- Email: [email protected]
molybdenum-matrix composites: Strength, fracture toughness and oxidation resistance
Journal Pre-proof Yttrium-aluminium-perovskite-fibre/molybdenum-matrix composites: Strength, fracture toughness and oxidation resistance S.T. Mileiko, A.A. Kolchin, V.M. Prokopenko PII:
S1359-8368(18)32771-9
DOI:
https://doi.org/10.1016/j.compositesb.2019.107604
Reference:
JCOMB 107604
To appear in:
Composites Part B
Received Date: 25 August 2018 Revised Date:
8 October 2019
Accepted Date: 13 November 2019
Please cite this article as: Mileiko ST, Kolchin AA, Prokopenko VM, Yttrium-aluminium-perovskite-fibre/ molybdenum-matrix composites: Strength, fracture toughness and oxidation resistance, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107604. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Yttrium-aluminium-perovskite-fibre/molybdenum-matrix composites: strength, fracture toughness and oxidation resistance S.T. Mileiko,1 A.A. Kolchin, V.M. Prokopenko Institute of Solid State Physics of RAS, Chernogolovka Moscow distr., 142432 Russia
Abstract Thermal efficiency of a gas turbine depends on the highest temperature of the thermal cycle in the a machine. Nickel supperalloys used in modern gas turbines have approached their physical limit. Hence, transportation and energy technologies call for new generations of heat resistant materials. Among four families of future heat resistant materials, metal matrix composites (MMCs), ceramic matrix composites, alloys based on refractory metals, and high entropy alloys, materials of the first family, MMCs, are characterized by an attractive balance between strength and fracture toughness. The only choice of the matrix for really high temperature composites is a refractory metal, for example, molybdenum. Since composites are to be exploited in extremely hard environments and the requirements to various important properties of future materials of a practical importance are often contradict to each other, a the choice of a particular fibre/matrix combination is to be based on the results of a studies of a large number of the variants. In the present work paper, molybdenum matrix composites reinforced with fibres based on yttrium-aluminium perovskite (YAP) have been obtained for the first time by using the internal crystallization method. The fabrication technology and fibres microstructures are briefly discussed. Strength, fracture toughness and oxidation resistance are discussed in details. The composites proved occur to be sufficiently strong at temperatures up to 1400oC and of are characterized by low notch sensitivity at room temperature.
1
Corresponding author: [email protected]; 2 Ossipyan str., Chernogolovka, Moscow distr 142432 Russia
1
Oxidation resistance of the molybdenum matrix of the composites at temperatures 1000 and 1100oC is much better than that of pure molybdenum. Keywords: A. Metal-matrix composites (MMCs); A. Oxides; B. Fracture toughness; B. Strength; Oxidation 1. Introduction
Thermal efficiency of a gas turbine depends on the highest temperature of the thermal cycle in the a machine. Nickel supperalloys used in modern gas turbines have approached their physical limit, which is about 1100oC, to resist the complicated loading of the crucial structured elements of the machine such as the turbine blades. Hence, transportation and energy technologies call for new generations of heat resistant materials. There are known four families of the materials for the high temperature use, those being ceramic matrix composites, high entropy alloys (HEA), alloys based on refractory metals, and metal matrix composites (MMC). Ceramic matrix composites, in particulars, SiCf/SiCm composites, are now used for elements of the stationary parts of gas turbines at temperatures up to about 1300o C [1]. A further increase in the use temperature of such composites is possible provided an appropriate protective coating has been developed [2]. Since Jeh discovered a possibility to make metal alloys composed of five or more elements in equiatomic ratios, the so-called high entropy alloys [3], a weighty number of papers describing compositions and microstructures of HEAs developed for the high temperature use have been published (see, i.e., [4,5,6]). However, as far as the present authors are aware, no systematic study of creep of refractory HEAs at really high temperatures has been carried out. Some researchers
2
explained it by a huge number of the HEAs and a necessity to choose most prospective compositions to be studied. Heat resistant alloys based on refractory metals, niobium and molybdenum, have been attracting an the attention of materials scientists during the last two decades. In particular, molybdenum alloys that are now under development are characterized by high creep properties at temperature up to 1300oC [7,8] and good oxidation resistance [9]. However, their fracture toughness is too low for turbine blades materials [10,11]. This is a result of the way of strengthening of metal alloys by limiting their plasticity, a the property that determines fracture toughness [12]. Unlike metal alloys, metal matrix composites can be designed in such a manner as to achieve simultaneously increase the in strength and fracture toughness (damage tolerance) with increasing the volume fraction of strong and brittle fibres [12,13]. Hence, fibrous composites are to be considered as promising prospective heat resistant materials. An appropriate balance between three main characteristics of heat resistant materials, those being strength – creep resistance – fracture toughness (damage tolerance) – oxidation resistance, is a key issue for heat resistance materials. The first three properties can be balanced in fibrous composites [12,13]. Actually, to obtain a Metal Matrix Composite (MMC) of high strength and high creep resistance it is necessary to organize a sufficiently strong fibre/matrix interface should be organized [12,14]. To obtain an the MMC with the use temperature much higher than that of nickel superalloys a refractory metal is obviously necessary to select prospective as the matrix. It is not to be excluded The usage of HEAs as the matrix materials for MMCs cannot be excluded either [14,15]. The internal crystallization method (ICM) to produce oxide-fibre/molybdenum-matrix composites, which provided automatically the strong interface, was invented in the last century [16,17], and was first considered either as a method to make model MMCs or as a way to crystallize various oxide fibres from the melts [18,19,20,21]. The method can be seen as a way method to produce a heat 3
resistant material if a composite microstructure is being developed to enhance the oxidation resistance of the matrix. The recently found finding of a possibility to reduce the oxidation rate of the molybdenum matrix by the orders of the magnitudes as compared with that of pure molybdenum [22,23] has opened a way to develop ultra-heat resistant composites. The improved oxidation resistance of the molybdenum matrix is improves due to the formation of molybdates of either yttrium or lanthanoid present existing in the fibre reinforcing the matrix on the composite surface. At the present time The development of ultra-heat resistant molybdenum matrix composites is on now at the starting stage. The first thing to be done was is to study mechanical behaviour of the composites reinforced with various fibres. In particular, strength and creep resistance of the composites reinforced with Al2O3 -Y3Al5O12 eutectic fibres was had been studied at the very beginning of the experiments with the internal crystallization method [17], before a reduced oxidation rate of molybdenum matrix was discovered. Strength, creep resistance, and damage tolerance of Mo3Si-fibre/Mo-matrix composites were presented in Refs. [24,25]. Strength of the composites reinforced with mullite-zirconia fibres is described in Refs [26,27]. This In the present paper presents, for the first time, molybdenum matrix composites reinforced with fibres based on yttrium-aluminium perovskite (YAP) have been obtained by using the internal crystallization method. The fabrication technology and fibres microstructures will be briefly discussed. Strength, fracture toughness and oxidation resistance are discussed in details. 2. Fabrication and microstructure Fabrication of the composite specimens includes the following main steps: (i) preparation of a raw material; (ii) melting the raw material; (iii) crystallization of the fibre in the channels of molybdenum matrix; (iv) preparation of composite specimens to for evaluate ing fracture toughness and strength properties.
