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DOI: 10.1016/j.jmrt.2019.09.044
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Available online 6 October 2019
Nanostructured oxide dispersion strengthened Mo alloys from Mo nanopowder doping with oxide nanoparticles
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Guo-Dong Sun, Guo-Hua Zhang
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ghzhang0914@ustb.edu.cn

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, Kuo-Chih Chou
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
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Abstract

A simple and efficient pathway was developed to synthesize ultrafine grained oxide dispersion strengthened (ODS) Mo alloys doped with nano-scaled oxide particles. Firstly, dispersed La(NO3)3 or Al(NO3)3 nanoparticles were introduced into commercial MoO3 via a solution spraying method. Then, Mo nanopowders containing different amounts of oxides nanoparticles (La2O3 (0%, 0.5%, 1% and 2%) or Al2O3 (0.5%, 1%)) were successfully prepared by reducing the doped MoO3 with carbon black at 600 °C and 1050 °C, and hydrogen at 800 °C. Due to the high sintering activity of prepared doped Mo nanopowder, after sintering in H2 at 1300 °C for 3 h, the relative density of sintered products containing 0.5% La2O3 or Al2O3 reached to above 95.4%. Nanostructured ODS Mo alloys with Mo and La2O3 (or Al2O3) grain sizes of about 0.5 μm and 75 nm (or 50 nm for Al2O3) were successfully prepared at around 1300 °C. The obtained nanostructured Mo-oxides alloys had much high hardness values. For the Mo-La2O3 alloys, the highest value of hardness can reach to 338 HV (1% La2O3, Mo grain size, 0.51 μm; La2O3 grain size, 71.3 nm; density, 93.0%), while the highest hardness of Mo-Al2O3 alloys reached to 385 HV (1% Al2O3, Mo grain size, 0.55 μm; Al2O3 grain size, 55.9 nm; density, 94.6%), which were much higher than the ODS-Mo alloys prepared from the traditional methods. This method could also have great potential for industrially producing nanostructured ODS Mo alloys and W alloys.

Keywords:
Mo alloy
Nanoparticles
Microstructure
Dispersion strengthening
Hardness
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1Introduction

Molybdenum (Mo) is one of the most common-used refractory metal that has high melting point, high strength, high creep and corrosion resistance, low thermal expansion coefficient, and excellent thermal and electronic conductivities [1–9]. Therefore, Mo alloys are very attractive for many critical fields, such as aerospace, electronics, chemical, military, electronics, metallurgy and nuclear industries [6,7,10–15]. With the rapid development of these industries, Mo materials with dramatically improved mechanical properties are desperately needed. Nanostructure refractory metal materials with ultrafine grain and large volume fraction of grain boundary show noticeably improved mechanical properties, such as high strength, hardness and wear resistance [1,16–20]. Normally, a powder metallurgy approach, involving the synthesis, compaction and sintering of nanoparticles, is employed to prepared ultrafine grain refractory materials. However, due to the much high driving force of grain growth for ultrafine particles/grains during the sintering or heat treatment process, it is hard to obtain ultrafine grain pure Mo material [21]. The introduction of second-phase particles to form oxide dispersion strengthened (ODS) refractory alloys (W and Mo) is an effective and wide-used approach to hinder the growth of grain (refine the grain size) and improve their strength,ductility, fracture toughness characteristics [1–3,11,16,19,20,22].

Various methods have been developed to prepare the precursor of ODS Mo composites, which can be divided into three categories: mixing of solid Mo source with solid oxide powder (S–S mixing) [3,9,23], or with liquid solution (S–L mixing) [2,22], as well as liquid–liquid mixing (L–L mixing) [1,24]. Nowadays, the S–S and S–L mixing methods are predominantly used in industrial applications [2,9]. Normally, the S–L mixing has a better mixing result than the S–S [2]. However, the oxide dopers added to Mo oxide as aqueous solutions in the previous S–L doping process always still have a large particle size due to the difficulty of controlling the nucleation, growth and particle size in the evaporation of solutions [25]. Additionally, the precursors prepared by these methods are reduced by H2 to obtain the mixed powder of oxide and Mo. However, Mo particles produced by hydrogen reduction processes always have a large particle size of a few microns [22,26]. Therefore, a sintering temperature of above 1850 °C for several hours is always needed to obtain a dense compact [1–3,9,22,24]. For example, the Mo powder doped with 0.1% La2O3, produced by the L–S mixing of MoO2 and lanthanum nitrate solution followed by hydrogen reduction, were sintered at 1960 °C for 6 h to reach a relative density of 95.6% [22]. This high temperature and long time lead to large Mo grains size of tens of microns and coarse oxide particles [22]. One efficient and attractive method to obtain dense compact with small grains size at low temperatures is the use of nanoparticle due to their much higher sintering activity compared to micron-sized Mo particles [6,10,27–29]. However, a crucial obstacle for the use of Mo nanoparticles is the difficulty of producing Mo nanoparticles in a large scale via an efficient, low-cost and industrially feasible method.

