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The effect of lamellar structure in Mo5Si3–MoSi2 alloy refined by annealing on the Vickers hardness at room temperature
Journal of Alloys and Compounds 392 (2005) 87–95
The effect of lamellar structure in Mo5Si3–MoSi2 alloy refined by annealing on the Vickers hardness at room temperature Haibo Yang∗ , Wei Li, Aidang Shan, Jiansheng Wu Key Laboratory for High Temperature Materials and Tests of Ministry of Education, P. R. China, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Received 29 May 2003; received in revised form 17 May 2004; accepted 10 September 2004 Available online 28 November 2004
Abstract Mo5 Si3 –MoSi2 eutectic alloy, hypoeutectic alloy and hypereutectic alloy were prepared by arc-melting pure raw materials of Mo and Si. Annealing at 1200 ◦ C for different times was conducted on all of the alloys to investigate the effect of annealing on the microstructure and mechanical property including Vickers hardness and indentation toughness at room temperature. Lamellar structure consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase was observed in all the alloys. For Mo5 Si3 –MoSi2 eutectic alloy, a typical eutectic microstructure with primary Mo5 Si3 phase and Mo5 Si3 /MoSi2 lamellar structure was observed, and the primary Mo5 Si3 phase was eliminated when annealing at 1200 ◦ C for over 24 h. For Mo5 Si3 –MoSi2 hypoeutectic alloy, the lamellar structure was found only after annealing. A full lamellar structure throughout the alloy developed well with fine spacing on the order of 100 nm after annealing at 1200 ◦ C for 48 h. For Mo5 Si3 –MoSi2 hypereutectic alloy, the lamellar structure was found both before and after annealing. Primary MoSi2 was observed in the alloy even after annealing at 1200 ◦ C for 96 h, although the volume fraction of lamellar structure was increased with prolonging annealing time. The effect of the formation, development and destruction of lamellar structure on Vickers hardness and indentation toughness of alloys was investigated. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy show a same regularity, that is, they increase with prolonging annealing time and development of lamellar structure. The highest Vickers hardness and indentation toughness are about 1470 HVN and 4.8 MPa m1/2 , respectively. The Vickers hardness and indentation of Mo5 Si3 –MoSi2 eutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy are all roughly constant with prolonging annealing time, that is, about 1400 and 1270 HVN for Vickers hardness and 4 and 3.2 MPa m1/2 for indentation toughness. © 2004 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Mo–Si; Lamellar structure; Anneal; Microstructure; Mechanical property
1. Introduction In order to meet the ever-increasing demand for operating temperature capability of structural components, a considerable amount of research has been focused on developing new materials. Over the past 20 years, many alloy systems and processing routes were investigated to select potential intermetallic compound for applications at high temperatures. Nickel aluminide and titanium aluminide are attractive because these materials exhibit significant room temperature ductility compared to other intermetallics. In terms of candi∗
Corresponding author. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.029
date materials for high temperature, oxidation-resistant structural applications, there is a temperature cut off at ≈1000 ◦ C. Below 1000 ◦ C, it is possible to use nickel and cobalt-base superalloys and aluminide intermetallics. However, above 1000 ◦ C, for oxidation and strength reasons, one must shift to the silicon-based ceramics, advanced intermetallics, and silicide-based materials [1–3]. Recent studies show that the refractory metal silicides are of great potential to be candidates for structural material at ultra high temperature [1–3]. Among the refractory metal silicides, the intermetallic phases in the Mo–Si system show promising potentials because Mo has a lower density compared to other refractory metals and Mo does not embrittle with oxygen and nitrogen volatilization [4]. In the Mo–Si
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system, MoSi2 with the Cllb structure has been studied extensively and has found several significant applications. MoSi2 with the Cllb structure is a promising candidate as a matrix phase because of its superior oxidation resistance [5,6] and potential plastic deformability at low temperature [7,8]. Unfortunately, the inadequacy of low-temperature fracture toughness and high-temperature strength above 1200 ◦ C in the monolithic condition limits it for a practical structural application. Recently, another molybdenum silicide, Mo5 Si3 with the D8m structure is attracting more attention of researchers because of its higher melting point, better high temperature creep property than MoSi2 [9,10] and the dramatically improved oxidation resistance with the addition of B [11,12]. Mason et al. [13,14] investigated the microstructures and crystallography as well as the creep property of directionally solidified MoSi2 –Mo5 Si3 eutectics. The Mo5 Si3 lamellae were found to grow with an inclination of 15◦ relative to the axis of growth and the creep resistance of directionally solidified MoSi2 –Mo5 Si3 eutectics was found to increase with decreasing Mo5 Si3 lamellar spacing. In fact, the study on Ti–Al alloys has suggested that the enhanced fracture toughness and creep properties are attributed to the fine fully lamellar (FL) microstructure of the alloys [15]. The study [16] shows that MoSi2 -based duplex silicides consisted of MoSi2 (Cllb)/NbSi2 (C40) lamellae have higher strength at high temperature than binary MoSi2 since the lamellar boundary and/or lamellar colony act as an effective barrier to the motion of dislocations. Thus, the knowledge of the lamellar structure in MoSi2 –Mo5 Si3 alloys is helpful to understand their mechani-
cal properties. In this research, microstructures of arc-melted Mo5 Si3 –MoSi2 hypoeutectic alloy and hypereutectic alloy were investigated focusing on formation of the lamellar structure. The effect of the lamellar structure on the Vickers hardness and indentation toughness at room temperature of the alloys was also studied.