4
2.1. Preparation of the raw material At First, powders of alumina (purity > 99.5 wt.%, ~4µm) and yttria (purity > 99.9 wt.%, ~3 µm) were heated at 900oC for 5 h in air. Possible agglomerations of the powders were disintegrated by a grinding in an agate mortar. Then an equi-molar mixture of the oxides in the equi-molar ratio was placed in a fluoroplastic vessel and an isopropyl/polyvinyl solution was added to the mixture to form a suspension. A high-speed blender was then used to homogenize the suspension; the processing time was 15 min. Then the mixture was dried drayed at a temperature of about 100oC. After drying the mixture was sintered at temperatures of either 1250oC or 1650oC. X-ray spectra of the sintered oxide mixtures are presented in Fig. 1.
Fig. 1. X-ray spectra of the oxide blanks sintered in under various temperatures regimes. 5
It can be seen that the sintering regimes used do not give a one-phase material. The content of perovskite is rather low even at the largest temperature/time combination. There exists A large quantity of monoclinic phase Y4Al2O9 (YAM) is present. The presence of The raw oxides, alumina and yttria, are should also found be pointed out. The result was not unexpected as the raw oxide particles are were too large for the raw oxide mixture to react completely to produce perovskite. Nevertheless, Still, melting the oxide mixtures in the process based on the internal crystallization method of producing composites should produce a more homogeneous oxide. 2.2. Crystallizing the fibres in the molybdenum matrix As mentioned in the Introduction, composite specimens were produced by using the internal crystallization method (ICM) [16,18]. The ICM for producing oxide-fibre/molybdenum-matrix composites consists of three main steps. First, a molybdenum carcass with continuous channels is prepared by the diffusion bonding of an assembly of molybdenum wire and foil. The process is performed in such a way as to prevent the gaps between neighbouring fibres to be being filled with the foil material. Consequently, the channel cross-section is formed by two planar and two concave lines; this is the shape of the resulting fibres. Then, the carcass is infiltrated with an oxide melt driven by capillary force. The third 3rd step is crystallizing the melt to form fibres in the channels of the molybdenum carcass by pulling the block up into the cold zone of the furnace. In the previous works on ICM cited in the Introduction, the oxide raw material was melted in a molybdenum crucible located below the molybdenum carcass. In the present work, the raw material was placed in a disposable molybdenum hopper located at the top of the carcass. In the present experiments, the molybdenum block configuration was such that the fibre volume fraction in the specimens was between 0.35 and 0.40. The pulling up rate was between 2 and 250 mm/min. This value is approximately equal to the fibre crystallization rate. Original composite
6
specimens were produced with a shape shown schematically in Fig. 2. Their size was ~5×15×65 mm3. 2.3. Preparing specimens for mechanical testing The specimen shown in Fig. 2 marked as VXYZW, where X,Y,Z,W are digits, was notched and used for measuring the value of critical stress intensity factor K*. Two parts of a the specimen obtained as a result of fracture test were cut along their length as shown in Fig. 2 to make six subspecimens to be used in 3-point bending. The sub-specimens preserved the names of the original one with an addition of digits 1 to 6. The sub-specimens marked by 1 to 3 present the upper part of the original specimen one, those marked by 4 to 6 are located in the bottom of the original specimen.
Fig. 2. Schematic of specimen VXYZW with its sub-specimens. FS: Fracture and failure surfaces. 2.4. The microstructure of the composites and fibres Crystallizing oxide fibres under the conditions of the internal crystallization method has special features. In particular, crystallization of the sapphire fibres starts in the upper part of the oxide/molybdenum block with a spontaneous process that yields a zone of polycrystalline alumina.
7
Then alumina grains at the solid/liquid interface serve as seeds for single crystalline fibres that grow in the rest volume of the channels [16]. In the case of crystallizing mullite fibres, a polycrystalline zone also occurs at the top of an oxide/molybdenum block. As the crystallization proceeds the largest mullite grain is growing to occupy the whole fibre cross-section with the exception of the coldest zones of the melt located in the corners of the channels where an excess of silica occurs [20]. Hence, the microstructure of the fibres in the upper part of an original specimen is changing along their length until a steady crystallization occurs. The formation of the microstructures of the YAPbased fibre has similar features. Here In the present paper, we will only give just a description of the fibre microstructure important for the discussion of the composite mechanical behaviour. A full detailed account of that will be published elsewhere. Some examples of the microstructure of fibres from the upper part of the original specimens that have been crystallized at various rates are shown in Fig. 3. It can be seen that the fibre consists of YAP with inclusions of YAM. The same phases are revealed by X-ray spectroscopy, Fig. 4. The occurrence of YAM phase can also be seen, although its quantities are sufficiently small, and in some specimens (see specimen V11565) this phase is absent. Also, it is important to note that the Xray analysis shows that the YAP phase is single crystalline.