Recently, we developed a low-cost, efficient and industrially feasible method for large-scale preparation of Mo nanoparticles via the reduction of MoO3 with carbon black and hydrogen [21,30,31]. Additionally, it was found in our previous reports [8,32,33] that the spraying of salt solution (such as NaCl) into MoO2 powder by a sprayer can effectively introduce a great number of dispersed nano-sized salt nanoparticles on the surface of MoO2 particles, which were used as nucleation aid for preparing Mo. This pretty simple and efficient spraying method (S–L mixing) may also have great potential to introduce oxide nanoparticles into MoO3.

In the present study, we developed an efficient and simple method for producing oxide-doped (La2O3 or Al2O3) Mo nanoparticles via the spray mixing of MoO3 and oxide solution (La(NO3)3 or Al(NO3)3), followed by reducing with carbon black at 600 °C and 1050 °C, and hydrogen at 800 °C. Mo nanopowders doped different amounts of La2O3 or Al2O3 nanoparticles were successfully synthesized. Then, these nanopowders were sintered at 1200–1500 °C in dry H2 atmosphere to investigate their sintering behaviors and prepare nanostructured ODS Mo alloys. The effects of the introduced La2O3 and Al2O3 nanoparticles on the carbothermic reduction of MoO3 with carbon black, the grain growth during the sintering of Mo nanoparticles, as well as the microstructure and hardness were investigated.

2Experimental section

Commercial MoO3 powder (1.42 μm, >99.9% purity), purchased form Jinduicheng Molybdenum Co., Ltd., Xi’an, China, was used as raw molybdenum source. Lanthanum nitrate hexahydrate and aluminum nitrate nonahydrate (99% purity, Shanghai Aladdin Biochemical Technology Co., Ltd.) were used as dopers. Carbon black (24 nm, MA100, Mitsubishi Chemical Corporation) and hydrogen (99.999% purity) were used as the reducing agents.

A solution spraying method was used to introduce La(NO3)3 and Al(NO3)3 nanoparticles into MoO3. Firstly, a certain amount of lanthanum nitrate hexahydrate or aluminium nitrate nonahydrate was dissolved in deionized water, and then a certain amount of solution was uniformly sprayed into 50 g MoO3 powders by a sprayer. After that, the sample was dried at 100 °C for 6 h to remove the water. A strategy of pre-carbothermic reduction followed with hydrogen deep reduction was used to produce doped Mo nanopowder. Firstly, the doped MoO3 powder was thoroughly mixed with carbon black with a C/MoO3 molar ratio of 1.9. Then, 60 g mixed sample with a thickness of about 2 cm was heated to 600 °C (heating rate, 5 °C/min) and held for 2 h to reduce the MoO3 to MoO2 under the protection of flowing argon. Then, the sample was further heated to 1050 °C (heating rate, 5 °C/min) and held for 4 h to reduce the most MoO2 to Mo. Then, the pre-reduced doped Mo powders containing a small amount of MoO2 was deeply reduced by H2 at 800 °C for 2 h to obtain pure doped Mo powder. Finally, Mo nanopowders with different amounts of La2O3 (0%, 0.5%, 1% and 2%) or Al2O3 (0.5%, 1%) were obtained. The prepared doped Mo nanopowders were pressed into a cylindrical compact (25 mm in diameter) with a uniaxial pressure of 150 MPa. Then, the green compacts were sintered at 1200 °C, 1300 °C, 1400 °C and 1500 °C for 3 h in hydrogen atmosphere.

