High-energy ball milling and spark plasma sintering of molybdenum - lanthanum oxide (Mo-La2O3) and molybdenum - lanthanum zirconate (Mo-La2Zr2O7) composite powders

International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 Available online 5 October 2021 0263-4368/© 2021 Elsevier Ltd. All rights reserved. High-energy ball milling and spark plasma sintering of molybdenum - lanthanum oxide (Mo-La 2 O 3 ) and molybdenum – lanthanum zirconate (Mo-La 2 Zr 2 O 7 ) composite powders Ladislav ˇ Celko a , Serhii Tkachenko a , * , Mariano Casas-Luna a , Lucie Dyˇ ckov´ a a , Vendula Bednaˇ ríkov´ a a , Michaela Remeˇ sov´ a a , Pavel Komarov a , Andrea De´ ak b , Matej Bal´ aˇ z c , Deborah Crawford d , Sebastian Diaz-de-la-Torre e , Ede Bodoki f , Jaroslav Cihl´ aˇ r a a Central European Institute of Technology, Brno University of Technology, Purkyˇ nova 123, 612 00 Brno, Czech Republic b Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry, Supramolecular Chemistry Research Group, E¨ otv¨ os Lor´ and Research Network (ELKH), Magyar Tud´ osok K¨ orútja 2, 1117 Budapest, Hungary c Department of Mechanochemistry, Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 04001 Koˇ sice, Slovakia d School of Chemistry and Biosciences, University of Bradford, Richmond Road, BD7 1DP Bradford, UK e CIITEC - Centro de Investigaci´ on e Innovaci´ on Tecnol´ ogica, Instituto Polit´ ecnico Nacional, Mexico City, Mexico f Department of Analytical Chemistry, Iuliu Hat¸ieganu University of Medicine and Pharmacy, Louis Pasteur Street 4, RO-400349 Cluj-Napoca, Romania ARTICLE INFO Keywords: Molybdenum Lanthanum oxide Lanthanum zirconium oxide High-energy milling Thermal stability Spark plasma sintering ABSTRACT The current study is focused on the preparation of Mo-10 vol%La 2 O 3 and Mo-10 vol% La 2 Zr 2 O 7 composite powders via lowand high-energy ball milling approaches as potential candidates for near-future high-temperature structural applications. The mechanical milling parameters play a critical role on the final powder’s microstructure. When using the high-energy milling mode (using 800 rpm, ball-to-powder ratio (BPR) 100: 6), the homogeneous powder agglomerates are formed with refined laminated microstructure and more uniform ceramic phase distribution in both Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 systems compared to the powders produced by means of the low-energy milling mode (using 350 rpm, BPR 100: 6), where inhomogeneous powder mixture with less embedding of ceramic phases into Mo agglomerates was obtained. This study also focuses on the evaluation of high-temperature phase and microstructural stability of the produced composite powders treated at the temperature of 1300 ◦C under the different gaseous environments, including ambient, inert and reducing atmospheres. The Mo-10 vol% La 2 Zr 2 O 7 composite powder exhibited better thermal stability during the hightemperature exposure in all tested atmospheres in comparison with the Mo-La 2 O 3 composite powder, since it revealed less intensive formation of the intermediate phases, such as lanthanum oxymolybdates. Therefore, the Mo-10 vol% La 2 Zr 2 O 7 composite powder was used further for consolidation by means of spark plasma sintering at 1600 ◦C. The successful production of Mo-La 2 Zr 2 O 7 composite with homogeneous distribution of ceramic phase, the grain size about of 5 μ m, and hardness of 3.4 GPa was not reported so far. 1. Introduction In recent years, alloys and composites based on refractory metals are receiving more attention because of they can play, together with ceramics, important role in the near future high-temperature structural applications and energy harvesting technologies due to their unique combinations of superior chemical, physical and mechanical properties. Among the most utilized refractory metals, molybdenum (Mo) became especially attractive in the areas of aerospace, energy, electronics, telecommunication, medicine, and defense [1]. This is because Mo has one of the highest melting points among other metals (2623 ◦C), good creep strength at high temperatures, relatively high thermal conductivity (138 W⋅m −1 ⋅C −1 ) and low specific heat (25.1–28.4 J ⋅ K - 1 ⋅ mol −1 in the range of 127–727 ◦C), low coefficient of thermal expansion (4.8 ×10 −6 ⋅ K - 1 at 25 ◦C), good electric conductivity, excellent heat resistance, and density of about 20% higher than nickel superalloys (10.2 g ⋅ cm −3 vs. 8.2–8.5 g⋅cm −3 ) [2]. Molybdenum alloys and composites represent a prospective alternative to nickel-based superalloys, * Corresponding author. E-mail address: [email protected] (S. Tkachenko). Contents lists available at ScienceDirect International Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM https://doi.org/10.1016/j.ijrmhm.2021.105717 Received 29 June 2021; Received in revised form 2 October 2021; Accepted 3 October 2021 International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 2 which are widely used in aerospace engines or stationary gas turbines, and can at the expanse of increase in part’s weight provide a remarkable increase in power generator energy gain and the reduction of CO 2 emissions in order to reach the extremely desirable outputs for the sustainable and green society of the future. This can be accomplished by significant improvement of low oxidation resistance of Mo, decrease in its ductile-to-brittle transition temperature (DBTT) defined by the bodycentered cubic (bcc) crystal structure, and by the improvement of the high-temperature mechanical properties including recrystallization temperature [3]. To provide a substantial improvement of molybdenum hightemperature strength and fracture toughness at ambient temperature, the alloying (e.g., with Si and/or B) and/or second phase strengthening with the different types of particles, such as rare-earth oxides (ZrO 2 , La 2 O 3 ), carbides (TiC, TaC, ZrC, etc.), or other types of particles (e.g., Mo 3 Si, Mo 5 SiB 2 ) [4–12] approaches are used. Rare earth oxide particles were found to be the most effective reinforcements of Mo-based alloys (so-called oxide dispersed Mo alloys – ODS-Mo) and Mo metal matrix composites for its enhanced service temperature capabilities, since the rare-earth oxides provide a purifying effect of metallic materials grain boundaries due to the strong metal-rare-earth‑oxygen interaction. Rareearth oxide particles also effectively prevent the grain size growth, thus delivering reliable ductility, toughness, high strength and elastic modulus, and also beneficially contributing to the decrease in DBTT [11,12]. Nevertheless, according to the available literature, the potential of lanthanum zirconate La 2 Zr 2 O 7 as a hardening phase in molybdenum alloys has not been studied yet. The pyrochlore-structure of rare earth zirconate La 2 Zr 2 O 7 have excellent thermal stability up to its melting point of 2300 ◦C, low thermal conductivity (1.56 W⋅m −1 ⋅ K −1 ), and better corrosion resistance than zirconium oxide, which is frequently used for Mo strengthening [13]. To our best knowledge, the production of Mo-La 2 Zr 2 O 7 composite powders, sintering and characterization of Mo-La 2 Zr 2 O 7 composites were not published by now. In case of Mo-based composites processing, it is extremely important to obtain a fine-grained structure, which is the key condition to obtain adequate mechanical performance. Achieving this condition is only possible by employing state-of-the-art composite powder preparation and consolidation methods, such as for example ball milling and spark plasma sintering. Lowand high-energy ball milling is a rapidly developing technology that has already proved to be a viable way of producing of Mo-based alloys and its composites [7,14–16]. Enhanced reaction rates, which are achieved and dynamically maintained in solid state during such processing, results in the microstructural refinement and/or mixing processes that accompany repeated fracture, welding and deformation of particles during their collisions. The synthesized powders could be carefully controlled in terms of chemical and phase composition, microstructure, size, particle morphology, and secondary phase distribution. For example, the nanosized reinforcement particles (e.g., La 2 O 3 , MoSi 3 , TiC etc.) can be incorporated into Mo by mechanical milling, which, being homogeneously dispersed in the submicron size grains interior of molybdenum, and are able to promote substantial grain size reduction, improvement in strength, and decrease in the DBTT up to 150 K [4,6,17]. For such prepared powders consolidation, in contrast to the conventional