The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System Do Hyung Kim1;*1 , Ju Youn Lee1;*1 , Hyun Kyu Lim1;*1 , Joon Seok Kyeong1;*1 , Won Tae Kim2 and Do Hyang Kim1;*2 1 Center for Noncrystalline Materials, Dept. of Metallurgical Eng., Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea 2 Applied Science Division, Cheongju University, 36 Naedok-dong, Sangdang-gu, Cheongju, Chongbuk 360-764, Korea The e?ect of microstructural evolution on the creep properties in Mg-Sn-Ca system has been investigated. As-cast microstructure of Mg- Sn-Ca alloy consists of two or three phases depending on the Ca/Sn ratio, i.e. Mg2Sn, CaMgSn and Mg2Ca phases. Ternary CaMgSn phase has two types of morphology by its pseudo hyper-eutectic reaction with -Mg; coarse rod-like primary or feather-like eutectic phase. Primary solidi?ed CaMgSn phase exhibit negative e?ect on the tensile properties in spite of its high thermal stability up to 500 C. According to the creep test results, apparent stress exponent value (n ? 7) indicates climb controlled creep mechanism by core di?usion above 150 C. Activation energy of Mg-5Sn-2Ca alloy (74 kJ/mol) is close to grain boundary di?usion for pure magnesium, 92 kJ/mol. Creep resistance is remarkably improved with the presence of Mg2Ca phase. [doi:10.2320/matertrans.MER2008140] (Received April 25, 2008; Accepted August 4, 2008; Published September 25, 2008) Keywords: magnesium-tin-calcium, creep, CaMgSn, Mg2Ca 1. Introduction Mg alloys have the great potential for high performance structural applications due to their inherent properties such as low density, high speci?c strength, superior damping capac- ity and good castability etc. Commercial high temperature Mg alloys are often classi?ed by their processing methods. There are Mg-Ag-RE, Mg-Zn-Zr and Mg-Y-RE series alloys which have been mainly fabricated by sand casting process. Though these alloys show high performance above 200 C,1,2) their high cost by using rare earth elements restricts wider applications. Die cast processing is well known for high productivity. To improve die castability, most alloys contain Al. Although Al signi?cantly increases castability, discon- tinuous precipitation of -phase (Mg17Al12) signi?cantly decreases high temperature strength.3) Therefore, usage of Mg alloys containing Al is limited below 200 C. Final goal for development of creep resistance Mg alloy is to design alloys with low cost, but maintaining its high performance. One of candidate for achieving this goal is Mg-Sn-Ca system. Basically, Sn and Ca have better cost e?ectiveness than rare earth elements. Moreover, according to the binary phase diagrams,4) their inter-metallic com- pounds formed with Mg exhibit high thermal stability. Especially, there are a lot of successful developments on the enhancement of creep resistance in Mg-Al series alloy with Ca addition.5–7) In Mg-Sn binary system, Mg2Sn phase has a high melting point ($770 C). In addition, this phase can be easily precipitated because it has a relatively high solubility limit (14.48 mass%) at 560 C and little solubility at ambient temperature. Lately, the alloys based on the binary Mg-Sn system have been received much attention for creep resistance alloy due to the high thermal stability of Mg2Sn phase. Kang et al. reported that Mg2Sn phase plays an important role in resisting matrix deformation during creep in Mg-8Sn-3Al-1Si alloy.8,9) Kainer et al. have reported that ternary CaMgSn phase not only shows highly thermal stability up to 500 C, but also the alloys containing CaMgSn phase show favorable creep resistance in compressive mode.10,11) More recently, A. Kozlov et al. have studied on the phase equilibria of Mg-Sn-Ca system using CALPHAD method.12,13) However, close examination on the role of respective 2nd phases on high temperature mechanical properties has not been performed in detail. Therefore, in this study, the e?ect of microstructural evolution on the high temperature properties in Mg-Sn-Ca system has been investigated. 2. Experimental Procedure All materials investigated in the present study are listed in Table 1. High purity metals with nominal compositions were prepared in an electrical resistance furnace under a SF6 + CO2 protective gas atmosphere. Molten metal was poured into a preheated ($100 C) rectangular steel mold with a dimension of 1.5 cm in thickness, 6 cm in width, and 10 cm in height. The phases in as-cast microstructure of the alloys listed in Table 1 were analyzed preliminary via X-ray di?raction (Rigaku, CN2301) using monochromatic CuK radiation to construct the phase selection map in as-cast state. For microstructural observations, the specimens were etched with a solution of 2% nital solution (2 ml nitric acid and 98 ml ethanol). The microstructures were observed by optical microscopy (OM; Leica DMRM) and scanning electron microscopy (SEM; JEOL 5310). A detailed analysis for phase identi?cation was performed by transmission electron microscopy (TEM; JEM 2000 EX). Specimens for TEM were prepared by an ion milling method (Gatan, model 600) after mechanical grinding. *1 Graduate Student, Yonsei University *2 Corresponding author, E-mail: dohkim@yonsei.ac.kr Materials Transactions, Vol. 49, No. 10 (2008) pp. 2405 to 2413 #2008 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Uniaxial tensile tests were carried out on dog-bone specimens with constant cross head speed at an initial strain rate of 1 ? 10?3 ?s?1 . Tensile creep tests were conducted by using direct loading type creep equipments. Cylindrical type creep specimens are precisely machined with a diameter of 6 mm and 25 mm gauge length. 