8
V11792, transverse section
V11552, longitudinal section
Specimen→ V/mm/min→ Points↓
V11792 V11562 V11552 2
50
250
Y:Al atomic ratio ↓
1
0.98
1.03
1.00
2
2.02
1.76
2.00
V11562, longitudinal section Fig. 3. SEM micrographs of the YAP-based fibres crystallized from the raw material sintered at 1250oC. In the Table, the values of Y:Al atomic ratio in the points shown on the micrographs are presented. 9
Fig. 4. X-ray spectra of the fibres from the bottom of original specimens containing the fibres crystallized at various rates. 3. Strength and fracture toughness We start with the methodology of mechanical testing and then proceed with the results. 3.1. Methodology Then The specimen is tested in 3-point bending and an apparent value of critical stress intensity factor K* was is calculated according to ASTM-399: ∗
3
⁄
(1)
2
where 10
1.96 − 2.75 + 13.66
− 23.98
+ 25.22
.
Here L is the distance between the supports, c is the notch length, h and w are the height and width of the specimen, respectively. These values sizes in the present experiments are L ≈ 60 mm, h ≈ 15 mm, w ≈ 5 mm, c ≈ (0.45 – 0.55)h. The notch was cut by diamond disks so that the radius of the notch tip was ~0.15 mm. The value of Q corresponds to a maximum load. Obviously, such this procedure developed to for characterize metals cannot be fully justified while dealing with composites; that is why we treat the value of K* obtained in such a way as the apparent critical stress intensity factor. Note that this test gives also strength
∗
of a the notch specimen.
The sub-specimens were tested in 3-point bending at room and high temperatures. Normally, subspecimens 2 and 5 were tested at room temperature to measure strength
∗
of the unnotched
specimen. 3.2. Strength The dependencies of composite strength on fibre crystallization rate are presented in Fig. 5. It can be seen that 1) Room temperature strength of all composites has a maximum at the crystallization rate 50 mm/min. 2) Composite strength at 1400oC does not seem to be dependent of fibre crystallization rate. 3) At low crystallization rates, 2 and 10 mm/min, composite strength at room temperatures does not noticeable change along the length of an the original specimen. At higher crystallization rates, 50 and 250 mm/min, the strength is slightly increasing from the top to bottom of the original specimen. This means that a steady state growth of the fibre and stabilization of its microstructure occurs earlier at low crystallization rates.
11
4) Room temperature strength does not depend of sintering temperature of the oxide mixture. On the other hand, high temperature strength depends on sintering temperature, the higher sintering temperature yields the larger higher temperature strength. 5) Nevertheless, even at low sintering temperature the composite strength at temperatures from 1000 to 1400oC is sufficiently high, Fig. 6.
(a) (b) Fig. 5. Strength of the composites versus crystallization rate of the fibres. (a) Room temperature; open and solid points correspond to the sub-specimens from the top and bottom of the original specimens, ones respectively. (b) Test temperature 1400oC.
Fig. 6. High temperature strength of the composites. 12
3.3. Fracture toughness and damage tolerance Typical load/displacement curves recorded in testing the specimens shown in Fig. 2 are presented in Fig. 7. The curves are characteristic of for materials with of the non-brittle behaviour. It should be noted that the molybdenum matrix in the composites is are nearly brittle (K* ≈ 9 MPa·m1/2) since it has been subjected undergone to heating up to ~ 1950oC in the fabrication process of the composites and, hence, is recrystallized. Reinforcing such a matrix with YAP-based fibres gives provides the a composite with a high apparent critical stress intensity factor (Fig. 8). No dependence of the K* value of on crystallization rate of the reinforcing fibres is can be observed. The average value of K* calculated by taking into account all the data plotted in Fig. 8 is 20.1 MPa·m1/2. Hence, as in many metal matrix composites, in the YAP-based-fibre/molybdenum-matrix composite fibrous macrostructure serves as a magnifier of fracture toughness of the matrix. We treat call the K* value obtained by testing a metal matrix composite as the apparent critical stress intensity factor because a large fracture zone in front of the crack tip in the composites makes the usage of linear fracture mechanics questionable [13]. Therefore, characterizing the fracture behaviour of the composite by notch sensitivity given by the ratio σN*/σo* of the strength values of specimens with and without the notch allows a more correct comparison of the materials with various macro- and micro-structures. However, this value, unlike the critical stress intensity factor in the linear fracture mechanics, cannot be used for estimating the failure loads of structural elements containing defects. The notch sensitivity of the composites is plotted in Fig. 9 versus crystallization rate Vcr of the reinforcing fibres. Again, no σN*/σo*(Vcr) dependence can be seen. Statistical characteristics of the notch sensitivity are presented in Table 1. Calculating the value of σN*/σo* by taking into account all data yields 0.69±0.14. Hence, the composites are characterized by a sufficiently low 13
notch sensitivity and, consequently, a large damage tolerance. A rather large scatter of the σN*/σo* value can be explained by that the composite specimens were produced by a laboratory fabrication technology that should be improved. However, Still, the values of σN*/σo* for some specimens are around unity 1, which means that composites of nearly non-sensitive to defects can be obtained in future.
Fig. 7. Typical load/displacement curves obtained in testing specimens with the notch to measure
the apparent critical stress intensity factor.
14
Fig. 8. The apparent stress intensity factor of the composites versus fibre crystallization rate
during fabrication of the specimens.
15
Fig. 9. Notch sensitivity versus crystallization rate of the fibres. Table 1. Statistical characteristics of notch sensitivity of the composites and molybdenum matrix. σN*/σo*
Fibres crystallization rate Mean value
Std
10
0.70
0.19
50
0.66
0.10
2+10+50+250
0.69
0.14
Molybdenum matrix
~ 0.5
mm/min
4. Oxidation resistance Oxide fibres containing yttrium make the molybdenum matrix much more resistant to oxidation as compared to pure molybdenum due to the formation of molybdates on the composite surface, yttrium molybdate Y2Mo3O12 being an example [22,23]. Saturated vapour pressures of the yttrium 16
molybdates are by orders of the magnitude less than that of molybdenum oxide MoO3. Hence, the oxidation rate of such oxide-fibre/molybdenum-matrix composites is sufficiently low, Fig. 10.
