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Effects of lanthanum oxide content on mechanical properties of mechanical alloying Mo–12Si–8.5B (at.%) alloys
Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of lanthanum oxide content on mechanical properties of mechanical alloying Mo–12Si–8.5B (at.%) alloys Guojun Zhang a,b,⁎, Yang Zha a, Bin Li b, Wei He a, Jun Sun b a b
School of Materials Science & Engineering, Xi'an University of Technology, Xi'an 710048, China State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
a r t i c l e
i n f o
Article history: Received 6 December 2012 Accepted 22 July 2013 Keywords: Mo–12Si–8.5B alloys Lanthanum oxide Mechanical alloying Mechanical properties
a b s t r a c t Mo–12Si–8.5B alloys with different mass fraction (0 wt.%, 0.3 wt.%, 0.9 wt.%, 1.5 wt.%, 2.5 wt.%) lanthanum oxide (La2O3) additive were fabricate by using mechanical alloying and hot pressing sintering techniques. XRD results indicate that the doped Mo–12Si–8.5B alloys are consisted mainly of α-Mo, Mo3Si and Mo5SiB2 (T2) phases. It is found that addition of La2O3 has little effect on both the grains and intermetallic sizes. The intermetallic particles are more homogeneously dispersed within α-Mo matrix by increasing the La2O3 content. Compression test and three-point bending test showed that both compression and flexure strength of alloys were obviously improved by the addition of La2O3. The predominant strengthening mechanisms are fine-grain strengthening and particles dispersion strengthening. In addition, fracture toughness tests showed that the fracture toughness values of all alloys were on the order of 9 MPa·m1/2 that revealed the addition of La2O3 had little effect on improving the alloys toughness. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Structural materials used in high-temperature environment should have adequate strength, good oxidation resistance, good creep resistance, and good fracture toughness. In order to achieve all these desired properties, new materials must be developed. For decades Mo–Si–B alloys have been extensively studied because of their adequate strength, good oxidation resistance, and good creep resistance [1–3]. At present Mo–Si–B alloys mainly refer to Mo3Si, Mo5SiB2 (T2), Mo5Si3 (T1) or αMo solid solution, as well as Mo3Si and Mo5SiB2 (T2) three phase alloys. Depending on the exact composition, the alloys consisting of Mo3Si, T1 and T2 have higher oxidation resistance than that consisting of α-Mo, Mo3Si and T2. However, the fracture toughness of Mo3Si–T1–T2 alloys is likely to exhibit low (2–4 MPa·m1/2) [2,4] due to three brittle phases. On the other hand, the alloys in the α-Mo–Mo3Si–T2 system are expected to have higher fracture toughness (5–21 MPa·m1/2) [2] than the Mo3Si–T1–T2 alloys because of the presence of a ductile α-Mo phase. The recent work aims to increase the fracture toughness of Mo–Si–B alloy and keep their original good strength and oxidation resistance. In order to maintain a good oxidation resistance, the volume fraction of intermetallics should be up to 50 vol.% [5]. Mo–12Si–8.5B (at.%) consisting
⁎ Corresponding author at: School of Materials Science & Engineering, Xi'an University of Technology, Xi'an 710048, China. Tel.: +86 2982312592. E-mail addresses: [email protected], [email protected] (G. Zhang). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.07.011
of approximately 38 vol.% α-Mo, 32 vol.% Mo3Si and 30 vol.% T2 is a candidate alloy for this study where the volume fraction of intermetallic phases is up to 62%, and this alloy exhibits a good oxidation resistance studied by Mendiratta et al. [5]. Besides excellent oxidation resistance, the alloys should also have adequate strength and good fracture toughness. The room temperature fracture toughness of Mo–12Si–8.5B alloy was on the order of 5–7 MPa·m1/2 in Choe et al.'s work [6] which used both ingot and power metallurgy methods. The room temperature flexure strength and fracture toughness of Mo–12Si–8.5B were 384 MPa and 14 MPa·m1/2 respectively in Schneibel et al.'s work which used a powder metallurgical production method [1]. It indicated that the alloys with the α-Mo matrix showed higher fracture toughness than the alloys with the intermetallic matrix. Moreover, Mo–Si–B alloys with finegrained microstructure have good oxidation resistance as studied by Rioult et al. [7]. The fracture toughness and the oxidation resistance of alloys should also be improved if the alloys showed such a microstructure [8,9]. It has been known that the doping of some reactive element in Nibased superalloys can enhance the mechanical properties remarkably. As one class of the reactive elements, the rare earth oxides have been given special attention and are widely used [10–12]. In Zhang et al.'s work [13], La2O3 doped Mo–12Si–8.5B alloys have good oxidation resistance. Whereas, there were limited reports on the effects of La2O3 on the microstructure and the mechanical properties of Mo–12Si–8.5B alloys. In this paper, mechanical alloying (MA) and hot pressing sintering technique were used to prepare different mass fraction of La2O3 doped Mo–12Si–8.5B alloys. The present work will investigate the effects of doped La2O3 on the mechanical properties of Mo–12Si–8.5B alloys and discuss the strengthening and toughening mechanisms.