3. Results 3.1 As-cast microstructure analysis Figure 1 describes as-cast state phase selection map for the alloys listed in Table 1. The alloy numbers corresponding to 14 alloys are also marked in the phase selection map. This map was constructed on the basis of phase identi?cation results via X-ray di?raction method. As-cast microstructures of Mg-Sn-Ca alloy system consist of two or three phases depending on Ca/Sn ratio. From the X-ray di?raction result, constitutive phases were indenti?ed as Mg2Sn, CaMgSn and Mg2Ca phases. The results of the phase identi?cation are also listed Table 1. Two-phase regions of -Mg + CaMgSn, -Mg + Mg2Sn, -Mg + Mg2Ca and three-phase regions of -Mg + CaMgSn + Mg2Sn and -Mg + CaMgSn + Mg2Ca are schematically outlined in Fig. 1 based on the result shown in Table 1. The Optical microscopic images shown in Fig. 1 stand for typical microstructures observed for the alloys listed in Table 1. As expected, T5 (Mg- 5 mass%Sn) alloy consisted of two phase microstructure, -Mg primary dendrite and interdendritic Mg2Sn phases. With small addition of Ca, CaMgSn phase newly formed together with Mg2Sn phase in TX51 (Mg-5 mass%Sn- 1 mass%Ca) alloy. Noticeably, the ternary CaMgSn phase was present in two types of morphology; coarse rod-like phase in the grain interior and ?ne feather-like phase near inter-dendritic region. To identify these two types of the Table 1 Chemical composition of all alloys prepared for this study. No. Composition (mass%) Constitutive Abbrev. Remarks Mg Sn Ca Phases 1 95 5 — -Mg, Mg2Sn T5 2 94 5 1 -Mg, CaMgSn, Mg2Sn TX51 Mechanical 3 93 5 2 -Mg, CaMgSn, Mg2Ca TX52 properties 4 92 5 3 -Mg, CaMgSn, Mg2Ca TX53 5 91 5 4 -Mg, CaMgSn, Mg2Ca TX54 6 97.58 1.20 1.22 -Mg, CaMgSn, Mg2Ca TX11 7 88.15 11.10 0.75 -Mg, CaMgSn, Mg2Sn TX111 8 87.88 6.78 5.34 -Mg, CaMgSn, Mg2Ca TX75 9 93.23 1.18 5.59 -Mg, Mg2Ca TX16 Phase 10 98.70 0.48 0.82 -Mg, CaMgSn, Mg2Ca TX01 construction 11 88.75 8.98 2.72 -Mg, CaMgSn, Mg2Sn TX93 map 12 85.33 10.97 3.70 -Mg, CaMgSn TX114 13 92.70 6.91 0.39 -Mg, CaMgSn, Mg2Sn TX70 14 93.38 1.21 0.41 -Mg, CaMgSn TX10 [9] TX16 [8] TX75 [13] TX70 [1] T5 [2] TX51 CaMgSn CaMgSn Fig. 1 Phase selection map for > 80 mass% Mg rich region obtained from as-cast state specimens. 2406 D. H. Kim et al. phase carefully, TEM analysis was performed. Figure 2(a) is the stereogram centered at [100] zone axis constructed by several di?raction patterns obtained from feather-like phase, clearly con?rming the primitive orthorhombic structure of CaMgSn phase. Besides, selected area di?raction pattern obtained from coarse rod-like phase corresponded to [011] zone axis of CaMgSn phase, as shown in Fig. 2(b). Energy dispersive spectrum (EDS) results obtained from coarse rod-like (#1 position in Fig. 2(b)) and ?ne feather-like (#2 position in Fig. 2(b)) phases are listed in Table 2, con?rming that the composition of two-types of phase is nearly similar. TEM and EDS analysis support that two types of phase have same crystal structure in spite of the di?erent morphologies. When the amount of Ca increased further or exceeds the amount of Sn, eutectic Mg2Ca phase forms in inter-dendritic region as shown in TX16 (Mg-1 mass%Sn-6 mass%Ca) alloy. In the regions between three two-phase regions of -Mg + CaMgSn, -Mg + Mg2Sn and -Mg + Mg2Ca, there are two three-phase regions of -Mg + CaMgSn + Mg2Sn and -Mg + CaMgSn + Mg2Ca as shown in TX70 (Mg-7 mass%Sn-0.5 mass%Ca) and TX75 (Mg-7 mass%Sn- 5 mass%Ca) alloy, respectively. Among the microstructures in Fig. 1, it is, in particular, noticeable that the ternary CaMgSn phase is present in two types of morphology when the alloy compositions are located in the two phase region of -Mg and CaMgSn phases, coarse rod-like phase in the grain interior and ?ne feather-like phase near inter-dendritic region as shown in TX51 alloy. For detailed analysis on the solidi?cation behavior, thermody- namic calculation was performed by using Pandat? pro- gram. Figure 3(a) shows vertical-section between Mg and CaMgSn phase in ternary Mg-Sn-Ca system. There is a peudo-binary eutectic reaction between -Mg and CaMgSn phase near the composition of 1 mass% Ca. Considering the optical microscope image of TX51 alloy in Fig. 1, the coarse rod-like CaMgSn phase forms as a primary solidi?cation phase when passing through L + CaMgSn region in hyper- eutectic composition range during solidi?cation. Primary CaMgSn phase grows with rod-like shape in the liquid melt at initial solidi?cation stage. The optical microscope image of TX51 alloy in Fig. 1 also clearly shows that -Mg halo forms around the primary CaMgSn phase. In general, when the composition di?erence between primary and the other eutectic phase is high enough, solute depleted zone easily forms around the primary solidi?cation phase.14) At the later stage of solidi?cation, eutectic reaction occurs resulting in the formation of ?ne feather-like CaMgSn phase embedded in the -Mg matrix since the pseudo-eutectic reaction composition is quite close to Mg. Figure 3(a) also shows that the pseudo-binary eutectic temperature is about 635 C. To con?rm the thermal stability of the CaMgSn phase, TX51 alloy was annealed at 500 C for 24 h. For comparison, T5 alloy was annealed at the same condition. After heat treatment at 500 C, CaMgSn phase remained stable, but Mg2Sn phase dissolved into the matrix in TX51 alloy (Fig. 3(c)), as can be seen clearly from the comparison with the as-cast microstructure of TX51 alloy in Fig. 1 (eutectic temperature of -Mg and Mg2Sn: 561 C). The result is further supported from the case of T5 alloy (Fig. 3(b)) where Mg2Sn phase perfectly dissolves into the matrix. According to the thermodynamic data,12) two invariant ternary eutectic reaction in Mg-rich region exist as follows; L ! -Mg + CaMgSn + Mg2Sn at 561.81 C (Mg88:92Sn11:07Ca0:01), L ! -Mg + CaMgSn + Mg2Ca at 514.39 C (Mg88:93Sn0:20Ca10:87). After pseudo-binary eutectic reaction, solidi?cation is terminated by selecting one of two ternary eutectic reactions, depending on the alloy 1 2 (b) (a) 500nm Fig. 2 Phase identi?cation using transmission electron microscopy: (a) Stereogram at [100] zone axis of the CaMgSn phase constructed by several di?raction patterns obtained from feather-like phase; and (b) Selected area di?raction pattern obtained from coarse rod-like phase. Table 2 EDS results obtained from CaMgSn phase as shown in Fig. 2(b). EDS (at%) Mg Ca Sn #1 39.67 31.71 28.63 #2 38.97 27.09 33.94 The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2407 composition. To predict the microstructure evolution more clearly, the solidi?cation behavior was calculated by Scheil's model15,16) (complete mixing in liquid, no di?usion in solid). The results about TX51 and TX52 alloy are described in Fig. 4(a) and (b). In both alloys, primary CaMgSn phase solidi?es as a primary phase, and then pseudo binary eutectic reaction occurs at 638 C. However, ?nal stage of solid- i?cation of remaining liquid in TX51 and TX52 is quiet di?erent each other. In TX51 alloy, remained liquid solidi?es by the ternary eutectic reaction; L ! -Mg + CaMgSn + Mg2Sn at 561 C. On the other hand, in TX52 alloy, remained liquid solidi?es by the ternary eutectic reaction; L ! - Mg + CaMgSn + Mg2Ca at 514 C. Therefore, the phases which form at the ?nal stage of solidi?cation are Mg2Sn and Mg2Ca phases in TX51 and TX52 alloys, respectively. The solidi?cation behavior of TX51 and TX52 alloys can be clearly evidenced from the as-cast microstructures shown in Fig. 4(a) and (b). The ?nal solidi?ed phases in ternary eutectic region were identi?ed as Mg2Sn and Mg2Ca phase in TX51 and TX52 alloy, respectively, as marked by arrows in Fig. 4. 