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Microstructure and high temperature deformation behavior of the Mo-ZrO2 alloys
Journal of Alloys and Compounds 716 (2017) 321e329
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and high temperature deformation behavior of the Mo-ZrO2 alloys Chaopeng Cui a, *, Yimin Gao a, Shizhong Wei b, c, **, Guoshang Zhang b, Yucheng Zhou c, Xiangwei Zhu b a
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Engineering Research Center of Tribology and Materials Protection, Ministry of Education, Henan University of Science and Technology, Luoyang 471003, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2017 Received in revised form 23 April 2017 Accepted 2 May 2017 Available online 4 May 2017
The Mo-ZrO2 alloys were prepared by a hydrothermal synthesis and a powder metallurgy process. The grain size, density and hardness of the alloy sintered compacts were investigated. The deformation behavior of the alloy at high temperature was studied by using a Gleeble-1500D thermal simulated test machine. The strengthening effect of zirconia particles was found during the deformation process of the alloy at high temperature. The result shows the mechanical properties of the Mo-ZrO2 alloys were obviously improved due to the uniform distribution of the small second phase. After adding ZrO2, the hardness of the alloys has increased by 40%, and the high temperature strength of the alloy was increased. At the temperature of 1400 C, the strength of the alloys was increased by 35% compared with that of the pure molybdenum. The ZrO2 particles can effectively improve the deformation resistance, resulting in increase of the mechanical properties of the Mo-ZrO2 alloys. © 2017 Elsevier B.V. All rights reserved.
Keywords: High-temperature alloys Powder metallurgy Grain boundaries Microstructure True strain Deformation behavior
1. Introduction The melting point of molybdenum is as high as 2620 C, and molybdenum can keep good mechanical strength at high temperature. The molybdenum has good dimensional stability at high temperature due to its small linear expansion coefficient; The thermal conductivity of molybdenum is relatively high, which is several times higher than that of many high temperature alloys. Thus, molybdenum alloys are widely used in aerospace, electronic communication and electrical equipment as refractory metal above 1000 C [1e5]. However, when the temperature is at or above 1600 C, the strength of pure molybdenum will sharply decrease with brittle fracture behavior, which would limit its application [6,7]. Many researchers found that the high temperature properties of molybdenum alloys could be improved by introducing alloy elements or second phase particles [8,9]. Then many kinds of
molybdenum alloys with high-temperature mechanical properties were developed. Thus, the application range of molybdenum became wider. Elements Ti and Zr were added to molybdenum, called TZM alloy, which had solid solution reaction with elements and formed oxides and carbides at the same time [5]. TZM alloy has better performance than pure molybdenum. Later, after adding lanthanum oxide and alumina, the mechanical properties of molybdenum can be enhanced by controlling the grain size and the distribution of the second phase particles in the alloy [3,5,6]. In this paper, the Mo-ZrO2 alloys were prepared by a hydrothermal process and a powder metallurgy method [10]. The high temperature mechanical properties of the sintered compacts were tested. The effect of the second phase on the microstructure and mechanical properties of the alloys was investigated.
2. Material and methods * Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China. E-mail address: [email protected] (C. Cui). http://dx.doi.org/10.1016/j.jallcom.2017.05.013 0925-8388/© 2017 Elsevier B.V. All rights reserved.
2.1. Material preparation The precursor powders of ZrO2 and MoO3 were prepared by the
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hydrothermal method. The mixed powders of ZrO2 and MoO3 can be obtained by calcination. The composite powders of Mo and ZrO2 could be obtained by two stage reduction processes, and the MoZrO2 alloy was prepared by powder metallurgy technology. The pressure value was 280 MPa, holding 5 min (by cold and static pressing (CIP)). The samples with size of Ø8mm 12 mm were sintered at 2000 C under H2 atmosphere. 2.2. Experiment procedure
Fig. 1. Schematic diagram of thermal simulation compression experiment device.
The deformation behaviors of the sample at the temperatures of 900 C, 1100 C, 1200 C, 1300 C, and 1400 C were studied respectively on a Gleeble-1500D thermal simulated test machine, under the following conditions: heating rate: 10 C/s, holding time
Fig. 2. Microstructure and morphology of molybdenum alloy sintered compact.
C. Cui et al. / Journal of Alloys and Compounds 716 (2017) 321e329
at the test temperature: 3min, experimental atmosphere: high vacuum, deformation rate: 50% and strain rate: 0.1s1. The main structure of the testing machine was shown in Fig. 1. In the thermal simulation process, due to the large deformation resistance of the high temperature molybdenum alloy, pure tungsten plug with the dimension of Ø20 mm 40 mm can be adopted. Also, to avoid drumming belly problem during the compression, end-face friction should be reduced as much as possible. And the experiments were performed in a vacuum environment. The transverse strain sensor continuously measures the diameter of the middle part of the sample when the sample was tested, and automatically calculating the cross section area of the sample. Then true stress can be obtained by the force signal value divided by the instantaneous cross-sectional area. The test machine automatically recorded and drew the true stress-strain relation curve.
micro holes or micro pores, and increases the strength and toughness of the alloy. In Fig. 2, the contrast sample containing 1% ZrO2 was prepared, in which the ZrO2 particles were added by a solid-solid adding method. The ZrO2 particles and bigger holes were obviously observed, while the ZrO2 particles in the alloy made by hydrothermal method are smaller and relatively scattered, which could be seen from the following partial enlarged image. Compared with the traditional method, the new technology has a very great improvement, which can improve the distribution and decrease the size of ZrO2 particles. The second phase of the new type of molybdenum alloy can not be observed under a low magnification. In this research, the second phase studied by using SEM at 30000 . The morphology and distribution of ZrO2 particles are shown in Fig. 3. The bigger ZrO2 particles are distributed on the grain boundaries, and the smaller ZrO2 particles are in the grains. Compared with those prepared by
3. Microstructure The microstructure was shown in Fig. 2. With the increase of ZrO2 content, the grain size of the alloys decreased obviously. It was shown that the grain size of the alloys was evidently reduced with the increase of dopant content, thus increasing the grain boundary area per unit volume and reducing the concentration of C, N, O and other impurities at the grain boundaries. In addition, a large number of uniformly distributed ZrO2 particles can adsorb impurity elements. Therefore, the concentration of impurity elements at the grain boundary is further reduced, which hinders the growth of
Fig. 3. Energy spectrum analysis of the second phase in the alloy.
