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Composition of the Mo-Mo3Si alloys obtained via various methods
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Composition of the Mo-Mo3Si alloys obtained via various methods ⁎
T
Ivan Gnesin , Boris Gnesin Institute of Solid State Physics of Russian Academy of Sciences (ISSP RAS), Chernogolovka, Moscow District, 2 Academician Ossipyan str., 142432, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum silicides Reaction sintering Solubility limit Mo-Si phase diagram Retrograde solidus
The phase and chemical composition of an MoeSi system alloy with a silicon content of 25 at.% was investigated via scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The samples were obtained by both sintering and melting of the components. The synthesis of Mo3Si from elemental Mo and Si via a sintering process at ≤1480 °C was significantly less complete than the synthesis of MoSi2, Mo5Si3 silicides via same process. To increase the efficiency of the synthesis, the reaction sintering of Mo3Si should be carried out at significantly higher temperatures than in the cases of MoSi2 and Mo5Si3 silicides synthesis. This offers a sufficiently phase-pure Mo3Si via reaction sintering at temperatures below the solidus. X-ray microanalysis data on Mo3Si chemical composition indicated a Mo3Si exists in a small compositional range. The average silicon content in Mo3Si was < 25 at.%. In addition, experimental data indicated that the solubility of silicon in molybdenum may be higher than previously thought. Furthermore, the composition of the melt-quenched samples suggests that the solidus line for a Si solution in Mo may be retrograde.
1. Introduction The search for new high-temperature materials that combine hightemperature strength, good resistance to high-temperature creep, high fracture toughness, and resistance to high-temperature oxidation remains one of the most pressing problems in materials science. In the overwhelming majority of cases, complex alloys based on nickel and chromium are used for temperatures < 1200 °C [1]. However, the possibility for them to be further improved has been almost exhausted, it is clear that other materials are required for higher operating temperatures. Numerous attempts were made to use refractory metals silicides - primarily molybdenum silicides. They are attractive due their high heat resistance up to 1800 °C and above. This is obvious in the case of high-temperature electric heaters [2] and in protective coatings for refractory metals and carbon materials [3–5]. Attempts to develop high-temperature composite materials using the high heat resistance of these silicides are often associated with the creation of a silicide matrix strengthened by dispersed particles of carbides, oxides, and some other refractory compounds [6,7] due to poor silicide strength above ~1300 °C. Significant progress has been achieved in the development of new alloys containing silicides of refractory metals and Mo-based solid solutions (Moss) [8,9]. Due to the success in the development of these alloys, Mo3Si silicide has attracted the attention of researchers as a potential component of high-temperature composites. The behavior of
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the Mo3Si phase during deformation at room temperature is somewhat unexpected. Related work [10] noted that the strength and ductility are higher than in pure molybdenum in Mo doped with 0.3 wt% Si (1.02 at. % Si); the ductility of Mo doped with 0.1 wt% (0.34 at.%) Si was lower than that of unalloyed Mo. Prior work [11] focused on Mo-Mo3SiMo5SiB2 alloys and showed that a crack moving through a structure with significant volume fractions of Moss and Mo3Si can stop its propagation. Others [12] studied Mo3Si + Mo5Si3 composites via transmission electron microscopy and showed that there are dislocations at the tip of the crack exactly in the Mo3Si phase whereas dislocations have not been found in the Mo5Si3 phase. These data allow us to consider the introduction of the Moss and Mo3Si combinations into hightemperature composites as an opportunity to increase their crack resistance at low temperatures while maintaining an acceptable level of high-temperature strength. It is important that materials containing carbides, Mo, and Mo3Si phases can be protected from high-temperature oxidation by coatings containing, for example, MoSi2 and Mo5Si3. A similar approach when the heat resistance of the high-temperature base material is achieved by applying protective coatings has been demonstrated in many cases [13,14]. The MoeSi binary system is usually considered as a well-studied [15–22] with a known phase diagram [16,19] (Fig. 1). To date, a considerable amount of experimental data has been published [23,24], but these data do not always correspond to the phase diagrams in the literature [16,19]. In one study [23], the authors
Corresponding author. E-mail address: [email protected] (I. Gnesin).
https://doi.org/10.1016/j.ijrmhm.2020.105188 Received 19 March 2019; Received in revised form 19 November 2019; Accepted 3 January 2020 Available online 07 January 2020 0263-4368/ © 2020 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
Fig. 1. Phase diagram for MoeSi binary system [19].
process. Thus, this work is devoted to the study of phase transformations features and changes in the chemical composition during the Mo3Si synthesis via reaction sintering and crucible-less melting. It is necessary to clarify the chemical composition of the product obtained, the presence of other phases in it, and the features of the synthesis kinetics at different temperatures. These results can aid in the development of high-temperature composite materials containing silicides.