3.2 Mechanical properties at elevated temperature Figure 5 describes the tensile test results of these alloys at room temperature and 200 C, respectively. T5 and TX51 alloys exhibited signi?cant yield strength decrease at 200 C. Form the results of stress variations as shown in Fig. 5(c), it can be noticed that the high temperature yield strength remarkably improved above 2 mass% Ca. Compared with T5 alloy, the yield strength of TX51 alloy was also deteriorated signi?cantly in spite of the existence of CaMgSn phase, indicating that primary and eutectic CaMgSn phase does not play a favorable role in enhancing the tensile properties. However, with increasing volume fraction of Mg2Ca phase, high temperature strength was signi?cantly improved, but ductility decreased (Fig. 5(b)). Figure 6(a) shows the creep properties of T5 binary alloy. From the double logarithmic plot of stress as a function of minimum creep rate, stress exponent, n, is determined by following power law creep equation:17) _ " " ? An exp ? Q RT ; where _ " " is minimum creep rate (or secondary strain rate) and is applied stress, respectively. It can be observed that there is a signi?cant variation in the stress exponent with stress and temperature. At the low stress regime, from the stress exponents (n ? 1), it can be deduced that di?usional creep dominates the deformation behavior. At the medium stress regime, stress exponent of T5 alloy is increased from 3 to 7 Liquid L CaMgSn + L + α?Mg α?Mg + CaMgSn Mg CaMgSn T5 TX51 (a) (b) (c) Temperature, K Ca contents, wt% Fig. 3 (a) Cross-sectional phase diagram between -Mg and CaMgSn phase constructed by thermodynamic calculation using Pandat?; and (b) Optical micrographs obtained from T5 and TX51 alloys after heat treatment at 500 C for 24 h. 2408 D. H. Kim et al. above 150 C. From these apparent stress exponents, creep mechanism changes from glide to climb controlled creep above 150 C. Detailed TEM analysis after creep deformation showed that dynamic precipitation of Mg2Sn occurrs above 150 C as shown in Fig. 6(b). This precipitation may induce the increment of stress exponent. However, the e?ect of precipitation on the deformation behavior is not dealt with in detail in the present study. 500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mg2 Sn CaMgSn Liquid Volume fraction, % Temperature, °C α-Mg TX51 500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mg2 Ca CaMgSn Liquid Volume fraction, % Temperature, °C α-Mg TX52 (a) (b) Ternary Eutectic with Mg2Sn Ternary Eutectic with Mg2Ca 725 700 675 650 625 600 575 550 525 725 700 675 650 625 600 575 550 525 Fig. 4 Solidi?cation behavior calculated using Scheil's model and comparison with optical microstructures in (a) TX51; and (b) TX52 alloy, respectively. 0 0 20 40 60 80 100 Yield stess, σ ys / MPa Ca contents, wt% Room temperature 200°C Mg-5Sn-xCa cast alloy Initial strain rate : 10 -3 s-1 0.00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 T5 TX51 TX52 TX53 Engineering stress, σ / MPa Strain, ε 200 °C Initial strain rate : 10-3 s-1 (a) (b) (c) 0.00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 T5 TX51 TX52 TX53 Engineering stress, σ / MPa Strain, ε Room temperature Initial strain rate : 10-3 s-1 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.3 0.30 0.25 0.20 0.15 0.10 0.05 3 2 1 Fig. 5 Tensile test results of TX series alloys: (a) Stress-strain curves at room temperature; (b) Stress-strain curves at 200 C; and (c) Comparison of yield stress between room temperature and 200 C. The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2409 Figure 7(a) shows creep properties of T5, TX51 and TX52 alloys at 150 C. Generally, when the Ca content increased, minimum creep rate decreased. Especially, creep resistance is remarkably improved above 2 mass% Ca content. This behavior shows similar tendency with the tensile test results at high temperature regime. Stress exponents of all the alloys show dislocation creep climb controlled behavior similar to T5 alloy. Figure 7(b) shows temperature dependence of creep behavior for TX52 alloy under 50 MPa. The activation energy of TX52 alloy was found to be 74 kJ/mol under loading condition of 50 MPa. This value is close to that of grain boundary di?usion for pure magnesium, 92 kJ/mol.18) From the above results, high temperature mechanical proper- ties are remarkably improved when the Ca contents increases above 2 mass%. According to the pre-described microstruc- ture/phase analysis, this improvement attributed to the presence of Mg2Ca phase. 4. Discussion 4.1 The e?ect of morphology of CaMgSn phase on the mechanical properties As mentioned above, CaMgSn phase has two types of morphology due to its pseudo eutectic solidi?cation behav- ior. Though the CaMgSn phase shows high thermal stability, it does not provide bene?cial e?ects on the tensile mechan- ical properties. This fact can be attributed to its solidi?cation morphology of the CaMgSn primary phase. Figure 8 shows the fractured surface scanning electron microscopy (SEM) image after tensile test of TX51 alloy. It can be observed that primary CaMgSn phase acts as crack initiation sites during 15 1E-8 1E-7 1E-6 Mg-5Sn Solid solution Stress, σ / MPa 125°C 150°C 175°C Stain rate, ε / s -1 [0001] (a) (b) 500nm 40 35 30 25 20 13 10 7 3 1 Fig. 6 Creep properties of T5 alloy: (a) Double logarithmic plot of stress as a function of minimum creep rate by power-law equation; and (b) Dynamic precipitation behavior in matrix after creep deformation at 150 C under 40 MPa. 20 1E-8 1E-7 T5 TX51 TX52 Strain rate, ε / s -1 Stress, σ / MPa 150°C 7 2.46x10-4 2.49x10-4 2.52x10-4 2.55x10-4 2.58x10-4 2.60x10-4 2.63x10-4 2.66x10-4 -19.0 -18.5 -18.0 -17.5 -17.0 -16.5 -16.0 -15.5 144.55kJ/mol ln (Strain rate), ln( ε ) / s -1 1/RT, 1/kJ/mol x K 73.91kJ/mol TX52 under 50MPa ) b ( ) a ( 100 90 80 70 60 50 40 30 Fig. 7 Creep properties of TX series alloys: (a) Stress dependence behavior at 150 C; and (b) Temperature dependence behavior under 50 MPa in TX52 alloy. CaMgSn Fig. 8 Scanning electron microscope image obtained from fractured surface in TX51 alloy. 