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rare earth composite particles on microstructure and mechanical properties of molybdenum
Int. Journal of Refractory Metals and Hard Materials 29 (2011) 505–508
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum Xiuqi Zan, Dezhi Wang ⁎, Kaihua Shi, Aokui Sun, Bing Xu Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Changsha 410083, PR China School of Materials Science and Engineering, Central South University, No. 932, Lushan Nanlu, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2010 Accepted 16 February 2011 Keywords: MoSi2/rare earth Sintered molybdenum Microstructure Property
a b s t r a c t MoSi2/La2O3 and MoSi2/Y2O3 composite particles were prepared by mechanical milling and doped into molybdenum by solid–solid method, respectively. Rods with a diameter of 17 mm were made by pressing and sintering. The effects of different composite particles on microstructures and strength of the as-sintered molybdenum were investigated. Results show that the MoSi2/La2O3 and MoSi2/Y2O3 composite particles transformed to La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles due to the in situ reaction between Mo and MoSi2 during sintering process. Mo5Si3/La2O3 and Mo5Si3/Y2O3 composite particles can reduce the grain size and improve both strength and toughness of sintered molybdenum significantly. Mo5Si3/Y2O3 composite particles contribute more to the strength, while the effect of Mo5Si3/La2O3 on toughness is greater than that of Mo5Si3/Y2O3. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Molybdenum is characterized by high melting point, high strength at elevated temperature, good thermal and electrical conductivity as well as low coefficient of thermal expansion which are desired for many high temperature applications, especially in chemical industry, metallurgical industry, metal-processing industry, aerospace industry and nuclear energy technology [1–4]. However, because of high melting point, high hardness and poor oxidation resistance, molybdenum is difficult in melting and processing. Moreover, because of the disadvantages like recrystallization brittleness, poor high-temperature anti-oxidation ability and high ductile to brittle transition temperature (DBTT), the further applications of molybdenum are limited. A well-known solution for this problem is adding some dopants such as rare earth oxides or potassium, silicon and aluminum (AKS) [5–8]. In this work, detection methods such as bending test, compression test, fracture test, X-Ray Diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to analyze room temperature properties, microstructures of MoSi2/rare earth composite particles doped molybdenum and pure molybdenum. At last, the strengthening and toughening mechanism of the doped molybdenum were investigated.
The MoSi2 powder (d b 48 μm) and rare earth oxide power (pure of 99.99% La2O3 and Y2O3) with a weight ratio of 1:1 were loaded into steel vials respectively, with alcohol as process control agent. In order to prevent contamination during milling, purified argon gas was filled to evacuate air. The powder mixtures were milled in a planet ball mill for 30 h with a rotational speed of 390 rpm and a powder to ball weight ratio of 1:20. Then the ball-milled powders were detected by X-ray diffractometer, and the results were shown in Fig. 1. The composite powders (d b 100 nm) were doped into pure Mo power (2.8 μm b db3.2 μm) with a same mass fraction of 2%. After being mixed with a XP01 type three-dimension moving mixer for 20 h, the power was cold isostatically pressed into cylindrical compacts (the pressure of cold-press was 160 MPa for 4 min), and then sintered in hydrogen atmosphere at 1980 °C for 6 h. The densities of the samples were measured by the Archimedes method. Hardness was tested on a HW-187.5 hardness tester, and the result of each sample was the average value of five points. Electrodischarge machining was used to prepare specimens and the mechanical properties were assessed by three-point bend tests (the dimensions were 30 mm × 5 mm × 5 mm with a span of 20 mm), fracture toughness tests (50 mm × 10 mm × 5 mm with a span of 40 mm) and compressibility tests (Φ8 mm × 12 mm) at room temperature. Fracture toughness was measured in three-point bending at a loading rate of 0.5 mm/min at room temperature using an Instron 8032 hydraulic servo control testing machine. A support span of 25 mm was used for the bending specimens. The specimens were fatigueprecracked prior to fracture toughness testing.