found relatively small deviations from the stoichiometric composition in the Mo3Si phase: the Si content is 23.5–24 at.% but not 25 at.%. Unfortunately, this work does not provide data on the directly measured composition of the studied phases. Deviations from the stoichiometric Si content after crystallization have also been found in [24,25]. These were very significant, but the authors explained this fact via non-equilibrium crystallization conditions. Thus, there is still uncertainty in matters of compliance of the Mo3Si composition with the stoichiometric one as well as the existence of a composition range for this phase. Another study [26] found that the Mo3Si phase has an incommensurate structure, and the composition of this phase may not correspond to the stoichiometric one. Both the stoichiometry and synthesis of the Mo3Si phase are important to its application. For controlled introduction of certain phases into high-temperature composites, it is often preferable to work with pre-synthesized phases rather than with pure components. This approach eliminates the undesirable presence of unreacted starting components, e.g., free silicon when working with silicide materials. In addition, this approach helps to achieve a better control of the phase composition of the composite material. In many studies [24,26,27] Mo3Si silicide was obtained via arc melting in argon using a “non-consumable” tungsten electrode. The concentration of possible tungsten contamination and its influence on phase equilibria remain unclear. This factor becomes important with multiple rounds of sample remelting during arc melting. It is very interesting to consider the possibility of Mo3Si silicide synthesis via reaction sintering of molybdenum and silicon powders mixtures. This could allow one to obtain it in appreciable volumes with very small contaminant concentrations. Thus, one of the goals of this work is to investigate the possibilities of such an approach. In addition, this work highlights the potential to obtain Mo3Si via crucible-less melting in an induction furnace while levitated in a magnetic field. This can significantly reduce the risk of contamination during the alloy production
2. Materials and methods Powders of commercially available MCH-grade Mo and polycrystalline Si were used as components for silicides synthesis. Before mixing, the powders were sifted through a 100-μm mesh sieve. A small amount of liquid binder based on a saturated polyvinyl alcohol aqueous solution was added to the mixture of Mo and Si powders. Reaction sintering was performed after compaction. Some of the sintered samples were melted at an argon pressure of 35–45 kPa in a levitation conditions in a EMT 27–64 levitation melting system. Rotating electromagnetic field frequency was near 200 kHz. After holding the molten sample in the liquid state for 1–2 min, the field was removed, and the samples (weighing ~15–17 g) were drop-cast into a 224 g copper mold (~11 mm in inner diameter). Next, crosscut sections were made on the quenched samples with a diamond cutting disk, and polished surfaces were prepared using diamond powders with particle sizes whose main size fraction gradually decreased from 28/20 to 1/0 μm. The phase composition of the obtained samples was investigated via X-ray diffraction (XRD) using 40 kV accelerating voltage and an incident beam monochromatizated MoKα radiation. The powders of the obtained materials were fixed on glass plates via a trace mount of plasticin. Polished cross-sections of dense powder materials as well as polished cross-sections of samples after levitational melting were also 2
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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silicide did not contain any impurity phases detected using the XRD (Fig. 3a). Thus, the experimental data suggest that the solid phase synthesis at 1480 °C for the Mo5Si3 phase (Fig. 3a) was much more complete than for Mo3Si despite the fact that the melting point of Mo5Si3 silicide is higher (by ~150 °С) than the peritectic temperature of Mo3Si. The Mo3Si is traditionally depicted as a line compound on MoeSi system phase diagrams, but Mo5Si3 silicide has a noticeable composition range (~2–3 at.% Si) [16,19]. One would assume that the presence of a composition range could facilitate the synthesis of the Mo5Si3 phase relative to Mo3Si. However, a mixture of Mo and Si powders corresponding to MoSi2 (also a line compound) had a sufficiently complete synthesis at 1480 °C similar to Mo5Si3 (Fig. 3b). Along with tetragonal MoSi2, only a small amount of Mo5Si3 (~ 1–2 vol%) was found. Thus, of the three known molybdenum silicides, Mo3Si had insufficient synthesis completeness for the reaction-sintering regime with Tmax~1480 °C. However, the synthesis of MoSi2 and Mo5Si3 silicides was quite effective via this technological route. This is consistent with the previously obtained results concerning the kinetics of diffusion growth of layers of various silicides in the MoeSi system. Prior work [28] showed that Mo3Si has a significantly smaller (not less than an order of magnitude) effective diffusion coefficient in this silicide relative to Mo5Si3 and MoSi2. Most likely, when Mo3Si undergoes reaction sintering with a maximum temperature 1480 °C, the diffusion stage of the Mo3Si phase growth takes more time than for Mo5Si3 or MoSi2. To verify this hypothesis, we used the reaction sintering regime with one additional heat treatment stage: 1 h of annealing at 1700 °C. Thus, we noticeably increased both the temperature and the time of the component interactions during Mo3Si synthesis.
investigated using X-ray diffraction. The volume fraction of the phases was estimated via the ratio of corresponding line intensities for different phases using experimental and ICDD data. The samples' microstructure was studied with a scanning electron microscope (SEM; Tescan Vega II XMU) with an INCA Energy 450 energy spectrometer with a Si(Li) semiconductor detector. The accelerating voltage was 20 kV. The error of the silicon concentration was determined using X-ray microanalysis and was estimated by measuring the silicon content in a SiO2 crystal. This phase is characterized by a nearly constant composition (it is the line compound) corresponding to 33.33 at.% Si. To estimate the error, 10 parallel measurements of Si concentration were performed in a small region of the quartz crystal surface. The standard deviation was 0.28 at.%. But according to the data obtained, the average value of the silicon concentration measured was 33.38 at.%, which is only 0.05 at.% different from the known composition of SiO2. 3. Results 3.1. Reaction sintering The Mo3Si synthesis in this work used an initial mixture of Mo and Si powders at a mass ratio of 10.5:1; the exact stoichiometric ratio corresponds to 10.28:1. Thus, the initial mixture had a slight excess of Si (~0.7 at.%) versus 25 at.% corresponding to the exact Mo3Si stoichiometry [16,19]. Reaction sintering was performed via stepwise annealing in a vacuum furnace with molybdenum heaters. The maximum temperature of the reaction sintering was ~1480 °C. The temperature was controlled using a WRe5/WRe20 thermocouple. Exposure at the maximum temperature of the reaction sintering was 25 min after which the sintered billet was cooled with the furnace. The billet was ground into a powder and then mixed and sieved with a 100-μm mesh. Fig. 2 shows the X-ray diffraction patterns of such a powder. Three phases were revealed: Mo3Si, Moss - Si solid solution in Mo, and Mo5Si3. Neither free Si nor MoSi2 were detected. The Moss volume fraction estimation amounts to a few tens of percent. The Mo5Si3 phase at the same time was contained in a rather small amount—only about several vol%. Thus, it is obvious that the synthesis of the Mo3Si was incomplete. Further experiments showed that the process of solid-phase Mo3Si synthesis via reaction sintering differs significantly from the solid-phase synthesis of Mo5Si3 and MoSi2. As a control, we made Mo5Si3 via the same technological route as with Mo3Si described above. The resulting
3.2. Additional sintering Additional annealing (1 h at 1700 °C) after Mo3Si reaction sintering (with a maximum temperature of 1480 °C) was performed in a vacuum furnace with molybdenum heaters. The polished cross-section was prepared on the obtained sample; the XRD and SEM results for this sample are shown in Fig. 4. Increasing the duration and the temperature of the final reaction sintering stage noticeably affected the homogeneity of the phase composition of the sample. The presence of Mo3Si and a relatively small amount of Moss solid solution was seen, but no other phases were found. In this case, the Moss volume fraction decreased significantly versus samples that did not undergo additional annealing; this amounted to no more than a few vol%. The Mo5Si3 phase was not detected at all. Thus,
Fig. 2. X-ray diffraction pattern from Mo - 25 at.% Si powder sample after reaction-sintering at 1480 °C. 3
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
Fig. 3. Diffraction patterns of Mo5Si3 (a) and MoSi2 (b) silicides after reaction sintering at 1480 °C.