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Fig. 4. Morphology and diffraction pattern of the second phase in the alloy.
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the traditional solid-solid addition [10e13], the size of ZrO2 is smaller and their distribution is relatively uniform. Larger ZrO2 particles are situated at grain boundaries or sub-boundaries, while most of smaller ZrO2 particles are located within Mo grains. The microstructures of the sintered billets were gradually refined with the increase in ZrO2 addition, and there is an obvious grain refinement effect as well. The ZrO2 particles can block the motion of the dislocations and change the propagation direction of the main crack. For these reasons, the mechanical properties of the alloys have effectively improved [11]. In order to control the phase transition of ZrO2, the stabilizer was added in the preparation process of zirconia. From the TEM picture, it can be seen that the stabilizer has played a stabilizing role. The phase of ZrO2 has a tetragonal crystal structure, which will avoid the reversible phase change less than 2000 C, as well as the 30% of the volume change caused by the phase change. Thus, it ensures the stability of the processing and performance of Mo/ZrO2 alloy. It can be seen from the SEM picture that the grain size of the alloy can be effectively refined by adding fine ZrO2 particles, controlling the grain size of the molybdenum alloy within 10 mm. The alloys with fine grains have better strength and also can obtain a certainty of plasticity due to the increase of slip surface. The ZrO2 particles prevent slippage, restrain cracks and reduce the dislocation density and crack density, then ultimately improving the plasticity of materials [14,15]. When there is a large second phase particle on the grain boundary in the process of metal deformation, the deformation can not be synchronized due to the difference in elastic modulus, resulting in grain boundary detachment and second phase fragmentation and become a source of cracking. It will reduce tensile strength and elongation rate of the material. At the same time, the second phase on the grain boundary breaks during the deformation process, which worsen the material processing performance and mechanical properties. The broken second phase is distributed on the grain boundary, which is beneficial to raise recrystallization temperature and improves tensile strength of the material [15e18]. From the TEM picture, it can be seen that the ZrO2 particles were small and the size was only 200 nm. From Fig. 4, it can be found that the ZrO2 particles were smaller and more dispersed compared with traditional solid-solid additive. The dislocation of alloy was effectively prevented, resulting in the increase of mechanical properties of the alloy. In the TEM picture, there are a lot of dislocation around the ZrO2 particles. As the dislocation extended around the ZrO2 particles, the hardness of ZrO2 particles is high. It can effectively avoid the expansion of the dislocation. Dislocation pile-up and tangling around ZrO2 particles were also pointed out. Later, the deformation behavior of Mo-ZrO2 alloy sintered billet at high temperature was tested in order to simulate the deformation behavior of the alloy at high temperature.
Fig. 5. Grain size of alloy sintered compact.
Fig. 6. Real density and theoretical density of alloy sintered compact.
4. Results and discussion The statistical information of the grain size of the alloys is shown in Fig. 5. It is observed that with the increase of the second phase particles in the alloy, the grain size of the alloys decreases obviously. The grain size of the alloy with 1.5% ZrO2 is lower than 10 mm, which is 1/3 that of the pure molybdenum. ZrO2 particles can effectively refine the grain size of the alloy, improve the grain boundary strength, reduce the dislocation density and make plastic deformation more uniformly. It can also reduce the effective length of the dislocation slip surface, alleviate the grain boundary dislocation, decrease the density of dislocations near the grain boundaries and the slip bands and slow crack formation and expansion
Fig. 7. Hardness of alloy sintered compact.
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Fig. 8. The Stress strain curve of the alloy sintered billet at high temperature.