2410 D. H. Kim et al. deformation. Namely, ductile fracture is originated from the primary CaMgSn phase in grain interior. This means that primary CaMgSn phase can a?ect negatively on the tensile properties. In order to examine the e?ect of cooling rate on morphology of CaMgSn phase, rapidly solidi?ed specimen by injection casting was prepared for mechanical test. In injection casting method, inductively re-melted sample in a fused silica tube is injected through a nozzle into water- cooled copper mold, having cylindrical cavity having 5 mm diameter and 50 mm height. Rapid cooling rate can e?ec- tively suppress growth of primary CaMgSn phase in TX51 alloy as shown in Fig. 9(a). According to the stress-strain curves obtained from tensile test, injection cast alloy shows higher mechanical properties than mold cast alloy (Fig. 9(b)). This result shows that formation of coarse primary CaMgSn phase can be avoided by high cooling rate processing. Further works about rapid cooling rate e?ect on the mechanical properties are in progress. 4.2 The e?ect of constituent phases on the elevated temperature properties Figure 10 shows creep rupture behavior after deformation at 200 C. In the alloys containing Mg2Sn phase, creep rupture occurs along the inter-granular boundary as shown in 0.00 0 25 50 75 100 125 150 175 200 TX51 alloy Mold cast Injection cast Tensile test Initial strain rate : 10 -3 Room temperature Engineering Stress σ / MPa Strain, ε ) b ( ) a ( 0.15 0.10 0.05 Fig. 9 E?ect of cooling rate on tensile properties: (a) Optical microstructure of rapid solidi?ed TX51 alloy; and (b) Comparison of tensile properties depending on the processing conditions. (a) (b) (c) Fig. 10 Optical micrograph obtained after creep deformation at 200 C: (a) T5; (b) TX51; and (c) TX52 alloy, respectively. The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2411 Fig. 10(a) and (b). On the other hand, TX52 alloy containing Mg2Ca phase shows trans-granular fracture as shown in Fig. 10(c). This means that eutectic Mg2Ca phase shows better boundary strengthening than Mg2Sn phase. Therefore, it is considered that Mg2Ca phase e?ectively inhibits grain boundary sliding during creep deformation. Figure 11 shows precipitation behavior of T5 and TX53 alloy after creep deformation at 200 C under 40 MPa and 50 MPa loading, respectively. As mentioned above, T5 alloy shows dynamic precipitation behavior. Mostly, rod-like Mg2Sn precipitates lie along basal plane of -Mg. Compared to the dynamic precipitation at 150 C as shown in Fig. 6(b), theses precipitates appears to be coarser. (Fig. 11(a)). On the other hand, in TX53 alloy, there is precipitation behavior, too. However, it can be observed that these ?ne precipitates remain stable without coarsening after creep deformation at 200 C. According to the selected area di?raction pattern, theses precipitates were identi?ed as Mg2Ca phase as shown in Fig. 11(b). Mg2Ca precipitates exhibit various types of morphology; rod, spherical and short rod type. Rod-like precipitates have an orientation relationship with matrix; ?0001?Mg2Ca == ?0001?Mg and h2 1 1 1 10iMg2Ca == h10 1 10iMg. This relationship corresponds to that reported in the previous report.19) From these results, it can be deduced that Mg2Ca precipitates are more e?ective on creep resistance than Mg2Sn due to high thermal stability against coarsening. 5. Conclusions In the present study, we have investigated the relationship between microstructural evolution and mechanical properties in Mg-Sn-Ca alloys. Major conclusions can be summarized as follows. (1) As-cast microstructures of Mg-Sn-Ca alloy consist of two or three phase depending on the Ca/Sn ratio, i.e. Mg2Sn, CaMgSn and Mg2Ca phases. (2) CaMgSn phase has two types of morphology due to its pseudo hyper-eutectic reaction with -Mg, i.e. coarse rod- like primary phase and feather-like eutectic phase. (3) Primary solidi?ed CaMgSn phase exhibits negative e?ect on the tensile properties due to its solidi?cation morphology. (4) According to the creep test results of TX series alloys, apparent stress exponent value shows dislocation climb controlled mechanism by core di?usion (n ? 7) above 150 C. Activation energy of TX52 alloy (74 kJ/mol) is close to grain boundary di?usion for pure magnesium, 92 kJ/mol. (5) Creep resistance remarkably improves with the presence of Mg2Ca phase. The Mg2Ca phase changed the fracture behavior from inter-granular to trans-granular fracture mode. Mg2Ca precipitates show higher thermal stability against coarsening that Mg2Sn precipitates. Acknowledgements This study was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, Republic of Korea, and the Global Research Laboratory Program of Korea Ministry of Science and Technology. Ju Youn Lee, Joon Seok Kyeong and Hyun Kyu Lim are grateful for the support by Second Stage of Brain Korea 21 Program. REFERENCES 1) B. L. Mordike: Mater. Sci. Eng. A 324 (2002) 103–112. 2) K. U. Kainer and F. von Buch: Magnesium alloys and technology, ed. 100nm [0001] [0001] 100nm (a) (b) Fig. 11 Bight ?eld images obtained from as-crept at 200 C: (a) T5 under 40 MPa loading. (b) TX53 alloy under 50 MPa loading, respectively. 2412 D. H. Kim et al. by K. U. Kainer, (Wiley-VCH, 2003). 3) C. J. Bettles and M. A. Gibson: Magnesium Alloys and Their Applications, ed. by K. U. Kainer, (Weinheim, 2004). 4) A. A. Nayeb-Hashemi and J. B. 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Gordon: Principles of phase diagrams in materials systems, (McGraw-Hill, 1968). 15) E. Scheil: Z. Metallkde 34 (1942) 70. 16) D. Stefanescu: Science and Engineering of Casting Solidi?cation, (Kluwer Academic/Plenum Publishers, 2002). 17) T. H. Courtney: Mechanical behavior of Materials, (McGraw Hill Int. Ed. 2000). 18) H. J. Frost and M. F. Ashby: Deformation-mechanism maps, (Perga- mon press, 1982). 19) J. F. Nie and B. C. Muddle: Scr. Mater. 37 (1997) 1475. The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2413