metallurgical routes, the advanced sintering technique such as spark plasma sintering (SPS), allows to apply high heating rates and shorten the processing times to retain a small grain size, using protective vacuum/inert gas atmosphere and precise controlling of all the consolidation process parameters, such as temperature, pressure and time. Moreover, due to the possibility to sinter microand nano-sized powders, keeping their fine particle size, it is also possible to sinter the materials with higher amount of reinforcement phase. By now, only a several works are available on the sintering of disperse strengthened Mo alloys or Mo metal matrix composites using SPS method. For example Takida et al. [5] manufactured ZrC particledispersed molybdenum (1.4 vol% ZrC) and obtained an excellent mechanical properties and limited grain growth after annealing at the temperature of 1797 ◦C for 1 h. Ohser-Wiedemann et al. [9] manufactured Mo-TiC composites with minor grain growth and hardness reduction after annealing at the temperature of 1500 ◦C for 10 h in a hydrogen atmosphere. Danisman et al. [18] produced Mo-Ti-Zr (socalled TMZ) alloy reinforced by TiC and ZrC carbide precipitates, which promoted an inhibition of grain growth after high thermal exposure tests. Guo-Jun et al. [19] fabricated Mo–12Si–8.5B (wt%) alloy reinforced by 0.6 and 1.2 wt% La 2 O 3 by means of a combination of arc melting and SPS techniques, which showed that the compression strength and strain to fracture of both increased with the addition of La 2 O 3 . However, no works regarding high-energy ball mill preparation and SPS consolidation of Mo-La 2 Zr 2 O 7 composites were found in the available literature. In our contribution, we focus on the preparation of the composite powders of Mo-La 2 O 3 and Mo-La 2 Mo 2 O 7 systems by the means of lowenergy (LE) and high-energy (HE) ball milling, further sintering of the promising candidates of composite powders via SPS, and a detailed characterization of manufactured composites. Lanthanum oxide La 2 O 3 was selected due to its known thermodynamic stability at high temperatures and a pronounced effect on a ductility, workability and machinability of pure molybdenum [11,12], while La 2 Zr 2 O 7 was the experimentally synthetized reinforcement compound in Mo studied for the first time. The effects of LE and HE milling process parameters on the composition, distribution, texture and morphology of the second phase in the produced composite Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 powder systems were investigated in detail. A special attention was drawn to the study of thermal stability of the produced powders at the temperature of 1300 ◦C in different gaseous environments that provided a scope for the effect of mechanical activation on their thermal stability. Another goal of this work was to provide the information on the sintering of manufactured composite powder systems and a detailed analysis of resulting asproduced composite structures. 2. Materials and experimental procedure 2.1. Lowand high-energy milling processing of Mo-La 2 O 3 and MoLa 2 Zr 2 O 7 composite powders For the lowand high-energy kinetic processing of the molybdenumbased composites, a commercially available molybdenum powder (Mo; GTV, Germany) with the particle size 45–90 μ m was employed. 99.3% purity lanthanum oxide powder was purchased from Luoyang Golden Egret Ceotools Co. (China), while the lanthanum zirconate compound was synthesized by chemical precipitation technique according to the developed experimental protocol [20]. In this method, 500 mL of 0.1 M solution of lanthanum nitrate (La(NO 3 ) 3 ⋅ 6H 2 O, 99.9% purity, Alfa Aesar, Germany) was mixed with 500 mL of 0.1 M solution of zirconyl chloride (ZrOCl 2 ⋅7.5H 2 O, 99.9% purity, Alfa Aesar, Germany). The mixture was kept under stirring overnight, after that, it was added dropwise to 400 mL of an aqueous ammonia solution (25 wt% NH 4 OH, Sigma Aldrich, Germany), where white precipitate was formed. The Table 1 The processed Mo-based composite powders and a summary of their milling conditions. Composite powder designation Nominal composition [vol %] Milling speed [rpm] Milling time [min] BPR Mo-La 2 O 3 –350 Mo +10% La 2 O 3 350 60 100: 6 Mo-La 2 O 3 –800 Mo +10% La 2 O 3 800 60 100: 6 Mo-La 