2. Experimental procedure Mo5 Si3 –MoSi2 hypoeutectic alloy (Mo–45 at.% Si), eutectic alloy (Mo–54 at.% Si) and hypereutectic alloy (Mo–60 at.% Si) were prepared by arc-melting high-purity elements (Mo—99.9% and Si—99.99%) in a water-cooled copper crucible in an argon atmosphere. The alloys annealed at 1200 ◦ C under a high-purity argon gas flow for 12, 24, 48 and 96 h, respectively. After metallographic polishing, the specimens were etched with 20 vol.% hydrochloric acid + 20 vol.% nitric acid for 10 s. The etched specimens were observed in a scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) system for determining the phases present in the alloys and the chemical composition in constituent phases. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) operated at 200 kV were performed to analyze the microstructures of the alloys annealed at 1200 ◦ C for different times. The mechanical properties of Vickers hardness were evaluated by using a Vickers indenter at 300 g load for 15 s at the room temperature. Indentation toughness (Kc ) was determined by measuring the lengths of radial cracks emanating
Fig. 1. X-ray diffraction patterns of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy and (b) Mo5 Si3 –MoSi2 hypereutectic alloy.
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Fig. 2. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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from the Vickers indents, using the following relation [17]. 1/2 P E Kc = A H c3/2 where A is a constant of value 0.016 [17], E the Young’s modulus, H the hardness, P the load and c is half-crack length.
3. Results 3.1. Constituent phases in Mo5 Si3 –MoSi2 hypoeutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy Fig. 1 shows X-ray diffraction profiles from samples of arc-melted ingots with compositions of Mo–45 at.% Si and Mo–60 at.% Si. Only two phases, Mo5 Si3 with D8m structure and MoSi2 with Cllb structure, were identified in both alloys. The eutectic alloy is also consisted of Mo5 Si3 and MoSi2 phase, although the X-ray diffraction pattern is not represented here. All peaks correspond to those reflected from the D8m structure or the Cllb structure, while relative diffraction intensity of the D8m phase to the Cllb depends on the alloy composition. This data suggests that the volume fraction of MoSi2 phase in hypoeutectic alloy is less than that in hypereutectic alloy. 3.2. Microstructures Fig. 2 shows the scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively. These alloys were denoted as sample nos. 0–4, respectively. Some cracks were observed in sample no. 1. The microstructures of the sample no. 0 (Fig. 2(a)) were remarkably changed after annealing. Except the sample no. 1 annealed at 1200 ◦ C for 12 h (Fig. 2(b)), all other samples show a full lamellar structure. Sample nos. 2 and 3, annealed at 1200 ◦ C for 24 and 48 h, respectively, exhibited full lamellar structures with fine spacing
of 100 nm order, as shown in Fig. 3. However, their arrangement disturbed at the colony boundaries (Fig. 2(c and d)). The lamellar structure was consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase. However, with the increasing time of annealing, the full lamellar structure became coarse in some areas of the alloy, as shown in Fig. 2(e). Annealing at 1200 ◦ C for 0, 24, 48 and 96 h was conducted on Mo5 Si3 –MoSi2 eutectic alloy, and the samples were denoted as sample no. 0 –4 , respectively. A typical eutectic microstructure consisted of primary Mo5 Si3 phase and lamellar structure was observed in the sample no. 0 , as shown in Fig. 4(a). After annealing at 1200 ◦ C for 12 h, the grain size of primary Mo5 Si3 phase decreased, as shown in Fig. 4(b). Prolonging annealing time, the primary Mo5 Si3 phase disappeared, and microstructure of alloy shown a Mo5 Si3 /MoSi2 eutectic lamellar structure when annealing at 1200 ◦ C for 24, 48 and 96 h, as shown in Fig. 4(c–e). Fig. 5 shows scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypereutectic alloy annealed at 1200 ◦ C for 0, 24, 48 and 96 h. These samples were denoted as sample nos. 0 –4 , respectively. A lamellar structure consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase was found in the sample no. 0 (Fig. 5(a)) of Mo5 Si3 –MoSi2 hypereutectic alloy without annealing, which was different with the sample no. 0 of Mo5 Si3 –MoSi2 hypoeutectic alloy without annealing. After annealing at 1200 ◦ C for different times, the microstructure of hypereutectic alloy was not changed essentially; however, the volume fraction of lamellae increased with the increasing annealing time. Unlike hypoeutectic alloy, besides the Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellae, primary MoSi2 was still observed in hypereutectic alloy after annealing at 1200 ◦ C for 0, 24, 48 and 96 h. In addition, some cracks were observed in sample nos. 0 , 1 , 3 and 4 (Fig. 5(a, b, d and e)). 3.3. Mechanical properties Fig. 6 shows the Vickers hardness values of the (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2
Fig. 3. TEM micrograph of Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for 48 h.