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G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
Table 1 Composition of the commercial-purity Mo. Composition
Fe
Ni
Al
Si
Ca
Mg
P
C
O
N
Mo
Weight (ppm)
14
0.79
0.69
2.3
4.0
0.82
b10
9.6
34
2.4
Balance
2. Experimental procedures The Mo–12Si–8.5B alloys with different mass fraction La2O3 were fabricated by mechanical alloying (MA) and hot pressing sintering technology. Elemental powders of Mo, Si, B and La2O3 were 99.95 wt.%, 99.9 wt.%, 99.8 wt.%, and 99.9 wt.% purity, respectively. The composition analyses of commercial-purity Mo are shown in Table 1. These powders were firstly mixed in a planetary ball mill (Retch PM100) with a speed of 200 rpm with a powder to ball weight ratio of 2:1 under protective (argon) atmosphere for 10 h. Then these fine powders were used to conduct mechanical alloying for 20 h. Mechanical alloying used PM100 under protective (argon) atmosphere at a high turning speed (300 rpm) and a large powder to ball weight ratio (1:10). The powder morphologies were observed by scanning electron microscopy (SEM) before and after mechanical alloying shown respectively in Fig. 1(a) and (b), which shown a typical microstructure with complete homogenization [9] after mechanical alloying. The relevant statistical analysis of the particle sizes was used to identify the particle size of powders by laser light scattering analyzer as shown in Fig. 1(c) and (d). The mechanical alloying refined the powder particles which decreased the median diameter of particles from 5.88 μm to 2.77 μm after 20 h mechanical alloying. After mechanical alloying, the powders
were prepared by hot pressing sintering (pressure = 50 MPa, vacuum degree b 10−3 Pa) at 1600 °C for 3 h. Microstructure analysis was carried out by optical metallography of polished specimens etched with Murakami's etch (an aqueous solution of potassium ferricyanide and sodium hydroxide). Phases were identified by X-ray diffraction (XRD). The average densities of the alloys by Archimedes drainage method were listed in Table 2. Room temperature compressive tests were performed at a strain rate of 5 × 10− 4 s− 1 using an HT-2402 universal testing machine. At least three specimens were tested and the sample sizes were 5 mm × 5 mm × 10 mm. The flexure strength values were determined by the three-point bending tests. Flexure specimens with a cross section of 3 mm × 4 mm and a length of 26 mm were electro-discharge machined and ground, subsequently tested in a universal testing machine (Instron 1185) in a three-point bend fixture with a 20 mm span at ambient temperature. The cross-hand speed in flexure strength test was 8 μm/s. Fracture toughness values were determined by three-point bending tests using tip notched specimens. The specimens with a cross section of 3 mm × 4 mm and a length of 26 mm with a 2 mm depth notch tip in the middle of the sample. The loading rate was 0.8 μm/s for fracture toughness test where the fracture toughness values were determined based on an energy criterion by integrating the load–displacement curve where W was the energy absorbed during the fracture of the area A swept out by the crack. The fracture toughness was determined as G = W / A. Assuming the material to be linearly elastic, the fracture energy (energy criterium) can also be expressed in terms of the stress intensity (stress criterium) by Kq = (G × E′)1/2, where
Fig. 1. The morphologies of powders with a composition of Mo–12Si–8.5B of (a) mixed powders and (b) MA for 15 h. (c) and (d) show the size distributions of the powder particle for mixed and MA powders respectively.