on a melt-quenched sample. The vast majority of its volume is represented by the Mo3Si phase (> 90 vol%). However, the sample contains both relatively small grains of Mo5Si3 phase (diameter of several microns) precipitated along the boundaries of larger Mo3Si grains as well as Moss crystallites up to 10–15 μm in diameter (Fig. 5a). The average Si concentration in Moss as a result of the quenching of the Mo3Si melt is 8.3 at.%. This is almost twice the solubility limit for Si in Moss for the peritectic temperature (~4–5 at.% [16,19]). The Si concentration measured in the Mo5Si3 phase was slightly lower than its known level (34.4–36.0. at.% instead of 37.2–39.2 at.% according to [19]). This is due to the very small diameter of the Mo5Si3 grains: analyzed backscattered electrons originated from surrounding Mo3Si phase also. The Si concentration in the Mo3Si phase itself was experimentally determined by X-ray microanalysis to be on average 22.6 at. %. This is even slightly lower than values obtained after additional sintering of Mo3Si at 1700 °C for 1 h. To control the phase composition of the melt-quenched sample, it was ground via cemented carbide disks. The resulting powder was used for XRD analysis (Fig. 5b), which showed that the main phase present in the sample was the Mo3Si (this was consistent with the microstructure data). The presence of Mo5Si3 on the diffraction pattern was manifested by only one separate line. There were no separate Moss lines on the
we confirmed experimentally that higher temperatures are required for sintering the Mo3Si phase versus other molybdenum silicides. It was also possible to estimate the Si content in both the Mo3Si and Moss phases. The data suggested that the Si content in Moss was about 2.7 at.% and 23.6 at.% in the Mo3Si phase. Importantly, we were still unable to obtain the Mo3Si phase with a 25 at.% Si content by increasing the maximum temperature of the sintering from 1480 to 1700 °C and the duration of maximal temperature stage from 25 min to 1 h. In connection to these experimental results, a question arose on the possibility of obtaining the Mo3Si phase with a composition as close as possible to the stoichiometric one. It was necessary to determine whether the observed deviations of the silicon concentration in Mo3Si are due to the use of solid-phase synthesis and incomplete diffusion processes or if it is from other sources. To do this, experiments on the synthesis of Mo3Si were performed by melting the components in an induction furnace in a levitation state. 3.3. Mo3Si melt-quenching The sample after the reaction sintering of a mixture of Mo and Si powders at 1480 °C was melted. Fig. 5 shows the results of the analysis of the composition and structure of the polished cross-section obtained 4
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Fig. 4. XRD pattern (a) and SEM (BSE) image (b) of the Mo–25 at.% Si sintered sample after additional annealing for 1 h at 1700 °C.
concentration in the Moss solid solution was about 5.1 at.%. The concentration was measured in Moss crystallites larger than 2 μm to avoid the effect of the surrounding Mo3Si phase on the analysis results. Even in the case of measurements in a such relatively large Moss crystallites, we cannot say with 100% certainty that we received a signal from this phase only. However, since the measured value of ~ 5.1 at.% is an estimation of the silicon content from above, we can definitely say that the silicon content in the Moss phase decreased markedly due to annealing. It approaches the equilibrium condition. The average measured Si content in Mo3Si was 22.3 at. %, which only slightly differs from the data obtained for the state before annealing. Thus, even after high-temperature annealing of the melted sample, it was not possible to obtain the Mo3Si phase with 25 at.% Si concentration corresponding to the state diagram. The silicon content in this phase remains at a level a few atomic percent below the stoichiometry.
diffraction pattern. This agrees well with its small volume fraction because the 100% Moss line {110} 2Θ = 18.35° is superimposed on the 100% Mo3Si line {210} 2Θ = 18.66°. A comparison of the intensity of the experimental lines with ICDD data suggests that a robust powder preparation technique could not completely overcome the crystallographic texture of the sample formed as a result of crystallization. Thus, melting under levitation conditions did not increase the Si concentration in Mo3Si up to a level of 25 at.% in accordance with the phase diagram [16,19]. We attempted to obtain the state of the sample closer to equilibrium using high-temperature annealing. The quenched sample was annealed at 1700 °C for 2 h in a vacuum. XRD analysis showed only one phase, Mo3Si, in the annealed sample (Fig. 6a). The samples' microstructure was noticeably changed as a result of the annealing. The Mo5Si3 phase was not found at all. The crystallites of the Moss phase were significantly smaller (1–3 μm in diameter). At the same time, the Moss volume fraction decreased to below ~ 1 vol%. The Si 5
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
Fig. 5. SEM (BSE) image of the structure (a) and the diffraction pattern (b) for Mo – 25 at.% Si sample after melt-quenching.