H. Yang et al. / Journal of Alloys and Compounds 392 (2005) 87–95
Fig. 4. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 eutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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Fig. 5. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypereutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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Fig. 6. The relationship between annealing time and Vickers hardness of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy after annealing at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively.
eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy at room temperature as a function of annealing time. Sample no. 3 annealed at 1200 ◦ C for 48 h in Mo5 Si3 –MoSi2 hypoeutectic alloy has the highest hardness value of 1457 HVN, which was improved about 19% compared with that of same no. 0 without annealing. The hardness of sample no. 0 is 1262 HVN. The Vickers hardness of eutectic alloy is nearly constant with increasing annealing time, and in the range of 1325–1360 HVN. Sample no. 3 annealed at 1200 ◦ C for 48 h in Mo5 Si3 –MoSi2 hypereutectic alloy has the highest hardness value about 1300 HVN, which was increased only about 2.7% compared with that of the sample no. 0 without annealing. The hardness of the sample no. 0 is 1266 HVN. The average Kc values of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy as a function of annealing time are shown in Fig. 7. A rapid increase in Kc is observed in Mo5 Si3 –MoSi2 hypoeutectic alloy from 2 to 4.8 MPa m1/2 . The Kc of Mo5 Si3 –MoSi2 eutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy are roughly constant in the range of 3.7–4.5 and 2–2.5 MPa m1/2 , respectively.
4. Discussion 4.1. Mo5 Si3 –MoSi2 hypoeutectic alloy The lamellar structure was formed in arc-melting Mo5 Si3 –MoSi2 hypoeutectic alloy during subsequent annealing. After annealing at 1200 ◦ C for 12 h, the volume fraction of D8m/Cllb lamellar structure in the alloy is very low. This may be due to 12 h at 1200 ◦ C is insufficient to promote atomic diffusion for the formation of lamellar structure. When annealing at 1200 ◦ C for 24 and 48 h, a full lamellar structure with fine spacing of the order of 100 nm was formed. However, when the annealing time is increased to 96 h, the full lamellar structure became coarse in some areas of the alloy. This phenomenon was also observed in duplex-phase NbSi2 (C40)/MoSi2 (Cllb) crystals [16]. When annealing at 1400 ◦ C for 6 h a well-developed lamellar structure consisted of NbSi2 (C40)/MoSi2 (Cllb) was formed while when annealing at 1400 ◦ C for 24 and 168 h the lamellae was destroyed [18]. Thermal stability of the lamellar structure may be closely related to lattice strain near lamellar interfaces in the duplex-phase microstructure [18]. Further study on the main factors which affect the formation, alteration and
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Fig. 7. The relationship between annealing time and indentation toughness of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy after annealing at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively.
coarsening of lamellar structure should be carried out in detail. This work is under way. The Vickers hardness values of the hypoeutectic alloy increased with the development of lamellar structure. When annealing at 1200 ◦ C for 96 h, corresponding to the coarsening of lamellar structure in some areas of the alloy, the Vickers hardness of the sample no. 4 begins decreasing. So it can be deduced that a well-developed lamellar structure can improve the Vickers hardness of Mo5 Si3 –MoSi2 hypoeutectic alloy at room temperature. The changing of indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy shows a same rule as its Vickers hardness. So it indicates that lamellar structure improves the indentation toughness of the alloy. 4.2. Mo5 Si3 –MoSi2 eutectic alloy The microstructure and creep of directionally solidified Mo5 Si3 –MoSi2 eutectic alloy were investigated [13,14]. Lamellar structure consisted of Mo5 Si3 /MoSi2 was found. Consistent with this texture an orientation relationship consisting of [1 1 0] MoSi2 //[1 1 0] Mo5 Si3 and (1 1¯ 1) MoSi2 //(0 0 2) Mo5 Si3 was observed in directionally solidified Mo5 Si3 –MoSi2 eutectic alloy grown at pull rates between 25 and 210 mm/h. In our study, a typical eutectic microstructure with Mo5 Si3 /MoSi2 lamellar structure and primary Mo5 Si3 phase was found in as-cast eutectic alloy. The presence of primary Mo5 Si3 phase may be due to the rapid cooling speed, because a water-cooled copper crucible was
