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Microstructure and mechanical properties of multi-components rare earth oxide-doped molybdenum alloys
Materials Science and Engineering A 483–484 (2008) 350–352
Microstructure and mechanical properties of multi-components rare earth oxide-doped molybdenum alloys Guo-jun Zhang ∗ , Yuan-jun Sun, Chao Zuo, Jian-feng Wei, Jun Sun State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China Received 6 June 2006; received in revised form 28 August 2006; accepted 29 August 2006
Abstract Pure molybdenum and molybdenum alloys doped with two- or three-components rare earth oxide particles were prepared by powder metallurgy. Both the tensile property and fracture toughness of the pure molybdenum and multi-components rare earth oxide-doped molybdenum alloys were determined at room temperature. The multi-components rare earth oxide-doped molybdenum alloys are fine grained and contain a homogeneous distribution of fine particles in the submicron and nanometer size ranges, which is why the molybdenum alloys have higher strength and fracture toughness than pure molybdenum. Quantitative analysis is used to explain the increase in yield strength with respect to grain size and second phase strengthening. Furthermore, the relationship between the tensile properties and microstructural parameters is quantitatively established. © 2007 Elsevier B.V. All rights reserved. Keywords: Multi-components; Rare earth oxide; Molybdenum alloy; Mechanical property
1. Introduction With their high melting point, high-temperature strength, low thermal-expansion coefficient and high thermal/electrical conductivity, molybdenum and its alloys have become a popular material for high-temperature structural parts, power semiconductor components and glass-melting electrodes. However, further applications are limited by brittleness. Many new molybdenum alloys with improved ductility have been developed among which the oxide-doped molybdenum alloy is of special interest. This alloy, prepared by adding a proper amount of rare earth oxide (La2 O3 , Y2 O3 , etc.) followed by substantial deformation, exhibits a much higher recrystallization temperature than commercial pure molybdenum [1,2]. Molybdenum alloys have not only superior strength and ductility but also superior fracture toughness as compared with pure molybdenum [3,4]. Whereas many studies have been carried out on the microstructure and mechanical properties of single component oxide-doped molybdenum alloys [1–6], little is known on the multi-component oxide-doped molybdenum alloy so far. This
∗
Corresponding author. Tel.: +86 135 1917 3936; fax: +86 29 8231 2617. E-mail address: [email protected] (G.-j. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.143
work is aimed at investigating molybdenum alloys doped with two or three components of rare earth oxide, and especially at establishing a relationship between microstructure parameters and mechanical properties, and to compare this with the behaviour of pure molybdenum alloy(s). Another objective of this work is to correlate the increase in tensile properties with changes in microstructure. 2. Experimental procedures The pure molybdenum and molybdenum alloys doped with two or three components amongst oxide lanthanum (La2 O3 ), yttria (Y2 O3 ) and ceria (CeO2 ) particles were prepared by powder metallurgy. All the doping oxides have the same total mass fraction in spite of different combinations and relative content(s), as shown in Table 1. The rare earth oxide was added to molybdenum oxide via an aqueous solution of nitrates. The doped molybdenum oxide powders were reduced into doped molybdenum powder in dry hydrogen. The doped molybdenum powder was cold-isostatically pressed into a cylinder 17 mm in diameter, and then sintered at 1850 ◦ C for 4 h in flowing dry hydrogen. Finally, the cylindrical compact was thermomechanically processed into a rod with a diameter of 7.8 mm. The rod was carefully annealed at 1250 ◦ C in a dry hydrogen atmosphere for 1 h.