2 Zr 2 O 7 –350 Mo +10% La 2 Zr 2 O 7 350 60 100: 6 Mo-La 2 Zr 2 O 7 –800 Mo +10% La 2 Zr 2 O 7 800 60 100: 6 L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 3 resulting suspension, with a pH above 10, was kept under stirring for 24 h. Finally, the precipitate was filtered and washed with distilled water until the filtrate solution had a neutral pH. The wet precipitate was then dried and calcined at the temperature of 1000 ◦C for 5 h. Lanthanum oxide La 2 O 3 and lanthanum zirconate La 2 Zr 2 O 7 powders were used for further composite powder preparation using high-energy milling technology. Composite powder mixtures with 10 vol% (which is about 7 wt% of ceramic phase) of lanthanum oxide or lanthanum zirconate powders were prepared by measuring of tap density of ceramic powders, considering the volume of 140 g of molybdenum powder. The summary on processed powders and selected milling conditions are as listed in Table 1. The procedure was duplicated for each ceramic phase in order to perform two different milling conditions. The mixtures were separately processed in a horizontal rotary ball mill (Simoloyer CM01; Zoz Gmbh, Germany) in air, using Fe 500 grade steel balls (diameter of 4.7 mm) with a ball-to-powder ratio (BPR) of 100: 6. After loading the mill with the powder mixture and the balls, 20 drops of isopropanol were added to avoid agglomeration and increase the milling efficiency. Initially, the powders were pre-milled/activated and homogenized using a rotation speed of 700 rpm for 5 min, followed by 60 min of the set milling conditions. Low-energy (LE) and high-energy (HE) ball milling milling process conditions, selected based on previous optimization of process parameters [21] to obtain the difference in produced powder’s secondary phase distribution and morphology, were tested, namely 350 rpm and 800 rpm, respectively. 2.2. Thermal exposure The composite powders after low-energy and high-energy milling were subjected to the thermal exposure, which was performed in the furnace GSL-1700×(MTI, USA) under controlled atmosphere and consisted of a heating up to 1300 ◦C with a heating rate of 5 ◦C/min, isothermal dwell for 5 h under the different atmospheres, including oxidizing ambient, vacuum inert or 99.999% pure argon (Ar) inert atmosphere, or reducing (5% hydrogen in Ar) environments, and subsequent cooling down to the room temperature in the furnace. For this test, 3 g of each powder were put into alumina crucibles and inserted to the furnace. After the thermal exposure, the phase composition and morphology of the powders were investigated in detail, using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD) analyses. 2.3. Spark plasma sintering of the composite powders Produced composite powders were consolidated in vacuum using a spark plasma sintering (SPS) system (Dr. Sinter 1050, Sumitomo Coal Mining Co., Japan). Samples of the powders of ~5 g in weight were placed in a cylindrical graphite die and sintered under the following process parameters: heating rate of 100 ◦C/min, sintering temperature of 1600 ◦C, dwell time of 10 min and uniaxial compact pressure of 50 MPa. The external power source of the SPS system provided a direct electrical current discharge with on–off pulse duration of 12 and 2 ms, respectively. To protect the graphite die and the plungers, and to make the compact release easier after the sintering, a 0.13 mm thick graphite foil was used to line the die and the plungers. After sintering, the samples were cooled down inside the vacuum chamber until the temperature of 200 ◦C was reached, before its extraction out of the die. At least three samples of each composite with a diameter of 20 mm and a thickness of 5 mm were made. These were used for subsequent structural, chemical Fig. 1. (a) XRD patterns and SEM analysis of initial powders of (b-c) Mo, (d-e) lanthanum oxide La 2 O 3 , and (f-g) lanthanum zirconate La 2 Zr 2 O 7 . L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 4 and phase characterization as well as for hardness measurements. The bulk density of SPS fabricated composites was measured using Archimedes’ principle. The samples were weighed in air and when immersed in distilled water, and the masses of the thoroughly wetted sample were also measured. An electronic balance Discovery DV314C (Ohaus, USA) with an accuracy of 0.0001g was used for recording the weights. Theoretical densities of the samples were calculated using the rule-ofmixture principle. 