The phase compositions of samples were analyzed by X-ray diffraction technology (XRD) (TTR III, Rigaku Corporation, Japan). The morphology, microstructure and size of particles/grains were detected by field emission scanning electron microscope (FE-SEM) (ZEISS SUPRA 55, Oberkochen, Germany) with an energy dispersive X-ray spectroscopy (EDS). The residual carbon was measured by an infrared carbon-sulfur analyzer (EMIA-920V2, HORIBA, Japan). The Archimedes method was used to determine the densities of the sintered samples. Vickers hardness was measured by a microhardness tester with a load of 100 g for 10 s, and at least five measurements were performed to obtain the average value.

3Results and discussion3.1Preparation of La2O3 and Al2O3 doped Mo nanopowders

As mentioned above, it was difficult for the traditional mixing methods and hydrogen reduction method to prepare Mo nanoparticle containing oxides nanoparticles due to the difficult of controlling the nucleation, growth and particle size. For example, the traditional L–S mixing method of MoO2 and the lanthanum nitrate solution involved the agitation, heating and evaporation processes, in which the size of doped oxide could not be controlled, leading the coarser oxide lanthanum particles [25]. This problem can be well solved by the spraying method, in which the aqueous solutions of La(NO3)3 was atomized into abundant much small droplets, and dispersedly attached to the surface of MoO3 particles. Then, during drying process, the water in the dispersed small droplets was gradually removed, and meanwhile, La(NO3)3 in the droplets was precipitated to nanoparticles. Fig. 1(a) and (b) shows the FE-SEM micrographs of the raw MoO3 and MoO3 doped with La(NO3)3 by the spraying method (1% La2O3 compared to Mo), respectively. It can be seen that a lot of small nanoparticles appeared on the smooth surface of MoO3 particles, which were identified to be La compound by EDS, as shown in Fig. 1(c). Therefore, the MoO3 attached with La(NO3)3 nanoparticles was successfully produced by the spraying method. When the doped MoO3 was mixed with carbon black in an agate mortar for 30 min, as shown in Fig. 1(d), the doped MoO3 particles were thoroughly surrounded by abundant much small carbon black nanoparticles, which could supply quite a lot of nucleation points to form a great deal of dispersed MoO2/Mo nuclei, making it possible to control the particle size of MoO2 and Mo. [30,34] Similarly, MoO3 powders doped with different amounts of La(NO3)3 or Al(NO3)3 nanoparticles were successfully prepared.

Fig. 1.

The FE-SEM micrographs of (a) the raw MoO3 and (b) MoO3 doped with La(NO3)3 nanopaticles by the spraying method (1% La2O3 compared to Mo). (c) EDS pattern of the La(NO3)3 nanopaticles on the large MoO3 particles in Fig. 1(b). (d) FE-SEM image of the mixture of doped MoO3 and carbon black.

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Fig. 2 shows the XRD patterns of MoO2 and pre-reduced doped Mo nanopowder produced by reducing doped MoO3 with carbon black, as well as the pure doped Mo nanopowder obtained after deep reduction with hydrogen. From Fig. 2(a) and (b), it can be seen that the MoO3 powders doped with La or Al were reduced to MoO2 after reacted at 600 °C for 2 h, and further reduced to Mo with a few excessive MoO2 at 1050 °C for 4 h. After deep reduction with H2, pure doped Mo powders were obtained. From the high resolution XRD patterns in Fig. 2(b) and (d), some weak XRD diffraction peaks of La2O3 and Al2O3 were found. Furthermore, the contents of residual carbon in the produced Mo powders were detected by the infrared carbon-sulfur analyzer, which were approximately 0.02%.

Fig. 2.

(a) XRD patterns products containing 1% La2O3 (compared to Mo) after different reaction stages. (b) High resolution XRD pattern of pure Mo powder containing 1% La2O3. (c) XRD patterns products containing 1% Al2O3 (compared to Mo) after different reaction stages. (b) High resolution XRD pattern of pure Mo powder containing 1% Al2O3.