G.-j. Zhang et al. / Materials Science and Engineering A 483–484 (2008) 350–352
351
Table 1 The rare earth oxides mass fraction of materials studied
Table 2 Microstructural parameters and calculated strength of the materials examined
Materials
Materials
Particle volume fraction
Grain
Particle
σ or (MPa)
σ HP (MPa)
Mo-LY1 Mo-LY2 Mo-LC1 Mo-LC2 Mo-LC3 Mo-YC1 Mo-YC2 Mo-LCY
0.730 0.721 0.686 0.694 0.690 0.733 0.708 0.715
2.1 2.4 2.3 1.9 2.1 1.3 1.6 2.2
90 102 105 84 76 73 87 82
97.9 87.9 83.5 100.6 108.6 116.0 98.9 104.2
208.1 193.1 203.6 177.5 175.5 214.0 187.2 195.8
Mo-LY1 Mo-LY2 Mo-LC1 Mo-LC2 Mo-LC3 Mo-YC1 Mo-YC2 Mo-LCY
Oxide mass fraction (%) La2 O3
Y2 O3
CeO2
0.3 0.4 0.2 0.4 0.3 – – 0.2
0.3 0.2 – – – 0.4 0.2 0.2
– – 0.4 0.2 0.3 0.2 0.4 0.2
At room temperature, dog-bone shaped tensile specimens, with a gage size of 5 mm in diameter and 25 mm in length, were tested at a constant strain rate of 1.3 × 10−3 s−1 in a servohydraulic Instron-1195 testing machine. Fracture toughness was measured in three-point bending at a slow loading rate of 0.0025 mm/s in accordance with the ASTM E399 method at room temperature. A support span of 20 mm was used for the bend specimens. The specimens were fatigue-precracked prior to fracture toughness testing. The microstructure of the alloy was observed by transmission electron microscopy (TEM). The foils were produced by electropolishing in a twin-jet apparatus with a solution of 12.5 vol.% H2 SO4 in ethanol at −15 ◦ C. A voltage of 10 V produced a current density of approximately 3 mA/mm2 . A computer image analyzer was also used to measure the size and distribution of microstructural parameters of the alloys studied. 3. Results and discussions 3.1. Microstructure Grain sizes and morphology of the pure molybdenum and molybdenum alloys were investigated by TEM, as typically illustrated in Fig. 1a and b, respectively. It is clearly seen that the multi-components rare earth oxide-doped molybdenum alloys have a finer grain size than pure molybdenum. In the molybdenum alloys, the oxide ellipsoidal particles, with submicron or nanometer in size, are homogeneously distributed (Fig. 1c). The average grain size (d), volume fraction (fv ) and average diameter (φ) of the oxide particles have been determined
(Table 2). At least 100 grains and 200 oxide particles were measured within random sections of each alloy. 3.2. Mechanical properties Fig. 2 shows that different molybdenum alloys exhibit varying yield strengths (measured at 0.2%), all larger than that of the pure molybdenum. The fracture toughness, which is also shown in Fig. 2, is similarly larger than that of pure molybdenum. Since the samples are limited in size and cannot meet the geometry requirements for plane–strain constraint, only KQ values are used for comparison and analysis in the present paper. 3.3. Strengthening mechanisms We expect two main contributions to the yield strength: (a) a strengthening due to a reduction in grain size via a Hall–Petch mechanism (σ HP ) and (b) some particle strengthening via the Orowan mechanism (σ or ). These have been assumed to be additive in nature and the yield strength σ y can be decomposed into: σy = σm + σHP + σor = σm +
k + σor d 1/2
(1)
where σ m is the matrix strength of the material (very largegrained molybdenum) in the absence of any other strengthening mechanism and has been evaluated to 417 MPa [4], k a constant, i.e., the Hall–Petch slope and d is the matrix grain size. TEM examinations observations (Fig. 3) have confirmed that when dislocations pass by the oxide particles, they tend to bow out between the particles leaving Orowan loops around the par-
Fig. 1. The molybdenum grain sizes and morphology of (a) pure molybdenum and (b) yttria and ceria bi-components doped molybdenum alloy (Mo-YC1) in radial direction and (c) the oxide particles sizes and morphology of the oxide lanthanum, yttria and ceria tri-components doped molybdenum alloy (Mo-LCY).