2.4. Structural, chemical and phase characterization, hardness measurements The structural and chemical analysis was performed on the initial powders, on the composite powders produced by LE and HE ball milling, on the composite powders after the thermal exposure, and on the SPS sintered compacts. The study was aimed at the detailed characterization of present phases, their size, morphology, microstructure, and spatial distribution. The SPS sintered samples of Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 systems were cross-cut, put into polymer resin, mechanically ground with silicon carbide (SiC) abrasive papers of 320–1200 grit, and finely polished with 1 μ m diamond paste using a linen disc, cleaned in ethanol ultrasonically and dried in air. Structural analysis was done by means of scanning electron microscope Lyra 3 (SEM; Tescan, Czech Republic) equipped with energy dispersive spectroscopy (EDX) unit XFlash 5010 (Bruker, USA). To prevent electrostatic charging of the studied powders, a thin carbon layer of 25 nm in thickness was deposited to their surface using EM ACE600 instrument (Leica Microsystems, Germany). For powder X-ray diffraction (XRD) analysis, to determine the crystalline phases, the SmartLab 3 kW diffractometer (Rigaku, Japan) was used, using Cu K α radiation (λ =0.154 nm) operated at the current of 30 mA and the voltage of 40 kV. The diffraction patterns were collected from 10◦to 90◦with the step size of 0.02◦and the scanning speed of 4◦/min. The Rietveld refinement of the obtained XRD patterns was performed using X’Pert Highscore v.2a software and the crystallographic models belonging to the detected phases. The average hardness of the sintered composite was characterized with Vickers microhardness test, which was performed on Duramin-100 AC3 machine (Struers ApS, Denmark) with 500 g load (HV 0.5) and dwell time of 10 s. 3. Results and their discussion 3.1. Structure, chemical and phase composition of initial powders As shown in XRD diffractograms (see Fig. 1a), the commercial molybdenum and the synthesized La 2 Zr 2 O 7 powders were found in its initial stage as a single-phase material (Fig. 1a). On the other hand, the commercial La 2 O 3 powder contained except prevailing La 2 O 3 phase of about 10 wt% of lanthanum hydroxide (La(OH) 3 , ICSD: 245674), which Fig. 2. X-ray diffraction patterns of low and high energy milled Mo composite powders containing a) 10 vol% of La 2 O 3 and b) 10 vol% of La 2 Zr 2 O 7 . Mo* are the k α peaks from the main diffraction peaks of molybdenum. Note that the diffraction peaks of Mo were scaled down due to the low intensity of ceramic phase patterns. L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 5 Fig. 3. The appearance of (a-c) Mo-La 2 O 3 –350 and (d-f) Mo-La 2 O 3 –800 composite powders, where (c) and (f) are the EDX spectra images from (b) and (e), respectively. Fig. 4. The appearance of (a-c) Mo-La 2 Zr 2 O 7 –350 and (d-f) Mo-La 2 Zr 2 O 7 –800 composite powders, where (c) and (f) are the EDX spectra images from (b) and (e), respectively. L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 6 is typically formed easily by the ambient hydrolysis of La 2 O 3 due to its high hygroscopicity. Mo powder was of a body-centred cubic (BCC) crystal structure (ICSD: 98–007-6279), while lanthanum oxide and lanthanum zirconate La 2 Zr 2 O 7 powders were composed of La 2 O 3 and La 2 Zr 2 O 7 compounds with hexagonal (ICSD: 98–015-4586) and cubic pyrochlore (ICSD: 98–015-0206) crystal structures, respectively. The morphologies of the initial powders are shown in Fig. 1 b-g. Initial Mo particles (Fig. 1 b-c) were large agglomerates of spherical shape, which were made up of fine particles (1–5 μ m) of irregular morphology. In contrast, lanthanum oxide (Fig. 1 c-d) and lanthanum zirconate (Fig. 1 ef) powders were of irregular morphology, consisting of micron-sized and submicron particles. 