⁎ Corresponding author at: School of Materials Science and Engineering, Central South University, No. 932, Lushan Nanlu, Changsha 410083, PR China. Tel.: + 86 731 88877221. E-mail address: [email protected] (D. Wang). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.02.011
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deformation can be dispersed to more grains, which makes the plastic deformation relatively homogeneous and decreases stress concentration; secondly, grain boundaries hinder the movement of dislocation. When dislocations move in polycrystalline metals, the slip resistance is increased because of changes in direction of slip plane and disorder at grain boundary. Generally, the more grain boundaries exist, the more difficult it is for dislocations to move, and as a result, the harder and stronger the material is. More over, grain size also has a greater impact on toughness, which is the amount of energy per volume that a material can absorb before rupturing. According to Cottrell [9], the relationship between grain diameter d and the critical stress of crack propagation is as follows: σc =
Fig. 1. XRD patterns of MoSi2/La2O3 and MoSi2/Y2O3 composite particles prepared by mechanical milling.
3. Results and discussion 3.1. Density and hardness From Table 1, we can see that the density of the doped molybdenum is lower than that of pure Mo due to the relatively low density of the doped phases (La2O3/MoSi2 and Y2O3/MoSi2). In the other side, the hardness of the doped molybdenum is higher. It is mainly because the grains of doped Mo were refined through the addition of MoSi2/ rare earth composite particles. Fig. 2 shows the metallographs of doped molybdenum and pure molybdenum. It can be seen clearly that both of the doped Mo had a finer grain size than pure Mo. 3.2. Strength and ductility Table 1 shows the strength and ductility of doped molybdenum and pure molybdenum. It is obviously that both of the properties were improved through doping composite particles. We believe that it's the result of strengthening and toughening by doping composite particles. There are two mechanisms: the first one is grain-refinement strengthening and toughening and the other one is second phase particles strengthening and toughening. From Fig. 2, we can see that the doped Mo had a finer grain size and that the grain size has a closed relationship with yield strength. The relationship follows Hall–Petch relation: −1 = 2
σy = σi + ky d
ð1Þ
where σy is yield strength, σi and ky are constants for a particular material and d is the average grain diameter. According to the Hall– Petch equation, the finer the grains are, the higher the strength becomes, and the strength of polycrystal is higher than that of single crystal. The main reason of grain-refinement having an influence on strength has two aspects. Firstly, the external force induced plastic
Table 1 List of properties of as-sintered Mo after doping with MoSi2/La2O3 (1#) and MoSi2/Y2O3 (2#).
1# 2# PM
Density
Hardness
Bending strength
g/cm3
(HV)
MPa
9.898 9.912 9.95
194.25 217.54 164.47
530.9 602.9 395.6
Compressive strength MPa 1681.43 N 1780.67 1036.24
Fracture toughness MPa·m1/2 9.36 8.72 6.15
2Gγp −1 = 2 d Ky
ð2Þ
where G is shear modulus, Ky is Petch slop and γp is effective surface energy. From formula (2), the critical crack spreading stress can be increased by grain-refinement. Meanwhile, the area of grain boundaries that impedes dislocation motion is also increased, and because of the disorder of atomic arrangement at grain boundaries, the dislocation structure becomes relatively complex. The above reasons make dislocation motion more difficult, thus the deformation needs more energy. So, grain-size reduction usually improves toughness as well. In order to discuss the second mechanism, a TEM test for La2O3/ MoSi2 and Y2O3/MoSi2 doped sintered molybdenum had been taken (Fig. 3). From the TEM images, we knew that the composite particles are spherical in shape and distributed dispersively in Mo matrix. In our previous work [10], we have proved that because of the in situ reaction between Mo and MoSi2 during sintering which generates Mo5Si3, the MoSi2/La2O3 and MoSi2/Y2O3 composite particles transformed to La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles. La2O3/ Mo5Si3 and Y2O3/Mo5Si3 composite particles are difficult to deform, so the dislocations can not cut through but only bypass them. This bypassing mechanism is called Orowan mechanism [11]. According to Orowan-Ashby, the increase in yield strength σp of sintered Mo by adding La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles can be determined by the following equation: σp = σOR =