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Fig. 3 shows the FE-SEM micrographs of MoO2 and pre-reduced Mo nanopowder produced by reducing doped MoO3 with carbon black, and the pure doped Mo nanopowder obtained after deep reduction with hydrogen. The kinds of particles were identified by XRD (Fig. 2), EDS as well as their typical morphologies. It can be seen from Fig. 3 (a), (d) and (g) that the pure MoO3 powder, as well as MoO3 doped La and Al was reduced to much smaller MoO2 nanosheets at 600 °C. After the further reduction of MoO2 at 1050 °C, near spherical Mo particles with an average particle size of about 100 nm were produced. For the pre-reduced Mo nanoparticles doped with La, as shown in Fig. 3(e), a lot of smaller La2O3 particles with an average size of 41.2 nm appeared nearby the Mo particles. For the case of doped with Al2O3 particles, as shown in Fig. 3(h), the average particle size of Al2O3 was about 23.4 nm. After the deep reduction by H2, morphology and particle size of both the Mo and doped La2O3/Al2O3 nanoparticles didn’t have obvious change. Accordingly, Mo nanoparticles contained different amounts of nano-sized La2O3/Al2O3 particles were successfully prepared via carbothermic reduction of the spraying-doped MoO3, and the schematic diagram of this developed pathway was drawn according to the experimental results and shown in Fig. 4. Compared to the methods based on S–S, S–L and L–L mixing followed by hydrogen reduction processes, the present method had the advantages as follows: tens of times smaller particles size of Mo, smaller oxides particles, simpler route and more efficiency. Accordingly, this developed method could have great potential for industrially producing oxide nanoparticles doped Mo nanopowder.

Fig. 3.

Products without addition after reaction at different stages, (a) 600 °C for 2 h, (b) 1050 °C–4 h and (c) deep reduction with H2. Products with 1% La2O3 after reaction at different stages, (d) 600 °C for 2 h, (e) 1050 °C–4 h and (f) deep reduction with H2. Products with 1% Al2O3 after reaction at different stages, (g) 600 °C for 2 h, (h) 1050 °C–4 h and (i) deep reduction with H2.

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Fig. 4.

The schematic diagram of the developed pathway for prepared oxide-doped Mo nanopowder and nanostructured ODS Mo alloy.

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3.2Sintering of La2O3 and Al2O3 doped Mo nanopowders3.2.1Densification analyses

Fig. 5 shows the relatively densities of samples after sintering at different temperatures for 3 h. It can be seen that for the pure Mo nanopowder without doping, the density can reach to 95.8% at a sintering temperature of 1200 °C, and gradually increased to 99.2% when the temperature increased to 1500 °C. However, the doped oxide nanoparticles had an obstructive effect on the densification of Mo nanopowder. For example, at 1200 °C, with the increase of the amount of doped La2O3 from 0 to 2%, the relative density of product gradually decreased from 95.8% to 81.4%. With the increase of temperature, the relative density gradually increased. At 1300 °C, the density of sample with 0.5% La2O3 or 0.5% Al2O3 can reach to above 95.4%. At 1500 °C, the densities of all the samples were above 97.4%. However, for the Mo–La2O3 (0.5, 1.0, and 2.0 wt%) alloys prepared by using the traditional solid–solid mixing/doping method and sintering at 1900 °C for 6 h, their relatively densities were only approximately 94.5% [3]. Additionally, the Mo powder doped with 0.1% La2O3, produced by the L–S mixing of MoO2 and lanthanum nitrate solution followed by hydrogen reduction, were sintered at 1960 °C for 6 h to reach a relative density of 95.6% [22]. The Mo-Al2O3 (1.1 wt%) alloys, produced by sintering Al2O3-doped Mo powder from hydrogen reduction of L–S mixing of MoO2 and Al(NO3)3 aqueous solution, had a relative density of about 95.5% (1840 °C, 6.5 h) [26]. Therefore, compared to the oxide-doped Mo powders prepared via traditional methods, the present oxide-doped Mo nanopowders can be sintered to a high relative density at much lower temperatures.

Fig. 5.

Relative density of samples with different additions after sintering at different temperatures.