G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
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Table 2 The densities of La2O3 doped Mo–12Si–8.5B alloys. Alloy composition
Mo–12Si–8.5B
Mo–12Si–8.5B + 0.3 wt.% La2O3
Mo–12Si–8.5B + 0.9 wt.% La2O3
Mo–12Si–8.5B + 1.5 wt.% La2O3
Mo–12Si–8.5B + 2.5 wt.% La2O3
Compression strength (MPa) Flexure strength (MPa) Fracture toughness (MPa·m1/2)
2549
2726
2705
2710
2732
1203 9.132
1325 9.337
1337 8.836
1444 9.486
1300 9.259
E′ = E / (1 − ν2) is the plane strain Young's modulus and ν is Poisson's ratio (E = 327 GPa and ν = 0.29) [1]. 3. Result and discussion 3.1. XRD analysis and microstructure characterization The results of X-ray diffraction analysis are shown in Fig. 2. These alloys are all composed of α-Mo, Mo3Si and T2. No other phases are found in these patterns which indicate that the phase composition is not obviously affected by doping the La2O3 particles. Representative micrographs are given in Fig. 3 for the alloys. The αMo, Mo3Si and T2 are the brightest appearing phase, the slightly darker gray phase and the dark gray appearing phase, respectively. One can clearly see that the alloys have a continuous α-Mo matrix with dispersive intermetallic (gray or black) particles distributing within matrix, which is similar to Kruger et al.'s study [9]. The grain sizes of intermetallics and α-Mo were reduced by increasing the La2O3 contents, and the intermetallic phases were more homogeneously distributed within the α-Mo matrix with the La2O3 contents increased from 0.0 wt.% to 2.5 wt.%. 3.2. Mechanical properties 3.2.1. Compression and flexure strength During compression test, the alloys display negligible ductility and fracture in a brittle manner. The results of the room temperature compression strengths are presented in Table 3. These La2O3 doped alloys exhibit a higher compression strength than the Mo–Si–B alloy. The Mo–12Si–8.5B + 2.5 wt.% La2O3 alloy exhibits the highest compression strength (2732 MPa) compared with what other alloys exhibit.
Fig. 2. XRD patterns of Mo–12Si–8.5B alloys with different mass fraction La2O3.
The flexure strengths are also increased by doping La2O3 into Mo–12Si–8.5B alloys (Table 3). The flexure strengths increase from 1203 MPa for the Mo–12Si–8.5B alloy to 1444 MPa for the Mo– 12Si–8.5B + 1.5 wt.% La2O3 alloy. Schneibel et al. [1] had reported that the flexural strengths of Mo–12Si–8.5B alloys were in the range of 380 MPa–539 MPa prepared by arc-melting and powdermetallurgical processing, which were lower than the present results. 3.2.2. Fracture toughness During fracture toughness testing at room temperature, prior to outright fracture no stable crack growth was observed in all fine-grained alloys. The samples fractured immediately at the onset of crack initiation. The fracture surface morphologies show the intergranular fracture to be the dominant failure mechanism in Fig. 4. The fracture toughness value in the present work is designated as Kq rather than KIC for the reason of that fracture toughness KIC supposed to be overestimated by approximately 25% in the chevron-notch technique [1]. The fracture toughness values (Kq) of the alloys are shown in Table 3. One can see that the fracture toughness values did not increase with the doping of La2O3. The reason may be that all the three phases in Mo–12Si–8.5B alloy are brittle at room temperature and the effect of La2O3 on improving the fracture toughness is limited. As a comparison, these alloys exhibit higher (~30%) room temperature fracture toughness than that of Choe et al.'s alloys (7 MPa·m1/2) [6]. 3.3. Strengthening and toughing mechanisms 3.3.1. Strengthening mechanisms Refer to the microstructure characteristics, the grain sizes of both intermetallics and α-Mo are reduced by the doping of La2O3. The reason responsible for the grain refinement may be that the La2O3 particles can act as nucleation sites and hence increase the nucleation rate during the sintering process, and some La2O3 particles may also exist at the molybdenum grain boundaries to hinder the grain growth via pinning effect. Both the pinning and the nucleation effects in La2O3 doped alloys decrease the grain size of α-Mo. According to the effect of fine grain strengthening mechanism, the La2O3 doped alloys exhibit a higher strength. The well-known Hall–Petch equation also indicates that the contribution of the grain size to strength will be improved with the reduction in grain size. In addition, the intermetallic particles of the alloys are more dispersely and uniformly distributed in the matrix by the doping of the La2O3. The particles distributed in the α-Mo matrix as well as the La2O3 particles can play the role of dispersion strengthening by hindering grain slip. Both the intermetallic and the La2O3 particles as dispersion phases increase the alloy strengths. 3.3.2. Toughing mechanisms The α-Mo phase is known well as a ductile phase compared with the two intermetallic Mo3Si and T2 in Mo–12Si–8.5B alloy. The toughening of the brittle intermetallic phases with a ductile phase can be achieved through extrinsic toughening mechanisms such as crack trapping and bridging. Choe et al. [2,6] studied the crack expansion behavior of Mo– 12Si–8.5B alloys and showed that the main toughening mechanism of Mo–12Si–8.5B alloys was ‘crack trapping’ by ductile α-Mo phase at
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Fig. 3. Optical micrographs of alloys: (a) Mo–12Si–8.5B; (b) Mo–12Si–8.5B + 0.3 wt.% La2O 3; (c) Mo–12Si–8.5B + 0.9 wt.% La2O3; (d) Mo–12Si–8.5B + 1.5 wt.% La2O3 and (e) Mo–12Si–8.5B + 2.5 wt.% La2O3.