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process [17e19]. The tested density and the theoretical density of the alloy is illustrated in Fig. 6. From the figure, the addition of ZrO2 has little effect on the actual density of the alloys. However, due to the sintering defects, the actual density of the alloy is less than the theoretical density, which is inevitable in the process of powder metallurgy. The actual density of the alloys moved closer and closer to the theoretical density with the increase of deformation. The hardness of the alloys was measured in Fig. 7. In the process of measuring micro hardness, the pressure exerted by the pressure head is shared by both the grain and grain boundary. With the increase of ZrO2, the microhardness of the alloys was increased by 40% due to dislocation strengthening and grain boundary strengthening as well as the second phase strengthening. The second phase particles (about 200e300 nm in size) are evenly distributed around the Mo grains, which can restrain the accumulation and growth of Mo grains during the sintering process. The fine particles were determined to be the ZrO2 particles. Finally, the tiny grains are acquired, which helps to obtain a higher strength for the Mo alloys. Through a series of studies on the microstructure, phase distribution, grain size, density and hardness of the alloy sintered compacts, it is found that the new alloys made by the new method have fine grain size and uniform size. With the increase of ZrO2, the grain size of the alloys decreases obviously and the second phase in the alloy is small and uniformly distributed. The size of the second phase in the alloy prepared by the traditional method decreased greatly. The problem of the second phase enrichment is serious, which seriously affects the mechanical properties and the plasticity of the alloy [16e20]. After adding ZrO2 particles, the density of the alloy decreases while the hardness increases. Deformation behavior of the alloy sintered billets at high temperature was studied. Fig. 8 is the true stress-strain relation curve of Mo-ZrO2 alloys at high temperature. It has simulated the stress and deformation conditions of alloy head at high temperature. By measuring the true stress-stain curve, it is found that the Mo-ZrO2 alloy has the great deformation resistance at a high temperature. With the increase of ZrO2, the deformation stress of the alloys increased. The alloys have obvious plastic deformation and yield in the compress process [15,16]. The ZrO2 particles can effectively improve the strength and plastic properties of the Mo-ZrO2 alloys at high temperature. It is vital to control the grain size of molybdenum alloys effectively for the overall mechanical properties of ZrO2 doped molybdenum alloy. ZrO2 as second phase was existed in molybdenum matrix, refining grains, increasing grain boundary area in unit volume, shortening the slippage of pile-up dislocation inside the grain, reducing quantity of dislocation pile-up, making the dislocation hardly occur and thus improving the material strength [20e24]. Depending on the analysis, there are two methods to reinforce ZrO2 doped molybdenum alloy: fine grain reinforcing and particulate reinforcing. After being doped with ZrO2, the grain size of molybdenum matrix becomes smaller, and the number of Mo substrate grains per unit volume increases. If the deformation is fixed, the deformation will take place in grains, and the deformation of grains will be more uniform. Firstly, amount of fine Mo grains can reduce the dislocation pile-up density and the possibility of cracking caused by stress concentration as well as cracks. Secondly, considerable dislocation pile-ups may occur during deformation of the material as there are many fine ZrO2 particles in the molybdenum alloys and the vicinity of such fine particles, resulting in stress concentration. When the stress concentration is enough great, fine cracks will appear at the joining interfaces, and even the molybdenum matrix and the ZrO2 particles will be separated,
deboned or pulled away from each other, relieving the stress concentration, passivating the crack tip and improving strength of the material [22e26]. Therefore, ZrO2 particles can refine grains, purify grain boundaries, and hinder the dislocation, thus improving the resistance to deformation and enhancing the strength of the molybdenum alloys. It can be observed that the nano-ZrO2 particles can effectively increase the strength, especially with the increase of ZrO2 addition. The strengthening effect of the second phase particles at high temperature was calculated, in order to study the strengthening effect of ZrO2 particles.
m¼(S1-S2)/S1 where m is the strengthening effect of ZrO2 particles, S1 is the maximum stress of the alloy and S2 is the maximum stress of the Pure molybdenum. In Fig. 9, the strengthening effect increased with the increase of temperature. The strengthening effect of ZrO2 particles is obvious when the temperature is above 1300 C. During the alloy deformation at high temperatures, the doping ZrO2 particles can improve the deformation resistance of the alloy sintered compacts by the second-phase-based reinforcement. With the increase of temperature, this reinforcement has shown a significant increase. The strengthening effect of Mo-1.5% ZrO2 alloy has reached 35% After the high-temperature deformation, the samples of pure molybdenum and Mo-1.0% ZrO2 are cut in the axial direction, shown in Fig. 10, Fig. 11 and Fig. 12. It can be seen in Fig. 10 that the cracks become more numerous and eventually larger with the increase in temperature. Nevertheless, there are a few cracks in Mo1.0%ZrO2 alloy. The plasticity of the alloy has greatly improved compared with pure molybdenum. This is because grains refined by ZrO2 particles, resulting in the increase of grain boundaries and dislocated slippage. With dislocation density and crack density decreased greatly, so few or no cracks are seen in the ZrO2 doped sintered alloy compacts which show a superior plasticity at high temperature. The displacement of ZrO2 particles hindering grain boundary slippage is obvious in the Mo-1.0% ZrO2 alloy. No obvious crack is seen in the alloy with raising temperature, but cavity begins to appear. During the deformation, the strength of Mo matrix is less than that of the second phase which imposes intense pinning on
Fig. 9. The strengthening effect of second phase particles at high temperature.
C. Cui et al. / Journal of Alloys and Compounds 716 (2017) 321e329
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Fig. 10. The morphology of pure molybdenum samples after experiment.
the displaced grains and gives rise to the cavities. With the comprehensive consideration of strength and performance of the alloy material, it can be concluded that the strength and plasticity of the alloy can be improved at high temperatures by adding ZrO2 particles, reducing crack propagation, decreasing the possibility of presence of cracks by refining grains and retarding cracks. It can be noted that ZrO2 in the alloy can effectively hinder dislocation and crack propagation.
5. Conclusion 1. The prepared Mo-ZrO2 alloy have fine grain size. The small size of the second phase in the alloy is only 100e200 nm. Big ZrO2 particles are at grain boundaries, while the small ZrO2 particles are in grains. The size and distribution of ZrO2 have been greatly improved compared with those prepared by the traditional solid-solid addition.
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Fig. 11. The morphology of Mo-1.0% ZrO2 alloy after experiment.
2. After adding ZrO2 into the alloy, the grains can be refined effectively. The minimum grain size of the alloy sintered compact is only 1/3 that of the pure molybdenum. Also, with the addition of ZrO2, the density of the alloy decreased and the hardness of the alloy increased by 40%. 3. The high temperature strength of the sintered compacts was improved by adding ZrO2 into the alloys. The strength of the
alloy decreases with the increase of the temperature and the high temperature strength increases with the increase of ZrO2. The strength of the alloys is 35% higher than that of the pure molybdenum at the temperature of 1400 C. The addition of ZrO2 can effectively improve the deformation resistance of the alloy.
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Fig. 12. Analysis of the second phase in the microstructure of Mo-0.1%ZrO2 samples after experiment.
