Microstructure of Mo-La2O3 Composite Powder Prepared Using Two Different High Energy Ball Milling Systems

Microstructure of Mo-La2O3 Composite Powder Prepared Using Two Different High Energy Ball Milling Systems Serhii Tkachenko1,a*, Ladislav Čelko1,b, Michaela Remešová1,c, Vendula Bednaříková1,d, Karel Dvořák2,e, Pavel Komarov1,f, Karel Zabranský1,g 1Central European Institute of Technology, Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech Republic 2Institute of Technology of Building Materials and Components, Faculty of Civil Engineering, Brno University of Technology, Veveří 331/95, 602 00, Brno, Czech Republic aserhii.tk[email protected], [email protected], [email protected], d[email protected], e[email protected]tbr.cz, f[email protected], [email protected] Keywords: Molybdenum, Composite, Lanthanum, Oxide dispersion, High-energy milling, Planetary mill, Attritor Abstract. The present study was focused on the high-energy ball milling preparation of the oxide dispersed Mo-10 vol.%La2O3 composite powder by using two high energy ball milling systems, including an attritor mill with horizontally operating rotor and a planetary mill. Microstructural investigations using scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) found out that composite powders with a refined laminated microstructure can be obtained using both milling systems. Under selected milling conditions, Mo-La2O3 powder with better particle size homogeneity and more uniform and dispersed ceramic phase distribution was received in the planetary mill that can be connected with more balanced contribution of breaking and cold welding processes during high energy milling. Introduction The development of new materials suitable for application temperatures beyond the capability limit of nickel-based superalloys (higher than 1200 °C) is of high importance worldwide. Alloys and composites based on refractory metals (Nb, Ta, Mo, W, and Re) are receiving considerable attention because they are potential candidates for incoming high-temperature applications and energy engineering technologies due to unique combinations of superior chemical, physical and mechanical properties. Among other refractory metals, molybdenum (Mo) is highly attractive because of high melting point (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·K1·mol-1 in the range of 127 - 727 °C), low coefficient of thermal expansion, good electric conductivity, excellent heat resistance, and density comparable to nickel superalloys (10.2 g · cm-3 vs. 8.2 – 8.5 g·cm-3) [1, 2]. However, some of the major drawbacks of Mo and its alloys, such as low oxidation resistance and relatively high ductile-to-brittle transition temperature (DBTT) have to be minimized; high-temperature mechanical properties and recrystallization temperature have to be improved as well [3,4]. Recently, oxide dispersion strengthened (ODS) molybdenum alloys have been developed to enhance high temperature strength and to decrease DBTT. Homogeneous dispersion of micro/nanometric-sized stable rare-earths oxide particles (e.g., La2O3) in the matrix of Mo-based ODS alloys can act as effective barriers against dislocation motion and improve high temperature mechanical properties including creep strength [5, 6]. The commonly adopted technological route for ODS alloys‘ manufacturing is powder metallurgy that includes mechanical milling of the precursor powders and the following hot consolidation. As the homogeneous dissolution of the rare-earths oxides during mechanical milling is considered as the key of success in the fabrication of Mo-based ODS alloys, the selection of the Solid State Phenomena Submitted: 2021-11-12 ISSN: 1662-9779, Vol. 334, pp 109-114 Revised: 2022-01-31 doi:10.4028/p-a18wu2 Accepted: 2022-02-01 © 2022 Trans Tech Publications Ltd, All Rights Reserved Online: 2022-07-15 All rights reserved. No part of contents of this paper may be reproduced, processed or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (#726856005, Brno University of Technology, Brno, Czech Republic-30/10/25,14:08:45) appropriate high energy (HE) milling system is of great importance. Among available HE ball milling systems, the Simoloyer system with horizontally operating rotor shows the highest achievable relative velocity of grinding balls in milling jar (up to 14 m/s), which results in high activation energy and very fast processing of the material [4]. The other advantages of the system also include the possibilities for the production of industrially relevant amounts of powder products and easiness upscaling of the technology. In contrast, modern laboratory planetary ball mill systems feature moderate maximum relative velocity of grinding balls, which results in longer processing times and lower impact energy (up to 5 m/s). This type of milling system is designed for processing of laboratory scale amounts only with no possibilities for the upscaling. The focus of the present study was to investigate the effects of the above-mentioned HE milling systems (horizontal attritor vs. planetary mill) on the microstructure of the Mo–10 vol.% La2O3 composite powder. A special attention was put on the detailed characterization of present phases, their size, morphology, microstructure, and spatial distribution. Experimental Raw materials. The commercially available molybdenum powder (Mo; GTV, Germany) with the particle size 45-90 µm was employed. 99.3% purity lanthanum oxide powder was purchased from Luoyang Golden Egret Ceotools Co. (China). Milling procedures. The composite powder mixture with 10 vol. % (which is about 7 wt.% of ceramic phase) of lanthanum oxide powders was prepared by measuring of tap density of ceramic powders, considering the volume of 140 g of molybdenum powder. The summary of the used milling systems and milling conditions are listed in Table 1. In case of every milling system, 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. The milling parameters were selected on the basis of the milling system’s producer recommendations, authors’ previous experience, and after some preliminary trials. Table 1. Summary of the milling systems/conditions applied to the Mo–10 vol.%La2O3 powder mixture. Milling system Milling equipment / producer Milling media Milling conditions Attritor milling Simoloyer CM01, Zoz Gmbh, Germany Stainless steel balls of diameter of 4.7 mm BPR 100 : 6 Air atmosphere Premilling/activation at 700 rpm for 5 min Final milling at 800 rpm for 60 min Planetary milling Retsch PM 100, Retsch, Germany Stainless steel balls of diameter of 25 mm BPR 10 : 1 Air atmosphere Milling at 350 rpm for 20 h of milling in cycles (each cycle includes 20 min of milling + 5 min of cooling) Microstructural examination. The structural and chemical analysis was performed on the initial powders and on the composite powders produced by HE milling. 