used for preparing the alloy. However, the primary can be eliminated by subsequent annealing. A full lamellar structure consisted of Mo5 Si3 /MoSi2 can be got when annealing at 1200 ◦ C for over 24 h. The mechanical properties including Vickers hardness and indentation toughness are both roughly constant with prolonging annealing time. For the same materials, its properties mainly depend on its microstructure, and the annealing hardly changed the microstructure of Mo5 Si3 –MoSi2 eutectic alloy. 4.3. Mo5 Si3 –MoSi2 hypereutectic alloy Being different with the Mo5 Si3 –MoSi2 hypoeutectic alloy, the lamellar structure was observed both before and after annealing. A full lamellar structure was not formed in hypereutectic alloy even after annealing for 96 h. The microstructure of hypereutectic alloy consisted of Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellae and monolithic MoSi2 , although the volume fraction of lamellar structure in the alloy increased with prolonging the annealing time. The Vickers hardness of hypereutectic alloy hardly changed with increasing annealing time. It can be found that the hypoeutectic alloy has higher Vickers hardness than hypereutectic alloy when a full lamellar structure was formed in the hypoeutectic alloy after annealing at 1200 ◦ C for 24, 48 and 96 h. In Mo5 Si3 –MoSi2 hypereutectic alloy, besides lamellar structure, primary MoSi2 (Cllb) phase existed in the alloy. This indicates that Mo5 Si3 (D8m)/MoSi2 (Cllb)
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lamellar structured has higher Vickers hardness than monolithic MoSi2 (Cllb) phase. According to the reports, the hardness of monolithic MoSi2 at room temperature is from 8380 to 1390 HVN with grain size from 30 to 1 m, respectively [19–23]. The existence of MoSi2 phase in Mo5 Si3 –MoSi2 hypereutectic alloy may lead its lower Vickers hardness than Mo5 Si3 –MoSi2 hypoeutectic alloy at the same condition. The Kc of the Mo5 Si3 –MoSi2 hypereutectic alloy shows no significant difference with increasing annealing time, which is the same as the Mo5 Si3 –MoSi2 eutectic alloy. But the Kc value is lower than the corresponding one of Mo5 Si3 –MoSi2 eutectic alloy. The microstructure with partial lamellar structure and MoSi2 phase of Mo5 Si3 –MoSi2 hypereutectic alloy leads to lower Kc than the Mo5 Si3 –MoSi2 eutectic alloy which almost contains a full lamellar structure.
5. Conclusions 1. Lamellar structure was observed in Mo5 Si3 –MoSi2 hypoeutectic alloy, Mo5 Si3 –MoSi2 eutectic alloy and hypereutectic alloy prepared by arc melting. However, in Mo5 Si3 –MoSi2 hypoeutectic alloy, it was only observed after annealing while in eutectic alloy and hypereutectic alloy, it was observed both before and after annealing. 2. Lamellar structure in Mo5 Si3 –MoSi2 hypoeutectic alloy developed with the prolonging of annealing time. When annealing at 1200 ◦ C for 48 h, a full lamellar structure with fine spacing of 100 nm order was formed. But when the annealing time was up to 96 h, the well-developed lamellar structure became coarse in some areas of the alloy. The microstructure of eutectic alloy was a typical eutectic one with Mo5 Si3 /MoSi2 lamellar structure and Mo5 Si3 primary phase. The Mo5 Si3 primary phase disappeared with prolonging annealing time to over 24 h at 1200 ◦ C. A full lamellar structure was not formed in Mo5 Si3 –MoSi2 hypereutectic alloy and primary MoSi2 was still observed in the alloy even after annealing at 1200 ◦ C for 96 h, although the volume fraction of lamellar structure in hypereutectic alloy increased with prolonging annealing time. 3. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy increased with the development of Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellar structure when prolonging the annealing time. But the Vickers hardness and indentation toughness begin decreasing
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when the lamellar structure become coarse in some areas of the alloy when annealing at 1200 ◦ C for 96 h. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 eutectic alloy and hypereutectic alloy were roughly constant with increasing annealing time.
Acknowledgment This work is supported by the National Natural Science Foundation of China under contract no. 50131030.
References [1] J.J. Petrovic, A.K. Vasudevan, Mater. Sci. Eng., A 261 (1999) 1. [2] T. Nakano, W. Azuma, Y. Umakoshi, Acta Mater. 50 (2002) 3731. [3] H.A. Zhang, P. Chen, M.J. Wang, X.Y. Liu, Rare Met. 21 (4) (2002) 304. [4] D.A. Berztiss, R.R. Cerchiara, E.A. Gulbransen, F.S. Pettit, G.H. Meier, Mater. Sci. Eng., A 155 (1992) 165. [5] T. Maruyama, K. Yanagihara, Mater. Sci. Eng., A 239/240 (1997) 828. [6] Y. Umakoshi, T. Sakagami, T. Hirano, T. Yamane, Acta Metall. 38 (6) (1990) 909. [7] S.A. Maloy, T.E. Mitchell, A.H. Heuer, Acta Metal. Mater. 43 (1995) 657. [8] A.B. Gokhale, G.J. Abbashian, J. Phase Equilib. 12 (1991) 493. [9] M.K. Meyer, M.J. Kramer, Intermetallics 4 (1996) 273. [10] M.K. Meyer, M. Akinc, J. Am. Ceram. Soc. 79 (1996) 938. [11] M.K. Meyer, M. Akinc, J. Am. Ceram. Soc. 79 (1996) 2763. [12] F. Chu, D.J. Thoma, K.J. Mcclellan, P. Peralta, Y. He, Intermetallics 7 (1999) 611. [13] D.P. Mason, D.C. Van Aken, J.F. Mansfield, Acta Metal. Mater. 3 (1995) 1189. [14] D.P. Mason, D.C. Van Aken, J.F. Mansfield, Acta Metal. Mater. 3 (1995) 1201. [15] J.Y. Jung, J.K. Park, Scripta Mater. 34 (10) (1996) 1581. [16] T. Nakano, M. Azuma, Y. Umakoshi, Intermetallics 6 (1998) 715. [17] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533. [18] T. Nakano, Y. Nakai, S. Maeda, Y. Umakoshi, Acta Mater. 50 (2002) 1781. [19] R. Giblal, A.K. Ghosh, D.C. Van Aken, D.J. Srolovitz, A. Basu, H. Chang, D.P. Mason, W. Yang, Mater. Sci. Eng., A 155 (1992) 147. [20] S. Maloy, A.H. Heuer, J. Lewandoski, J. Petrovic, J. Am. Ceram. Soc. 74 (1991) 2704. [21] R.K. Wade, J. Am. Ceram. Soc. 75 (1992) 1682. [22] R.B. Schwartz, S.R. Srinivasan, J.J. Petrovic, C.J. Maggiore, Mater. Sci. Eng., A 155 (1992) 75. [23] A. Bhattacharya, J. Petrovic, J. Am. Ceram. Soc. 74 (1991) 2700.