Acknowledgments This work was supported by Program for Chang Jiang Scholars and Innovative Research Team in University(IRT1234), National Natural Science Foundation of China(50972039) and Plan for Scientific Innovation Talent of Henan Province, China [No. 2017JQ0012]. References [1] E. Ahmadi, M. Malekzadeh, S.K. Sadrnezhaad, Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering, Int. J.
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Refract. Met. Hard Mater 29 (2011) 141e145. [2] G. Bai, R.W. Gao, Y. Sun, Study of high-coercivity sintered NdFeB magnets, J. Magn. Magn. MATER 308 (2007) 20e23. [3] S.P. Chakraborty, S. Banerjee, K. Singh, Studies on the development of protective coating on TZM alloy and its subsequent characterization, J. Mater. Process. Tech. 207 (2008) 240e247. [4] B.V. Cockeram, The role of stress state on the fracture toughness and toughening mechanisms of wrought molybdenum and molybdenum alloys, Mat. Sci. Eng. A Struct. A 528 (2010) 288e308. [5] B.V. Cockeram, W. Smith, T.S. Byun, The change in the hardness of LCAC, TZM, and ODS molybdenum in the post-irradiated and annealed conditions, J. Nucl. Mater. 393 (2009) 12e21. [6] B.V. Cockeram, R.W. Smith, N. Hashimoto, The swelling, microstructure, and hardening of wrought LCAC, TZM, and ODS molybdenum following neutron irradiation, J. Nucl. Mater. 418 (2011) 121e136. [7] B. Gludovatz, S. Wurster, A. Hoffmann, Fracture toughness of polycrystalline tungsten alloys, Int. J. Refract. Met. Hard Mater. 28 (2010) 674e678. [8] W.F. Ho, S.C. Wu, S.K. Hsu, Effects of molybdenum content on the structure and mechanical properties of as-cast Tie10Zr-based alloys for biomedical applications, Mat. Sci. Eng. C Mater. 32 (2012) 517e522. [9] G. Liu, G.J. Zhang, F. Jiang, Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater 12 (2013) 344e350. [10] T.M. Li, W. Zeng, Y.Y. Zhang, Nanobelt-assembled nest-like MoO3 hierarchical structure: hydrothermal synthesis and gas-sensing properties, Mater. Lett. 160 (2015) 476e479. [11] C.P. Cui, Y.M. Gao, S.Z. Wei, Preparation and properties of ZrO2/Mo alloys, High. Temp. Mat. PR-ISR 36 (2017) 163e166. [12] N.M.D. Vitorino, A.V. Kovalevsky, J.C.C. Abrantes, Hydrothermal synthesis of boehmite in cellular alumina monoliths for catalytic and separation applications, J. Eur. Ceram. Soc. 35 (2015) 3119e3125. [13] X.M. Liu, C.G. Niu, X.P. Zhen, Novel approach for synthesis of boehmite nanostructures and their conversion to aluminum oxide nanostructures for remove Congo red, J. Colloid Interf. Sci. 452 (2015) 116e125. [14] G. Rani, P.D. Sahare, Effect of temperature on structural and optical properties of boehmite nanostructure, Int. J. Appl. Ceram. Tec. 12 (2015) 124e132. [15] M. Aziz, D.C. Hudson, S. Russo, Molybdenum-rhenium superconducting suspended nanostructures, Appl. Phys. Lett. 104 (2014) 297e346. [16] S.H. Kim, S.J. Kang, M.H. Park, Vacancy-mediated omega-assisted alpha-phase formation mechanism in titanium-molybdenum alloy, Acta Mater. 83 (2015) 499e506. [17] D. Lang, C. Poehl, J. Schatte, On the precipitation mechanism in the molybdenum based alloy MHC (Mo-Hf-C), Powder Metall. 57 (2014) 311e313. [18] C. Poehl, J. Schatte, H. Leitner, Solid solution hardening of molybdenumhafnium alloys: experiments and modeling, Mat. Sci. Eng. A Struct. A 559 (2013) 643e650. [19] D. Lang, C. Poehl, D. Holec, On the chemistry of the carbides in a molybdenum base Mo-Hf-C alloy produced by powder metallurgy, J. Alloy Compd. 654 (2016) 445e454. [20] C. Poehl, J. Schatte, H. Leitner, Solid solution softening of polycrystalline molybdenum-hafnium alloys[J], J. Alloy Compd. 576 (2013) 250e256. [21] H.C. Cheng, J.L. Fan, Z. Qian, Effects of trace ZrC on the microstructure and properties of molybdenum alloy, J. Mater. Res. 785 (2013) 76e80. [22] P.F. Feng, Q.L. Yang, X.M. Dang, Reaction mechanism of lanthanum nitratedoped Mo-La material during solid-liquid spray doping processing, Rare Met. 34 (2015) 814e817. [23] H.M. Hu, C.H. Deng, J.C. Xu, Metastable h-MoO3 and stable alpha-MoO3 microstructures: controllable synthesis, growth mechanism and their enhanced photocatalytic activity, J. Exp. Nanosci. 10 (2015) 1336e1346. [24] A. Phuruangrat, J.S. Chen, X.W. Lou, Hydrothermal synthesis and electrochemical properties of alpha-MoO3 nanobelts used as cathode materials for Li-ion batteries, Appl. Phys. A Mater. 107 (2012) 249e254. [25] Z.Y. Wang, S. Madhavi, X.W. Lou, Ultralong alpha-MoO3 nanobelts: synthesis and effect of binder choice on their lithium storage properties, J. Phys. Chem. C 116 (2012) 12508e12513. [26] R. Nadimicherla, W. Chen, X. Guo, Synthesis and characterization of alphaMoO3 nanobelt composite positive electrode materials for lithium battery application, Mater. Res. Bull. 66 (2015) 140e146.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and high temperature deformation behavior of the Mo-ZrO2 alloys Chaopeng Cui a, *, Yimin Gao a, Shizhong Wei b, c, **, Guoshang Zhang b, Yucheng Zhou c, Xiangwei Zhu b a
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Engineering Research Center of Tribology and Materials Protection, Ministry of Education, Henan University of Science and Technology, Luoyang 471003, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2017 Received in revised form 23 April 2017 Accepted 2 May 2017 Available online 4 May 2017
The Mo-ZrO2 alloys were prepared by a hydrothermal synthesis and a powder metallurgy process. The grain size, density and hardness of the alloy sintered compacts were investigated. The deformation behavior of the alloy at high temperature was studied by using a Gleeble-1500D thermal simulated test machine. The strengthening effect of zirconia particles was found during the deformation process of the alloy at high temperature. The result shows the mechanical properties of the Mo-ZrO2 alloys were obviously improved due to the uniform distribution of the small second phase. After adding ZrO2, the hardness of the alloys has increased by 40%, and the high temperature strength of the alloy was increased. At the temperature of 1400 C, the strength of the alloys was increased by 35% compared with that of the pure molybdenum. The ZrO2 particles can effectively improve the deformation resistance, resulting in increase of the mechanical properties of the Mo-ZrO2 alloys. © 2017 Elsevier B.V. All rights reserved.