ambient temperature. In the crack trapping mechanism when a crack tip propagates to α-Mo grain, some plastic deformation occurred and the stress concentration of the crack tip was released and therefore the alloy toughness was improved [14,15]. In Zhang et al.'s work [12], La2O3 was doped into the Mo alloys and the KIC was improved to 24.76 MPa·m1/2 which was obviously better than 9.78 MPa·m1/2 of the pure Mo. In this work, the intermetallic phases are refined and distributed more homogeneously with the doping of La2O3 and when a crack occurred in the more homogeneously dispersed intermetallic particle, the crack is more easily trapped by ductile α-Mo matrix which leads to the increase of the fracture toughness. Because the three phases in Mo–12Si–8.5 alloys are all brittle at room temperature, consequently the effect of extrinsic crack trapping mechanism on improving the fracture toughness is limited.
4. Conclusions Different mass fraction La2O3 doped Mo–12Si–8.5B alloys were fabricated by mechanical alloying and hot pressing sintering techniques. It was a feasible technique for producing Mo–12Si–8.5B alloy with a continuous α-Mo matrix. The following conclusions may be made. (1) The La2O3 doped Mo–12Si–8.5B alloys exhibit a finer and more uniform microstructure. During doping the La2O3 into alloys, the grain size and the intermetallic particles size were refined and distributed more homogeneously within the matrix. (2) Both the compression and the flexure strength of Mo–12Si–8.5B are increased with the doping of La2O3. The major strengthening mechanisms are the fine grain strengthening and the particle dispersion strengthening.
G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
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Table 3 The mechanical properties of La2O3 doped Mo–12Si–8.5B alloys at room temperature. Alloy composition
Mo–12Si–8.5B
Mo–12Si–8.5B + 0.3 wt.% La2O3
Mo–12Si–8.5B + 0.9 wt.% La2O3
Mo–12Si–8.5B + 1.5 wt.% La2O3
Mo–12Si–8.5B + 2.5 wt.% La2O3
Density ρ (kg·m−3)
8.9105 × 103
8.8350 × 103
8.9116 × 103
8.8159 × 103
8.8187 × 103
No. 2012BAE06B02) and the State Key Laboratory for Mechanical Behavior of Materials. References
Fig. 4. The fracture surface morphology of Mo–12Si–8.5B + 2.5 wt.% La2O3.
(3) The fracture toughness values of the alloys are on the order of 9 MPa·m1/2 and addition of La2O3 has little effect on improving the fracture toughness, which is restricted by the lower ductility of the three phases in Mo–12Si–8.5B alloy at room temperature. Acknowledgments This subject was supported by the National Natural Science Foundation (Grant Nos. 51171149 and 50801051), the Program for New Century Excellent Talents in University (Grant No. NCET10-0876), the 973 Program of China (Grant No. 2012CB619600), the National Science Technology Supporting Program of China (Grant
[1] Schneibel JH, Kramer MJ, Unal O, Wright RN. Processing and mechanical properties of a molybdenum silicide with the composition Mo–12Si–8.5B. Intermetallics 2001;9:25–31. [2] Choe H, Chen D, Schneibel JH, Ritchie RO. Ambient to high temperature fracture toughness and fatigue-crack propagation behavior in a Mo–12Si–8.5B (at.%) intermetallic. Intermetallics 2001;9:319–29. [3] Yoshimi K, Nakatani S, Suda T, Hanada S, Habazaki H. Oxidation behavior of Mo5SiB2-based alloy at elevated temperature. Intermetallics 2002;10:407–14. [4] Thom AJ, Summers E, Akinc M. Oxidation behavior of extruded Mo5Si3Bx–MoSi2–MoB intermetallics from 600 °C–1600 °C. Intermetallics 2002;10:555–70. [5] Mendiratta MG, Parthasarathy TA, Dimiduk DM. Oxidation behavior of αMo– Mo3Si–Mo5SiB2 (T2) three phase system. Intermetallics 2002;10:225–32. [6] Choe H, Schneibel JH, Ritchie RO. On the fracture and fatigue properties of Mo– Mo3Si–Mo5SiB2 refractory intermetallic alloys at ambient to elevated temperature (25 °C to 1300 °C). Metall Mater Trans A 2003;34:225–39. [7] Rioult FA, Imhoff SD, Sakidja R, Perepezko JH. Transient oxidation of Mo–Si–B alloys: effect of the microstructure size scale. Acta Mater 2009;57:4600–13. [8] Abbasi AR, Shamanian M. Synthesis of α-Mo–Mo5SiB2–Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment. J Alloys Compd 2011;509:8097–104. [9] Krüger M, Franz S, Saage H, Heilmaier M, Schneibel JH, Jéhanno P, et al. Mechanically alloyed Mo–Si–B alloys with a continuous α-Mo matrix and improved mechanical properties. Intermetallics 2008;16:933–41. [10] Katrych S, Grytsiv A, Bondar A, Rogl P, Velikanova T, Bohn M. Structural materials: metal–silicon–boron: on the melting behavior of Mo–Si–B alloys. J Alloys Compd 2002;347:94–100. [11] Zhang GJ, Dang Q, Kou H, Wang RH, Liu G, Sun J. Microstructure and mechanical properties of lanthanum oxide-doped Mo–12Si–8.5B(at%) alloys. J Alloys Compd 2013 http://dx.doi.org/10.1016/j.jallcom.2012.03.037 [in press]. [12] Zhang JX, Liu L, Zhou ML, Hu YC, Zuo TY. Fracture toughness of sintered Mo–La2O3 alloy and the toughening mechanism. Int J Refract Met Hard Mater 1999;17:405–9. [13] Zhang GJ, Kou H, Dang Q, Liu G, Sun J. Microstructure and oxidation resistance behavior of lanthanum oxide-doped Mo–12Si–8.5B alloys. Int J Refract Met Hard Mater 2012;30: 6–11. [14] Schneibel JH, Kramer MJ, Easton DS. A Mo–Si–B intermetallic alloy with a continuous α-Mo matrix. Scr Mater 2002;46:217–21. [15] Joseph LA, Michael RM, Tobias W, Bernd G, Joe KC, Robert OR. On the fracture toughness of fine-grained Mo–3Si–1B (wt.