3.2. The effect of low-energy and high-energy kinetic milling on the formation of Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 composite powders The X-ray diffraction analyses (see Fig. 2) confirm that the phase composition of the produced composite powders of Mo-La 2 O 3 and MoLa 2 Zr 2 O 7 were composed from the mixture of the phases observed in the initial powders. It means that no chemical reaction occurred during milling that could for instance result in the formation of other undesirable phases such as simple (i.e., MoO 3 ) and/or complex oxides (e.g., La 2 Mo 2 O 9 , La 2 (MoO 4 ) 3 , etc.). The effect of plastic deformation on the microstructure of metal and ceramic phases was that the intensities and the shapes of the characteristic peaks got shortened and widened for both high-energy milling (800 rpm) and low-energy (350 rpm) conditions in comparison with the initial non-milled molybdenum powder. This is related to the intensive strain hardening of the materials during deformation, since both pure Mo and the rare-earth oxides are known by their easy mechanical deformability [8]. Hence, such an intense plastic deformation at higher speeds can promote improved grain refinement and strain hardening of Mo matrix as well as high interaction between the metallic and ceramic phases, consequently, promoting the formation of smaller ceramic particles embedded in the interior of fine-grained Mo matrix. The effect of rotational speed on the crystal structure of metal and ceramic phases in the composite powders was insignificant and manifested itself in only a slightly lower intensity of the peaks in HE milled powders as compared to that of LE ones. The Rietveld refinement pointed out the weight percentage of the phases in the composite powders; for Mo containing 10 vol% of La 2 O 3 , the diffraction pattern showed the presence of lanthanum hydroxide (La (OH) 3 , ICSD: 245674) besides the hexagonal lanthanum oxide (La 2 O 3 , ICSD: 98–015-4586). The presence of the hydroxide is typical in the chemistry of the lanthanum oxide, moreover, the hydroxide easily Fig. 5. Schematic illustration of the effect of milling conditions on the microstructure of Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 composite powders, which are processed under (a) low-energy milling conditions, which lead to the formation of coarse Mo particles with some ceramic laminate inclusions that are surrounded by smaller ceramic phase particles, or (b) high-energy milling conditions, which leads to the formation of composite particles with higher amount of finer ceramic inclusions inside Mo agglomerates matrix. Table 2 EDX analysis of cross-sections of the composite powders’ agglomerates. Composite powder designation Element (wt%) Mo O La Zr Fe Mo-La 2 O 3 –350 81.6 15.8 2.6 – – Mo-La 2 O 3 –800 84.1 11.0 4.9 – – Mo-La 2 Zr 2 O 7 –350 73.7 18.1 4.9 3.2 – Mo-La 2 Zr 2 O 7 –800 70.2 15.0 9.0 5.8 0.1 Fig. 6. X-ray diffraction patterns of the high-energy milled Mo-La 2 O 3 –800 composite powder after thermal exposure at 1300 ◦C for 5 h under the vacuum, argon, and hydrogen-containing atmospheres. The diffraction peaks of Mo are pointed as Mo and Mo*, while the secondary ceramic phases correspond to 1 =monoclinic LaMo 5 O 8 ; 2 =orthorhombic La 3 MoO 7 or La 2 Mo 2 O 7 ; 3 =hexagonal La 2 O 3 ; and 4 =hexagonal La(OH) 3 . L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 7 decomposes to the oxide by heat treatment of about the temperature of 350 ◦C. On the other hand, synthesized Mo-La 2 Zr 2 O 7 composite powder showed only a content of cubic La 2 Zr 2 O 7 phase within 7–8 wt%, which only with a slight deviation corresponded to the desired powder composition. The SEM/EDS analysis (shown in Figs. 3 and 4) corroborates the observed XRD features regarding the effect of the delivered milling energy on the composite powder morphology. The morphologies of powders after mechanochemical processing showed that for both Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 systems a low-energy kinetic milling of 350 rpm (Fig. 3a-c, Fig. 4a-c) resulted in the formation of composite agglomerates of ~20–40 μ m in size and consisted of the deformed and welded Mo powder agglomerates and crushed La 2 O 3 particles. Due to the plastic deformation, the microstructure of these flake-like agglomerates was lamellar with some La 2 O 3 particles sandwiched between Mo deformed plates, although many La 2 O 3 debris surrounded the composite agglomerates and were on the surface without getting inside the grains. With