The effect of lamellar structure in Mo5Si3–MoSi2 alloy refined by annealing on the Vickers hardness at room temperature Haibo Yang∗ , Wei Li, Aidang Shan, Jiansheng Wu Key Laboratory for High Temperature Materials and Tests of Ministry of Education, P. R. China, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Received 29 May 2003; received in revised form 17 May 2004; accepted 10 September 2004 Available online 28 November 2004
Abstract Mo5 Si3 –MoSi2 eutectic alloy, hypoeutectic alloy and hypereutectic alloy were prepared by arc-melting pure raw materials of Mo and Si. Annealing at 1200 ◦ C for different times was conducted on all of the alloys to investigate the effect of annealing on the microstructure and mechanical property including Vickers hardness and indentation toughness at room temperature. Lamellar structure consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase was observed in all the alloys. For Mo5 Si3 –MoSi2 eutectic alloy, a typical eutectic microstructure with primary Mo5 Si3 phase and Mo5 Si3 /MoSi2 lamellar structure was observed, and the primary Mo5 Si3 phase was eliminated when annealing at 1200 ◦ C for over 24 h. For Mo5 Si3 –MoSi2 hypoeutectic alloy, the lamellar structure was found only after annealing. A full lamellar structure throughout the alloy developed well with fine spacing on the order of 100 nm after annealing at 1200 ◦ C for 48 h. For Mo5 Si3 –MoSi2 hypereutectic alloy, the lamellar structure was found both before and after annealing. Primary MoSi2 was observed in the alloy even after annealing at 1200 ◦ C for 96 h, although the volume fraction of lamellar structure was increased with prolonging annealing time. The effect of the formation, development and destruction of lamellar structure on Vickers hardness and indentation toughness of alloys was investigated. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy show a same regularity, that is, they increase with prolonging annealing time and development of lamellar structure. The highest Vickers hardness and indentation toughness are about 1470 HVN and 4.8 MPa m1/2 , respectively. The Vickers hardness and indentation of Mo5 Si3 –MoSi2 eutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy are all roughly constant with prolonging annealing time, that is, about 1400 and 1270 HVN for Vickers hardness and 4 and 3.2 MPa m1/2 for indentation toughness. © 2004 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Mo–Si; Lamellar structure; Anneal; Microstructure; Mechanical property
1. Introduction In order to meet the ever-increasing demand for operating temperature capability of structural components, a considerable amount of research has been focused on developing new materials. Over the past 20 years, many alloy systems and processing routes were investigated to select potential intermetallic compound for applications at high temperatures. Nickel aluminide and titanium aluminide are attractive because these materials exhibit significant room temperature ductility compared to other intermetallics. In terms of candi∗
Corresponding author. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.029
date materials for high temperature, oxidation-resistant structural applications, there is a temperature cut off at ≈1000 ◦ C. Below 1000 ◦ C, it is possible to use nickel and cobalt-base superalloys and aluminide intermetallics. However, above 1000 ◦ C, for oxidation and strength reasons, one must shift to the silicon-based ceramics, advanced intermetallics, and silicide-based materials [1–3]. Recent studies show that the refractory metal silicides are of great potential to be candidates for structural material at ultra high temperature [1–3]. Among the refractory metal silicides, the intermetallic phases in the Mo–Si system show promising potentials because Mo has a lower density compared to other refractory metals and Mo does not embrittle with oxygen and nitrogen volatilization [4]. In the Mo–Si
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system, MoSi2 with the Cllb structure has been studied extensively and has found several significant applications. MoSi2 with the Cllb structure is a promising candidate as a matrix phase because of its superior oxidation resistance [5,6] and potential plastic deformability at low temperature [7,8]. Unfortunately, the inadequacy of low-temperature fracture toughness and high-temperature strength above 1200 ◦ C in the monolithic condition limits it for a practical structural application. Recently, another molybdenum silicide, Mo5 Si3 with the D8m structure is attracting more attention of researchers because of its higher melting point, better high temperature creep property than MoSi2 [9,10] and the dramatically improved oxidation resistance with the addition of B [11,12]. Mason et al. [13,14] investigated the microstructures and crystallography as well as the creep property of directionally solidified MoSi2 –Mo5 Si3 eutectics. The Mo5 Si3 lamellae were found to grow with an inclination of 15◦ relative to the axis of growth and the creep resistance of directionally solidified MoSi2 –Mo5 Si3 eutectics was found to increase with decreasing Mo5 Si3 lamellar spacing. In fact, the study on Ti–Al alloys has suggested that the enhanced fracture toughness and creep properties are attributed to the fine fully lamellar (FL) microstructure of the alloys [15]. The study [16] shows that MoSi2 -based duplex silicides consisted of MoSi2 (Cllb)/NbSi2 (C40) lamellae have higher strength at high temperature than binary MoSi2 since the lamellar boundary and/or lamellar colony act as an effective barrier to the motion of dislocations. Thus, the knowledge of the lamellar structure in MoSi2 –Mo5 Si3 alloys is helpful to understand their mechani-
cal properties. In this research, microstructures of arc-melted Mo5 Si3 –MoSi2 hypoeutectic alloy and hypereutectic alloy were investigated focusing on formation of the lamellar structure. The effect of the lamellar structure on the Vickers hardness and indentation toughness at room temperature of the alloys was also studied.