4. Discussion
presence of a region of existence of a Mo3Si phase. Moreover, the average Si concentration in the Mo3Si phase should be < 25 at.%. We cannot say at the moment what is exact configuration of Mo3Si region of existence on the phase diagram. But results obtained allow us to assume that Mo3Si is not a line compound and its lower silicon solubility limit is lower than 23 at.%. The described above difficulties in the synthesis of Mo3Si in comparison with other molybdenum silicides should also be noted. This may be due to the hindered diffusion in the Mo3Si crystal lattice—researchers have previously noted its complex structure [26]. This is consistent with data on diffusion in the Mo3Si phase published previously [28]. The presence of a solid solution based on Mo among the products of Mo3Si synthesis allows one to trace some interesting features in the chemical composition of this phase. The Si concentration within the Moss was subject to more significant changes than in the Mo3Si phase
The measured Si content for all studied samples is shown in Table 1. As noted above (Section 2), the Si concentration measurements were verified via a SiO2 etalon. The results showed a relatively high accuracy of the average Si concentrations measured here. The difference between the measured and known silicon concentrations in SiO2 is < 0.15% (about 0.05 at.%). The data in Table 1 shows that the average concentration of Si in the Mo3Si phase is 22.3–23.6 at.% for all the samples investigated. Samples after quenching have a slightly lower silicon concentration than samples obtained by sintering. This may be because of some loss of silicon as a result of the higher temperature treatment. It should be noted also that samples after additional annealing contain no Mo5Si3 phase. Thus, high temperature annealing could result in additional loss of silicon by the samples. In general, these results confirm the hypothesis of a
6
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
Fig. 6. The diffraction pattern (a) and SEM (BSE) image of the microstructure (b) of the melt-quenched sample after annealing at 1700 °C 2 h.
splat quenching). This follows from the fact that all measured Si concentrations in Moss were significantly higher than the solubility limit for room temperature [16,19]. Given the relatively slow diffusion in the phases studied here—even at high temperatures—this is not surprising. Another feature discovered during the work was that after quenching from a liquid state, the Si content in the Moss phase reaches > 8 at. %. As a result of subsequent annealing at 1700 °C the concentration of Si in the Moss phase decreased significantly and reached about 5 at.% or less. The excess silicon content in molybdenum decreases during annealing as a result of activation of the diffusion processes. The main question is what led to the appearance of such a high (> 8 at.%) concentration of silicon in molybdenum immediately after crystallization. In our experiments, we did not find any signs of amorphization of the melt. Based on the XRD data that were obtained, the high melting temperature of the alloy under study, and the relatively large mass of samples (15–17 g), amorphization of the melt seems unlikely. This is not surprising because the quenching in our work was performed by a
Table 1 Silicon concentrations in the molybdenum-based solid solution (Moss) and in the Mo3Si phase according to the results of microanalysis. Technological route
Additional sintering 1700 °С 1 h Levitation melting and quenching Levitation melting and quenching, 1700 °С 2h annealing
Moss
Mo3Si
CSi
σ
n
CSi
σ
n
2,70 8,32 5,12
0,44 0,24 0,08
5 8 4
23,60 22,58 22,33
0,56 0,7 0,34
6 14 25
CSi – Si average concentration in at.%, σ - estimation of a standard deviation, n – number of measurements.
discussed above. The results suggest that in the framework of the experiments provided here, all heat treatments have preserved the high temperature state at room temperature despite the absence of special measures to increase the cooling rate of the samples (e.g. melt spinning, 7
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
The work was partially supported by the RFBR, project 18-3300471.
simple casting of the melt into a massive copper mold, without using any measures to increase the cooling rate (e.g. melt spinning, splat quenching). During quenching, significantly supersaturated solutions are usually formed from solid phases, and not from liquid phases. This is because in solid phases, diffusion is easily suppressed by quenching, while in the liquid phase, the mobility of atoms is orders of magnitude higher. Thus, even during rapid cooling, the deviations from local equilibrium at the liquid–solid interface, are most often small. After crystallization, the average composition of the solid phase may differ from the equilibrium one because the diffusion processes in the solid phase are suppressed. Currently, there are no data that allow us to determinate the exact reason for supersaturation of the Si solid solution in Mo up to > 8 at.%. We cannot exclude the possibility of completely nonequilibrium primary crystallization. However, taking into account the information presented above, the probability of such a case seems to be very low. A significant supersaturation of the Moss phase with silicon, in our estimation, is caused by the features of the phase diagram. Here, we suggest two possible explanations for this phenomenon. The first one: the solubility limit of silicon in a molybdenum-based solid solution at peritectic equilibrium Moss + L = Mo3Si may be significantly higher than previously assumed. The second one: the solidus of the molybdenum-based solid solution may be a retrograde one [29,30]. The solubility limit of silicon in molybdenum near the peritectic temperature is considered to be known and experimentally determined as ~4 at. % [31]. At the same time, the position of the solidus line above the peritectic temperature is still not established [16,18,19]. Thus, we consider the retrograde solidus as more probable explanation of the results obtained. But for now, this is just an assumption. To provide an exact answer to the question about the reasons for the obtaining the Moss phase with a silicon content of > 8 at.%, further studies are needed.
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5. Conclusion We show successful solid phase synthesis of Mo3Si. In addition to high productivity, this method can produce relatively pure silicides. To obtain Mo3Si via the reaction sintering process, it is necessary to use higher temperature (about 1700 °C and above) and longer exposure (about several hours) than for the synthesis of MoSi2 and Mo5Si3 silicides. We believe that this is the consequence of significantly lower rates of diffusion during the Mo3Si silicide synthesis than in the case of MoSi2 and Mo5Si3. The data obtained for silicon content in the Mo3Si are consistent with the hypothesis that the Mo3Si phase is not the line compound. It has region of existence at ~22–24 at.% Si. There were no data obtained here with Si concentrations in Mo3Si exceeding 24 at.%. However, it is still an open question as to what is the exact configuration of Mo3Si single phase region. A solid solution based on molybdenum with a silicon content exceeding the known solubility limit by almost two times has been obtained. Possible causes of this phenomenon are discussed. The data obtained for the silicon content in the Moss phase in the melt-quenched samples indicate the probable retrograde character of the solidus line for a molybdenum-based solid solution. Declaration of Competing Interest None. Acknowledgements The authors gratefully acknowledge the assistance of Alexey Nekrasov (IEM RAS) for his help with obtaining scanning electron microscopy and microanalysis data.