Fig. 10. Mass change of YAP- fibre/Mo-matrix composite specimens versus time of their exposure in air. It is clear that any heat resistant materials are to be coated with appropriate materials to protect a structural element from oxidation. The appropriate materials for the composites under consideration are a series of the yttrium molybdates to make the molybdate appeared occurred as a result of the a reaction between molybdenum oxide and yttria to serve as be a healing agent for possible defects in the coating. The Y2O3-MoO3 phase diagram [28] shows a number of the molybdates, in particular Y2Mo3O12, Y2MoO6, Y6MoO12. The melting temperature of only the first of them one is known, that is 1310oC. All the rest others have higher melting temperatures.
17
Such coatings have been made by ion-plasma deposition [29]. Preliminary tests of the coated composite specimens in a gas flow with a temperature up to ~1400oC gave an encouraging result [30]. 5. Conclusions 1. For the first time, molybdenum matrix composites with YAP-based reinforcing fibres have been obtained. 2. The composites are sufficiently strong at temperatures up to 1400oC and are characterized by low notch sensitivity at room temperature. 3. Oxidation resistance of molybdenum matrix of the composites at temperatures 1000 and 1100oC is much better than that of pure molybdenum. 4. A scatter of the mechanical properties of the composites means that the adopted fabrication technology, which is based on the internal crystallization method, needs to be optimisation ed. Aknowledgments Financial support of Russian Science Foundation, project #16-19-10624-P is gratefully acknowledged. The authors thank their colleagues NA Prokopenko, OF Shachlevich, S.A. Abashkin and VA Chumichev for their help in the experiments and Mrs. A.A. Serebryakova for proofreading the manuscript. Referencies
1 Kiser JD, David KE, Curtis D, Andrulonis R, Ashforth C, Updating Composite Materials Handbook-17 Volume 5—Ceramic Matrix Composites, in: M. Singh, T. Ohji, Sh. Dong, D. Koch, K. Shimamura, B. Clauss, B. Heidenreich, J. Akedo (Eds.), Temperature Ceramic Matrix Composites and Materials for Sustainable Development, Volume 263, Willey, New York, 2017, pp. 413-426.
18
2 Nasiri NA, Patra N, Ni N, Jayaseelan DD, Lee WE. Oxidation behaviour of SiC/SiC ceramic matrix composites in air. J Eur Ceram Soc. 2016; 36:3293–3302. https\\:doi.org/10.1016/j.jeurceramsoc.2016.05.051. 3 Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, Tsau CH, Chang SY. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes, Adv Eng Mater. 2004;6:299–303. http://DOI:10.1002/adem.200300567 4 Lin CM, Juan CC, Chang CH, Tsai CW, Yeh JW. Effect of Al addition on mechanical properties and microstructure of refractory HfNbTaTiZr alloys. J Alloys Compd. 2015;624:100–107. http://doi:10.1016/j.jallcom.2014.11.064. 5 Senkov ON, Senkova SV, Woodward C. Effect of aluminum on the microstructure and properties of two refractory high-entropy alloys. Acta Mater. 2014; 68:214–228. http://doi.org/10.1016/j.actamat. 2014.01.029. 6 Gorr B, Azim M, Christ H-J, Chen H, Szabo D V, Kauffmann A, and Heilmajer M. Microstructure Evolution in a New Refractory High-Entropy Alloy W-Mo-Cr-Ti-Al. Metall Mater Trans. 2016; 47A:961 – 970. DOI: 10.1007/s11661-015-3246-0 7 Jain P, Kumar KS. Tensile creep of Mo–Si–B alloys. Acta Mater. 2010; 58:2124–2142. doi:10.1016/j.actamat.2009.11.054 8 Tanaka R, Kasama A, Fujikura M, Iwanaga I, Tanaka H and Matsumura Y. Research and development of niobium-based superalloys for hot components of gas turbines. Proc. of the Intern. Gas Turbine Congress 2003 Tokyo, pp. 1-5. 9 Schliephake D, Azim M, von Klinski-Wetzel K, Gorr B, Christ H-Jr, H Bei, George EP , Heilmaier M. High-temperature creep and oxidation behavior of Mo-Si-B alloys with high Ti contents. Metall Mater Trans. 2014; 45A:1102—1111. DOI: 10.1007/s11661-013-1944-z 19
10 Lemberg JA, Middlemas MR, Weingärtner T, Gludovatz B, Cochran JK, Ritchie RO. On the fracture toughness of fine-grained Mo-3Si-1B (wt.%) alloys at ambient to elevated (1300oC) temperatures. Intermetallics. 2012; 20:141-154. doi:10.1016/j.intermet.2011.09.003 11 Krüger M, Jain P, Kumar KS, Heilmaier M. Correlation between microstructure and properties of fine grained Mo-Mo3Si-Mo5SiB2 alloys. Intermetallics. 2014; 48:10-18. DOI: 10.1016/j.intermet.2013.10.025 12 Mileiko ST, Metal and Ceramic Based Composites, Elsevier, Amsterdam, 1997 13 Mileiko ST. Fracture-toughness/notch-sensitivity correlation for metal- and ceramic-based fibrous composites. Composites Part B. 2017; 116:1-6. doi: 10.1016/j.compositesb.2016.12.060 14 Mileiko ST, Firstov SA, Novokhatskaya NA, Gorban’ VF, Krapivka NP. Oxide-fibre/highentropy-alloy-matrix composites. Composites Part A. 2015; 76 :131-134. DOI: 10.1016/j.compositesa.2015.05.023. 15 Mileiko ST. High temperature oxide-fibre/metal matrix composites. Mater Chem Phys. 2017; 210:353-361. http://dx.doi.org/10.1016/j.matchemphys 16 Mileiko ST, Kazmin VI. Crystallization of fibres inside a matrix: a new way of fabrication of composites. J Mater Sci. 1992; 27:2165-2172. https://doi.org/10.1007/BF01117932 17 Mileiko ST, Kazmin VI. Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the internal crystallization method. Compos Sci Technol. 1992; 45:209-220. http://doi.10.1016/0266-3538(92)90081-D 18 Mileiko ST. Single crystalline oxide fibres for heat-resistant composites. Compos Sci Technol. 2005;65:2500-2513. DOI: 10.1016/j.compscitech. 2005.05.029