mμb ϕ In ð1:18Þ ⋅ 2π ⋅ ðλ−ϕÞ 2b
ð3Þ
where m is Taylor factor, μ is the shear modulus, b is the Burgers vector, ϕ is the particle size and λ is the distance of the composite particles. From formula (3), the strengthening effect of La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles is mainly related to Burgers vector, the diameter and distance of particles. The closer and larger of the particles, the better the strengthening effect is. A lot of dislocations slip to grain boundaries and cause high stress concentration, which is the main reason of brittle fracture of molybdenum at low temperature. The hard and brittle composite particles that dispersively distributed in the matrix resulted in the change of dislocation distribution, which make dislocation distribution more even. Thus, on the one hand, the deformation of sintered Mo is more uniform, shorten the effective length of slip plane and lighten the pileup of dislocation near grain boundaries. On the other hand, pinning effect of the composite particles to dislocations decreased dislocation density near grain boundaries, which is beneficial to delay the formation of along grain microcracks, or even transfer intergranular fracture to transgranular fracture. The fracture morphology of doped Mo and pure Mo after three-point bending test are shown in Fig. 4. It can be observed that the fracture mode of pure Mo is intergranular fracture Fig. 4(PM), whereas after addition of the MoSi2/rare earth composite particles, transgranular fracture mode was found (Fig. 4a and b). Generally, microcracks emerge at grain boundaries and propagate to
X. Zan et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 505–508
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Fig. 2. Optical microstructure of pure molybdenum (PM), MoSi2/La2O3 doped molybdenum alloy (a) and MoSi2/Y2O3 doped molybdenum alloy (b).
Fig. 3. Transmission electron microscope images of La2O3/Mo5Si3 and Y2O3/Mo5Si3 doped sintered molybdenum. (a) The shape of second phase particle—La2O3/Mo5Si3; (b) the shape of second phase particle Y2O3/Mo5Si3; (c) the grain boundaries' morphology of molybdenum matrix doped with La2O3/Mo5Si3; (d) the grain boundaries' morphology of molybdenum matrix doped with Y2O3/Mo5Si3.
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X. Zan et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 505–508
Fig. 4. Fracture morphology of pure molybdenum (PM), MoSi2/La2O3 doped molybdenum (a) and MoSi2/Y2O3 doped molybdenum (b).
the inner. The existence of La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles in sintered Mo may change the orientation of crack growth, which make the crack tip blunted [12]. The change of crack morphology and the formation of new fracture surface will both absorb more energy. Thereby, the addition of MoSi2/rare earth composite particles makes the sintered Mo obtain outstanding strength and toughness. 4. Conclusions Effect of the MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum was studied. SEM investigations showed that the addition of MoSi2/rare earth to molybdenum leads to the refinement of grain and also transfers intergranular fracture to transgranular fracture. TEM results showed that the MoSi2/rare earth composite particles exist in the matrix, which bring about strengthening and toughening. Compared with pure molybdenum, the addition of MoSi2/La2O3 and MoSi2/Y2O3 composite particles to molybdenum decreases the density and enhances the hardness of sintered molybdenum, and the bending strength and fracture toughness could be increased by up to 135.3 MPa, 207.3 MPa and 52.2%, 41.8% respectively. Taking density, hardness, strength and toughness into consideration, MoSi2/Y2O3-Mo has the better comprehensive properties than MoSi2/La2O3-Mo. Acknowledgments The financial support of the National High Technology Research and Development Program of China (863 Program) Grant No.