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Composition of the Mo-Mo3Si alloys obtained via various methods ⁎
T
Ivan Gnesin , Boris Gnesin Institute of Solid State Physics of Russian Academy of Sciences (ISSP RAS), Chernogolovka, Moscow District, 2 Academician Ossipyan str., 142432, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum silicides Reaction sintering Solubility limit Mo-Si phase diagram Retrograde solidus
The phase and chemical composition of an MoeSi system alloy with a silicon content of 25 at.% was investigated via scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The samples were obtained by both sintering and melting of the components. The synthesis of Mo3Si from elemental Mo and Si via a sintering process at ≤1480 °C was significantly less complete than the synthesis of MoSi2, Mo5Si3 silicides via same process. To increase the efficiency of the synthesis, the reaction sintering of Mo3Si should be carried out at significantly higher temperatures than in the cases of MoSi2 and Mo5Si3 silicides synthesis. This offers a sufficiently phase-pure Mo3Si via reaction sintering at temperatures below the solidus. X-ray microanalysis data on Mo3Si chemical composition indicated a Mo3Si exists in a small compositional range. The average silicon content in Mo3Si was < 25 at.%. In addition, experimental data indicated that the solubility of silicon in molybdenum may be higher than previously thought. Furthermore, the composition of the melt-quenched samples suggests that the solidus line for a Si solution in Mo may be retrograde.
1. Introduction The search for new high-temperature materials that combine hightemperature strength, good resistance to high-temperature creep, high fracture toughness, and resistance to high-temperature oxidation remains one of the most pressing problems in materials science. In the overwhelming majority of cases, complex alloys based on nickel and chromium are used for temperatures < 1200 °C [1]. However, the possibility for them to be further improved has been almost exhausted, it is clear that other materials are required for higher operating temperatures. Numerous attempts were made to use refractory metals silicides - primarily molybdenum silicides. They are attractive due their high heat resistance up to 1800 °C and above. This is obvious in the case of high-temperature electric heaters [2] and in protective coatings for refractory metals and carbon materials [3–5]. Attempts to develop high-temperature composite materials using the high heat resistance of these silicides are often associated with the creation of a silicide matrix strengthened by dispersed particles of carbides, oxides, and some other refractory compounds [6,7] due to poor silicide strength above ~1300 °C. Significant progress has been achieved in the development of new alloys containing silicides of refractory metals and Mo-based solid solutions (Moss) [8,9]. Due to the success in the development of these alloys, Mo3Si silicide has attracted the attention of researchers as a potential component of high-temperature composites. The behavior of
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the Mo3Si phase during deformation at room temperature is somewhat unexpected. Related work [10] noted that the strength and ductility are higher than in pure molybdenum in Mo doped with 0.3 wt% Si (1.02 at. % Si); the ductility of Mo doped with 0.1 wt% (0.34 at.%) Si was lower than that of unalloyed Mo. Prior work [11] focused on Mo-Mo3SiMo5SiB2 alloys and showed that a crack moving through a structure with significant volume fractions of Moss and Mo3Si can stop its propagation. Others [12] studied Mo3Si + Mo5Si3 composites via transmission electron microscopy and showed that there are dislocations at the tip of the crack exactly in the Mo3Si phase whereas dislocations have not been found in the Mo5Si3 phase. These data allow us to consider the introduction of the Moss and Mo3Si combinations into hightemperature composites as an opportunity to increase their crack resistance at low temperatures while maintaining an acceptable level of high-temperature strength. It is important that materials containing carbides, Mo, and Mo3Si phases can be protected from high-temperature oxidation by coatings containing, for example, MoSi2 and Mo5Si3. A similar approach when the heat resistance of the high-temperature base material is achieved by applying protective coatings has been demonstrated in many cases [13,14]. The MoeSi binary system is usually considered as a well-studied [15–22] with a known phase diagram [16,19] (Fig. 1). To date, a considerable amount of experimental data has been published [23,24], but these data do not always correspond to the phase diagrams in the literature [16,19]. In one study [23], the authors
Corresponding author. E-mail address: [email protected] (I. Gnesin).
https://doi.org/10.1016/j.ijrmhm.2020.105188 Received 19 March 2019; Received in revised form 19 November 2019; Accepted 3 January 2020 Available online 07 January 2020 0263-4368/ © 2020 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Fig. 1. Phase diagram for MoeSi binary system [19].
process. Thus, this work is devoted to the study of phase transformations features and changes in the chemical composition during the Mo3Si synthesis via reaction sintering and crucible-less melting. It is necessary to clarify the chemical composition of the product obtained, the presence of other phases in it, and the features of the synthesis kinetics at different temperatures. These results can aid in the development of high-temperature composite materials containing silicides.