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3.2.2Microstructure and grain size analyses

Figs. 6 and 7 show the FE-SEM micrographs of fracture surfaces of the sintered compacts at different temperatures, and the measured grain sizes of Mo and doped La2O3 or Al2O3 are shown in Fig. 8. It can be seen from Fig. 6(a) to (d) that when oxide doper was not introduced, the Mo compacts had dense structures. However, the pure Mo compact had a relatively large grain size of 1.88 μm at 1200 °C, and dramatically increased from 1.88 μm to 16.3 μm with the increase of temperature from 1200 to 1500 °C, indicating that ultrafine Mo grains/particles were unstable and had great tendency to grow. This problem can be well solved by introducing a small amount of oxide particles. As shown in Fig. 6(e)–(h), when 0.5 wt% of La2O3 nanoparticles were added, the grain size of Mo alloy produced at 1200 °C dramatically decreased from 1.88 μm to 0.46 μm. As the temperature further increased to 1300 °C, as shown in Fig. 6(f), the grain size increased to 0.69 μm, but the product had a denser structure with few pores compared to that of 1200 °C (Fig. 6(e)). With further increasing the temperature, the grain size of Mo increased to 1.08 μm at 1400 °C, and dramatically increased to 2.82 μm at 1500 °C, which was still much smaller than that without the addition of La2O3 at 1500 °C (16.3 μm). When the added La2O3 increased, the grain size of Mo gradually decreased, but the relative density also gradually decreased. For example, when Mo nanopowder containing 2% La2O3 was sintered at 1200 °C, the grain size of Mo was 0.30 μm, while its relative density was only 81.4%. With the increase of temperature to 1400 °C, the grain size of Mo product containing 2% La2O3 was 0.74 μm with a relative density of 94.3%, while the average grain size of Mo containing 1% La2O3 was 0.83 μm with a relative density of 96.1%. As shown in Fig. 6(e)–(p), a lot of smaller La2O3 nanoparticles homogeneously distributed on the boundaries of Mo, and their average particles sizes at different temperatures and different amounts of La2O3 are shown in Fig. 8(c). It can be found that the sintering temperature rather than the addition amount of La2O3 had a more obvious effect on the particle size of La2O3. At 1200 °C, the average sizes of La2O3 particles for different addition amounts of La2O3 were around 65 nm, and gradually increased gradually to above 143 nm when the temperature increased to 1500 °C.

Fig. 6.

FE-SEM micrographs of fracture surfaces of products. (a) pure Mo-1200 °C, (b) pure Mo-1300 °C, (c) pure Mo-1400 °C, (d) pure Mo-1500 °C, (e) 0.5% La2O3-1200 °C, (f) 0.5% La2O3-1300 °C, (g) 0.5% La2O3-1400 °C, (h) 0.5% La2O3-1500 °C, (i) 1.0% La2O3-1200 °C, (j) 1.0% La2O3-1300 °C, (k) 1.0% La2O3-1400 °C, (l) 1.0% La2O3-1500 °C, (m) 2.0% La2O3-1200 °C, (n) 2.0% La2O3-1300 °C, (o) 2.0% La2O3-1400 °C, (p) 2.0% La2O3-1500 °C.

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Fig. 7.

FE-SEM micrographs of fracture surfaces of products. (a) 0.5% Al2O3-1200 °C, (b) 0.5% Al2O3-1300 °C, (c) 0.5% Al2O3-1400 °C, (d) 0.5% Al2O3-1500 °C, (e)1.0% Al2O3-1200 °C, (f) 1.0% Al2O3-1300 °C, (g) 1.0% Al2O3-1400 °C, (h) 1.0% Al2O3-1500 °C.

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Fig. 8.

(a)–(b) Grain sizes of Mo of products prepared from different temperatures, (c) Grain sizes of La2O3 and Al2O3 of different products.

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As for other oxide dopers, such as Al2O3, similar results can also be obtained, as shown in Figs. 7 and 8. At 1200 °C, as shown in Fig. 7(a) and (e), with the additions of 0.5% and 1% Al2O3, nanostructured Mo-Al2O3 alloys with the average Mo grain size and Al2O3 size of 0.52 μm(Mo)–48.8 nm (0.5% Al2O3; density, 93.6%), and 0.43 μm(Mo)–46.5 nm (1% Al2O3; density, 89.1%) were prepared. At 1300 °C, the average grain sizes of Mo and Al2O3 increased to 0.70 μm (Mo)–51.3 nm (0.5% Al2O3; density, 96.1%), and 0.55 μm(Mo)–55.9 nm (1% Al2O3; density, 94.6%). When the temperature further increased to above 1400 °C, denser Mo-Al2O3 alloys with an average Mo grain size of 0.94–1.76 μm and an average Al2O3 particle size of 65.1–82.3 nm were successfully prepared. Therefore, the present method can be potential for producing various ODS-Mo alloys. Additionally, this method could also have great potential for producing ODS W alloys.

