain refiners into metal melts, where the formation of fine and uniform grain structures can be enhanced. Such grain-refined microstructures provide casting soundness and improved mechanical properties; further use of two basic refinement processes, deformation and/or heat treatment, promotes the service performance of the alloys. It has been reported that the addition of rare-earth (RE) elements can significantly improve the mechanical properties of Mg alloys at elevated temperature via solid so-

lution strengthening and precipitate hardening[9,10]. Because of the high cost of RE elements, the strengthening mechanisms of RE elements are complex. Samarium (Sm), as one of the least expensive light RE elements with a large solubility in solid Mg that decreases with a reduction in temperature, can form a supersaturated solid solution based on Mg-Sm, whose decomposition during aging is accompanied by strengthening[11]. Rokhlin et al.[12] observed that the optimum addition of Sm in Mg-Sm alloys was 2 wt.%–4 wt.%, which results in good mechanical properties; the alloys can be used for industrial applications. The introduction of a minor amount of low-cost Ca as a grain refiner has also been extensively studied in Mg alloys; this ability is attributed to its large atomic radius. Some researchers[13–15] have reported that Ca dramatically refined Mg grains when the addition was below 1.0 wt.% by forming a stable Mg2Ca compound. Further additions had no effect on the grain size. A high-strength Mg-5.3Zn-0.2Ca-0.5Ce alloy was developed using the conventional working process[16–18]. In this work, the Mg-4.0Sm alloy was selected as the base alloy (close to the maximum solubility of Sm in solid Mg), and low-cost Ca (1.0 wt.%) was added with the aim of developing refined Mg alloys. The microstructure and mechanical properties of an Mg-4.0Sm1.0Ca alloy were systematically investigated during thermomechanical treatments consisting of hot extrusion, hot rolling, and subsequent artificial aging process, to compare which is very valuable for Mg alloy community.

Foundation item: Project supported by the Nature Science Foundation of Shanxi Province (201401115-3, 2015011038) * Corresponding authors: FANG Daqing (E-mail: [email protected]; Tel.: +86-351-2161126) DOI: 10.1016/S1002-0721(16)60145-X

LUO Xiaoping et al., Microstructure and mechanical properties of an Mg-4.0Sm-1.0Ca alloy during …

1 Experimental An Mg-4.0Sm-1.0Ca alloy ingot was prepared from high-purity Mg (>99.95%), Mg-25 wt.% Ca, and Mg-25 wt.% Sm master alloy by melting in an electric furnace at 1023 K under a mixed gas including CO2 and SF6. The thermomechanical treatment is schematically illustrated in Fig. 1. The as-cast Mg-4.0Sm-1.0Ca alloys were homogenized at 773 K for 20 h followed by quenching in water. Then, the homogenized samples were forward extruded at 733 K into rods of 16 mm in diameter with an extrusion ratio of 25:1; the extrusion velocity was set as 5 mm/s. The extruded samples were heated to 623 K, held at this temperature for 30 min, and then rolled in a four-roll mill with two passes at 623 K; each pass reduction was 15%. Finally, the hot-rolled specimens were aged at 473 K for 48 h. The microstructure was examined using optical microscopy (OM), scanning electron microscopy (SEM, S4800) equipped with energy-dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM). The grain sizes were determined using the linear intercept method; the average of the measured values was used as the grain size of the alloy. All the specimens were polished and etched with a solution of 10 mL of acetic acid, 5 g picric acid, and 10 mL of water. Thin foils for TEM were prepared after mechanical grinding to 0.1 mm, followed by twin-jet electrical polishing under cooling by liquid nitrogen. Standard tensile tests were performed at a strain rate of 1.0×10–3 s–1. The ultimate tensile strength, yield strength, and elongation to fracture were determined using three samples. The tensile fractured surfaces of the specimens were examined using SEM. The Vickers hardness (HV) was measured using a loading force of 100 g and a holding time of 20 s.