Keywords: High-temperature alloys Powder metallurgy Grain boundaries Microstructure True strain Deformation behavior
1. Introduction The melting point of molybdenum is as high as 2620 C, and molybdenum can keep good mechanical strength at high temperature. The molybdenum has good dimensional stability at high temperature due to its small linear expansion coefficient; The thermal conductivity of molybdenum is relatively high, which is several times higher than that of many high temperature alloys. Thus, molybdenum alloys are widely used in aerospace, electronic communication and electrical equipment as refractory metal above 1000 C [1e5]. However, when the temperature is at or above 1600 C, the strength of pure molybdenum will sharply decrease with brittle fracture behavior, which would limit its application [6,7]. Many researchers found that the high temperature properties of molybdenum alloys could be improved by introducing alloy elements or second phase particles [8,9]. Then many kinds of
molybdenum alloys with high-temperature mechanical properties were developed. Thus, the application range of molybdenum became wider. Elements Ti and Zr were added to molybdenum, called TZM alloy, which had solid solution reaction with elements and formed oxides and carbides at the same time [5]. TZM alloy has better performance than pure molybdenum. Later, after adding lanthanum oxide and alumina, the mechanical properties of molybdenum can be enhanced by controlling the grain size and the distribution of the second phase particles in the alloy [3,5,6]. In this paper, the Mo-ZrO2 alloys were prepared by a hydrothermal process and a powder metallurgy method [10]. The high temperature mechanical properties of the sintered compacts were tested. The effect of the second phase on the microstructure and mechanical properties of the alloys was investigated.
2. Material and methods * Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China. E-mail address: [email protected] (C. Cui). http://dx.doi.org/10.1016/j.jallcom.2017.05.013 0925-8388/© 2017 Elsevier B.V. All rights reserved.
2.1. Material preparation The precursor powders of ZrO2 and MoO3 were prepared by the
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hydrothermal method. The mixed powders of ZrO2 and MoO3 can be obtained by calcination. The composite powders of Mo and ZrO2 could be obtained by two stage reduction processes, and the MoZrO2 alloy was prepared by powder metallurgy technology. The pressure value was 280 MPa, holding 5 min (by cold and static pressing (CIP)). The samples with size of Ø8mm 12 mm were sintered at 2000 C under H2 atmosphere. 2.2. Experiment procedure
Fig. 1. Schematic diagram of thermal simulation compression experiment device.
The deformation behaviors of the sample at the temperatures of 900 C, 1100 C, 1200 C, 1300 C, and 1400 C were studied respectively on a Gleeble-1500D thermal simulated test machine, under the following conditions: heating rate: 10 C/s, holding time
Fig. 2. Microstructure and morphology of molybdenum alloy sintered compact.
C. Cui et al. / Journal of Alloys and Compounds 716 (2017) 321e329
at the test temperature: 3min, experimental atmosphere: high vacuum, deformation rate: 50% and strain rate: 0.1s1. The main structure of the testing machine was shown in Fig. 1. In the thermal simulation process, due to the large deformation resistance of the high temperature molybdenum alloy, pure tungsten plug with the dimension of Ø20 mm 40 mm can be adopted. Also, to avoid drumming belly problem during the compression, end-face friction should be reduced as much as possible. And the experiments were performed in a vacuum environment. The transverse strain sensor continuously measures the diameter of the middle part of the sample when the sample was tested, and automatically calculating the cross section area of the sample. Then true stress can be obtained by the force signal value divided by the instantaneous cross-sectional area. The test machine automatically recorded and drew the true stress-strain relation curve.
micro holes or micro pores, and increases the strength and toughness of the alloy. In Fig. 2, the contrast sample containing 1% ZrO2 was prepared, in which the ZrO2 particles were added by a solid-solid adding method. The ZrO2 particles and bigger holes were obviously observed, while the ZrO2 particles in the alloy made by hydrothermal method are smaller and relatively scattered, which could be seen from the following partial enlarged image. Compared with the traditional method, the new technology has a very great improvement, which can improve the distribution and decrease the size of ZrO2 particles. The second phase of the new type of molybdenum alloy can not be observed under a low magnification. In this research, the second phase studied by using SEM at 30000 . The morphology and distribution of ZrO2 particles are shown in Fig. 3. The bigger ZrO2 particles are distributed on the grain boundaries, and the smaller ZrO2 particles are in the grains. Compared with those prepared by
3. Microstructure The microstructure was shown in Fig. 2. With the increase of ZrO2 content, the grain size of the alloys decreased obviously. It was shown that the grain size of the alloys was evidently reduced with the increase of dopant content, thus increasing the grain boundary area per unit volume and reducing the concentration of C, N, O and other impurities at the grain boundaries. In addition, a large number of uniformly distributed ZrO2 particles can adsorb impurity elements. Therefore, the concentration of impurity elements at the grain boundary is further reduced, which hinders the growth of
Fig. 3. Energy spectrum analysis of the second phase in the alloy.