20
19 Rüscher C H, Mileiko ST, Schneider H. Mullite single cyrystal fibres produced by the Internal Crystallization Method (ICM). J Eur Ceram Soc. 2003; 23:3113–3117. https://doi.org/10.1016/S0955-2219(03)00118-3 20 Mileiko ST, Serebryakov AV, Kiiko VM, Kolchin AA, Kurlov VN, Novokhatskaya NI. Single crystalline mullite fibres obtained by the internal crystallization method: microstructure and creep resistance. J Europ Ceram Soc. 2009; 29: 337-345. DOI: 10.1016/j.jeurceramsoc.2008.06.022 21 Mileiko ST, Kurlov VN, Kolchin AA and Kiiko VM. Fabrication, properties and usage of single-crystalline YAG fibres. J Eur Ceram Soc. 2002;22:1831-1837. DOI:10.1016/S09552219(01)00505-2 22 Mileiko ST, Novokhatskaya NI. On a possibility to make heat resistant composites of high gas corrosion resistance based on refractory metal matrix. Composites and Nanostructures. 2012:4:5-14 (in Russian). 23 Mileiko ST, Novokhatskaya NI. High temperature oxide-fibre/molybdenum-matrix composites of improved oxidation resistance. J Mater Eng Perform. 2015;24:2836-2840 . DOI: 10.1007/s11665014-1305-0. 24 Mileiko ST, Novokhatskaya NI, Stolin AM, Bazhin PM. Producing silicide/molybdenum composites by using internal crystallization method. Composites and Nanostructures. 2014;6:185197 (in Russian) 25 Mileiko ST, Chumichev VA, Novokhatskaya NI, Shakhlevich OF, Stolin AM, Bazhin PM and Prokopenko VM. Silicide-molybdenum fibrous composites: fracture toughness and creep resistance, Res Rep Metals 1:2 (2017).
21
26 Mileiko ST, Kolchin AA, Chumichev VA, Novokhatskaya NI, Prokopenko NA, Shakhlevich OF. Mullite-zirconia-fibre/molybdenum-matrix composites: fabrication, microstructure and mechanical properties. Composites and Nanostructures. 2016;8:211-222 (in Russian) 27 Mileiko ST, Chumichev VA. Mullite-zirconia-fibre/molybdenum-matrix composites: Strength and damage tolerance. Composites Part A. 2018;112:365-370. https://doi.org/10.1016/j.compositesa.2018.06.023 28 Phase Equilibria Diagrams, Acer – NIST, Version 3, CD-ROM Database, 2003. 29 Vardanyan EL, Nazarov AYu, Galyshev SN, Gallyamova RF, Mileiko SТ, Yttrium molybdates coating deposition of oxide-fibre/molybdenum-matrix composites, IOP Conf. Series: Materials Science and Engineering. 2018; 387:1-5. doi:10.1088/1757-899X/387/1/012082 30 Galyshev SN, Vardanyan EL, Gallyamova R, Nazarov Al, Miktybekov B, Ramazanov K, Mileiko ST. Yttrium molybdates coating of oxide-fibre/molybdenum-matrix composites. Submitted to Composites Part B.
22
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
S1359-8368(18)32771-9
DOI:
https://doi.org/10.1016/j.compositesb.2019.107604
Reference:
JCOMB 107604
To appear in:
Composites Part B
Received Date: 25 August 2018 Revised Date:
8 October 2019
Accepted Date: 13 November 2019
Please cite this article as: Mileiko ST, Kolchin AA, Prokopenko VM, Yttrium-aluminium-perovskite-fibre/ molybdenum-matrix composites: Strength, fracture toughness and oxidation resistance, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107604. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Yttrium-aluminium-perovskite-fibre/molybdenum-matrix composites: strength, fracture toughness and oxidation resistance S.T. Mileiko,1 A.A. Kolchin, V.M. Prokopenko Institute of Solid State Physics of RAS, Chernogolovka Moscow distr., 142432 Russia
Abstract Thermal efficiency of a gas turbine depends on the highest temperature of the thermal cycle in the a machine. Nickel supperalloys used in modern gas turbines have approached their physical limit. Hence, transportation and energy technologies call for new generations of heat resistant materials. Among four families of future heat resistant materials, metal matrix composites (MMCs), ceramic matrix composites, alloys based on refractory metals, and high entropy alloys, materials of the first family, MMCs, are characterized by an attractive balance between strength and fracture toughness. The only choice of the matrix for really high temperature composites is a refractory metal, for example, molybdenum. Since composites are to be exploited in extremely hard environments and the requirements to various important properties of future materials of a practical importance are often contradict to each other, a the choice of a particular fibre/matrix combination is to be based on the results of a studies of a large number of the variants. In the present work paper, molybdenum matrix composites reinforced with fibres based on yttrium-aluminium perovskite (YAP) have been obtained for the first time by using the internal crystallization method. The fabrication technology and fibres microstructures are briefly discussed. Strength, fracture toughness and oxidation resistance are discussed in details. The composites proved occur to be sufficiently strong at temperatures up to 1400oC and of are characterized by low notch sensitivity at room temperature.