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G.-j. Zhang et al. / Materials Science and Engineering A 483–484 (2008) 350–352
fraction and sizes. From Eq. (1), we then derive the strength contribution caused by the reduction in grain size, σ HP (Table 2). The grain size strengthening contributes about 175–214 MPa to hardening, actually more than the oxide particle strengthening. This result can also be simply obtained from Eq. (3) indicating that the Orowan stress increases with the increase in particle volume fraction and/or the decrease in particle size. Since in the present alloys, the volume fraction is low (about 0.7%) and does not change remarkably, the increase in yield stress with respect to particle free alloys is thus correlated with grain size when one considers the peak for the alloy of Mo-YC1 and Mo-YC2 in Fig. 2. 4. Conclusion Fig. 2. The multi-components rare earth oxide-doped molybdenum alloys have higher strength and fracture toughness than pure molybdenum.
ticles. As a result, an additional stress is needed as described in the Orowan model [7–9]. For quantitative purposes, the traditional Orowan–Ashby equation [9] is used to assess the increase in yield strength due to the presence of oxide particles: mμb φ σor = ln (2) 1.18 × 2π(λ − φ) 2b where m is the Taylor factor, μ the shear modulus, b the magnitude of the Burgers vector, φ the particle size and λ is the interparticle spacing in the shear plane of the dislocation. Generally, the particle size and interparticle spacing are related to the volume fraction, fv , as λ = φ(6fv /π)−1/3 [10], so that Eq. (2) can be rewritten as: mμb φ √ ln σor = (3) 2b 1.18 × 2πφ π/6f − 1 The Taylor factor is taken as 2.5 for the bcc crystal structure [11], the shear modulus of molybdenum alloys as 140 GPa [12], and the Burgers vector (a/2 1 1 1) as 2.72 × 10−10 m. Table 2 shows that Orowan strengthening contributes about 83–116 MPa for the investigated range of oxide particles volume
Fig. 3. Dislocations pass by oxide particles and tend to bend like bow between particles.
(i) Multi-components rare earth oxide-doped molybdenum alloys have been successfully prepared with a finely grained microstructure (1.3–2.4 m in grain size) and uniformly distributed oxide particles (73–105 nm in size). (ii) The tensile strength and fracture toughness exhibited by the doped molybdenum alloys are much higher than those of pure molybdenum. (iii) Quantitative analyses indicate that while, both Hall–Petch and Orowan mechanisms play a significant role in strengthening the multi-components rare earth oxide-doped molybdenum alloys, this strengthening is dominantly governed by grain size reduction. Acknowledgement This study was supported by the National High Technology Research and Development Program of China (Grant No. 2003AA33X040). References [1] A.J. Mueller, R. Bianco, R.W. Buckman, Int. J. Refract. Met. Hard Mater. 18 (2000) 205–211. [2] A. Crowson, E.S. Chen, J.A. Shields, P.R. Subramanian, Molybdenum and Molybdenum Alloys, TMS, San Antonio, 1998. [3] J.X. Zhang, L. Liu, M.L. Zhou, et al., Int. J. Refract. Met. Hard Mater. 17 (1999) 405–409. [4] G.J. Zhang, Y.J. Sun, R.M. Niu, J. Sun, et al., Adv. Eng. Mater. 6 (2004) 943–948. [5] L.E. Iorio, B.P. Bewlay, M. Larsen, Int. J. Refract. Met. Hard Mater. 24 (2006) 306–310. [6] G.J. Zhang, Y.J. Sun, R.M. Niu, J. Sun, et al., The 16th International Plansee Seminar on “Powder Metallurgical High Performance Materials”, Reutee, 2005, pp. 1089–1095. [7] A. Kelly, R.B. Nicholson, Strengthening Methods in Crystals, Elsevier Press, New York, 1971. [8] A.W. Zhu, E.A. Starke Jr., Acta Mater. 47 (1999) 3263–3269. [9] A.W. Zhu, J. Chen, E.A. Starke Jr., Acta Mater. 48 (2000) 2239– 2246. [10] Z.Y. Ma, Y.L. Li, Y. Liang, F. Zheng, J. Bi, S.C. Tjong, Mater. Sci. Eng. A 219 (1996) 229–231. [11] M.A. Munoz-Morris, C.G. Oca, D.G. Morris, Acta Mater. 50 (2002) 2825–2836. [12] R.W. Cahn, P. Haasen, E.J. Kramer, Materials Science and Technology: Structure and Properties of Nonferrous Alloys, vol. 8, VCH, Weinheim, 1996.