%) alloys at ambient to elevated (1300 °C) temperatures. Intermetallics 2012;20:141–54.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of lanthanum oxide content on mechanical properties of mechanical alloying Mo–12Si–8.5B (at.%) alloys Guojun Zhang a,b,⁎, Yang Zha a, Bin Li b, Wei He a, Jun Sun b a b
School of Materials Science & Engineering, Xi'an University of Technology, Xi'an 710048, China State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
a r t i c l e
i n f o
Article history: Received 6 December 2012 Accepted 22 July 2013 Keywords: Mo–12Si–8.5B alloys Lanthanum oxide Mechanical alloying Mechanical properties
a b s t r a c t Mo–12Si–8.5B alloys with different mass fraction (0 wt.%, 0.3 wt.%, 0.9 wt.%, 1.5 wt.%, 2.5 wt.%) lanthanum oxide (La2O3) additive were fabricate by using mechanical alloying and hot pressing sintering techniques. XRD results indicate that the doped Mo–12Si–8.5B alloys are consisted mainly of α-Mo, Mo3Si and Mo5SiB2 (T2) phases. It is found that addition of La2O3 has little effect on both the grains and intermetallic sizes. The intermetallic particles are more homogeneously dispersed within α-Mo matrix by increasing the La2O3 content. Compression test and three-point bending test showed that both compression and flexure strength of alloys were obviously improved by the addition of La2O3. The predominant strengthening mechanisms are fine-grain strengthening and particles dispersion strengthening. In addition, fracture toughness tests showed that the fracture toughness values of all alloys were on the order of 9 MPa·m1/2 that revealed the addition of La2O3 had little effect on improving the alloys toughness. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Structural materials used in high-temperature environment should have adequate strength, good oxidation resistance, good creep resistance, and good fracture toughness. In order to achieve all these desired properties, new materials must be developed. For decades Mo–Si–B alloys have been extensively studied because of their adequate strength, good oxidation resistance, and good creep resistance [1–3]. At present Mo–Si–B alloys mainly refer to Mo3Si, Mo5SiB2 (T2), Mo5Si3 (T1) or αMo solid solution, as well as Mo3Si and Mo5SiB2 (T2) three phase alloys. Depending on the exact composition, the alloys consisting of Mo3Si, T1 and T2 have higher oxidation resistance than that consisting of α-Mo, Mo3Si and T2. However, the fracture toughness of Mo3Si–T1–T2 alloys is likely to exhibit low (2–4 MPa·m1/2) [2,4] due to three brittle phases. On the other hand, the alloys in the α-Mo–Mo3Si–T2 system are expected to have higher fracture toughness (5–21 MPa·m1/2) [2] than the Mo3Si–T1–T2 alloys because of the presence of a ductile α-Mo phase. The recent work aims to increase the fracture toughness of Mo–Si–B alloy and keep their original good strength and oxidation resistance. In order to maintain a good oxidation resistance, the volume fraction of intermetallics should be up to 50 vol.% [5]. Mo–12Si–8.5B (at.%) consisting
⁎ Corresponding author at: School of Materials Science & Engineering, Xi'an University of Technology, Xi'an 710048, China. Tel.: +86 2982312592. E-mail addresses: [email protected], [email protected] (G. Zhang). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.07.011
of approximately 38 vol.% α-Mo, 32 vol.% Mo3Si and 30 vol.% T2 is a candidate alloy for this study where the volume fraction of intermetallic phases is up to 62%, and this alloy exhibits a good oxidation resistance studied by Mendiratta et al. [5]. Besides excellent oxidation resistance, the alloys should also have adequate strength and good fracture toughness. The room temperature fracture toughness of Mo–12Si–8.5B alloy was on the order of 5–7 MPa·m1/2 in Choe et al.'s work [6] which used both ingot and power metallurgy methods. The room temperature flexure strength and fracture toughness of Mo–12Si–8.5B were 384 MPa and 14 MPa·m1/2 respectively in Schneibel et al.'s work which used a powder metallurgical production method [1]. It indicated that the alloys with the α-Mo matrix showed higher