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The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System Do Hyung Kim1;*1 , Ju Youn Lee1;*1 , Hyun Kyu Lim1;*1 , Joon Seok Kyeong1;*1 , Won Tae Kim2 and Do Hyang Kim1;*2 1 Center for Noncrystalline Materials, Dept. of Metallurgical Eng., Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea 2 Applied Science Division, Cheongju University, 36 Naedok-dong, Sangdang-gu, Cheongju, Chongbuk 360-764, Korea The e?ect of microstructural evolution on the creep properties in Mg-Sn-Ca system has been investigated. As-cast microstructure of Mg- Sn-Ca alloy consists of two or three phases depending on the Ca/Sn ratio, i.e. Mg2Sn, CaMgSn and Mg2Ca phases. Ternary CaMgSn phase has two types of morphology by its pseudo hyper-eutectic reaction with -Mg; coarse rod-like primary or feather-like eutectic phase. Primary solidi?ed CaMgSn phase exhibit negative e?ect on the tensile properties in spite of its high thermal stability up to 500 C. According to the creep test results, apparent stress exponent value (n ? 7) indicates climb controlled creep mechanism by core di?usion above 150 C. Activation energy of Mg-5Sn-2Ca alloy (74 kJ/mol) is close to grain boundary di?usion for pure magnesium, 92 kJ/mol. Creep resistance is remarkably improved with the presence of Mg2Ca phase. [doi:10.2320/matertrans.MER2008140] (Received April 25, 2008; Accepted August 4, 2008; Published September 25, 2008) Keywords: magnesium-tin-calcium, creep, CaMgSn, Mg2Ca 1. Introduction Mg alloys have the great potential for high performance structural applications due to their inherent properties such as low density, high speci?c strength, superior damping capac- ity and good castability etc. Commercial high temperature Mg alloys are often classi?ed by their processing methods. There are Mg-Ag-RE, Mg-Zn-Zr and Mg-Y-RE series alloys which have been mainly fabricated by sand casting process. Though these alloys show high performance above 200 C,1,2) their high cost by using rare earth elements restricts wider applications. Die cast processing is well known for high productivity. To improve die castability, most alloys contain Al. Although Al signi?cantly increases castability, discon- tinuous precipitation of -phase (Mg17Al12) signi?cantly decreases high temperature strength.3) Therefore, usage of Mg alloys containing Al is limited below 200 C. Final goal for development of creep resistance Mg alloy is to design alloys with low cost, but maintaining its high performance. One of candidate for achieving this goal is Mg-Sn-Ca system. Basically, Sn and Ca have better cost e?ectiveness than rare earth elements. Moreover, according to the binary phase diagrams,4) their inter-metallic com- pounds formed with Mg exhibit high thermal stability. Especially, there are a lot of successful developments on the enhancement of creep resistance in Mg-Al series alloy with Ca addition.5–7) In Mg-Sn binary system, Mg2Sn phase has a high melting point ($770 C). In addition, this phase can be easily precipitated because it has a relatively high solubility limit (14.48 mass%) at 560 C and little solubility at ambient temperature. Lately, the alloys based on the binary Mg-Sn system have been received much attention for creep resistance alloy due to the high thermal stability of Mg2Sn phase. Kang et al. reported that Mg2Sn phase plays an important role in resisting matrix deformation during creep in Mg-8Sn-3Al-1Si alloy.8,9) Kainer et al. have reported that ternary CaMgSn phase not only shows highly thermal stability up to 500 C, but also the alloys containing CaMgSn phase show favorable creep resistance in compressive mode.10,11) More recently, A. Kozlov et al. have studied on the phase equilibria of Mg-Sn-Ca system using CALPHAD method.12,13) However, close examination on the role of respective 2nd phases on high temperature mechanical properties has not been performed in detail. Therefore, in this study, the e?ect of microstructural evolution on the high temperature properties in Mg-Sn-Ca system has been investigated. 2. Experimental Procedure All materials investigated in the present study are listed in Table 1. High purity metals with nominal compositions were prepared in an electrical resistance furnace under a SF6 + CO2 protective gas atmosphere. Molten metal was poured into a preheated ($100 C) rectangular steel mold with a dimension of 1.5 cm in thickness, 6 cm in width, and 10 cm in height. The phases in as-cast microstructure of the alloys listed in Table 1 were analyzed preliminary via X-ray di?raction (Rigaku, CN2301) using monochromatic CuK radiation to construct the phase selection map in as-cast state. For microstructural observations, the specimens were etched with a solution of 2% nital solution (2 ml nitric acid and 98 ml ethanol). The microstructures were observed by optical microscopy (OM; Leica DMRM) and scanning electron microscopy (SEM; JEOL 5310). A detailed analysis for phase identi?cation was performed by transmission electron microscopy (TEM; JEM 2000 EX). Specimens for TEM were prepared by an ion milling method (Gatan, model 600) after mechanical grinding. *1 Graduate Student, Yonsei University *2 Corresponding author, E-mail: dohkim@yonsei.ac.kr Materials Transactions, Vol. 49, No. 10 (2008) pp. 2405 to 2413 #2008 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Uniaxial tensile tests were carried out on dog-bone specimens with constant cross head speed at an initial strain rate of 1 ? 10?3 ?s?1 . Tensile creep tests were conducted by using direct loading type creep equipments. Cylindrical type creep specimens are precisely machined with a diameter of 6 mm and 25 mm gauge length. 