Fig. 1 Thermomechanical treatment Hom.=homogenizing; H.R.=hot rolling; Ext.=extrusion

2 Results and discussion 2.1 Microstructure and phase distribution

1135

Fig. 2(a) shows the as-cast microstructure of the Mg4.0Sm-1.0Ca alloy with an average grain size of 45 μm and two types of phases distributed along the grain boundaries and within the grains. The dispersed second particle distribution is not uniform. Fig. 2(b) shows the second intermetallic precipitate phases of the Mg-4.0Sm1.0Ca alloy, which consist of Mg41Sm5 and Mg2Ca based on the EDS results of points A and B presented in Table 1. This result occurs because the Ca addition has a low solubility in the Mg matrix and tends to aggregate in the diffusion layer before the solid/liquid interface. The phenomena can reduce the diffusion rate of Sm atoms and provide a more heterogeneous core, which can inhibit the continuous growth of the Mg41Sm5 phase while easily forming the Mg2Ca phase. It is known that the non-equilibrium phase is harmful to the subsequent deformation process and limits improvement of the mechanical performance of the alloy. Therefore, homogenization treatment is necessary to eliminate the non-equilibrium phase. Fig. 2(c) shows the microstructure of the solution-treated Mg-4.0Sm-1.0Ca alloy with an average grain size of 59 μm. Most of the secondary phase in the alloy was dissolved into the Mg matrix after solution treatment; the average grain size increased slightly and the second phase in the grain boundary was significantly reduced compared with those of the as-cast alloy. Fig. 3 shows the microstructure of the hot-extruded alloy. The grain size of the alloy was greatly refined during the extrusion process, and the alloy consisted of mostly equiaxial grains, which were remarkably uniform. The average grain size decreased to approximately 4.2 μm because of the microstructure heredity. This result occurred because the forward extrusion temperature mainly affects the nucleation and growth of dynamic recrystallization, implying that dynamic recrystallization has fully occurred during the extrusion process. Fig. 4 shows the microstructure of the hot-rolled Mg-4.0Sm-1.0Ca alloy. Compared with the extruded alloys, there are new dynamic recrystallization grains, leading to an average grain size of 3.7 μm after the hot-rolling process. Because the grain boundary diffusion speed is fast in the hot-rolling process, dynamic recovery

Fig. 2 Microstructure of as-cast and homogenized Mg-4.0Sm-1.0Ca alloy (a) As-cast, OM; (b) As-cast, SEM; (c) Homogenized OM

1136

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016

Table 1 EDS results of the experimental alloys (at.%)

degree. Fig. 5 presents images of the peak-aged alloy, which is composed of an Mg matrix and dispersed precipitates. Compared with the hot-rolled alloy, the average grain size after the peak-aging process increased slightly, and the grain size became more uniform because of static recrystallization, and the new secondary phases of Mg3Sm containing Ca precipitated along the grain boundaries besides major Mg41Sm5 phases. However, particle-like Mg3Sm phases were observed in the interior of grains. In particular, needle/rod-like Mg2Ca appears.

Positions

Element

Total

Mg

Sm

Ca

Fig. 2(b)-A

84.44

13.44

2.12

100

Fig. 2(b)-B

91.71

0.13

8.16

100

2.2 Tensile properties

Fig. 3 Microstructure of hot extruded Mg-4.0Sm-1.0Ca alloy

Tensile tests were performed to determine the effect of extrusion, rolling, and aging process on the mechanical properties of the alloy, and the results are presented in Table 2. Both the ultimate tensile strength (UTS) and yield strength (YS) clearly increased compared with those of the as-cast alloy. The as-cast Mg-4.0Sm-1.0Ca alloy exhibited a YS of 99 MPa, a UTS of 158 MPa, and

Fig. 4 Microstructure of hot rolled Mg-4.0Sm-1.0Ca alloy (a) OM; (b) SEM

does not easily occur; however, dynamic recrystallization does easily occur. The full discontinuous dynamic recrystallization becomes more dominant because of the lower stacking fault energy under the accumulated deformation degree including the hot-extrusion and rolling processes. In addition, no edge cracks and coarse dynamic recovery grains are observed because of the higher hot rolling temperature, and each rolling pass deformation degree (15%) is lower than the critical deformation