3.2.3Mechanisms analyses

In the previous pathways for prepared ODS Mo alloys via S–S, S–L or L–L mixing and hydrogen reduction, the sintering temperature and time were always above 1850 °C and 5 h due to the large particle size of Mo or oxides. In the present method, Mo nanopowder with dispersed nanosized oxides particle can be successfully produced by reducing commercial MoO3 with carbon black and deep reduction with hydrogen. These Mo nanopowders with nanosized dispersed oxides were successfully sintered to dense materials at around 1300 °C for 3 h, which was more than 500 °C lower than the previous methods. This noticeably decrease of sintering temperature was crucial for obtaining nanostructured ODS-Mo alloys with ultrafine Mo grains and nano-sized dispersed oxides. It is well known that the driving force of sintering is mainly determined by the surface energy (broken bonds) [27,35]. The smaller the particle size, the higher the surface energy (broken bonds) would be. Therefore, compared to conventional micron-sized powder, Mo nanopowder has much higher driving force of sintering [29]. Therefore, Mo nanoparticles have great driving force to form neck and coarsen to larger particles. The coarsening of the Mo particles would make the doped Mo nanopowder be densified at the much lower temperature than traditional micron-sized Mo [36].

From Figs. 6 and 7, it was found the introduced nanosized oxide particles homogeneously distributed in the Mo matrix. The doped nano-sized oxides could have a great influence on the mechanism of the grain growth. In the initial and immediate stages of sintering pure Mo nanoparticles, due to the high sintering activity of Mo nanoparticles, the main ways for the grain growth of Mo were grain boundary migration after the coalescent of particles [21,37]. However, the presence of oxide nanoparticles distributed among the Mo grains would pin the grain boundary and hinder its migration [11]. Therefore, the grain size of Mo dramatically decreased when a small amount of oxide nanoparticles were doped. Inversely, the oxides nanoparticles were also separated apart by the Mo matrix, making it also difficult for oxides nanoparticle to grow. Finally, this mutual restraint enabled both of them to stably exist in ultrafine grains.

The kind of oxide also had effect on the densification and grain size of Mo. For the same additive amount, the volume of Al2O3 was about 1.75 times than La2O3 due to its smaller molecular mass. However, it can be seen from Fig. 5 that Mo-Al2O3 alloys had a higher relative density than Mo-La2O3 alloys. Additionally, when the temperature was below 1500 °C, the Mo grain sizes of Mo-La2O3 alloys were smaller than those of Mo-Al2O3 alloys. As mentioned above, for the same sintering temperature and additive amount, the grain size of the introduced La2O3 nanoparticles was larger than Al2O3. These phenomena could indicate that La2O3 particles had more obvious obstructive effect on both the coalescent of particles and grain boundary migration than Al2O3. The detailed reason is unknown, but may be related to the existence of d-orbital electrons which lead to a strong interaction with Mo.