3. Results 3.1 As-cast microstructure analysis Figure 1 describes as-cast state phase selection map for the alloys listed in Table 1. The alloy numbers corresponding to 14 alloys are also marked in the phase selection map. This map was constructed on the basis of phase identi?cation results via X-ray di?raction method. As-cast microstructures of Mg-Sn-Ca alloy system consist of two or three phases depending on Ca/Sn ratio. From the X-ray di?raction result, constitutive phases were indenti?ed as Mg2Sn, CaMgSn and Mg2Ca phases. The results of the phase identi?cation are also listed Table 1. Two-phase regions of -Mg + CaMgSn, -Mg + Mg2Sn, -Mg + Mg2Ca and three-phase regions of -Mg + CaMgSn + Mg2Sn and -Mg + CaMgSn + Mg2Ca are schematically outlined in Fig. 1 based on the result shown in Table 1. The Optical microscopic images shown in Fig. 1 stand for typical microstructures observed for the alloys listed in Table 1. As expected, T5 (Mg- 5 mass%Sn) alloy consisted of two phase microstructure, -Mg primary dendrite and interdendritic Mg2Sn phases. With small addition of Ca, CaMgSn phase newly formed together with Mg2Sn phase in TX51 (Mg-5 mass%Sn- 1 mass%Ca) alloy. Noticeably, the ternary CaMgSn phase was present in two types of morphology; coarse rod-like phase in the grain interior and ?ne feather-like phase near inter-dendritic region. To identify these two types of the Table 1 Chemical composition of all alloys prepared for this study. No. Composition (mass%) Constitutive Abbrev. Remarks Mg Sn Ca Phases 1 95 5 — -Mg, Mg2Sn T5 2 94 5 1 -Mg, CaMgSn, Mg2Sn TX51 Mechanical 3 93 5 2 -Mg, CaMgSn, Mg2Ca TX52 properties 4 92 5 3 -Mg, CaMgSn, Mg2Ca TX53 5 91 5 4 -Mg, CaMgSn, Mg2Ca TX54 6 97.58 1.20 1.22 -Mg, CaMgSn, Mg2Ca TX11 7 88.15 11.10 0.75 -Mg, CaMgSn, Mg2Sn TX111 8 87.88 6.78 5.34 -Mg, CaMgSn, Mg2Ca TX75 9 93.23 1.18 5.59 -Mg, Mg2Ca TX16 Phase 10 98.70 0.48 0.82 -Mg, CaMgSn, Mg2Ca TX01 construction 11 88.75 8.98 2.72 -Mg, CaMgSn, Mg2Sn TX93 map 12 85.33 10.97 3.70 -Mg, CaMgSn TX114 13 92.70 6.91 0.39 -Mg, CaMgSn, Mg2Sn TX70 14 93.38 1.21 0.41 -Mg, CaMgSn TX10 [9] TX16 [8] TX75 [13] TX70 [1] T5 [2] TX51 CaMgSn CaMgSn Fig. 1 Phase selection map for > 80 mass% Mg rich region obtained from as-cast state specimens. 2406 D. H. Kim et al. phase carefully, TEM analysis was performed. Figure 2(a) is the stereogram centered at [100] zone axis constructed by several di?raction patterns obtained from feather-like phase, clearly con?rming the primitive orthorhombic structure of CaMgSn phase. Besides, selected area di?raction pattern obtained from coarse rod-like phase corresponded to [011] zone axis of CaMgSn phase, as shown in Fig. 2(b). Energy dispersive spectrum (EDS) results obtained from coarse rod-like (#1 position in Fig. 2(b)) and ?ne feather-like (#2 position in Fig. 2(b)) phases are listed in Table 2, con?rming that the composition of two-types of phase is nearly similar. TEM and EDS analysis support that two types of phase have same crystal structure in spite of the di?erent morphologies. When the amount of Ca increased further or exceeds the amount of Sn, eutectic Mg2Ca phase forms in inter-dendritic region as shown in TX16 (Mg-1 mass%Sn-6 mass%Ca) alloy. In the regions between three two-phase regions of -Mg + CaMgSn, -Mg + Mg2Sn and -Mg + Mg2Ca, there are two three-phase regions of -Mg + CaMgSn + Mg2Sn and -Mg + CaMgSn + Mg2Ca as shown in TX70 (Mg-7 mass%Sn-0.5 mass%Ca) and TX75 (Mg-7 mass%Sn- 5 mass%Ca) alloy, respectively. Among the microstructures in Fig. 1, it is, in particular, noticeable that the ternary CaMgSn phase is present in two types of morphology when the alloy compositions are located in the two phase region of -Mg and CaMgSn phases, coarse rod-like phase in the grain interior and ?ne feather-like phase near inter-dendritic region as shown in TX51 alloy. For detailed analysis on the solidi?cation behavior, thermody- namic calculation was performed by using Pandat? pro- gram. Figure 3(a) shows vertical-section between Mg and CaMgSn phase in ternary Mg-Sn-Ca system. There is a peudo-binary eutectic reaction between -Mg and CaMgSn phase near the composition of 1 mass% Ca. Considering the optical microscope image of TX51 alloy in Fig. 1, the coarse rod-like CaMgSn phase forms as a primary solidi?cation phase when passing through L + CaMgSn region in hyper- eutectic composition range during solidi?cation. Primary CaMgSn phase grows with rod-like shape in the liquid melt at initial solidi?cation stage. The optical microscope image of TX51 alloy in Fig. 1 also clearly shows that -Mg halo forms around the primary CaMgSn phase. In general, when the composition di?erence between primary and the other eutectic phase is high enough, solute depleted zone easily forms around the primary solidi?cation phase.14) At the later stage of solidi?cation, eutectic reaction occurs resulting in the formation of ?ne feather-like CaMgSn phase embedded in the -Mg matrix since the pseudo-eutectic reaction composition is quite close to Mg. Figure 3(a) also shows that the pseudo-binary eutectic temperature is about 635 C. To con?rm the thermal stability of the CaMgSn phase, TX51 alloy was annealed at 500 C for 24 h. For comparison, T5 alloy was annealed at the same condition. After heat treatment at 500 C, CaMgSn phase remained stable, but Mg2Sn phase dissolved into the matrix in TX51 alloy (Fig. 3(c)), as can be seen clearly from the comparison with the as-cast microstructure of TX51 alloy in Fig. 1 (eutectic temperature of -Mg and Mg2Sn: 561 C). The result is further supported from the case of T5 alloy (Fig. 3(b)) where Mg2Sn phase perfectly dissolves into the matrix. According to the thermodynamic data,12) two invariant ternary eutectic reaction in Mg-rich region exist as follows; L ! -Mg + CaMgSn + Mg2Sn at 561.81 C (Mg88:92Sn11:07Ca0:01), L ! -Mg + CaMgSn + Mg2Ca at 514.39 C (Mg88:93Sn0:20Ca10:87). After pseudo-binary eutectic reaction, solidi?cation is terminated by selecting one of two ternary eutectic reactions, depending on the alloy 1 2 (b) (a) 500nm Fig. 2 Phase identi?cation using transmission electron microscopy: (a) Stereogram at [100] zone axis of the CaMgSn phase constructed by several di?raction patterns obtained from feather-like phase; and (b) Selected area di?raction pattern obtained from coarse rod-like phase. Table 2 EDS results obtained from CaMgSn phase as shown in Fig. 2(b). EDS (at%) Mg Ca Sn #1 39.67 31.71 28.63 #2 38.97 27.09 33.94 The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2407 composition. To predict the microstructure evolution more clearly, the solidi?cation behavior was calculated by Scheil's model15,16) (complete mixing in liquid, no di?usion