Microstructure and mechanical properties of multi-components rare earth oxide-doped molybdenum alloys Guo-jun Zhang ∗ , Yuan-jun Sun, Chao Zuo, Jian-feng Wei, Jun Sun State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China Received 6 June 2006; received in revised form 28 August 2006; accepted 29 August 2006
Abstract Pure molybdenum and molybdenum alloys doped with two- or three-components rare earth oxide particles were prepared by powder metallurgy. Both the tensile property and fracture toughness of the pure molybdenum and multi-components rare earth oxide-doped molybdenum alloys were determined at room temperature. The multi-components rare earth oxide-doped molybdenum alloys are fine grained and contain a homogeneous distribution of fine particles in the submicron and nanometer size ranges, which is why the molybdenum alloys have higher strength and fracture toughness than pure molybdenum. Quantitative analysis is used to explain the increase in yield strength with respect to grain size and second phase strengthening. Furthermore, the relationship between the tensile properties and microstructural parameters is quantitatively established. © 2007 Elsevier B.V. All rights reserved. Keywords: Multi-components; Rare earth oxide; Molybdenum alloy; Mechanical property
1. Introduction With their high melting point, high-temperature strength, low thermal-expansion coefficient and high thermal/electrical conductivity, molybdenum and its alloys have become a popular material for high-temperature structural parts, power semiconductor components and glass-melting electrodes. However, further applications are limited by brittleness. Many new molybdenum alloys with improved ductility have been developed among which the oxide-doped molybdenum alloy is of special interest. This alloy, prepared by adding a proper amount of rare earth oxide (La2 O3 , Y2 O3 , etc.) followed by substantial deformation, exhibits a much higher recrystallization temperature than commercial pure molybdenum [1,2]. Molybdenum alloys have not only superior strength and ductility but also superior fracture toughness as compared with pure molybdenum [3,4]. Whereas many studies have been carried out on the microstructure and mechanical properties of single component oxide-doped molybdenum alloys [1–6], little is known on the multi-component oxide-doped molybdenum alloy so far. This
∗
Corresponding author. Tel.: +86 135 1917 3936; fax: +86 29 8231 2617. E-mail address: [email protected] (G.-j. Zhang).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.143
work is aimed at investigating molybdenum alloys doped with two or three components of rare earth oxide, and especially at establishing a relationship between microstructure parameters and mechanical properties, and to compare this with the behaviour of pure molybdenum alloy(s). Another objective of this work is to correlate the increase in tensile properties with changes in microstructure. 2. Experimental procedures The pure molybdenum and molybdenum alloys doped with two or three components amongst oxide lanthanum (La2 O3 ), yttria (Y2 O3 ) and ceria (CeO2 ) particles were prepared by powder metallurgy. All the doping oxides have the same total mass fraction in spite of different combinations and relative content(s), as shown in Table 1. The rare earth oxide was added to molybdenum oxide via an aqueous solution of nitrates. The doped molybdenum oxide powders were reduced into doped molybdenum powder in dry hydrogen. The doped molybdenum powder was cold-isostatically pressed into a cylinder 17 mm in diameter, and then sintered at 1850 ◦ C for 4 h in flowing dry hydrogen. Finally, the cylindrical compact was thermomechanically processed into a rod with a diameter of 7.8 mm. The rod was carefully annealed at 1250 ◦ C in a dry hydrogen atmosphere for 1 h.