1
Corresponding author: [email protected]; 2 Ossipyan str., Chernogolovka, Moscow distr 142432 Russia
1
Oxidation resistance of the molybdenum matrix of the composites at temperatures 1000 and 1100oC is much better than that of pure molybdenum. Keywords: A. Metal-matrix composites (MMCs); A. Oxides; B. Fracture toughness; B. Strength; Oxidation 1. Introduction
Thermal efficiency of a gas turbine depends on the highest temperature of the thermal cycle in the a machine. Nickel supperalloys used in modern gas turbines have approached their physical limit, which is about 1100oC, to resist the complicated loading of the crucial structured elements of the machine such as the turbine blades. Hence, transportation and energy technologies call for new generations of heat resistant materials. There are known four families of the materials for the high temperature use, those being ceramic matrix composites, high entropy alloys (HEA), alloys based on refractory metals, and metal matrix composites (MMC). Ceramic matrix composites, in particulars, SiCf/SiCm composites, are now used for elements of the stationary parts of gas turbines at temperatures up to about 1300o C [1]. A further increase in the use temperature of such composites is possible provided an appropriate protective coating has been developed [2]. Since Jeh discovered a possibility to make metal alloys composed of five or more elements in equiatomic ratios, the so-called high entropy alloys [3], a weighty number of papers describing compositions and microstructures of HEAs developed for the high temperature use have been published (see, i.e., [4,5,6]). However, as far as the present authors are aware, no systematic study of creep of refractory HEAs at really high temperatures has been carried out. Some researchers
2
explained it by a huge number of the HEAs and a necessity to choose most prospective compositions to be studied. Heat resistant alloys based on refractory metals, niobium and molybdenum, have been attracting an the attention of materials scientists during the last two decades. In particular, molybdenum alloys that are now under development are characterized by high creep properties at temperature up to 1300oC [7,8] and good oxidation resistance [9]. However, their fracture toughness is too low for turbine blades materials [10,11]. This is a result of the way of strengthening of metal alloys by limiting their plasticity, a the property that determines fracture toughness [12]. Unlike metal alloys, metal matrix composites can be designed in such a manner as to achieve simultaneously increase the in strength and fracture toughness (damage tolerance) with increasing the volume fraction of strong and brittle fibres [12,13]. Hence, fibrous composites are to be considered as promising prospective heat resistant materials. An appropriate balance between three main characteristics of heat resistant materials, those being strength – creep resistance – fracture toughness (damage tolerance) – oxidation resistance, is a key issue for heat resistance materials. The first three properties can be balanced in fibrous composites [12,13]. Actually, to obtain a Metal Matrix Composite (MMC) of high strength and high creep resistance it is necessary to organize a sufficiently strong fibre/matrix interface should be organized [12,14]. To obtain an the MMC with the use temperature much higher than that of nickel superalloys a refractory metal is obviously necessary to select prospective as the matrix. It is not to be excluded The usage of HEAs as the matrix materials for MMCs cannot be excluded either [14,15]. The internal crystallization method (ICM) to produce oxide-fibre/molybdenum-matrix composites, which provided automatically the strong interface, was invented in the last century [16,17], and was first considered either as a method to make model MMCs or as a way to crystallize various oxide fibres from the melts [18,19,20,21]. The method can be seen as a way method to produce a heat 3
resistant material if a composite microstructure is being developed to enhance the oxidation resistance of the matrix. The recently found finding of a possibility to reduce the oxidation rate of the molybdenum matrix by the orders of the magnitudes as compared with that of pure molybdenum [22,23] has opened a way to develop ultra-heat resistant composites. The improved oxidation resistance of the molybdenum matrix is improves due to the formation of molybdates of either yttrium or lanthanoid present existing in the fibre reinforcing the matrix on the composite surface. At the present time The development of ultra-heat resistant molybdenum matrix composites is on now at the starting stage. The first thing to be done was is to study mechanical behaviour of the composites reinforced with various fibres. In particular, strength and creep resistance of the composites reinforced with Al2O3 -Y3Al5O12 eutectic fibres was had been studied at the very beginning of the experiments with the internal crystallization method [17], before a reduced oxidation rate of molybdenum matrix was discovered. Strength, creep resistance, and damage tolerance of Mo3Si-fibre/Mo-matrix composites were presented in Refs. [24,25]. Strength of the composites reinforced with mullite-zirconia fibres is described in Refs [26,27]. This In the present paper presents, for the first time, molybdenum matrix composites reinforced with fibres based on yttrium-aluminium perovskite (YAP) have been obtained by using the internal crystallization method. The fabrication technology and fibres microstructures will be briefly discussed. Strength, fracture toughness and oxidation resistance are discussed in details. 2. Fabrication and microstructure Fabrication of the composite specimens includes the following main steps: (i) preparation of a raw material; (ii) melting the raw material; (iii) crystallization of the fibre in the channels of molybdenum matrix; (iv) preparation of composite specimens to for evaluate ing fracture toughness and strength properties.
4
2.1. Preparation of the raw material At First, powders of alumina (purity > 99.5 wt.%, ~4µm) and yttria (purity > 99.9 wt.%, ~3 µm) were heated at 900oC for 5 h in air. Possible agglomerations of the powders were disintegrated by a grinding in an agate mortar. Then an equi-molar mixture of the oxides in the equi-molar ratio was placed in a fluoroplastic vessel and an isopropyl/polyvinyl solution was added to the mixture to form a suspension. A high-speed blender was then used to homogenize the suspension; the processing time was 15 min. Then the mixture was dried drayed at a temperature of about 100oC. After drying the mixture was sintered at temperatures of either 1250oC or 1650oC. X-ray spectra of the sintered oxide mixtures are presented in Fig. 1.