Fig. 5 SEM and TEM photographs of peak aging treatment (a) SEM; (b) TEM

LUO Xiaoping et al., Microstructure and mechanical properties of an Mg-4.0Sm-1.0Ca alloy during …

1137

Table 2 Mechanical property of Mg-4.0Sm-1.0Ca in different conditions State

Yield strength/

Tensile strength/

Elongation/

MPa

MPa

%

As-cast

99

158

10.3

Extrusion

177

250

27

Rolling

222

293

9.2

Peak aging

235

313

6.2

an elongation of 10.3%, and the further hot-extruded Mg-4.0Sm-1.0Ca alloy exhibited a YS, UTS, and elongation of 177 MPa, 250 MPa, and 27%, respectively. Increasing the refined microstructure led to a large improvement in the ductility of the alloy. The hot-rolled Mg-4.0Sm-1.0Ca alloy exhibited comparable YS and UTS (222 and 293 MPa, respectively) but inferior elongation (9.2%). The UTS increased further, which indicates that the deformation strength became much more apparent. The peak aging process further increased the UTS (313 MPa) and YS (235 MPa); however, a lower elongation (6.2%) was attained. The age-hardening response significantly increased the YS at the expenses of the elongation to fracture. The decrease in elongation might be due to the increased amount of second phase with prolonged aging time, which reaches a maximum value at 12 h and then declines because of the gradual coarsening of the precipitation phases. At the peak aging time, the hardness is 83 HV (as shown in Fig. 6), which displays better age-hardening that is attributed to grain-refinement strengthening and the high content of second phases. Therefore, it is credible that the mechanical properties of the Mg-4.0Sm-1.0Ca alloy in-

Fig. 6 Aging hardening curve of the extruded Mg-4.0Sm-1.0Ca alloy

creased sharply because of the accumulated deformation of hot extrusion and hot rolling and peak-aging treatment to prevent the edge cracking of the rolling process. 2.3 Fracture properties Fig. 7 presents SEM micrographs of the fracture surfaces of the Mg-4.0Sm-1.0Ca alloy under various conditions. In Figs. 7(a, b), many dimples and tearing ridges are observed on the fracture surface of the peak-aged alloy. Some second phase particles are observed within these dimples. These deep dimples, which are associated with the drawing of particles, indicate that a certain amount of plastic deformation occurred before rupture. Thus, the fracture mode of the peak-aged alloy was quasi-cleavage with a large amount of dimples. In Fig. 7(c), cleavage steps and a river pattern are observed in the fracture surfaces of the as-cast aged speci-

Fig. 7 Tensile fracture micrographs of the peak-aged (a), the high magnification images of (a) (b), as-cast (c), as-extruded (d) alloys

1138

JOURNAL OF RARE EARTHS, Vol. 34, No. 11, Nov. 2016

mens, indicating the occurrence of the typical cleavage fracture mechanism. Many dimples and tearing ridges are also observed. Some precipitate particles are observed in the dimples in the extruded alloy in Fig. 7(d). The deeper dimples are related to the tensile plastic deformation of particles before rupture. In addition, the Mg2Ca belongs to the brittle phase, which will decrease the plastic and increase the cracking tendency.