2008AA031003 is gratefully acknowledged. We also thank Molybdenum Department of Zhuzhou Cemented Carbide Group Co., Ltd. and Key Laboratory of Non-ferrous Materials Science and Engineering (Central South University), Ministry of Education, for the sample preparation and research facilities. References [1] Xiang TG. Molybdenum metallurgy. 3rd ed. Changsha: Central South University Press; 2002. [2] Freeman RR. Properties and application of commercial molybdenum and molybdenum alloys. In: Harwood JJ, editor. The metal molybdenum. Cleveland: ASM; 1958. p. 10–1. [3] Zhang WZ. Annual review of molybdenum in 2007. China Molybdenum Ind 2007;31(6):3–9. [4] Lv Z, Yin JS. Refractory metal science and engineering. Proceedings of the 8th Refractory Metal Academic Exchange Conference. Shanghai: Shanghai Science and Technology Press; 1998. p. 234–6. [5] Zhang GJ, Sun YJ, Zuo C, Wei JF, Sun J. Microstructure and mechanical properties of multi-components rare earth oxide-doped molybdenum alloys. Mater Sci Eng A 2008;483–484:350–2. [6] Yi YP, Gao JQ. The study of molybdenum wire doped with Y2O3/CeO2(MYC). Rare Met Mat Eng 2005;34(2):271–4. [7] Iorio LE, Bewlay BP, Larsen M. Analysis of AKS- and lanthana-doped molybdenum wire. Int J Refract Met Hard Mat 2006;24:306–10. [8] Yong W, Gao JC, Chen GM, Lin WQ, Zhou YG, Zhang W. Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum. Int J Refract Met Hard Mat 2008;26:9–13. [9] Wang L. Mechanical properties of materials. Shenyang: Northeastern University Press; 2007. p. 71. [10] Wu RJ. Composite materials. Tianjin: Tianjin University Press; 2000. [11] Kelly A, Nicholson RB. Strengthening methods in crystals. New York: Elsevier; 1971. p. 16. [12] Zhang JX, Wang YJ, Liu DM, Zhou ML. The influence of rare earth oxide on the mechanical properties of molybdenum. J Beijing Polytechnic Univ 1998;24(2):1–6.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum Xiuqi Zan, Dezhi Wang ⁎, Kaihua Shi, Aokui Sun, Bing Xu Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Changsha 410083, PR China School of Materials Science and Engineering, Central South University, No. 932, Lushan Nanlu, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2010 Accepted 16 February 2011 Keywords: MoSi2/rare earth Sintered molybdenum Microstructure Property
a b s t r a c t MoSi2/La2O3 and MoSi2/Y2O3 composite particles were prepared by mechanical milling and doped into molybdenum by solid–solid method, respectively. Rods with a diameter of 17 mm were made by pressing and sintering. The effects of different composite particles on microstructures and strength of the as-sintered molybdenum were investigated. Results show that the MoSi2/La2O3 and MoSi2/Y2O3 composite particles transformed to La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles due to the in situ reaction between Mo and MoSi2 during sintering process. Mo5Si3/La2O3 and Mo5Si3/Y2O3 composite particles can reduce the grain size and improve both strength and toughness of sintered molybdenum significantly. Mo5Si3/Y2O3 composite particles contribute more to the strength, while the effect of Mo5Si3/La2O3 on toughness is greater than that of Mo5Si3/Y2O3. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Molybdenum is characterized by high melting point, high strength at elevated temperature, good thermal and electrical conductivity as well as low coefficient of thermal expansion which are desired for many high temperature applications, especially in chemical industry, metallurgical industry, metal-processing industry, aerospace industry and nuclear energy technology [1–4]. However, because of high melting point, high hardness and poor oxidation resistance, molybdenum is difficult in melting and processing. Moreover, because of the disadvantages like recrystallization brittleness, poor high-temperature anti-oxidation ability and high ductile to brittle transition temperature (DBTT), the further applications of molybdenum are limited. A well-known solution for this problem is adding some dopants such as rare earth oxides or potassium, silicon and aluminum (AKS) [5–8]. In this work, detection methods such as bending test, compression test, fracture test, X-Ray Diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to analyze room temperature properties, microstructures of MoSi2/rare earth composite particles doped molybdenum and pure molybdenum. At last, the strengthening and toughening mechanism of the doped molybdenum were investigated.