Fig. 1. X-ray spectra of the oxide blanks sintered in under various temperatures regimes. 5
It can be seen that the sintering regimes used do not give a one-phase material. The content of perovskite is rather low even at the largest temperature/time combination. There exists A large quantity of monoclinic phase Y4Al2O9 (YAM) is present. The presence of The raw oxides, alumina and yttria, are should also found be pointed out. The result was not unexpected as the raw oxide particles are were too large for the raw oxide mixture to react completely to produce perovskite. Nevertheless, Still, melting the oxide mixtures in the process based on the internal crystallization method of producing composites should produce a more homogeneous oxide. 2.2. Crystallizing the fibres in the molybdenum matrix As mentioned in the Introduction, composite specimens were produced by using the internal crystallization method (ICM) [16,18]. The ICM for producing oxide-fibre/molybdenum-matrix composites consists of three main steps. First, a molybdenum carcass with continuous channels is prepared by the diffusion bonding of an assembly of molybdenum wire and foil. The process is performed in such a way as to prevent the gaps between neighbouring fibres to be being filled with the foil material. Consequently, the channel cross-section is formed by two planar and two concave lines; this is the shape of the resulting fibres. Then, the carcass is infiltrated with an oxide melt driven by capillary force. The third 3rd step is crystallizing the melt to form fibres in the channels of the molybdenum carcass by pulling the block up into the cold zone of the furnace. In the previous works on ICM cited in the Introduction, the oxide raw material was melted in a molybdenum crucible located below the molybdenum carcass. In the present work, the raw material was placed in a disposable molybdenum hopper located at the top of the carcass. In the present experiments, the molybdenum block configuration was such that the fibre volume fraction in the specimens was between 0.35 and 0.40. The pulling up rate was between 2 and 250 mm/min. This value is approximately equal to the fibre crystallization rate. Original composite
6
specimens were produced with a shape shown schematically in Fig. 2. Their size was ~5×15×65 mm3. 2.3. Preparing specimens for mechanical testing The specimen shown in Fig. 2 marked as VXYZW, where X,Y,Z,W are digits, was notched and used for measuring the value of critical stress intensity factor K*. Two parts of a the specimen obtained as a result of fracture test were cut along their length as shown in Fig. 2 to make six subspecimens to be used in 3-point bending. The sub-specimens preserved the names of the original one with an addition of digits 1 to 6. The sub-specimens marked by 1 to 3 present the upper part of the original specimen one, those marked by 4 to 6 are located in the bottom of the original specimen.
Fig. 2. Schematic of specimen VXYZW with its sub-specimens. FS: Fracture and failure surfaces. 2.4. The microstructure of the composites and fibres Crystallizing oxide fibres under the conditions of the internal crystallization method has special features. In particular, crystallization of the sapphire fibres starts in the upper part of the oxide/molybdenum block with a spontaneous process that yields a zone of polycrystalline alumina.
7
Then alumina grains at the solid/liquid interface serve as seeds for single crystalline fibres that grow in the rest volume of the channels [16]. In the case of crystallizing mullite fibres, a polycrystalline zone also occurs at the top of an oxide/molybdenum block. As the crystallization proceeds the largest mullite grain is growing to occupy the whole fibre cross-section with the exception of the coldest zones of the melt located in the corners of the channels where an excess of silica occurs [20]. Hence, the microstructure of the fibres in the upper part of an original specimen is changing along their length until a steady crystallization occurs. The formation of the microstructures of the YAPbased fibre has similar features. Here In the present paper, we will only give just a description of the fibre microstructure important for the discussion of the composite mechanical behaviour. A full detailed account of that will be published elsewhere. Some examples of the microstructure of fibres from the upper part of the original specimens that have been crystallized at various rates are shown in Fig. 3. It can be seen that the fibre consists of YAP with inclusions of YAM. The same phases are revealed by X-ray spectroscopy, Fig. 4. The occurrence of YAM phase can also be seen, although its quantities are sufficiently small, and in some specimens (see specimen V11565) this phase is absent. Also, it is important to note that the Xray analysis shows that the YAP phase is single crystalline.
8
V11792, transverse section
V11552, longitudinal section
Specimen→ V/mm/min→ Points↓
V11792 V11562 V11552 2
50
250
Y:Al atomic ratio ↓
1
0.98
1.03
1.00
2
2.02
1.76
2.00
V11562, longitudinal section Fig. 3. SEM micrographs of the YAP-based fibres crystallized from the raw material sintered at 1250oC. In the Table, the values of Y:Al atomic ratio in the points shown on the micrographs are presented. 9
Fig. 4. X-ray spectra of the fibres from the bottom of original specimens containing the fibres crystallized at various rates. 3. Strength and fracture toughness We start with the methodology of mechanical testing and then proceed with the results. 3.1. Methodology Then The specimen is tested in 3-point bending and an apparent value of critical stress intensity factor K* was is calculated according to ASTM-399: ∗
3
⁄
(1)
2
where 10
1.96 − 2.75 + 13.66
− 23.98
+ 25.22
.
Here L is the distance between the supports, c is the notch length, h and w are the height and width of the specimen, respectively. These values sizes in the present experiments are L ≈ 60 mm, h ≈ 15 mm, w ≈ 5 mm, c ≈ (0.45 – 0.55)h. The notch was cut by diamond disks so that the radius of the notch tip was ~0.15 mm. The value of Q corresponds to a maximum load. Obviously, such this procedure developed to for characterize metals cannot be fully justified while dealing with composites; that is why we treat the value of K* obtained in such a way as the apparent critical stress intensity factor. Note that this test gives also strength
∗
of a the notch specimen.
The sub-specimens were tested in 3-point bending at room and high temperatures. Normally, subspecimens 2 and 5 were tested at room temperature to measure strength
∗
of the unnotched
specimen. 3.2. Strength The dependencies of composite strength on fibre crystallization rate are presented in Fig. 5. It can be seen that 1) Room temperature strength of all composites has a maximum at the crystallization rate 50 mm/min. 2) Composite strength at 1400oC does not seem to be dependent of fibre crystallization rate. 3) At low crystallization rates, 2 and 10 mm/min, composite strength at room temperatures does not noticeable change along the length of an the original specimen. At higher crystallization rates, 50 and 250 mm/min, the strength is slightly increasing from the top to bottom of the original specimen. This means that a steady state growth of the fibre and stabilization of its microstructure occurs earlier at low crystallization rates.
11
4) Room temperature strength does not depend of sintering temperature of the oxide mixture. On the other hand, high temperature strength depends on sintering temperature, the higher sintering temperature yields the larger higher temperature strength. 5) Nevertheless, even at low sintering temperature the composite strength at temperatures from 1000 to 1400oC is sufficiently high, Fig. 6.
(a) (b) Fig. 5. Strength of the composites versus crystallization rate of the fibres. (a) Room temperature; open and solid points correspond to the sub-specimens from the top and bottom of the original specimens, ones respectively. (b) Test temperature 1400oC.