the increase in rotating speed Mo-La 2 O 3 and MoLa 2 Zr 2 O 7 composites produced at high-energy kinetic milled conditions, i.e. at 800 rpm, exhibited much higher level of microstructural refinement, showing more developed lamellar microstructure of Mo matrix and lower content of ceramic phases located at the grain boundaries (Fig. 3df, Fig. 4d-f). Also, a lot of brittle La 2 O 3 and La 2 Zr 2 O 7 particles were found fragmented and embedded within the grain interior of molybdenum particles. Schematically, the resulting effect of process conditions on the formation of the different type of composites of both Mo-La 2 O 3 and MoLa 2 Zr 2 O 7 systems is shown in Fig. 5. For both cases, at initial phase of milling, Mo particles break, decreasing the particle size, however, while increasing the time of the milling or increasing the applied energy of the collisions, which is linked to the rotation speed, the malleability of Mo allows the welding of the particle, starting the formation of flake-like particles. In case of the low-energy milling (e.g., at lower rotation speed), a composite powder system is produced with the ceramic particles localized in two main areas: predominantly (1) surroundings or covering of the Mo agglomerates, and in minority 2) embedding into Mo agglomerates due to the welding effect promoted by the collisions during the milling process. On the contrary, high-energy milling delivers higher energy to the systems during the milling that results in more pronounced refinement and strain-hardening of Mo matrix and more intense incorporation of ceramic phase as inclusions inside the Mo welded agglomerates. As a result, low-energy milling leads to the low presence of ceramic inclusions in the welded molybdenum particles, while due to high-energy condition the ceramic phases are predominantly embedded into the deformed Mo flake-like particles. This was also confirmed additionally by the EDX analysis of the composite powder agglomerates, which showed higher La and Zr concentration for high-energy milling condition (see Table 2). Also, the important fact was that the iron contamination due to the erosion of the milling jar and the balls measured by EDX in the powders of both Mo-La 2 O 3 and MoLa 2 Zr 2 O 7 systems in both milling modes was either not detected or was very low (0.1 wt%). Summarizing this section, it is important to consider that the apparent difference in the microstructure of composite powders will reflect the final properties of materials produced for specific application. In this way, high-energy ball milling conditions are expected to produce Mo-based composites with better mechanical properties, which are based on the refined Mo-matrix and the toughening effect of the Laceramic systems in Mo-metallic matrix. Homogeneity in the distribution of ceramic phase in molybdenum matrix can also affect the ductile to brittle transition temperature (DBTT), which is one of the most challenging goals in the multiple design studies of Mo-metal matrix composite systems. 3.3. Thermal stability of selected powder compositions It is well-known that the thermal stability of Mo-based composites is crucial for many structural applications of these refractory materials [22]. According to the literature, the formation of lanthanum‑molybdenum oxides such as lanthanum molybdates is expected during the thermal exposure of La 2 O 3 -MoO 2 or La 2 O 3 -MoO 3 material systems [23]. In our case, the high-energy ball milling did not promote directly the formation of intermediate or transient oxides during composite powders processing. On the other hand, it is also well-known that the mechanical activation can activate chemical reactions, especially at high temperatures. Therefore, in this section, the main focus was invested to the examination of thermal stability of both obtained composite powder systems produced by high-energy milling only (using 800 rpm and BPR 100: 6), which are of high interest for the potential high-temperature structural application. The thermal stability of the composites based on pure Mo-matrix under oxidative atmospheres is very low since Mo readily oxidizes to MoO 3 compound in the temperature interval of 650–750 ◦C according to the following reaction (1): Mo +3/2 O2→MoO3(1) The volatilization of MoO 3 occurs