G.-j. Zhang et al. / Materials Science and Engineering A 483–484 (2008) 350–352
351
Table 1 The rare earth oxides mass fraction of materials studied
Table 2 Microstructural parameters and calculated strength of the materials examined
Materials
Materials
Particle volume fraction
Grain
Particle
σ or (MPa)
σ HP (MPa)
Mo-LY1 Mo-LY2 Mo-LC1 Mo-LC2 Mo-LC3 Mo-YC1 Mo-YC2 Mo-LCY
0.730 0.721 0.686 0.694 0.690 0.733 0.708 0.715
2.1 2.4 2.3 1.9 2.1 1.3 1.6 2.2
90 102 105 84 76 73 87 82
97.9 87.9 83.5 100.6 108.6 116.0 98.9 104.2
208.1 193.1 203.6 177.5 175.5 214.0 187.2 195.8
Mo-LY1 Mo-LY2 Mo-LC1 Mo-LC2 Mo-LC3 Mo-YC1 Mo-YC2 Mo-LCY
Oxide mass fraction (%) La2 O3
Y2 O3
CeO2
0.3 0.4 0.2 0.4 0.3 – – 0.2
0.3 0.2 – – – 0.4 0.2 0.2
– – 0.4 0.2 0.3 0.2 0.4 0.2
At room temperature, dog-bone shaped tensile specimens, with a gage size of 5 mm in diameter and 25 mm in length, were tested at a constant strain rate of 1.3 × 10−3 s−1 in a servohydraulic Instron-1195 testing machine. Fracture toughness was measured in three-point bending at a slow loading rate of 0.0025 mm/s in accordance with the ASTM E399 method at room temperature. A support span of 20 mm was used for the bend specimens. The specimens were fatigue-precracked prior to fracture toughness testing. The microstructure of the alloy was observed by transmission electron microscopy (TEM). The foils were produced by electropolishing in a twin-jet apparatus with a solution of 12.5 vol.% H2 SO4 in ethanol at −15 ◦ C. A voltage of 10 V produced a current density of approximately 3 mA/mm2 . A computer image analyzer was also used to measure the size and distribution of microstructural parameters of the alloys studied. 3. Results and discussions 3.1. Microstructure Grain sizes and morphology of the pure molybdenum and molybdenum alloys were investigated by TEM, as typically illustrated in Fig. 1a and b, respectively. It is clearly seen that the multi-components rare earth oxide-doped molybdenum alloys have a finer grain size than pure molybdenum. In the molybdenum alloys, the oxide ellipsoidal particles, with submicron or nanometer in size, are homogeneously distributed (Fig. 1c). The average grain size (d), volume fraction (fv ) and average diameter (φ) of the oxide particles have been determined
(Table 2). At least 100 grains and 200 oxide particles were measured within random sections of each alloy. 3.2. Mechanical properties Fig. 2 shows that different molybdenum alloys exhibit varying yield strengths (measured at 0.2%), all larger than that of the pure molybdenum. The fracture toughness, which is also shown in Fig. 2, is similarly larger than that of pure molybdenum. Since the samples are limited in size and cannot meet the geometry requirements for plane–strain constraint, only KQ values are used for comparison and analysis in the present paper. 3.3. Strengthening mechanisms We expect two main contributions to the yield strength: (a) a strengthening due to a reduction in grain size via a Hall–Petch mechanism (σ HP ) and (b) some particle strengthening via the Orowan mechanism (σ or ). These have been assumed to be additive in nature and the yield strength σ y can be decomposed into: σy = σm + σHP + σor = σm +
k + σor d 1/2
(1)
where σ m is the matrix strength of the material (very largegrained molybdenum) in the absence of any other strengthening mechanism and has been evaluated to 417 MPa [4], k a constant, i.e., the Hall–Petch slope and d is the matrix grain size. TEM examinations observations (Fig. 3) have confirmed that when dislocations pass by the oxide particles, they tend to bow out between the particles leaving Orowan loops around the par-
Fig. 1. The molybdenum grain sizes and morphology of (a) pure molybdenum and (b) yttria and ceria bi-components doped molybdenum alloy (Mo-YC1) in radial direction and (c) the oxide particles sizes and morphology of the oxide lanthanum, yttria and ceria tri-components doped molybdenum alloy (Mo-LCY).