3.2.4Hardness analyses

Fig. 9 shows the Vickers hardness of the sintered products obtained from different oxide dopers and temperatures. For the pure Mo, the hardness had the highest value of 254 HV at 1200 °C (grain size, 1.88µm; density, 95.8%), and gradually decreased with the increase of temperatures. When 0.5% or 1.0% La2O3 was introduced, the hardness had the highest value at 1300 °C, which were 315 HV and 338 HV, respectively. These values were much higher than the 243 HV of pure Mo at 1300 °C. Furthermore, the hardness gradually decreased with the increase of temperature. However, for the case of 2.0% La2O3, when the temperature increased, the hardness gradually increased, and reach the highest value of 325 HV at 1500 °C (grain size, 1.20 μm; density, 97.7%). When 0.5% or 1.0% Al2O3 was introduced, at the same temperature, the hardness values were always higher than those cases of adding La2O3. Even at 1200 °C, the values of hardness were 351 HV (0.5% Al2O3) and 342 HV (1.0% Al2O3). When the temperature increased to 1300 °C, the values of hardness increased to 364 HV (0.5% Al2O3) and 385 HV (1.0% Al2O3, grain size, 0.55 μm; relative density, 94.6%). Then, hardness gradually decreased with increasing the temperature from 1300 °C to 1500 °C, while they were still above 322 HV at 1500 °C. These hardness values of ODS-Mo were much higher than those of previous reports. For example, the hardness of Mo-Al2O3 (1.1 wt%) alloys produced by sintering Al2O3-doped Mo powder from hydrogen reduction of L–S mixing of MoO2 and Al(NO3)3 aqueous solution, was only about 210 HV (Mo grain size, ∼8 μm; Al2O3 grain size, 2–4 μm; relative density, 95.5%) [26]. Additionally, the hardness of Mo-Al2O3 (3.02 wt%) alloy prepared from the powder prepared via L–L mixing and hydrogen reduction processes was about 225 HV (Mo grain size, >5 μm, Al2O3 grain size, 1–3 μm; density, 96%) [24].

Fig. 9.

Vickers hardness of different sintered samples.

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As analyzed above, it can be found that the addition of oxide doper can dramatically increase the hardness value of product. The mechanisms for this result could be lied in the noticeable change of microstructure due to the introduction of nano-sized oxide dopers. The above microstructural analyses of the prepared ODS Mo alloys have shown that the grain size of Mo dramatically decreased after a small amount of nano-sized oxide particles. According to the Hall–Petch relationship [11,25], the refining of grain size can increase strength. Additionally, Mo alloy with a smaller grain size would have more volume fraction of grain boundary, which can reduce the concentration of deleterious solutes in the lattice and increase the path of crack propagation and energy dissipation [1,25]. Furthermore, according to the Orowan Strengthening Mechanism (dispersion strengthening) [11,25], the dispersed oxides nanoparticles in the Mo matrix can increased the resistance of dislocation movement, which also improved the strength and microhardness of Mo alloys. When the ODS-Mo alloys were subjected to a force, such as the hardness test, the oxide nanoparticles distributed on the grain boundaries of Mo can hinder the sliding of grain boundaries. The dispersion strengthening mechanism is mainly determined by the particle size and volume fraction of the doped oxides particles. The decrease of the particle size or increase of volume fraction of La2O3 or Al2O3 nanoparticles is beneficial to increase the hardness. Additionally, the relative density also had a great influence on the hardness value. For most of the cases, even though the grain size of Mo at 1200 °C was smaller than those at 1300 °C, the hardness at 1200 °C had smaller values, due to the lower relative densities. What’s more, when 2% La2O3 was added, the grain size increased with increasing temperature, but there was an increase in hardness, benefiting from the increased relative density.

4Conclusions

  • (1)

    Mo nanopowders containing different amounts of oxides nanoparticles were successfully prepared via the spray mixing of MoO3 and oxide solution (La(NO3)3 or Al(NO3)3), followed by reducing with carbon black at 600 °C and 1050 °C, and hydrogen at 800 °C.

  • (2)

    The doping of oxides had a slightly obstructive effect on the densification of Mo. However, due to the high sintering activity of prepared doped Mo nanopowder, after sintering at 1300 °C for 3 h, the relative density of sintered products containing 0.5% La2O3 or Al2O3 can reached to above 95.4%.

  • (3)

    Nanostructured Mo-oxide alloys with Mo and La2O3 (or Al2O3) grain sizes of about 0.5 μm and 75 nm (or 50 nm for Al2O3) were successfully prepared. The oxide nanoparticles homogeneously distributed among the Mo grains, which pinned the grain boundaries and hindered their migration, limiting the growth of Mo grains during the sintering process.

  • (4)

    The obtained nanostructured Mo-oxides alloys had very high hardness values. For the Mo-La2O3 alloys, the highest value of hardness can reach to 338 HV (1% La2O3, Mo grain size, 0.51 μm; La2O3 grain size, 71.3 nm; density, 93.0%), while the highest hardness of Mo-Al2O3 alloys reached to 385 HV (1% Al2O3, Mo grain size, 0.55 μm; Al2O3 grain size, 55.9 nm; density, 94.6%).

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51734002).

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Journal of Materials Research and Technology

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