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Fig. 4. Morphology and diffraction pattern of the second phase in the alloy.
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the traditional solid-solid addition [10e13], the size of ZrO2 is smaller and their distribution is relatively uniform. Larger ZrO2 particles are situated at grain boundaries or sub-boundaries, while most of smaller ZrO2 particles are located within Mo grains. The microstructures of the sintered billets were gradually refined with the increase in ZrO2 addition, and there is an obvious grain refinement effect as well. The ZrO2 particles can block the motion of the dislocations and change the propagation direction of the main crack. For these reasons, the mechanical properties of the alloys have effectively improved [11]. In order to control the phase transition of ZrO2, the stabilizer was added in the preparation process of zirconia. From the TEM picture, it can be seen that the stabilizer has played a stabilizing role. The phase of ZrO2 has a tetragonal crystal structure, which will avoid the reversible phase change less than 2000 C, as well as the 30% of the volume change caused by the phase change. Thus, it ensures the stability of the processing and performance of Mo/ZrO2 alloy. It can be seen from the SEM picture that the grain size of the alloy can be effectively refined by adding fine ZrO2 particles, controlling the grain size of the molybdenum alloy within 10 mm. The alloys with fine grains have better strength and also can obtain a certainty of plasticity due to the increase of slip surface. The ZrO2 particles prevent slippage, restrain cracks and reduce the dislocation density and crack density, then ultimately improving the plasticity of materials [14,15]. When there is a large second phase particle on the grain boundary in the process of metal deformation, the deformation can not be synchronized due to the difference in elastic modulus, resulting in grain boundary detachment and second phase fragmentation and become a source of cracking. It will reduce tensile strength and elongation rate of the material. At the same time, the second phase on the grain boundary breaks during the deformation process, which worsen the material processing performance and mechanical properties. The broken second phase is distributed on the grain boundary, which is beneficial to raise recrystallization temperature and improves tensile strength of the material [15e18]. From the TEM picture, it can be seen that the ZrO2 particles were small and the size was only 200 nm. From Fig. 4, it can be found that the ZrO2 particles were smaller and more dispersed compared with traditional solid-solid additive. The dislocation of alloy was effectively prevented, resulting in the increase of mechanical properties of the alloy. In the TEM picture, there are a lot of dislocation around the ZrO2 particles. As the dislocation extended around the ZrO2 particles, the hardness of ZrO2 particles is high. It can effectively avoid the expansion of the dislocation. Dislocation pile-up and tangling around ZrO2 particles were also pointed out. Later, the deformation behavior of Mo-ZrO2 alloy sintered billet at high temperature was tested in order to simulate the deformation behavior of the alloy at high temperature.
Fig. 5. Grain size of alloy sintered compact.
Fig. 6. Real density and theoretical density of alloy sintered compact.
4. Results and discussion The statistical information of the grain size of the alloys is shown in Fig. 5. It is observed that with the increase of the second phase particles in the alloy, the grain size of the alloys decreases obviously. The grain size of the alloy with 1.5% ZrO2 is lower than 10 mm, which is 1/3 that of the pure molybdenum. ZrO2 particles can effectively refine the grain size of the alloy, improve the grain boundary strength, reduce the dislocation density and make plastic deformation more uniformly. It can also reduce the effective length of the dislocation slip surface, alleviate the grain boundary dislocation, decrease the density of dislocations near the grain boundaries and the slip bands and slow crack formation and expansion
Fig. 7. Hardness of alloy sintered compact.
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Fig. 8. The Stress strain curve of the alloy sintered billet at high temperature.