in solid). The results about TX51 and TX52 alloy are described in Fig. 4(a) and (b). In both alloys, primary CaMgSn phase solidi?es as a primary phase, and then pseudo binary eutectic reaction occurs at 638 C. However, ?nal stage of solid- i?cation of remaining liquid in TX51 and TX52 is quiet di?erent each other. In TX51 alloy, remained liquid solidi?es by the ternary eutectic reaction; L ! -Mg + CaMgSn + Mg2Sn at 561 C. On the other hand, in TX52 alloy, remained liquid solidi?es by the ternary eutectic reaction; L ! - Mg + CaMgSn + Mg2Ca at 514 C. Therefore, the phases which form at the ?nal stage of solidi?cation are Mg2Sn and Mg2Ca phases in TX51 and TX52 alloys, respectively. The solidi?cation behavior of TX51 and TX52 alloys can be clearly evidenced from the as-cast microstructures shown in Fig. 4(a) and (b). The ?nal solidi?ed phases in ternary eutectic region were identi?ed as Mg2Sn and Mg2Ca phase in TX51 and TX52 alloy, respectively, as marked by arrows in Fig. 4. 3.2 Mechanical properties at elevated temperature Figure 5 describes the tensile test results of these alloys at room temperature and 200 C, respectively. T5 and TX51 alloys exhibited signi?cant yield strength decrease at 200 C. Form the results of stress variations as shown in Fig. 5(c), it can be noticed that the high temperature yield strength remarkably improved above 2 mass% Ca. Compared with T5 alloy, the yield strength of TX51 alloy was also deteriorated signi?cantly in spite of the existence of CaMgSn phase, indicating that primary and eutectic CaMgSn phase does not play a favorable role in enhancing the tensile properties. However, with increasing volume fraction of Mg2Ca phase, high temperature strength was signi?cantly improved, but ductility decreased (Fig. 5(b)). Figure 6(a) shows the creep properties of T5 binary alloy. From the double logarithmic plot of stress as a function of minimum creep rate, stress exponent, n, is determined by following power law creep equation:17) _ " " ? An exp ? Q RT ; where _ " " is minimum creep rate (or secondary strain rate) and is applied stress, respectively. It can be observed that there is a signi?cant variation in the stress exponent with stress and temperature. At the low stress regime, from the stress exponents (n ? 1), it can be deduced that di?usional creep dominates the deformation behavior. At the medium stress regime, stress exponent of T5 alloy is increased from 3 to 7 Liquid L CaMgSn + L + α?Mg α?Mg + CaMgSn Mg CaMgSn T5 TX51 (a) (b) (c) Temperature, K Ca contents, wt% Fig. 3 (a) Cross-sectional phase diagram between -Mg and CaMgSn phase constructed by thermodynamic calculation using Pandat?; and (b) Optical micrographs obtained from T5 and TX51 alloys after heat treatment at 500 C for 24 h. 2408 D. H. Kim et al. above 150 C. From these apparent stress exponents, creep mechanism changes from glide to climb controlled creep above 150 C. Detailed TEM analysis after creep deformation showed that dynamic precipitation of Mg2Sn occurrs above 150 C as shown in Fig. 6(b). This precipitation may induce the increment of stress exponent. However, the e?ect of precipitation on the deformation behavior is not dealt with in detail in the present study. 500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mg2 Sn CaMgSn Liquid Volume fraction, % Temperature, °C α-Mg TX51 500 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mg2 Ca CaMgSn Liquid Volume fraction, % Temperature, °C α-Mg TX52 (a) (b) Ternary Eutectic with Mg2Sn Ternary Eutectic with Mg2Ca 725 700 675 650 625 600 575 550 525 725 700 675 650 625 600 575 550 525 Fig. 4 Solidi?cation behavior calculated using Scheil's model and comparison with optical microstructures in (a) TX51; and (b) TX52 alloy, respectively. 0 0 20 40 60 80 100 Yield stess, σ ys / MPa Ca contents, wt% Room temperature 200°C Mg-5Sn-xCa cast alloy Initial strain rate : 10 -3 s-1 0.00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 T5 TX51 TX52 TX53 Engineering stress, σ / MPa Strain, ε 200 °C Initial strain rate : 10-3 s-1 (a) (b) (c) 0.00 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 T5 TX51 TX52 TX53 Engineering stress, σ / MPa Strain, ε Room temperature Initial strain rate : 10-3 s-1 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.3 0.30 0.25 0.20 0.15 0.10 0.05 3 2 1 Fig. 5 Tensile test results of TX series alloys: (a) Stress-strain curves at room temperature; (b) Stress-strain curves at 200 C; and (c) Comparison of yield stress between room temperature and 200 C. The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2409 Figure 7(a) shows creep properties of T5, TX51 and TX52 alloys at 150 C. Generally, when the Ca content increased, minimum creep rate decreased. Especially, creep resistance is remarkably improved above 2 mass% Ca content. This behavior shows similar tendency with the tensile test results at high temperature regime. Stress exponents of all the alloys show dislocation creep climb controlled behavior similar to T5 alloy. Figure 7(b) shows temperature dependence of creep behavior for TX52 alloy under 50 MPa. The activation energy of TX52 alloy was found to be 74 kJ/mol under loading condition of 50 MPa. This value is close to that of grain boundary di?usion for pure magnesium, 92 kJ/mol.18) From the above results, high temperature mechanical proper- ties are remarkably improved when the Ca contents increases above 2 mass%. According to the pre-described microstruc- ture/phase analysis, this improvement attributed to the presence of Mg2Ca phase. 