simultaneously to its melting Fig. 7. X-ray diffraction patterns of the high-energy milled Mo-La 2 O 3 –800 composite powder after thermal exposure at 1300 ◦C for 5 h under the vacuum, argon, and hydrogen-containing atmospheres. The diffraction peaks of Mo are pointed as Mo and Mo*, while the secondary ceramic phases correspond to 1 =cubic La 2 Zr 2 O 7 ; and 2 =monoclinic LaMo 5 O 8 . L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 8 point, which is around 800 ◦C. Therefore, the stability of Mo is a function of the partial pressure of oxygen, however, even at a low partial pressure of oxygen, the oxidation and further volatilization of MoO 3 is increased with the temperature [24]. In case of the processed Mo-based composite powders, the oxidation of the major phase (Mo) proceeds prior any potential solid-state reaction with the lanthanum oxide, to form any intermediate compounds such as for example oxy-molybdates, during oxidation in ambient atmosphere. In case of the La 2 Zr 2 O 7 , due to the high chemical stability of the zirconate, no chemical interaction can be expected during the molybdenum oxidation. This was confirmed by trial experiments which revealed that very low amount of powder was obtained after the thermal exposure at high temperatures of the Mo-based composites under oxidizing environment as a consequence of oxidation of Mo-matrix and volatilization of MoO 3 . This fact confirms the impossibility of use of Mo-La 2 O 3 or La 2 Zr 2 O 7 composites with pure Mo-matrix directly under the oxidizing atmosphere; however, these composites could represent alternative refractory systems in the applications where partial pressure of oxygen is minimized. For this reason, the thermal stability evaluation was conducted as well in inert and reducing atmospheres with lower oxygen partial pressure. 3.3.1. Inert atmosphere The thermal stability of high-energy milled Mo-based composite powders was studied in two different inert environments: 1) vacuum (1 ×10 −7 Pa), and 2) 99.999% pure argon atmosphere. The XRD Fig. 8. The cross-sectional micrographs of Mo-La 2 O 3 –800 composite powder after the thermal exposure at 1300 ◦C for 5 h in (a-b) vacuum, (c-d) argon, and (e-f) hydrogen atmospheres; SEM-BSE. L. ˇ Celko et al. International Journal of Refractory Metals and Hard Materials 102 (2022) 105717 9 characterization confirmed that produced Mo-La 2 O 3 and Mo-La 2 Zr 2 O 7 composite powders after the thermal exposure in both inert atmospheres revealed an interaction between composite powders constituents via solid state-reaction resulted in formation of different lanthanum molybdate phases. The thermal exposure of Mo-La 2 O 3 composite powder in vacuum resulted in the complete transformation of La 2 O 3 into LaMo 5 O 8 phase (ICSD: 98–006-8219) with P12/c1 monoclinic crystal structure lattice due to the reaction of La 2 O 3 with Mo. The Rietveld refinement pointed out that this second phase represented up to the 21.7 wt% of the composite, complemented with a pure Mo phase (Fig. 6). On the other hand, the thermal exposure under the vacuum kept the integrity of the second studied Mo-La 2 Zr 2 O 7 system almost the same, and only slightly reacted to form the same LaMo 5 O 8 compound (Fig. 7). As compared to the Mo-La 2 O 3 system, the presence of the lanthanum oxymolybdate product was very low (of about 1.2 wt% of the composite), probably due to the higher chemical stability of the La 2 Zr 2 O 7 pyrochlore structure phase. The La 2 Zr 2 O 7 phase remained in the composite, representing almost the 7 wt%, while pure Mo took the rest of complementary percentage. The thermal exposure of the composites under the argon atmosphere promoted different chemical reactions. For the Mo-La 2 O 3 system, the lanthanum oxide fully interacted with Mo to form orthorhombic La 3 MoO 7 phase (ICSD: 98–008-4708) as secondary phase, which represented the 5.3 wt% of the composite with the Mo phase as the matrix (Fig. 6). The formation of La 3 MoO 7 has been previously registered above the temperature of 1300 ◦C and in a narrow range of partial pressure of Fig. 9. The cross-sectional micrographs of Mo-La 2 Zr 2 O 7 –800 composite powder after the thermal exposure at 1300 ◦C for 5 h in (a-b) vacuum, (c-d) argon, and (e-f) hydrogen atmospheres; SEM-BSE. L. ˇ Celko et al.