2. Experimental procedure Mo5 Si3 –MoSi2 hypoeutectic alloy (Mo–45 at.% Si), eutectic alloy (Mo–54 at.% Si) and hypereutectic alloy (Mo–60 at.% Si) were prepared by arc-melting high-purity elements (Mo—99.9% and Si—99.99%) in a water-cooled copper crucible in an argon atmosphere. The alloys annealed at 1200 ◦ C under a high-purity argon gas flow for 12, 24, 48 and 96 h, respectively. After metallographic polishing, the specimens were etched with 20 vol.% hydrochloric acid + 20 vol.% nitric acid for 10 s. The etched specimens were observed in a scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) system for determining the phases present in the alloys and the chemical composition in constituent phases. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) operated at 200 kV were performed to analyze the microstructures of the alloys annealed at 1200 ◦ C for different times. The mechanical properties of Vickers hardness were evaluated by using a Vickers indenter at 300 g load for 15 s at the room temperature. Indentation toughness (Kc ) was determined by measuring the lengths of radial cracks emanating
Fig. 1. X-ray diffraction patterns of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy and (b) Mo5 Si3 –MoSi2 hypereutectic alloy.
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Fig. 2. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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from the Vickers indents, using the following relation [17]. 1/2 P E Kc = A H c3/2 where A is a constant of value 0.016 [17], E the Young’s modulus, H the hardness, P the load and c is half-crack length.
3. Results 3.1. Constituent phases in Mo5 Si3 –MoSi2 hypoeutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy Fig. 1 shows X-ray diffraction profiles from samples of arc-melted ingots with compositions of Mo–45 at.% Si and Mo–60 at.% Si. Only two phases, Mo5 Si3 with D8m structure and MoSi2 with Cllb structure, were identified in both alloys. The eutectic alloy is also consisted of Mo5 Si3 and MoSi2 phase, although the X-ray diffraction pattern is not represented here. All peaks correspond to those reflected from the D8m structure or the Cllb structure, while relative diffraction intensity of the D8m phase to the Cllb depends on the alloy composition. This data suggests that the volume fraction of MoSi2 phase in hypoeutectic alloy is less than that in hypereutectic alloy. 3.2. Microstructures Fig. 2 shows the scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively. These alloys were denoted as sample nos. 0–4, respectively. Some cracks were observed in sample no. 1. The microstructures of the sample no. 0 (Fig. 2(a)) were remarkably changed after annealing. Except the sample no. 1 annealed at 1200 ◦ C for 12 h (Fig. 2(b)), all other samples show a full lamellar structure. Sample nos. 2 and 3, annealed at 1200 ◦ C for 24 and 48 h, respectively, exhibited full lamellar structures with fine spacing
of 100 nm order, as shown in Fig. 3. However, their arrangement disturbed at the colony boundaries (Fig. 2(c and d)). The lamellar structure was consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase. However, with the increasing time of annealing, the full lamellar structure became coarse in some areas of the alloy, as shown in Fig. 2(e). Annealing at 1200 ◦ C for 0, 24, 48 and 96 h was conducted on Mo5 Si3 –MoSi2 eutectic alloy, and the samples were denoted as sample no. 0 –4 , respectively. A typical eutectic microstructure consisted of primary Mo5 Si3 phase and lamellar structure was observed in the sample no. 0 , as shown in Fig. 4(a). After annealing at 1200 ◦ C for 12 h, the grain size of primary Mo5 Si3 phase decreased, as shown in Fig. 4(b). Prolonging annealing time, the primary Mo5 Si3 phase disappeared, and microstructure of alloy shown a Mo5 Si3 /MoSi2 eutectic lamellar structure when annealing at 1200 ◦ C for 24, 48 and 96 h, as shown in Fig. 4(c–e). Fig. 5 shows scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypereutectic alloy annealed at 1200 ◦ C for 0, 24, 48 and 96 h. These samples were denoted as sample nos. 0 –4 , respectively. A lamellar structure consisted of Mo5 Si3 (D8m) phase and MoSi2 (Cllb) phase was found in the sample no. 0 (Fig. 5(a)) of Mo5 Si3 –MoSi2 hypereutectic alloy without annealing, which was different with the sample no. 0 of Mo5 Si3 –MoSi2 hypoeutectic alloy without annealing. After annealing at 1200 ◦ C for different times, the microstructure of hypereutectic alloy was not changed essentially; however, the volume fraction of lamellae increased with the increasing annealing time. Unlike hypoeutectic alloy, besides the Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellae, primary MoSi2 was still observed in hypereutectic alloy after annealing at 1200 ◦ C for 0, 24, 48 and 96 h. In addition, some cracks were observed in sample nos. 0 , 1 , 3 and 4 (Fig. 5(a, b, d and e)). 3.3. Mechanical properties Fig. 6 shows the Vickers hardness values of the (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2
Fig. 3. TEM micrograph of Mo5 Si3 –MoSi2 hypoeutectic alloy annealed at 1200 ◦ C for 48 h.