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process [17e19]. The tested density and the theoretical density of the alloy is illustrated in Fig. 6. From the figure, the addition of ZrO2 has little effect on the actual density of the alloys. However, due to the sintering defects, the actual density of the alloy is less than the theoretical density, which is inevitable in the process of powder metallurgy. The actual density of the alloys moved closer and closer to the theoretical density with the increase of deformation. The hardness of the alloys was measured in Fig. 7. In the process of measuring micro hardness, the pressure exerted by the pressure head is shared by both the grain and grain boundary. With the increase of ZrO2, the microhardness of the alloys was increased by 40% due to dislocation strengthening and grain boundary strengthening as well as the second phase strengthening. The second phase particles (about 200e300 nm in size) are evenly distributed around the Mo grains, which can restrain the accumulation and growth of Mo grains during the sintering process. The fine particles were determined to be the ZrO2 particles. Finally, the tiny grains are acquired, which helps to obtain a higher strength for the Mo alloys. Through a series of studies on the microstructure, phase distribution, grain size, density and hardness of the alloy sintered compacts, it is found that the new alloys made by the new method have fine grain size and uniform size. With the increase of ZrO2, the grain size of the alloys decreases obviously and the second phase in the alloy is small and uniformly distributed. The size of the second phase in the alloy prepared by the traditional method decreased greatly. The problem of the second phase enrichment is serious, which seriously affects the mechanical properties and the plasticity of the alloy [16e20]. After adding ZrO2 particles, the density of the alloy decreases while the hardness increases. Deformation behavior of the alloy sintered billets at high temperature was studied. Fig. 8 is the true stress-strain relation curve of Mo-ZrO2 alloys at high temperature. It has simulated the stress and deformation conditions of alloy head at high temperature. By measuring the true stress-stain curve, it is found that the Mo-ZrO2 alloy has the great deformation resistance at a high temperature. With the increase of ZrO2, the deformation stress of the alloys increased. The alloys have obvious plastic deformation and yield in the compress process [15,16]. The ZrO2 particles can effectively improve the strength and plastic properties of the Mo-ZrO2 alloys at high temperature. It is vital to control the grain size of molybdenum alloys effectively for the overall mechanical properties of ZrO2 doped molybdenum alloy. ZrO2 as second phase was existed in molybdenum matrix, refining grains, increasing grain boundary area in unit volume, shortening the slippage of pile-up dislocation inside the grain, reducing quantity of dislocation pile-up, making the dislocation hardly occur and thus improving the material strength [20e24]. Depending on the analysis, there are two methods to reinforce ZrO2 doped molybdenum alloy: fine grain reinforcing and particulate reinforcing. After being doped with ZrO2, the grain size of molybdenum matrix becomes smaller, and the number of Mo substrate grains per unit volume increases. If the deformation is fixed, the deformation will take place in grains, and the deformation of grains will be more uniform. Firstly, amount of fine Mo grains can reduce the dislocation pile-up density and the possibility of cracking caused by stress concentration as well as cracks. Secondly, considerable dislocation pile-ups may occur during deformation of the material as there are many fine ZrO2 particles in the molybdenum alloys and the vicinity of such fine particles, resulting in stress concentration. When the stress concentration is enough great, fine cracks will appear at the joining interfaces, and even the molybdenum matrix and the ZrO2 particles will be separated,
deboned or pulled away from each other, relieving the stress concentration, passivating the crack tip and improving strength of the material [22e26]. Therefore, ZrO2 particles can refine grains, purify grain boundaries, and hinder the dislocation, thus improving the resistance to deformation and enhancing the strength of the molybdenum alloys. It can be observed that the nano-ZrO2 particles can effectively increase the strength, especially with the increase of ZrO2 addition. The strengthening effect of the second phase particles at high temperature was calculated, in order to study the strengthening effect of ZrO2 particles.
m¼(S1-S2)/S1 where m is the strengthening effect of ZrO2 particles, S1 is the maximum stress of the alloy and S2 is the maximum stress of the Pure molybdenum. In Fig. 9, the strengthening effect increased with the increase of temperature. The strengthening effect of ZrO2 particles is obvious when the temperature is above 1300 C. During the alloy deformation at high temperatures, the doping ZrO2 particles can improve the deformation resistance of the alloy sintered compacts by the second-phase-based reinforcement. With the increase of temperature, this reinforcement has shown a significant increase. The strengthening effect of Mo-1.5% ZrO2 alloy has reached 35% After the high-temperature deformation, the samples of pure molybdenum and Mo-1.0% ZrO2 are cut in the axial direction, shown in Fig. 10, Fig. 11 and Fig. 12. It can be seen in Fig. 10 that the cracks become more numerous and eventually larger with the increase in temperature. Nevertheless, there are a few cracks in Mo1.0%ZrO2 alloy. The plasticity of the alloy has greatly improved compared with pure molybdenum. This is because grains refined by ZrO2 particles, resulting in the increase of grain boundaries and dislocated slippage. With dislocation density and crack density decreased greatly, so few or no cracks are seen in the ZrO2 doped sintered alloy compacts which show a superior plasticity at high temperature. The displacement of ZrO2 particles hindering grain boundary slippage is obvious in the Mo-1.0% ZrO2 alloy. No obvious crack is seen in the alloy with raising temperature, but cavity begins to appear. During the deformation, the strength of Mo matrix is less than that of the second phase which imposes intense pinning on
Fig. 9. The strengthening effect of second phase particles at high temperature.
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Fig. 10. The morphology of pure molybdenum samples after experiment.
the displaced grains and gives rise to the cavities. With the comprehensive consideration of strength and performance of the alloy material, it can be concluded that the strength and plasticity of the alloy can be improved at high temperatures by adding ZrO2 particles, reducing crack propagation, decreasing the possibility of presence of cracks by refining grains and retarding cracks. It can be noted that ZrO2 in the alloy can effectively hinder dislocation and crack propagation.
5. Conclusion 1. The prepared Mo-ZrO2 alloy have fine grain size. The small size of the second phase in the alloy is only 100e200 nm. Big ZrO2 particles are at grain boundaries, while the small ZrO2 particles are in grains. The size and distribution of ZrO2 have been greatly improved compared with those prepared by the traditional solid-solid addition.
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Fig. 11. The morphology of Mo-1.0% ZrO2 alloy after experiment.
2. After adding ZrO2 into the alloy, the grains can be refined effectively. The minimum grain size of the alloy sintered compact is only 1/3 that of the pure molybdenum. Also, with the addition of ZrO2, the density of the alloy decreased and the hardness of the alloy increased by 40%. 3. The high temperature strength of the sintered compacts was improved by adding ZrO2 into the alloys. The strength of the
alloy decreases with the increase of the temperature and the high temperature strength increases with the increase of ZrO2. The strength of the alloys is 35% higher than that of the pure molybdenum at the temperature of 1400 C. The addition of ZrO2 can effectively improve the deformation resistance of the alloy.
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Fig. 12. Analysis of the second phase in the microstructure of Mo-0.1%ZrO2 samples after experiment.