found relatively small deviations from the stoichiometric composition in the Mo3Si phase: the Si content is 23.5–24 at.% but not 25 at.%. Unfortunately, this work does not provide data on the directly measured composition of the studied phases. Deviations from the stoichiometric Si content after crystallization have also been found in [24,25]. These were very significant, but the authors explained this fact via non-equilibrium crystallization conditions. Thus, there is still uncertainty in matters of compliance of the Mo3Si composition with the stoichiometric one as well as the existence of a composition range for this phase. Another study [26] found that the Mo3Si phase has an incommensurate structure, and the composition of this phase may not correspond to the stoichiometric one. Both the stoichiometry and synthesis of the Mo3Si phase are important to its application. For controlled introduction of certain phases into high-temperature composites, it is often preferable to work with pre-synthesized phases rather than with pure components. This approach eliminates the undesirable presence of unreacted starting components, e.g., free silicon when working with silicide materials. In addition, this approach helps to achieve a better control of the phase composition of the composite material. In many studies [24,26,27] Mo3Si silicide was obtained via arc melting in argon using a “non-consumable” tungsten electrode. The concentration of possible tungsten contamination and its influence on phase equilibria remain unclear. This factor becomes important with multiple rounds of sample remelting during arc melting. It is very interesting to consider the possibility of Mo3Si silicide synthesis via reaction sintering of molybdenum and silicon powders mixtures. This could allow one to obtain it in appreciable volumes with very small contaminant concentrations. Thus, one of the goals of this work is to investigate the possibilities of such an approach. In addition, this work highlights the potential to obtain Mo3Si via crucible-less melting in an induction furnace while levitated in a magnetic field. This can significantly reduce the risk of contamination during the alloy production
2. Materials and methods Powders of commercially available MCH-grade Mo and polycrystalline Si were used as components for silicides synthesis. Before mixing, the powders were sifted through a 100-μm mesh sieve. A small amount of liquid binder based on a saturated polyvinyl alcohol aqueous solution was added to the mixture of Mo and Si powders. Reaction sintering was performed after compaction. Some of the sintered samples were melted at an argon pressure of 35–45 kPa in a levitation conditions in a EMT 27–64 levitation melting system. Rotating electromagnetic field frequency was near 200 kHz. After holding the molten sample in the liquid state for 1–2 min, the field was removed, and the samples (weighing ~15–17 g) were drop-cast into a 224 g copper mold (~11 mm in inner diameter). Next, crosscut sections were made on the quenched samples with a diamond cutting disk, and polished surfaces were prepared using diamond powders with particle sizes whose main size fraction gradually decreased from 28/20 to 1/0 μm. The phase composition of the obtained samples was investigated via X-ray diffraction (XRD) using 40 kV accelerating voltage and an incident beam monochromatizated MoKα radiation. The powders of the obtained materials were fixed on glass plates via a trace mount of plasticin. Polished cross-sections of dense powder materials as well as polished cross-sections of samples after levitational melting were also 2
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silicide did not contain any impurity phases detected using the XRD (Fig. 3a). Thus, the experimental data suggest that the solid phase synthesis at 1480 °C for the Mo5Si3 phase (Fig. 3a) was much more complete than for Mo3Si despite the fact that the melting point of Mo5Si3 silicide is higher (by ~150 °С) than the peritectic temperature of Mo3Si. The Mo3Si is traditionally depicted as a line compound on MoeSi system phase diagrams, but Mo5Si3 silicide has a noticeable composition range (~2–3 at.% Si) [16,19]. One would assume that the presence of a composition range could facilitate the synthesis of the Mo5Si3 phase relative to Mo3Si. However, a mixture of Mo and Si powders corresponding to MoSi2 (also a line compound) had a sufficiently complete synthesis at 1480 °C similar to Mo5Si3 (Fig. 3b). Along with tetragonal MoSi2, only a small amount of Mo5Si3 (~ 1–2 vol%) was found. Thus, of the three known molybdenum silicides, Mo3Si had insufficient synthesis completeness for the reaction-sintering regime with Tmax~1480 °C. However, the synthesis of MoSi2 and Mo5Si3 silicides was quite effective via this technological route. This is consistent with the previously obtained results concerning the kinetics of diffusion growth of layers of various silicides in the MoeSi system. Prior work [28] showed that Mo3Si has a significantly smaller (not less than an order of magnitude) effective diffusion coefficient in this silicide relative to Mo5Si3 and MoSi2. Most likely, when Mo3Si undergoes reaction sintering with a maximum temperature 1480 °C, the diffusion stage of the Mo3Si phase growth takes more time than for Mo5Si3 or MoSi2. To verify this hypothesis, we used the reaction sintering regime with one additional heat treatment stage: 1 h of annealing at 1700 °C. Thus, we noticeably increased both the temperature and the time of the component interactions during Mo3Si synthesis.
investigated using X-ray diffraction. The volume fraction of the phases was estimated via the ratio of corresponding line intensities for different phases using experimental and ICDD data. The samples' microstructure was studied with a scanning electron microscope (SEM; Tescan Vega II XMU) with an INCA Energy 450 energy spectrometer with a Si(Li) semiconductor detector. The accelerating voltage was 20 kV. The error of the silicon concentration was determined using X-ray microanalysis and was estimated by measuring the silicon content in a SiO2 crystal. This phase is characterized by a nearly constant composition (it is the line compound) corresponding to 33.33 at.% Si. To estimate the error, 10 parallel measurements of Si concentration were performed in a small region of the quartz crystal surface. The standard deviation was 0.28 at.%. But according to the data obtained, the average value of the silicon concentration measured was 33.38 at.%, which is only 0.05 at.% different from the known composition of SiO2. 3. Results 3.1. Reaction sintering The Mo3Si synthesis in this work used an initial mixture of Mo and Si powders at a mass ratio of 10.5:1; the exact stoichiometric ratio corresponds to 10.28:1. Thus, the initial mixture had a slight excess of Si (~0.7 at.%) versus 25 at.% corresponding to the exact Mo3Si stoichiometry [16,19]. Reaction sintering was performed via stepwise annealing in a vacuum furnace with molybdenum heaters. The maximum temperature of the reaction sintering was ~1480 °C. The temperature was controlled using a WRe5/WRe20 thermocouple. Exposure at the maximum temperature of the reaction sintering was 25 min after which the sintered billet was cooled with the furnace. The billet was ground into a powder and then mixed and sieved with a 100-μm mesh. Fig. 2 shows the X-ray diffraction patterns of such a powder. Three phases were revealed: Mo3Si, Moss - Si solid solution in Mo, and Mo5Si3. Neither free Si nor MoSi2 were detected. The Moss volume fraction estimation amounts to a few tens of percent. The Mo5Si3 phase at the same time was contained in a rather small amount—only about several vol%. Thus, it is obvious that the synthesis of the Mo3Si was incomplete. Further experiments showed that the process of solid-phase Mo3Si synthesis via reaction sintering differs significantly from the solid-phase synthesis of Mo5Si3 and MoSi2. As a control, we made Mo5Si3 via the same technological route as with Mo3Si described above. The resulting
3.2. Additional sintering Additional annealing (1 h at 1700 °C) after Mo3Si reaction sintering (with a maximum temperature of 1480 °C) was performed in a vacuum furnace with molybdenum heaters. The polished cross-section was prepared on the obtained sample; the XRD and SEM results for this sample are shown in Fig. 4. Increasing the duration and the temperature of the final reaction sintering stage noticeably affected the homogeneity of the phase composition of the sample. The presence of Mo3Si and a relatively small amount of Moss solid solution was seen, but no other phases were found. In this case, the Moss volume fraction decreased significantly versus samples that did not undergo additional annealing; this amounted to no more than a few vol%. The Mo5Si3 phase was not detected at all. Thus,
Fig. 2. X-ray diffraction pattern from Mo - 25 at.% Si powder sample after reaction-sintering at 1480 °C. 3
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Fig. 3. Diffraction patterns of Mo5Si3 (a) and MoSi2 (b) silicides after reaction sintering at 1480 °C.