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Fig. 4. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 eutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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Fig. 5. Scanning electron micrographs of samples Mo5 Si3 –MoSi2 hypereutectic alloy annealed at 1200 ◦ C for (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h and (e) 96 h.
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Fig. 6. The relationship between annealing time and Vickers hardness of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy after annealing at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively.
eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy at room temperature as a function of annealing time. Sample no. 3 annealed at 1200 ◦ C for 48 h in Mo5 Si3 –MoSi2 hypoeutectic alloy has the highest hardness value of 1457 HVN, which was improved about 19% compared with that of same no. 0 without annealing. The hardness of sample no. 0 is 1262 HVN. The Vickers hardness of eutectic alloy is nearly constant with increasing annealing time, and in the range of 1325–1360 HVN. Sample no. 3 annealed at 1200 ◦ C for 48 h in Mo5 Si3 –MoSi2 hypereutectic alloy has the highest hardness value about 1300 HVN, which was increased only about 2.7% compared with that of the sample no. 0 without annealing. The hardness of the sample no. 0 is 1266 HVN. The average Kc values of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy as a function of annealing time are shown in Fig. 7. A rapid increase in Kc is observed in Mo5 Si3 –MoSi2 hypoeutectic alloy from 2 to 4.8 MPa m1/2 . The Kc of Mo5 Si3 –MoSi2 eutectic alloy and Mo5 Si3 –MoSi2 hypereutectic alloy are roughly constant in the range of 3.7–4.5 and 2–2.5 MPa m1/2 , respectively.
4. Discussion 4.1. Mo5 Si3 –MoSi2 hypoeutectic alloy The lamellar structure was formed in arc-melting Mo5 Si3 –MoSi2 hypoeutectic alloy during subsequent annealing. After annealing at 1200 ◦ C for 12 h, the volume fraction of D8m/Cllb lamellar structure in the alloy is very low. This may be due to 12 h at 1200 ◦ C is insufficient to promote atomic diffusion for the formation of lamellar structure. When annealing at 1200 ◦ C for 24 and 48 h, a full lamellar structure with fine spacing of the order of 100 nm was formed. However, when the annealing time is increased to 96 h, the full lamellar structure became coarse in some areas of the alloy. This phenomenon was also observed in duplex-phase NbSi2 (C40)/MoSi2 (Cllb) crystals [16]. When annealing at 1400 ◦ C for 6 h a well-developed lamellar structure consisted of NbSi2 (C40)/MoSi2 (Cllb) was formed while when annealing at 1400 ◦ C for 24 and 168 h the lamellae was destroyed [18]. Thermal stability of the lamellar structure may be closely related to lattice strain near lamellar interfaces in the duplex-phase microstructure [18]. Further study on the main factors which affect the formation, alteration and
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Fig. 7. The relationship between annealing time and indentation toughness of (a) Mo5 Si3 –MoSi2 hypoeutectic alloy, (b) Mo5 Si3 –MoSi2 eutectic alloy and (c) Mo5 Si3 –MoSi2 hypereutectic alloy after annealing at 1200 ◦ C for 0, 12, 24, 48 and 96 h, respectively.