The MoSi2 powder (d b 48 μm) and rare earth oxide power (pure of 99.99% La2O3 and Y2O3) with a weight ratio of 1:1 were loaded into steel vials respectively, with alcohol as process control agent. In order to prevent contamination during milling, purified argon gas was filled to evacuate air. The powder mixtures were milled in a planet ball mill for 30 h with a rotational speed of 390 rpm and a powder to ball weight ratio of 1:20. Then the ball-milled powders were detected by X-ray diffractometer, and the results were shown in Fig. 1. The composite powders (d b 100 nm) were doped into pure Mo power (2.8 μm b db3.2 μm) with a same mass fraction of 2%. After being mixed with a XP01 type three-dimension moving mixer for 20 h, the power was cold isostatically pressed into cylindrical compacts (the pressure of cold-press was 160 MPa for 4 min), and then sintered in hydrogen atmosphere at 1980 °C for 6 h. The densities of the samples were measured by the Archimedes method. Hardness was tested on a HW-187.5 hardness tester, and the result of each sample was the average value of five points. Electrodischarge machining was used to prepare specimens and the mechanical properties were assessed by three-point bend tests (the dimensions were 30 mm × 5 mm × 5 mm with a span of 20 mm), fracture toughness tests (50 mm × 10 mm × 5 mm with a span of 40 mm) and compressibility tests (Φ8 mm × 12 mm) at room temperature. Fracture toughness was measured in three-point bending at a loading rate of 0.5 mm/min at room temperature using an Instron 8032 hydraulic servo control testing machine. A support span of 25 mm was used for the bending specimens. The specimens were fatigueprecracked prior to fracture toughness testing.
⁎ Corresponding author at: School of Materials Science and Engineering, Central South University, No. 932, Lushan Nanlu, Changsha 410083, PR China. Tel.: + 86 731 88877221. E-mail address: [email protected] (D. Wang). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.02.011
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X. Zan et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 505–508
deformation can be dispersed to more grains, which makes the plastic deformation relatively homogeneous and decreases stress concentration; secondly, grain boundaries hinder the movement of dislocation. When dislocations move in polycrystalline metals, the slip resistance is increased because of changes in direction of slip plane and disorder at grain boundary. Generally, the more grain boundaries exist, the more difficult it is for dislocations to move, and as a result, the harder and stronger the material is. More over, grain size also has a greater impact on toughness, which is the amount of energy per volume that a material can absorb before rupturing. According to Cottrell [9], the relationship between grain diameter d and the critical stress of crack propagation is as follows: σc =
Fig. 1. XRD patterns of MoSi2/La2O3 and MoSi2/Y2O3 composite particles prepared by mechanical milling.
3. Results and discussion 3.1. Density and hardness From Table 1, we can see that the density of the doped molybdenum is lower than that of pure Mo due to the relatively low density of the doped phases (La2O3/MoSi2 and Y2O3/MoSi2). In the other side, the hardness of the doped molybdenum is higher. It is mainly because the grains of doped Mo were refined through the addition of MoSi2/ rare earth composite particles. Fig. 2 shows the metallographs of doped molybdenum and pure molybdenum. It can be seen clearly that both of the doped Mo had a finer grain size than pure Mo. 3.2. Strength and ductility Table 1 shows the strength and ductility of doped molybdenum and pure molybdenum. It is obviously that both of the properties were improved through doping composite particles. We believe that it's the result of strengthening and toughening by doping composite particles. There are two mechanisms: the first one is grain-refinement strengthening and toughening and the other one is second phase particles strengthening and toughening. From Fig. 2, we can see that the doped Mo had a finer grain size and that the grain size has a closed relationship with yield strength. The relationship follows Hall–Petch relation: −1 = 2
σy = σi + ky d
ð1Þ
where σy is yield strength, σi and ky are constants for a particular material and d is the average grain diameter. According to the Hall– Petch equation, the finer the grains are, the higher the strength becomes, and the strength of polycrystal is higher than that of single crystal. The main reason of grain-refinement having an influence on strength has two aspects. Firstly, the external force induced plastic
Table 1 List of properties of as-sintered Mo after doping with MoSi2/La2O3 (1#) and MoSi2/Y2O3 (2#).