fracture toughness than the alloys with the intermetallic matrix. Moreover, Mo–Si–B alloys with finegrained microstructure have good oxidation resistance as studied by Rioult et al. [7]. The fracture toughness and the oxidation resistance of alloys should also be improved if the alloys showed such a microstructure [8,9]. It has been known that the doping of some reactive element in Nibased superalloys can enhance the mechanical properties remarkably. As one class of the reactive elements, the rare earth oxides have been given special attention and are widely used [10–12]. In Zhang et al.'s work [13], La2O3 doped Mo–12Si–8.5B alloys have good oxidation resistance. Whereas, there were limited reports on the effects of La2O3 on the microstructure and the mechanical properties of Mo–12Si–8.5B alloys. In this paper, mechanical alloying (MA) and hot pressing sintering technique were used to prepare different mass fraction of La2O3 doped Mo–12Si–8.5B alloys. The present work will investigate the effects of doped La2O3 on the mechanical properties of Mo–12Si–8.5B alloys and discuss the strengthening and toughening mechanisms.
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G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
Table 1 Composition of the commercial-purity Mo. Composition
Fe
Ni
Al
Si
Ca
Mg
P
C
O
N
Mo
Weight (ppm)
14
0.79
0.69
2.3
4.0
0.82
b10
9.6
34
2.4
Balance
2. Experimental procedures The Mo–12Si–8.5B alloys with different mass fraction La2O3 were fabricated by mechanical alloying (MA) and hot pressing sintering technology. Elemental powders of Mo, Si, B and La2O3 were 99.95 wt.%, 99.9 wt.%, 99.8 wt.%, and 99.9 wt.% purity, respectively. The composition analyses of commercial-purity Mo are shown in Table 1. These powders were firstly mixed in a planetary ball mill (Retch PM100) with a speed of 200 rpm with a powder to ball weight ratio of 2:1 under protective (argon) atmosphere for 10 h. Then these fine powders were used to conduct mechanical alloying for 20 h. Mechanical alloying used PM100 under protective (argon) atmosphere at a high turning speed (300 rpm) and a large powder to ball weight ratio (1:10). The powder morphologies were observed by scanning electron microscopy (SEM) before and after mechanical alloying shown respectively in Fig. 1(a) and (b), which shown a typical microstructure with complete homogenization [9] after mechanical alloying. The relevant statistical analysis of the particle sizes was used to identify the particle size of powders by laser light scattering analyzer as shown in Fig. 1(c) and (d). The mechanical alloying refined the powder particles which decreased the median diameter of particles from 5.88 μm to 2.77 μm after 20 h mechanical alloying. After mechanical alloying, the powders
were prepared by hot pressing sintering (pressure = 50 MPa, vacuum degree b 10−3 Pa) at 1600 °C for 3 h. Microstructure analysis was carried out by optical metallography of polished specimens etched with Murakami's etch (an aqueous solution of potassium ferricyanide and sodium hydroxide). Phases were identified by X-ray diffraction (XRD). The average densities of the alloys by Archimedes drainage method were listed in Table 2. Room temperature compressive tests were performed at a strain rate of 5 × 10− 4 s− 1 using an HT-2402 universal testing machine. At least three specimens were tested and the sample sizes were 5 mm × 5 mm × 10 mm. The flexure strength values were determined by the three-point bending tests. Flexure specimens with a cross section of 3 mm × 4 mm and a length of 26 mm were electro-discharge machined and ground, subsequently tested in a universal testing machine (Instron 1185) in a three-point bend fixture with a 20 mm span at ambient temperature. The cross-hand speed in flexure strength test was 8 μm/s. Fracture toughness values were determined by three-point bending tests using tip notched specimens. The specimens with a cross section of 3 mm × 4 mm and a length of 26 mm with a 2 mm depth notch tip in the middle of the sample. The loading rate was 0.8 μm/s for fracture toughness test where the fracture toughness values were determined based on an energy criterion by integrating the load–displacement curve where W was the energy absorbed during the fracture of the area A swept out by the crack. The fracture toughness was determined as G = W / A. Assuming the material to be linearly elastic, the fracture energy (energy criterium) can also be expressed in terms of the stress intensity (stress criterium) by Kq = (G × E′)1/2, where
Fig. 1. The morphologies of powders with a composition of Mo–12Si–8.5B of (a) mixed powders and (b) MA for 15 h. (c) and (d) show the size distributions of the powder particle for mixed and MA powders respectively.