on a melt-quenched sample. The vast majority of its volume is represented by the Mo3Si phase (> 90 vol%). However, the sample contains both relatively small grains of Mo5Si3 phase (diameter of several microns) precipitated along the boundaries of larger Mo3Si grains as well as Moss crystallites up to 10–15 μm in diameter (Fig. 5a). The average Si concentration in Moss as a result of the quenching of the Mo3Si melt is 8.3 at.%. This is almost twice the solubility limit for Si in Moss for the peritectic temperature (~4–5 at.% [16,19]). The Si concentration measured in the Mo5Si3 phase was slightly lower than its known level (34.4–36.0. at.% instead of 37.2–39.2 at.% according to [19]). This is due to the very small diameter of the Mo5Si3 grains: analyzed backscattered electrons originated from surrounding Mo3Si phase also. The Si concentration in the Mo3Si phase itself was experimentally determined by X-ray microanalysis to be on average 22.6 at. %. This is even slightly lower than values obtained after additional sintering of Mo3Si at 1700 °C for 1 h. To control the phase composition of the melt-quenched sample, it was ground via cemented carbide disks. The resulting powder was used for XRD analysis (Fig. 5b), which showed that the main phase present in the sample was the Mo3Si (this was consistent with the microstructure data). The presence of Mo5Si3 on the diffraction pattern was manifested by only one separate line. There were no separate Moss lines on the
we confirmed experimentally that higher temperatures are required for sintering the Mo3Si phase versus other molybdenum silicides. It was also possible to estimate the Si content in both the Mo3Si and Moss phases. The data suggested that the Si content in Moss was about 2.7 at.% and 23.6 at.% in the Mo3Si phase. Importantly, we were still unable to obtain the Mo3Si phase with a 25 at.% Si content by increasing the maximum temperature of the sintering from 1480 to 1700 °C and the duration of maximal temperature stage from 25 min to 1 h. In connection to these experimental results, a question arose on the possibility of obtaining the Mo3Si phase with a composition as close as possible to the stoichiometric one. It was necessary to determine whether the observed deviations of the silicon concentration in Mo3Si are due to the use of solid-phase synthesis and incomplete diffusion processes or if it is from other sources. To do this, experiments on the synthesis of Mo3Si were performed by melting the components in an induction furnace in a levitation state. 3.3. Mo3Si melt-quenching The sample after the reaction sintering of a mixture of Mo and Si powders at 1480 °C was melted. Fig. 5 shows the results of the analysis of the composition and structure of the polished cross-section obtained 4
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Fig. 4. XRD pattern (a) and SEM (BSE) image (b) of the Mo–25 at.% Si sintered sample after additional annealing for 1 h at 1700 °C.
concentration in the Moss solid solution was about 5.1 at.%. The concentration was measured in Moss crystallites larger than 2 μm to avoid the effect of the surrounding Mo3Si phase on the analysis results. Even in the case of measurements in a such relatively large Moss crystallites, we cannot say with 100% certainty that we received a signal from this phase only. However, since the measured value of ~ 5.1 at.% is an estimation of the silicon content from above, we can definitely say that the silicon content in the Moss phase decreased markedly due to annealing. It approaches the equilibrium condition. The average measured Si content in Mo3Si was 22.3 at. %, which only slightly differs from the data obtained for the state before annealing. Thus, even after high-temperature annealing of the melted sample, it was not possible to obtain the Mo3Si phase with 25 at.% Si concentration corresponding to the state diagram. The silicon content in this phase remains at a level a few atomic percent below the stoichiometry.
diffraction pattern. This agrees well with its small volume fraction because the 100% Moss line {110} 2Θ = 18.35° is superimposed on the 100% Mo3Si line {210} 2Θ = 18.66°. A comparison of the intensity of the experimental lines with ICDD data suggests that a robust powder preparation technique could not completely overcome the crystallographic texture of the sample formed as a result of crystallization. Thus, melting under levitation conditions did not increase the Si concentration in Mo3Si up to a level of 25 at.% in accordance with the phase diagram [16,19]. We attempted to obtain the state of the sample closer to equilibrium using high-temperature annealing. The quenched sample was annealed at 1700 °C for 2 h in a vacuum. XRD analysis showed only one phase, Mo3Si, in the annealed sample (Fig. 6a). The samples' microstructure was noticeably changed as a result of the annealing. The Mo5Si3 phase was not found at all. The crystallites of the Moss phase were significantly smaller (1–3 μm in diameter). At the same time, the Moss volume fraction decreased to below ~ 1 vol%. The Si 5
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Fig. 5. SEM (BSE) image of the structure (a) and the diffraction pattern (b) for Mo – 25 at.% Si sample after melt-quenching.