processing designs for magnesium alloys – grain refining by repeated plastic working and solid-state synthesis of Mg2Si (review). Adv. Technol. Mater. Mater. Process, 2004, 6(2): 328. [7] Kang S H, Lee Y S, Lee J H. Effect of grain refinement of magnesium alloy AZ31 by severe plastic deformation on material characteristics. J. Mater. Process. Technol., 2008, 201(1–3): 436. [8] Ramirez A, Ma Qian, Davis B, Wilks T, StJohn D H. Potency of high-intensity ultrasonic treatment for grain refinement of magnesium alloys. Scr. Mater., 2008, 59(1): 19. [9] Song Y L, Liu Y H, Yu S R, Zhu X Y, Wang S H. Effect of neodymium on microstructure and corrosion resistance of AZ91 magnesium alloy. J. Mater. Sci., 2007, 42(12): 4435. [10] Nie J F, Muddle B C. Characterisation of strengthening precipitate phases in a Mg-Y-Nd alloy. Acta Mater., 2000, 48(8): 1691. [11] Xia X, Sanaty-Zadeh A, Zhang C, Luo A A. Thermodynamic modeling and experimental investigation of the magnesium-zinc-samarium alloys. J. Alloys Compd., 2014, 593: 71. [12] Rokhlin L L, Dobatkina Tatiana V, Nikitina N I. Constitution and properties of the ternary magnesium alloys containing two rare-earth metals of different subgroups. Mater. Sci. Forum, 2003, 419-422: 291. [13] Wang Q D, Chen W Z, Zeng X Q, Lu Y Z, Ding W J, Zhu Y P, Xu X P, Mabuchi M. Effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy. J. Mater. Sci., 2001, 36: 3035. [14] Kondori B, Mahmudi R. Effect of Ca additions on the microstructure, thermal stability and mechanical properties of a cast AM60 magnesium alloy. Mater. Sci. Eng.: A, 2010, 527(7-8): 2014. [15] Chen Y, Jin L, Fang D, Song Y, Ye R Y. Effects of calcium, samarium addition on microstructure and mechanical properties of AZ61 magnesium alloy. J. Rare Earths, 2015, 33(1): 66. [16] Zhou N, Zhang Z Y, Jin L, Dong J, Ding W J. High ductility of a Mg-Y-Ca alloy via extrusion. Mater. Sci. Eng. A, 2013, 560(10): 103. [17] Du Y Z, Qiao X G, Zheng M Y, Wu K, Xu S W. The microstructure, texture and mechanical properties of extruded Mg-5.3Zn-0.2Ca-0.5Ce (wt.%) alloy. Mater. Sci. Eng. A, 2015, 620(3): 164. [18] Li X, Qi W, Zheng K, Zhou N. Enhanced strength and ductility of Mg-Gd-Y-Zr alloys by secondary extrusion. J. Mag. Alloys, 2013, 1(1): 54.

3 Conclusions Thermomechanical treatments were performed on an Mg-4.0Sm-1.0Ca alloy. After hot extrusion of the as-cast Mg-4.0Sm-1.0Ca alloy, dynamic recrystallization occurred, the tensile strength increased, and the ductility improved. After the hot-rolling process, grain refinement and improvement of the tensile strength were observed. And after the peak-aging process, fine, needle-like precipitates were dispersed in the interior of the grains, the hardness reached a maximum value of 83 HV, and the tensile strength increased to 313 MPa. The solid solution and precipitation strengthening of the alloying elements Ca and Sm were the main strengthening mechanism in the Mg-4.0Sm-1.0Ca alloy. Acknowledgements: Authors would like to thank the Doctoral Foundation of Taiyuan University of Science and Technology (20132019) for providing financial support.

References: [1] Liu Z L, Shen Y F, Li Z Q, Wang L. Review of the grain refinement technology of cast magnesium alloys. J. Mater. Sci. Eng., 2004, 1: 8268. [2] Luo A A. Recent magnesium alloy development for elevated temperature applications. Int. Mater. Rev., 2004, 49: 13. [3] Mohri T, Mabuchi M, Satio N, Nakamura M. Microstructure and mechanical properties of a Mg-4Y-3RE alloy processed by thermo-mechanical treatment. Mater. Sci. Eng. A, 1998, 257: 287. [4] Chen T J, Wang W, Zhang D H, Ma Y, Hao Y. Development of a new magnesium alloy ZW21. Mater. Des., 2013, 44: 555. [5] Wang Y B, Liao X Z, Zhu Y T. Grain refinement and growth induced by severe plastic deformation. Int. J. Mater. Res., 2009, 100(12): 1632. [6] Kondoh K, Tsuzuki R, Du W B, Kamado S. Materials and