G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
587
Table 2 The densities of La2O3 doped Mo–12Si–8.5B alloys. Alloy composition
Mo–12Si–8.5B
Mo–12Si–8.5B + 0.3 wt.% La2O3
Mo–12Si–8.5B + 0.9 wt.% La2O3
Mo–12Si–8.5B + 1.5 wt.% La2O3
Mo–12Si–8.5B + 2.5 wt.% La2O3
Compression strength (MPa) Flexure strength (MPa) Fracture toughness (MPa·m1/2)
2549
2726
2705
2710
2732
1203 9.132
1325 9.337
1337 8.836
1444 9.486
1300 9.259
E′ = E / (1 − ν2) is the plane strain Young's modulus and ν is Poisson's ratio (E = 327 GPa and ν = 0.29) [1]. 3. Result and discussion 3.1. XRD analysis and microstructure characterization The results of X-ray diffraction analysis are shown in Fig. 2. These alloys are all composed of α-Mo, Mo3Si and T2. No other phases are found in these patterns which indicate that the phase composition is not obviously affected by doping the La2O3 particles. Representative micrographs are given in Fig. 3 for the alloys. The αMo, Mo3Si and T2 are the brightest appearing phase, the slightly darker gray phase and the dark gray appearing phase, respectively. One can clearly see that the alloys have a continuous α-Mo matrix with dispersive intermetallic (gray or black) particles distributing within matrix, which is similar to Kruger et al.'s study [9]. The grain sizes of intermetallics and α-Mo were reduced by increasing the La2O3 contents, and the intermetallic phases were more homogeneously distributed within the α-Mo matrix with the La2O3 contents increased from 0.0 wt.% to 2.5 wt.%. 3.2. Mechanical properties 3.2.1. Compression and flexure strength During compression test, the alloys display negligible ductility and fracture in a brittle manner. The results of the room temperature compression strengths are presented in Table 3. These La2O3 doped alloys exhibit a higher compression strength than the Mo–Si–B alloy. The Mo–12Si–8.5B + 2.5 wt.% La2O3 alloy exhibits the highest compression strength (2732 MPa) compared with what other alloys exhibit.
Fig. 2. XRD patterns of Mo–12Si–8.5B alloys with different mass fraction La2O3.
The flexure strengths are also increased by doping La2O3 into Mo–12Si–8.5B alloys (Table 3). The flexure strengths increase from 1203 MPa for the Mo–12Si–8.5B alloy to 1444 MPa for the Mo– 12Si–8.5B + 1.5 wt.% La2O3 alloy. Schneibel et al. [1] had reported that the flexural strengths of Mo–12Si–8.5B alloys were in the range of 380 MPa–539 MPa prepared by arc-melting and powdermetallurgical processing, which were lower than the present results. 3.2.2. Fracture toughness During fracture toughness testing at room temperature, prior to outright fracture no stable crack growth was observed in all fine-grained alloys. The samples fractured immediately at the onset of crack initiation. The fracture surface morphologies show the intergranular fracture to be the dominant failure mechanism in Fig. 4. The fracture toughness value in the present work is designated as Kq rather than KIC for the reason of that fracture toughness KIC supposed to be overestimated by approximately 25% in the chevron-notch technique [1]. The fracture toughness values (Kq) of the alloys are shown in Table 3. One can see that the fracture toughness values did not increase with the doping of La2O3. The reason may be that all the three phases in Mo–12Si–8.5B alloy are brittle at room temperature and the effect of La2O3 on improving the fracture toughness is limited. As a comparison, these alloys exhibit higher (~30%) room temperature fracture toughness than that of Choe et al.'s alloys (7 MPa·m1/2) [6]. 3.3. Strengthening and toughing mechanisms 3.3.1. Strengthening mechanisms Refer to the microstructure characteristics, the grain sizes of both intermetallics and α-Mo are reduced by the doping of La2O3. The reason responsible for the grain refinement may be that the La2O3 particles can act as nucleation sites and hence increase the nucleation rate during the sintering process, and some La2O3 particles may also exist at the molybdenum grain boundaries to hinder the grain growth via pinning effect. Both the pinning and the nucleation effects in La2O3 doped alloys decrease the grain size of α-Mo. According to the effect of fine grain strengthening mechanism, the La2O3 doped alloys exhibit a higher strength. The well-known Hall–Petch equation also indicates that the contribution of the grain size to strength will be improved with the reduction in grain size. In addition, the intermetallic particles of the alloys are more dispersely and uniformly distributed in the matrix by the doping of the La2O3. The particles distributed in the α-Mo matrix as well as the La2O3 particles can play the role of dispersion strengthening by hindering grain slip. Both the intermetallic and the La2O3 particles as dispersion phases increase the alloy strengths. 