Acknowledgments This work was supported by Program for Chang Jiang Scholars and Innovative Research Team in University(IRT1234), National Natural Science Foundation of China(50972039) and Plan for Scientific Innovation Talent of Henan Province, China [No. 2017JQ0012]. References [1] E. Ahmadi, M. Malekzadeh, S.K. Sadrnezhaad, Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering, Int. J.
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Refract. Met. Hard Mater 29 (2011) 141e145. [2] G. Bai, R.W. Gao, Y. Sun, Study of high-coercivity sintered NdFeB magnets, J. Magn. Magn. MATER 308 (2007) 20e23. [3] S.P. Chakraborty, S. Banerjee, K. Singh, Studies on the development of protective coating on TZM alloy and its subsequent characterization, J. Mater. Process. Tech. 207 (2008) 240e247. [4] B.V. Cockeram, The role of stress state on the fracture toughness and toughening mechanisms of wrought molybdenum and molybdenum alloys, Mat. Sci. Eng. A Struct. A 528 (2010) 288e308. [5] B.V. Cockeram, W. Smith, T.S. Byun, The change in the hardness of LCAC, TZM, and ODS molybdenum in the post-irradiated and annealed conditions, J. Nucl. Mater. 393 (2009) 12e21. [6] B.V. Cockeram, R.W. Smith, N. Hashimoto, The swelling, microstructure, and hardening of wrought LCAC, TZM, and ODS molybdenum following neutron irradiation, J. Nucl. Mater. 418 (2011) 121e136. [7] B. Gludovatz, S. Wurster, A. Hoffmann, Fracture toughness of polycrystalline tungsten alloys, Int. J. Refract. Met. Hard Mater. 28 (2010) 674e678. [8] W.F. Ho, S.C. Wu, S.K. Hsu, Effects of molybdenum content on the structure and mechanical properties of as-cast Tie10Zr-based alloys for biomedical applications, Mat. Sci. Eng. C Mater. 32 (2012) 517e522. [9] G. Liu, G.J. Zhang, F. Jiang, Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater 12 (2013) 344e350. [10] T.M. Li, W. Zeng, Y.Y. Zhang, Nanobelt-assembled nest-like MoO3 hierarchical structure: hydrothermal synthesis and gas-sensing properties, Mater. Lett. 160 (2015) 476e479. [11] C.P. Cui, Y.M. Gao, S.Z. Wei, Preparation and properties of ZrO2/Mo alloys, High. Temp. Mat. PR-ISR 36 (2017) 163e166. [12] N.M.D. Vitorino, A.V. Kovalevsky, J.C.C. Abrantes, Hydrothermal synthesis of boehmite in cellular alumina monoliths for catalytic and separation applications, J. Eur. Ceram. Soc. 35 (2015) 3119e3125. [13] X.M. Liu, C.G. Niu, X.P. Zhen, Novel approach for synthesis of boehmite nanostructures and their conversion to aluminum oxide nanostructures for remove Congo red, J. Colloid Interf. Sci. 452 (2015) 116e125. [14] G. Rani, P.D. Sahare, Effect of temperature on structural and optical properties of boehmite nanostructure, Int. J. Appl. Ceram. Tec. 12 (2015) 124e132. [15] M. Aziz, D.C. Hudson, S. Russo, Molybdenum-rhenium superconducting suspended nanostructures, Appl. Phys. Lett. 104 (2014) 297e346. [16] S.H. Kim, S.J. Kang, M.H. Park, Vacancy-mediated omega-assisted alpha-phase formation mechanism in titanium-molybdenum alloy, Acta Mater. 83 (2015) 499e506. [17] D. Lang, C. Poehl, J. Schatte, On the precipitation mechanism in the molybdenum based alloy MHC (Mo-Hf-C), Powder Metall. 57 (2014) 311e313. [18] C. Poehl, J. Schatte, H. Leitner, Solid solution hardening of molybdenumhafnium alloys: experiments and modeling, Mat. Sci. Eng. A Struct. A 559 (2013) 643e650. [19] D. Lang, C. Poehl, D. Holec, On the chemistry of the carbides in a molybdenum base Mo-Hf-C alloy produced by powder metallurgy, J. Alloy Compd. 654 (2016) 445e454. [20] C. Poehl, J. Schatte, H. Leitner, Solid solution softening of polycrystalline molybdenum-hafnium alloys[J], J. Alloy Compd. 576 (2013) 250e256. [21] H.C. Cheng, J.L. Fan, Z. Qian, Effects of trace ZrC on the microstructure and properties of molybdenum alloy, J. Mater. Res. 785 (2013) 76e80. [22] P.F. Feng, Q.L. Yang, X.M. Dang, Reaction mechanism of lanthanum nitratedoped Mo-La material during solid-liquid spray doping processing, Rare Met. 34 (2015) 814e817. [23] H.M. Hu, C.H. Deng, J.C. Xu, Metastable h-MoO3 and stable alpha-MoO3 microstructures: controllable synthesis, growth mechanism and their enhanced photocatalytic activity, J. Exp. Nanosci. 10 (2015) 1336e1346. [24] A. Phuruangrat, J.S. Chen, X.W. Lou, Hydrothermal synthesis and electrochemical properties of alpha-MoO3 nanobelts used as cathode materials for Li-ion batteries, Appl. Phys. A Mater. 107 (2012) 249e254. [25] Z.Y. Wang, S. Madhavi, X.W. Lou, Ultralong alpha-MoO3 nanobelts: synthesis and effect of binder choice on their lithium storage properties, J. Phys. Chem. C 116 (2012) 12508e12513. [26] R. Nadimicherla, W. Chen, X. Guo, Synthesis and characterization of alphaMoO3 nanobelt composite positive electrode materials for lithium battery application, Mater. Res. Bull. 66 (2015) 140e146.