4. Discussion 4.1 The e?ect of morphology of CaMgSn phase on the mechanical properties As mentioned above, CaMgSn phase has two types of morphology due to its pseudo eutectic solidi?cation behav- ior. Though the CaMgSn phase shows high thermal stability, it does not provide bene?cial e?ects on the tensile mechan- ical properties. This fact can be attributed to its solidi?cation morphology of the CaMgSn primary phase. Figure 8 shows the fractured surface scanning electron microscopy (SEM) image after tensile test of TX51 alloy. It can be observed that primary CaMgSn phase acts as crack initiation sites during 15 1E-8 1E-7 1E-6 Mg-5Sn Solid solution Stress, σ / MPa 125°C 150°C 175°C Stain rate, ε / s -1 [0001] (a) (b) 500nm 40 35 30 25 20 13 10 7 3 1 Fig. 6 Creep properties of T5 alloy: (a) Double logarithmic plot of stress as a function of minimum creep rate by power-law equation; and (b) Dynamic precipitation behavior in matrix after creep deformation at 150 C under 40 MPa. 20 1E-8 1E-7 T5 TX51 TX52 Strain rate, ε / s -1 Stress, σ / MPa 150°C 7 2.46x10-4 2.49x10-4 2.52x10-4 2.55x10-4 2.58x10-4 2.60x10-4 2.63x10-4 2.66x10-4 -19.0 -18.5 -18.0 -17.5 -17.0 -16.5 -16.0 -15.5 144.55kJ/mol ln (Strain rate), ln( ε ) / s -1 1/RT, 1/kJ/mol x K 73.91kJ/mol TX52 under 50MPa ) b ( ) a ( 100 90 80 70 60 50 40 30 Fig. 7 Creep properties of TX series alloys: (a) Stress dependence behavior at 150 C; and (b) Temperature dependence behavior under 50 MPa in TX52 alloy. CaMgSn Fig. 8 Scanning electron microscope image obtained from fractured surface in TX51 alloy. 2410 D. H. Kim et al. deformation. Namely, ductile fracture is originated from the primary CaMgSn phase in grain interior. This means that primary CaMgSn phase can a?ect negatively on the tensile properties. In order to examine the e?ect of cooling rate on morphology of CaMgSn phase, rapidly solidi?ed specimen by injection casting was prepared for mechanical test. In injection casting method, inductively re-melted sample in a fused silica tube is injected through a nozzle into water- cooled copper mold, having cylindrical cavity having 5 mm diameter and 50 mm height. Rapid cooling rate can e?ec- tively suppress growth of primary CaMgSn phase in TX51 alloy as shown in Fig. 9(a). According to the stress-strain curves obtained from tensile test, injection cast alloy shows higher mechanical properties than mold cast alloy (Fig. 9(b)). This result shows that formation of coarse primary CaMgSn phase can be avoided by high cooling rate processing. Further works about rapid cooling rate e?ect on the mechanical properties are in progress. 4.2 The e?ect of constituent phases on the elevated temperature properties Figure 10 shows creep rupture behavior after deformation at 200 C. In the alloys containing Mg2Sn phase, creep rupture occurs along the inter-granular boundary as shown in 0.00 0 25 50 75 100 125 150 175 200 TX51 alloy Mold cast Injection cast Tensile test Initial strain rate : 10 -3 Room temperature Engineering Stress σ / MPa Strain, ε ) b ( ) a ( 0.15 0.10 0.05 Fig. 9 E?ect of cooling rate on tensile properties: (a) Optical microstructure of rapid solidi?ed TX51 alloy; and (b) Comparison of tensile properties depending on the processing conditions. (a) (b) (c) Fig. 10 Optical micrograph obtained after creep deformation at 200 C: (a) T5; (b) TX51; and (c) TX52 alloy, respectively. The E?ect of Microstructure Evolution on the Elevated Temperature Mechanical Properties in Mg-Sn-Ca System 2411 Fig. 10(a) and (b). On the other hand, TX52 alloy containing Mg2Ca phase shows trans-granular fracture as shown in Fig. 10(c). This means that eutectic Mg2Ca phase shows better boundary strengthening than Mg2Sn phase. Therefore, it is considered that Mg2Ca phase e?ectively inhibits grain boundary sliding during creep deformation. Figure 11 shows precipitation behavior of T5 and TX53 alloy after creep deformation at 200 C under 40 MPa and 50 MPa loading, respectively. As mentioned above, T5 alloy shows dynamic precipitation behavior. Mostly, rod-like Mg2Sn precipitates lie along basal plane of -Mg. Compared to the dynamic precipitation at 150 C as shown in Fig. 6(b), theses precipitates appears to be coarser. (Fig. 11(a)). On the other hand, in TX53 alloy, there is precipitation behavior, too. However, it can be observed that these ?ne precipitates remain stable without coarsening after creep deformation at 200 C. According to the selected area di?raction pattern, theses precipitates were identi?ed as Mg2Ca phase as shown in Fig. 11(b). Mg2Ca precipitates exhibit various types of morphology; rod, spherical and short rod type. Rod-like precipitates have an orientation relationship with matrix; ?0001?Mg2Ca == ?0001?Mg and h2 1 1 1 10iMg2Ca == h10 1 10iMg. This relationship corresponds to that reported in the previous report.19) From these results, it can be deduced that Mg2Ca precipitates are more e?ective on creep resistance than Mg2Sn due to high thermal stability against coarsening. 5. Conclusions In the present study, we have investigated the relationship between microstructural evolution and mechanical properties in Mg-Sn-Ca alloys. Major conclusions can be summarized as follows. (1) As-cast microstructures of Mg-Sn-Ca alloy consist of two or three phase depending on the Ca/Sn ratio, i.e. Mg2Sn, CaMgSn and Mg2Ca phases. (2) CaMgSn phase has two types of morphology due to its pseudo hyper-eutectic reaction with -Mg, i.e. coarse rod- like primary phase and feather-like eutectic phase. (3) Primary solidi?ed CaMgSn phase exhibits negative e?ect on the tensile properties due to its solidi?cation morphology. (4) According to the creep test results of TX series alloys, apparent stress exponent value shows dislocation climb controlled mechanism by core di?usion (n ? 7) above 150 C. Activation energy of TX52 alloy (74 kJ/mol) is close to grain boundary di?usion for pure magnesium, 92 kJ/mol. (5) Creep resistance remarkably improves with the presence of Mg2Ca phase. The Mg2Ca phase changed the fracture behavior from inter-granular to trans-granular fracture mode. Mg2Ca precipitates show higher thermal stability against coarsening that Mg2Sn precipitates. Acknowledgements This study was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, Republic of Korea, and the Global Research Laboratory Program of Korea Ministry of Science and Technology. Ju Youn Lee, Joon Seok Kyeong and Hyun Kyu Lim are grateful for the support by Second Stage of Brain Korea 21 Program. REFERENCES 1) B. L. Mordike: Mater. Sci. Eng. A 324 (2002) 103–112. 2) K. U. Kainer and F. von Buch: Magnesium alloys and technology, ed. 100nm [0001] [0001] 100nm (a) (b) Fig. 11 Bight ?eld images obtained from as-crept at 200 C: (a) T5 under 40 MPa loading. (b) TX53 alloy under 50 MPa loading, respectively. 2412 D. H. Kim et al. by K. U. Kainer, (Wiley-VCH, 2003). 3) C. J. Bettles and M. A. Gibson: Magnesium Alloys and Their Applications, ed. by K. U. Kainer, (Weinheim, 2004). 4) A. A. Nayeb-Hashemi and J. B. 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