3.3.2. Toughing mechanisms The α-Mo phase is known well as a ductile phase compared with the two intermetallic Mo3Si and T2 in Mo–12Si–8.5B alloy. The toughening of the brittle intermetallic phases with a ductile phase can be achieved through extrinsic toughening mechanisms such as crack trapping and bridging. Choe et al. [2,6] studied the crack expansion behavior of Mo– 12Si–8.5B alloys and showed that the main toughening mechanism of Mo–12Si–8.5B alloys was ‘crack trapping’ by ductile α-Mo phase at
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Fig. 3. Optical micrographs of alloys: (a) Mo–12Si–8.5B; (b) Mo–12Si–8.5B + 0.3 wt.% La2O 3; (c) Mo–12Si–8.5B + 0.9 wt.% La2O3; (d) Mo–12Si–8.5B + 1.5 wt.% La2O3 and (e) Mo–12Si–8.5B + 2.5 wt.% La2O3.
ambient temperature. In the crack trapping mechanism when a crack tip propagates to α-Mo grain, some plastic deformation occurred and the stress concentration of the crack tip was released and therefore the alloy toughness was improved [14,15]. In Zhang et al.'s work [12], La2O3 was doped into the Mo alloys and the KIC was improved to 24.76 MPa·m1/2 which was obviously better than 9.78 MPa·m1/2 of the pure Mo. In this work, the intermetallic phases are refined and distributed more homogeneously with the doping of La2O3 and when a crack occurred in the more homogeneously dispersed intermetallic particle, the crack is more easily trapped by ductile α-Mo matrix which leads to the increase of the fracture toughness. Because the three phases in Mo–12Si–8.5 alloys are all brittle at room temperature, consequently the effect of extrinsic crack trapping mechanism on improving the fracture toughness is limited.
4. Conclusions Different mass fraction La2O3 doped Mo–12Si–8.5B alloys were fabricated by mechanical alloying and hot pressing sintering techniques. It was a feasible technique for producing Mo–12Si–8.5B alloy with a continuous α-Mo matrix. The following conclusions may be made. (1) The La2O3 doped Mo–12Si–8.5B alloys exhibit a finer and more uniform microstructure. During doping the La2O3 into alloys, the grain size and the intermetallic particles size were refined and distributed more homogeneously within the matrix. (2) Both the compression and the flexure strength of Mo–12Si–8.5B are increased with the doping of La2O3. The major strengthening mechanisms are the fine grain strengthening and the particle dispersion strengthening.
G. Zhang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 585–589
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Table 3 The mechanical properties of La2O3 doped Mo–12Si–8.5B alloys at room temperature. Alloy composition
Mo–12Si–8.5B
Mo–12Si–8.5B + 0.3 wt.% La2O3
Mo–12Si–8.5B + 0.9 wt.% La2O3
Mo–12Si–8.5B + 1.5 wt.% La2O3
Mo–12Si–8.5B + 2.5 wt.% La2O3
Density ρ (kg·m−3)
8.9105 × 103
8.8350 × 103
8.9116 × 103
8.8159 × 103
8.8187 × 103
No. 2012BAE06B02) and the State Key Laboratory for Mechanical Behavior of Materials. References
Fig. 4. The fracture surface morphology of Mo–12Si–8.5B + 2.5 wt.% La2O3.
(3) The fracture toughness values of the alloys are on the order of 9 MPa·m1/2 and addition of La2O3 has little effect on improving the fracture toughness, which is restricted by the lower ductility of the three phases in Mo–12Si–8.5B alloy at room temperature. Acknowledgments This subject was supported by the National Natural Science Foundation (Grant Nos. 51171149 and 50801051), the Program for New Century Excellent Talents in University (Grant No. NCET10-0876), the 973 Program of China (Grant No. 2012CB619600), the National Science Technology Supporting Program of China (Grant
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