1# 2# PM
Density
Hardness
Bending strength
g/cm3
(HV)
MPa
9.898 9.912 9.95
194.25 217.54 164.47
530.9 602.9 395.6
Compressive strength MPa 1681.43 N 1780.67 1036.24
Fracture toughness MPa·m1/2 9.36 8.72 6.15
2Gγp −1 = 2 d Ky
ð2Þ
where G is shear modulus, Ky is Petch slop and γp is effective surface energy. From formula (2), the critical crack spreading stress can be increased by grain-refinement. Meanwhile, the area of grain boundaries that impedes dislocation motion is also increased, and because of the disorder of atomic arrangement at grain boundaries, the dislocation structure becomes relatively complex. The above reasons make dislocation motion more difficult, thus the deformation needs more energy. So, grain-size reduction usually improves toughness as well. In order to discuss the second mechanism, a TEM test for La2O3/ MoSi2 and Y2O3/MoSi2 doped sintered molybdenum had been taken (Fig. 3). From the TEM images, we knew that the composite particles are spherical in shape and distributed dispersively in Mo matrix. In our previous work [10], we have proved that because of the in situ reaction between Mo and MoSi2 during sintering which generates Mo5Si3, the MoSi2/La2O3 and MoSi2/Y2O3 composite particles transformed to La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles. La2O3/ Mo5Si3 and Y2O3/Mo5Si3 composite particles are difficult to deform, so the dislocations can not cut through but only bypass them. This bypassing mechanism is called Orowan mechanism [11]. According to Orowan-Ashby, the increase in yield strength σp of sintered Mo by adding La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles can be determined by the following equation: σp = σOR =
mμb ϕ In ð1:18Þ ⋅ 2π ⋅ ðλ−ϕÞ 2b
ð3Þ
where m is Taylor factor, μ is the shear modulus, b is the Burgers vector, ϕ is the particle size and λ is the distance of the composite particles. From formula (3), the strengthening effect of La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles is mainly related to Burgers vector, the diameter and distance of particles. The closer and larger of the particles, the better the strengthening effect is. A lot of dislocations slip to grain boundaries and cause high stress concentration, which is the main reason of brittle fracture of molybdenum at low temperature. The hard and brittle composite particles that dispersively distributed in the matrix resulted in the change of dislocation distribution, which make dislocation distribution more even. Thus, on the one hand, the deformation of sintered Mo is more uniform, shorten the effective length of slip plane and lighten the pileup of dislocation near grain boundaries. On the other hand, pinning effect of the composite particles to dislocations decreased dislocation density near grain boundaries, which is beneficial to delay the formation of along grain microcracks, or even transfer intergranular fracture to transgranular fracture. The fracture morphology of doped Mo and pure Mo after three-point bending test are shown in Fig. 4. It can be observed that the fracture mode of pure Mo is intergranular fracture Fig. 4(PM), whereas after addition of the MoSi2/rare earth composite particles, transgranular fracture mode was found (Fig. 4a and b). Generally, microcracks emerge at grain boundaries and propagate to
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Fig. 2. Optical microstructure of pure molybdenum (PM), MoSi2/La2O3 doped molybdenum alloy (a) and MoSi2/Y2O3 doped molybdenum alloy (b).
Fig. 3. Transmission electron microscope images of La2O3/Mo5Si3 and Y2O3/Mo5Si3 doped sintered molybdenum. (a) The shape of second phase particle—La2O3/Mo5Si3; (b) the shape of second phase particle Y2O3/Mo5Si3; (c) the grain boundaries' morphology of molybdenum matrix doped with La2O3/Mo5Si3; (d) the grain boundaries' morphology of molybdenum matrix doped with Y2O3/Mo5Si3.