4. Discussion
presence of a region of existence of a Mo3Si phase. Moreover, the average Si concentration in the Mo3Si phase should be < 25 at.%. We cannot say at the moment what is exact configuration of Mo3Si region of existence on the phase diagram. But results obtained allow us to assume that Mo3Si is not a line compound and its lower silicon solubility limit is lower than 23 at.%. The described above difficulties in the synthesis of Mo3Si in comparison with other molybdenum silicides should also be noted. This may be due to the hindered diffusion in the Mo3Si crystal lattice—researchers have previously noted its complex structure [26]. This is consistent with data on diffusion in the Mo3Si phase published previously [28]. The presence of a solid solution based on Mo among the products of Mo3Si synthesis allows one to trace some interesting features in the chemical composition of this phase. The Si concentration within the Moss was subject to more significant changes than in the Mo3Si phase
The measured Si content for all studied samples is shown in Table 1. As noted above (Section 2), the Si concentration measurements were verified via a SiO2 etalon. The results showed a relatively high accuracy of the average Si concentrations measured here. The difference between the measured and known silicon concentrations in SiO2 is < 0.15% (about 0.05 at.%). The data in Table 1 shows that the average concentration of Si in the Mo3Si phase is 22.3–23.6 at.% for all the samples investigated. Samples after quenching have a slightly lower silicon concentration than samples obtained by sintering. This may be because of some loss of silicon as a result of the higher temperature treatment. It should be noted also that samples after additional annealing contain no Mo5Si3 phase. Thus, high temperature annealing could result in additional loss of silicon by the samples. In general, these results confirm the hypothesis of a
6
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
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Fig. 6. The diffraction pattern (a) and SEM (BSE) image of the microstructure (b) of the melt-quenched sample after annealing at 1700 °C 2 h.
splat quenching). This follows from the fact that all measured Si concentrations in Moss were significantly higher than the solubility limit for room temperature [16,19]. Given the relatively slow diffusion in the phases studied here—even at high temperatures—this is not surprising. Another feature discovered during the work was that after quenching from a liquid state, the Si content in the Moss phase reaches > 8 at. %. As a result of subsequent annealing at 1700 °C the concentration of Si in the Moss phase decreased significantly and reached about 5 at.% or less. The excess silicon content in molybdenum decreases during annealing as a result of activation of the diffusion processes. The main question is what led to the appearance of such a high (> 8 at.%) concentration of silicon in molybdenum immediately after crystallization. In our experiments, we did not find any signs of amorphization of the melt. Based on the XRD data that were obtained, the high melting temperature of the alloy under study, and the relatively large mass of samples (15–17 g), amorphization of the melt seems unlikely. This is not surprising because the quenching in our work was performed by a
Table 1 Silicon concentrations in the molybdenum-based solid solution (Moss) and in the Mo3Si phase according to the results of microanalysis. Technological route
Additional sintering 1700 °С 1 h Levitation melting and quenching Levitation melting and quenching, 1700 °С 2h annealing
Moss
Mo3Si
CSi
σ
n
CSi
σ
n
2,70 8,32 5,12
0,44 0,24 0,08
5 8 4
23,60 22,58 22,33
0,56 0,7 0,34
6 14 25
CSi – Si average concentration in at.%, σ - estimation of a standard deviation, n – number of measurements.
discussed above. The results suggest that in the framework of the experiments provided here, all heat treatments have preserved the high temperature state at room temperature despite the absence of special measures to increase the cooling rate of the samples (e.g. melt spinning, 7
International Journal of Refractory Metals & Hard Materials 88 (2020) 105188
I. Gnesin and B. Gnesin
The work was partially supported by the RFBR, project 18-3300471.
simple casting of the melt into a massive copper mold, without using any measures to increase the cooling rate (e.g. melt spinning, splat quenching). During quenching, significantly supersaturated solutions are usually formed from solid phases, and not from liquid phases. This is because in solid phases, diffusion is easily suppressed by quenching, while in the liquid phase, the mobility of atoms is orders of magnitude higher. Thus, even during rapid cooling, the deviations from local equilibrium at the liquid–solid interface, are most often small. After crystallization, the average composition of the solid phase may differ from the equilibrium one because the diffusion processes in the solid phase are suppressed. Currently, there are no data that allow us to determinate the exact reason for supersaturation of the Si solid solution in Mo up to > 8 at.%. We cannot exclude the possibility of completely nonequilibrium primary crystallization. However, taking into account the information presented above, the probability of such a case seems to be very low. A significant supersaturation of the Moss phase with silicon, in our estimation, is caused by the features of the phase diagram. Here, we suggest two possible explanations for this phenomenon. The first one: the solubility limit of silicon in a molybdenum-based solid solution at peritectic equilibrium Moss + L = Mo3Si may be significantly higher than previously assumed. The second one: the solidus of the molybdenum-based solid solution may be a retrograde one [29,30]. The solubility limit of silicon in molybdenum near the peritectic temperature is considered to be known and experimentally determined as ~4 at. % [31]. At the same time, the position of the solidus line above the peritectic temperature is still not established [16,18,19]. Thus, we consider the retrograde solidus as more probable explanation of the results obtained. But for now, this is just an assumption. To provide an exact answer to the question about the reasons for the obtaining the Moss phase with a silicon content of > 8 at.%, further studies are needed.
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5. Conclusion We show successful solid phase synthesis of Mo3Si. In addition to high productivity, this method can produce relatively pure silicides. To obtain Mo3Si via the reaction sintering process, it is necessary to use higher temperature (about 1700 °C and above) and longer exposure (about several hours) than for the synthesis of MoSi2 and Mo5Si3 silicides. We believe that this is the consequence of significantly lower rates of diffusion during the Mo3Si silicide synthesis than in the case of MoSi2 and Mo5Si3. The data obtained for silicon content in the Mo3Si are consistent with the hypothesis that the Mo3Si phase is not the line compound. It has region of existence at ~22–24 at.% Si. There were no data obtained here with Si concentrations in Mo3Si exceeding 24 at.%. However, it is still an open question as to what is the exact configuration of Mo3Si single phase region. A solid solution based on molybdenum with a silicon content exceeding the known solubility limit by almost two times has been obtained. Possible causes of this phenomenon are discussed. The data obtained for the silicon content in the Moss phase in the melt-quenched samples indicate the probable retrograde character of the solidus line for a molybdenum-based solid solution. Declaration of Competing Interest None. Acknowledgements The authors gratefully acknowledge the assistance of Alexey Nekrasov (IEM RAS) for his help with obtaining scanning electron microscopy and microanalysis data.
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