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G.-j. Zhang et al. / Materials Science and Engineering A 483–484 (2008) 350–352
fraction and sizes. From Eq. (1), we then derive the strength contribution caused by the reduction in grain size, σ HP (Table 2). The grain size strengthening contributes about 175–214 MPa to hardening, actually more than the oxide particle strengthening. This result can also be simply obtained from Eq. (3) indicating that the Orowan stress increases with the increase in particle volume fraction and/or the decrease in particle size. Since in the present alloys, the volume fraction is low (about 0.7%) and does not change remarkably, the increase in yield stress with respect to particle free alloys is thus correlated with grain size when one considers the peak for the alloy of Mo-YC1 and Mo-YC2 in Fig. 2. 4. Conclusion Fig. 2. The multi-components rare earth oxide-doped molybdenum alloys have higher strength and fracture toughness than pure molybdenum.
ticles. As a result, an additional stress is needed as described in the Orowan model [7–9]. For quantitative purposes, the traditional Orowan–Ashby equation [9] is used to assess the increase in yield strength due to the presence of oxide particles: mμb φ σor = ln (2) 1.18 × 2π(λ − φ) 2b where m is the Taylor factor, μ the shear modulus, b the magnitude of the Burgers vector, φ the particle size and λ is the interparticle spacing in the shear plane of the dislocation. Generally, the particle size and interparticle spacing are related to the volume fraction, fv , as λ = φ(6fv /π)−1/3 [10], so that Eq. (2) can be rewritten as: mμb φ √ ln σor = (3) 2b 1.18 × 2πφ π/6f − 1 The Taylor factor is taken as 2.5 for the bcc crystal structure [11], the shear modulus of molybdenum alloys as 140 GPa [12], and the Burgers vector (a/2 1 1 1) as 2.72 × 10−10 m. Table 2 shows that Orowan strengthening contributes about 83–116 MPa for the investigated range of oxide particles volume
Fig. 3. Dislocations pass by oxide particles and tend to bend like bow between particles.
(i) Multi-components rare earth oxide-doped molybdenum alloys have been successfully prepared with a finely grained microstructure (1.3–2.4 m in grain size) and uniformly distributed oxide particles (73–105 nm in size). (ii) The tensile strength and fracture toughness exhibited by the doped molybdenum alloys are much higher than those of pure molybdenum. (iii) Quantitative analyses indicate that while, both Hall–Petch and Orowan mechanisms play a significant role in strengthening the multi-components rare earth oxide-doped molybdenum alloys, this strengthening is dominantly governed by grain size reduction. Acknowledgement This study was supported by the National High Technology Research and Development Program of China (Grant No. 2003AA33X040). References [1] A.J. Mueller, R. Bianco, R.W. Buckman, Int. J. Refract. Met. Hard Mater. 18 (2000) 205–211. [2] A. Crowson, E.S. Chen, J.A. Shields, P.R. Subramanian, Molybdenum and Molybdenum Alloys, TMS, San Antonio, 1998. [3] J.X. Zhang, L. Liu, M.L. Zhou, et al., Int. J. Refract. Met. Hard Mater. 17 (1999) 405–409. [4] G.J. Zhang, Y.J. Sun, R.M. Niu, J. Sun, et al., Adv. Eng. Mater. 6 (2004) 943–948. [5] L.E. Iorio, B.P. Bewlay, M. Larsen, Int. J. Refract. Met. Hard Mater. 24 (2006) 306–310. [6] G.J. Zhang, Y.J. Sun, R.M. Niu, J. Sun, et al., The 16th International Plansee Seminar on “Powder Metallurgical High Performance Materials”, Reutee, 2005, pp. 1089–1095. [7] A. Kelly, R.B. Nicholson, Strengthening Methods in Crystals, Elsevier Press, New York, 1971. [8] A.W. Zhu, E.A. Starke Jr., Acta Mater. 47 (1999) 3263–3269. [9] A.W. Zhu, J. Chen, E.A. Starke Jr., Acta Mater. 48 (2000) 2239– 2246. [10] Z.Y. Ma, Y.L. Li, Y. Liang, F. Zheng, J. Bi, S.C. Tjong, Mater. Sci. Eng. A 219 (1996) 229–231. [11] M.A. Munoz-Morris, C.G. Oca, D.G. Morris, Acta Mater. 50 (2002) 2825–2836. [12] R.W. Cahn, P. Haasen, E.J. Kramer, Materials Science and Technology: Structure and Properties of Nonferrous Alloys, vol. 8, VCH, Weinheim, 1996.