coarsening of lamellar structure should be carried out in detail. This work is under way. The Vickers hardness values of the hypoeutectic alloy increased with the development of lamellar structure. When annealing at 1200 ◦ C for 96 h, corresponding to the coarsening of lamellar structure in some areas of the alloy, the Vickers hardness of the sample no. 4 begins decreasing. So it can be deduced that a well-developed lamellar structure can improve the Vickers hardness of Mo5 Si3 –MoSi2 hypoeutectic alloy at room temperature. The changing of indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy shows a same rule as its Vickers hardness. So it indicates that lamellar structure improves the indentation toughness of the alloy. 4.2. Mo5 Si3 –MoSi2 eutectic alloy The microstructure and creep of directionally solidified Mo5 Si3 –MoSi2 eutectic alloy were investigated [13,14]. Lamellar structure consisted of Mo5 Si3 /MoSi2 was found. Consistent with this texture an orientation relationship consisting of [1 1 0] MoSi2 //[1 1 0] Mo5 Si3 and (1 1¯ 1) MoSi2 //(0 0 2) Mo5 Si3 was observed in directionally solidified Mo5 Si3 –MoSi2 eutectic alloy grown at pull rates between 25 and 210 mm/h. In our study, a typical eutectic microstructure with Mo5 Si3 /MoSi2 lamellar structure and primary Mo5 Si3 phase was found in as-cast eutectic alloy. The presence of primary Mo5 Si3 phase may be due to the rapid cooling speed, because a water-cooled copper crucible was
used for preparing the alloy. However, the primary can be eliminated by subsequent annealing. A full lamellar structure consisted of Mo5 Si3 /MoSi2 can be got when annealing at 1200 ◦ C for over 24 h. The mechanical properties including Vickers hardness and indentation toughness are both roughly constant with prolonging annealing time. For the same materials, its properties mainly depend on its microstructure, and the annealing hardly changed the microstructure of Mo5 Si3 –MoSi2 eutectic alloy. 4.3. Mo5 Si3 –MoSi2 hypereutectic alloy Being different with the Mo5 Si3 –MoSi2 hypoeutectic alloy, the lamellar structure was observed both before and after annealing. A full lamellar structure was not formed in hypereutectic alloy even after annealing for 96 h. The microstructure of hypereutectic alloy consisted of Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellae and monolithic MoSi2 , although the volume fraction of lamellar structure in the alloy increased with prolonging the annealing time. The Vickers hardness of hypereutectic alloy hardly changed with increasing annealing time. It can be found that the hypoeutectic alloy has higher Vickers hardness than hypereutectic alloy when a full lamellar structure was formed in the hypoeutectic alloy after annealing at 1200 ◦ C for 24, 48 and 96 h. In Mo5 Si3 –MoSi2 hypereutectic alloy, besides lamellar structure, primary MoSi2 (Cllb) phase existed in the alloy. This indicates that Mo5 Si3 (D8m)/MoSi2 (Cllb)
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lamellar structured has higher Vickers hardness than monolithic MoSi2 (Cllb) phase. According to the reports, the hardness of monolithic MoSi2 at room temperature is from 8380 to 1390 HVN with grain size from 30 to 1 m, respectively [19–23]. The existence of MoSi2 phase in Mo5 Si3 –MoSi2 hypereutectic alloy may lead its lower Vickers hardness than Mo5 Si3 –MoSi2 hypoeutectic alloy at the same condition. The Kc of the Mo5 Si3 –MoSi2 hypereutectic alloy shows no significant difference with increasing annealing time, which is the same as the Mo5 Si3 –MoSi2 eutectic alloy. But the Kc value is lower than the corresponding one of Mo5 Si3 –MoSi2 eutectic alloy. The microstructure with partial lamellar structure and MoSi2 phase of Mo5 Si3 –MoSi2 hypereutectic alloy leads to lower Kc than the Mo5 Si3 –MoSi2 eutectic alloy which almost contains a full lamellar structure.
5. Conclusions 1. Lamellar structure was observed in Mo5 Si3 –MoSi2 hypoeutectic alloy, Mo5 Si3 –MoSi2 eutectic alloy and hypereutectic alloy prepared by arc melting. However, in Mo5 Si3 –MoSi2 hypoeutectic alloy, it was only observed after annealing while in eutectic alloy and hypereutectic alloy, it was observed both before and after annealing. 2. Lamellar structure in Mo5 Si3 –MoSi2 hypoeutectic alloy developed with the prolonging of annealing time. When annealing at 1200 ◦ C for 48 h, a full lamellar structure with fine spacing of 100 nm order was formed. But when the annealing time was up to 96 h, the well-developed lamellar structure became coarse in some areas of the alloy. The microstructure of eutectic alloy was a typical eutectic one with Mo5 Si3 /MoSi2 lamellar structure and Mo5 Si3 primary phase. The Mo5 Si3 primary phase disappeared with prolonging annealing time to over 24 h at 1200 ◦ C. A full lamellar structure was not formed in Mo5 Si3 –MoSi2 hypereutectic alloy and primary MoSi2 was still observed in the alloy even after annealing at 1200 ◦ C for 96 h, although the volume fraction of lamellar structure in hypereutectic alloy increased with prolonging annealing time. 3. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 hypoeutectic alloy increased with the development of Mo5 Si3 (D8m)/MoSi2 (Cllb) lamellar structure when prolonging the annealing time. But the Vickers hardness and indentation toughness begin decreasing
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when the lamellar structure become coarse in some areas of the alloy when annealing at 1200 ◦ C for 96 h. The Vickers hardness and indentation toughness of Mo5 Si3 –MoSi2 eutectic alloy and hypereutectic alloy were roughly constant with increasing annealing time.
Acknowledgment This work is supported by the National Natural Science Foundation of China under contract no. 50131030.
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