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). To investigate the crosssectional microstructure of the powder after milling, powder was put into fast-curing polymer resin at ambient temperature, and then obtained samples were ground (using 220 diamond disc), polished (using 9-1 µm diamond suspension), and mechanochemically etched (using OPS suspension). For powder X-ray diffraction (XRD) analysis, to determine the crystalline phases, the SmartLab 3kW 110 Structural and Functional Materials diffractometer (Rigaku, Japan) was used, using Cu Kα 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. Results and Discussion The morphologies of the initial powders are shown in Figure 1 a-b. Initial Mo particles (Fig. 1a) were large agglomerates of spherical shape of 40-100 µm, which were composed of fine particles (1-5 µm) of irregular morphology. In contrast, lanthanum oxide (Fig. 1b) powder was of irregular morphology, consisting of micron-sized and submicron particles. As found out by XRD analysis (Fig. 1c), the commercial molybdenum was found in its initial stage as a single-phase material, while the commercial La2O3 powder contained except prevailing La2O3 phase of about 10 wt.% of lanthanum hydroxide (La(OH)3, ICSD: 245674), which typically forms easily by the ambient hydrolysis of La2O3 due to its high hygroscopicity. Mo powder was of a body-centred cubic (BCC) crystal structure (ICSD: 98-007-6279), while lanthanum oxide powder was a La2O3 compound with hexagonal (ICSD: 98-015-4586) crystal structure. Fig. 1 The morphology of initial (a) molybdenum and (b) lanthanum oxide powder particles with (c) the corresponding diffraction patterns. The morphology of the Mo–10 vol.% La2O3 powder produced at high-energy kinetic milling conditions of the attritor mill is shown in Fig. 2. The powder was not entirely homogeneous, showing both large agglomerates of 20-50 µm in size as well as a large amount of small (Mo + La2O3) debris between them (Fig. 2a). Cross-sectional microstructure of Mo-La2O3 composite powder exhibited high level of microstructural refinement of cold-welded agglomerates, showing the developed lamellar microstructure of Mo matrix (Fig. 2b), while ceramic phase was located Solid State Phenomena Vol. 334 111 mainly at the grain boundaries. Also, a lot of brittle La2O3 particles were found fragmented and embedded within the grain interior of molybdenum particles. The X-ray diffraction analysis (Fig. 2d) confirmed that the phase composition of the produced composite powder was composed from the mixture of Mo and La2O3 phases. The intensities and the shapes of the characteristic peaks got shortened as compared to the raw materials as a result of the intensive strain hardening. Fig. 2 (a) The morphology of the Mo–10 vol. %La2O3 composite powder processed using an attritor mill, (b) cross-sectional microstructure of the powder particle, (c) elemental EDX maps from (b), and (d) the corresponding XRD pattern of this powder. As compared to the attrition milling, the Mo–10 vol.% La2O3 composite powder processed in a planetary mill showed also lamellar powder morphology, however, with more homogeneous particle distribution (Fig. 3a). The powder agglomerates were of 5-30 µm in size, showing almost no fine debris in the interparticle space. Cross-sectional microstructure showed similar microstructural refinement due to the intensive plastic deformation (Fig. 3b), where fine La2O3 particles were mostly located in the grain interiors (Fig. 3c). In contrast to the attrition milling, the higher refinement degree of ceramic phase was confirmed by XRD analysis, which detected the shortening and the widening of peaks of Mo matrix only with no peaks of La2O3 detected. Both, pure Mo and the rare-earth oxides are known by their easy mechanical deformability [8], so such an intense plastic deformation by planetary milling 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. 112 Structural and Functional Materials Fig. 3 (a) The morphology of the Mo–10 vol. %La2O3 composite powder processed using a planetary mill, (b) cross-sectional microstructure of the powder particle, (c) elemental EDX maps, and (d) the corresponding XRD pattern of this powder with reference initial Mo. The difference in the powder morphology and the distribution of ceramic phase in the powder produced by two different milling systems is schematically described in Fig. 4. The main reason can be connected with the difference in the operating processes of counteracting breaking and cold welding during high energy milling in selected HE milling systems. As known, the breaking of the particles is mainly caused by shear loads transferred by grinding balls colliding under a low angle, while cold welding is dominant for perpendicular impact of grinding balls [7]. Under selected milling condition, the contribution of the breaking in the attritor milling seems to be much higher then in the planetary ball milling, so stable particle agglomerates are not able to form due to cold welding. This means that further milling parameter optimization is needed for attritor milling systems in order to reach more homogeneous particle distribution and finer La2O3 phase dispersion. Fig. 4 Schematic description of the effect of used HE milling systems on the microstructure of Mo-10 vol.% La2O3 composite powder. Solid State Phenomena Vol. 334 113 Summary In this work, Mo-based composite powder with the laminated microstructure and reinforced with 10 vol. % of La2O3 ceramic phase was produced by using both the attritor and the planetary highenergy kinetic milling systems. Under selected milling conditions, more homogeneous distribution and finer La2O3 particle dispersion was achieved by using planetary ball milling that can be connected with more balanced contribution of breaking and cold welding processes occurring during high energy milling. Acknowledgements Authors acknowledge the Ministry of Education, Youth and Sports of the Czech Republic for funding the project LTC20068 “New materials for emerging energy technologies “under INTEREXCELLENCE program, sub-program INTER-COST. The work was also carried out with the support of European Union’s COST programme within the CA18112 action “Mechanochemistry for Sustainable Industry”. 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