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Fig. 4. Fracture morphology of pure molybdenum (PM), MoSi2/La2O3 doped molybdenum (a) and MoSi2/Y2O3 doped molybdenum (b).
the inner. The existence of La2O3/Mo5Si3 and Y2O3/Mo5Si3 composite particles in sintered Mo may change the orientation of crack growth, which make the crack tip blunted [12]. The change of crack morphology and the formation of new fracture surface will both absorb more energy. Thereby, the addition of MoSi2/rare earth composite particles makes the sintered Mo obtain outstanding strength and toughness. 4. Conclusions Effect of the MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum was studied. SEM investigations showed that the addition of MoSi2/rare earth to molybdenum leads to the refinement of grain and also transfers intergranular fracture to transgranular fracture. TEM results showed that the MoSi2/rare earth composite particles exist in the matrix, which bring about strengthening and toughening. Compared with pure molybdenum, the addition of MoSi2/La2O3 and MoSi2/Y2O3 composite particles to molybdenum decreases the density and enhances the hardness of sintered molybdenum, and the bending strength and fracture toughness could be increased by up to 135.3 MPa, 207.3 MPa and 52.2%, 41.8% respectively. Taking density, hardness, strength and toughness into consideration, MoSi2/Y2O3-Mo has the better comprehensive properties than MoSi2/La2O3-Mo. Acknowledgments The financial support of the National High Technology Research and Development Program of China (863 Program) Grant No.
2008AA031003 is gratefully acknowledged. We also thank Molybdenum Department of Zhuzhou Cemented Carbide Group Co., Ltd. and Key Laboratory of Non-ferrous Materials Science and Engineering (Central South University), Ministry of Education, for the sample preparation and research facilities. References [1] Xiang TG. Molybdenum metallurgy. 3rd ed. Changsha: Central South University Press; 2002. [2] Freeman RR. Properties and application of commercial molybdenum and molybdenum alloys. In: Harwood JJ, editor. The metal molybdenum. Cleveland: ASM; 1958. p. 10–1. [3] Zhang WZ. Annual review of molybdenum in 2007. China Molybdenum Ind 2007;31(6):3–9. [4] Lv Z, Yin JS. Refractory metal science and engineering. Proceedings of the 8th Refractory Metal Academic Exchange Conference. Shanghai: Shanghai Science and Technology Press; 1998. p. 234–6. [5] Zhang GJ, Sun YJ, Zuo C, Wei JF, Sun J. Microstructure and mechanical properties of multi-components rare earth oxide-doped molybdenum alloys. Mater Sci Eng A 2008;483–484:350–2. [6] Yi YP, Gao JQ. The study of molybdenum wire doped with Y2O3/CeO2(MYC). Rare Met Mat Eng 2005;34(2):271–4. [7] Iorio LE, Bewlay BP, Larsen M. Analysis of AKS- and lanthana-doped molybdenum wire. Int J Refract Met Hard Mat 2006;24:306–10. [8] Yong W, Gao JC, Chen GM, Lin WQ, Zhou YG, Zhang W. Properties at elevated temperature and recrystallization of molybdenum doped with potassium, silicon and aluminum. Int J Refract Met Hard Mat 2008;26:9–13. [9] Wang L. Mechanical properties of materials. Shenyang: Northeastern University Press; 2007. p. 71. [10] Wu RJ. Composite materials. Tianjin: Tianjin University Press; 2000. [11] Kelly A, Nicholson RB. Strengthening methods in crystals. New York: Elsevier; 1971. p. 16. [12] Zhang JX, Wang YJ, Liu DM, Zhou ML. The influence of rare earth oxide on the mechanical properties of molybdenum. J Beijing Polytechnic Univ 1998;24(2):1–6.