Fig. 6. High temperature strength of the composites. 12
3.3. Fracture toughness and damage tolerance Typical load/displacement curves recorded in testing the specimens shown in Fig. 2 are presented in Fig. 7. The curves are characteristic of for materials with of the non-brittle behaviour. It should be noted that the molybdenum matrix in the composites is are nearly brittle (K* ≈ 9 MPa·m1/2) since it has been subjected undergone to heating up to ~ 1950oC in the fabrication process of the composites and, hence, is recrystallized. Reinforcing such a matrix with YAP-based fibres gives provides the a composite with a high apparent critical stress intensity factor (Fig. 8). No dependence of the K* value of on crystallization rate of the reinforcing fibres is can be observed. The average value of K* calculated by taking into account all the data plotted in Fig. 8 is 20.1 MPa·m1/2. Hence, as in many metal matrix composites, in the YAP-based-fibre/molybdenum-matrix composite fibrous macrostructure serves as a magnifier of fracture toughness of the matrix. We treat call the K* value obtained by testing a metal matrix composite as the apparent critical stress intensity factor because a large fracture zone in front of the crack tip in the composites makes the usage of linear fracture mechanics questionable [13]. Therefore, characterizing the fracture behaviour of the composite by notch sensitivity given by the ratio σN*/σo* of the strength values of specimens with and without the notch allows a more correct comparison of the materials with various macro- and micro-structures. However, this value, unlike the critical stress intensity factor in the linear fracture mechanics, cannot be used for estimating the failure loads of structural elements containing defects. The notch sensitivity of the composites is plotted in Fig. 9 versus crystallization rate Vcr of the reinforcing fibres. Again, no σN*/σo*(Vcr) dependence can be seen. Statistical characteristics of the notch sensitivity are presented in Table 1. Calculating the value of σN*/σo* by taking into account all data yields 0.69±0.14. Hence, the composites are characterized by a sufficiently low 13
notch sensitivity and, consequently, a large damage tolerance. A rather large scatter of the σN*/σo* value can be explained by that the composite specimens were produced by a laboratory fabrication technology that should be improved. However, Still, the values of σN*/σo* for some specimens are around unity 1, which means that composites of nearly non-sensitive to defects can be obtained in future.
Fig. 7. Typical load/displacement curves obtained in testing specimens with the notch to measure
the apparent critical stress intensity factor.
14
Fig. 8. The apparent stress intensity factor of the composites versus fibre crystallization rate
during fabrication of the specimens.
15
Fig. 9. Notch sensitivity versus crystallization rate of the fibres. Table 1. Statistical characteristics of notch sensitivity of the composites and molybdenum matrix. σN*/σo*
Fibres crystallization rate Mean value
Std
10
0.70
0.19
50
0.66
0.10
2+10+50+250
0.69
0.14
Molybdenum matrix
~ 0.5
mm/min
4. Oxidation resistance Oxide fibres containing yttrium make the molybdenum matrix much more resistant to oxidation as compared to pure molybdenum due to the formation of molybdates on the composite surface, yttrium molybdate Y2Mo3O12 being an example [22,23]. Saturated vapour pressures of the yttrium 16
molybdates are by orders of the magnitude less than that of molybdenum oxide MoO3. Hence, the oxidation rate of such oxide-fibre/molybdenum-matrix composites is sufficiently low, Fig. 10.
Fig. 10. Mass change of YAP- fibre/Mo-matrix composite specimens versus time of their exposure in air. It is clear that any heat resistant materials are to be coated with appropriate materials to protect a structural element from oxidation. The appropriate materials for the composites under consideration are a series of the yttrium molybdates to make the molybdate appeared occurred as a result of the a reaction between molybdenum oxide and yttria to serve as be a healing agent for possible defects in the coating. The Y2O3-MoO3 phase diagram [28] shows a number of the molybdates, in particular Y2Mo3O12, Y2MoO6, Y6MoO12. The melting temperature of only the first of them one is known, that is 1310oC. All the rest others have higher melting temperatures.
17
Such coatings have been made by ion-plasma deposition [29]. Preliminary tests of the coated composite specimens in a gas flow with a temperature up to ~1400oC gave an encouraging result [30]. 5. Conclusions 1. For the first time, molybdenum matrix composites with YAP-based reinforcing fibres have been obtained. 2. The composites are sufficiently strong at temperatures up to 1400oC and are characterized by low notch sensitivity at room temperature. 3. Oxidation resistance of molybdenum matrix of the composites at temperatures 1000 and 1100oC is much better than that of pure molybdenum. 4. A scatter of the mechanical properties of the composites means that the adopted fabrication technology, which is based on the internal crystallization method, needs to be optimisation ed. Aknowledgments Financial support of Russian Science Foundation, project #16-19-10624-P is gratefully acknowledged. The authors thank their colleagues NA Prokopenko, OF Shachlevich, S.A. Abashkin and VA Chumichev for their help in the experiments and Mrs. A.A. Serebryakova for proofreading the manuscript. Referencies
1 Kiser JD, David KE, Curtis D, Andrulonis R, Ashforth C, Updating Composite Materials Handbook-17 Volume 5—Ceramic Matrix Composites, in: M. Singh, T. Ohji, Sh. Dong, D. Koch, K. Shimamura, B. Clauss, B. Heidenreich, J. Akedo (Eds.), Temperature Ceramic Matrix Composites and Materials for Sustainable Development, Volume 263, Willey, New York, 2017, pp. 413-426.
18
2 Nasiri NA, Patra N, Ni N, Jayaseelan DD, Lee WE. Oxidation behaviour of SiC/SiC ceramic matrix composites in air. J Eur Ceram Soc. 2016; 36:3293–3302. https\\:doi.org/10.1016/j.jeurceramsoc.2016.05.051. 3 Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, Tsau CH, Chang SY. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes, Adv Eng Mater. 2004;6:299–303. http://DOI:10.1002/adem.200300567 4 Lin CM, Juan CC, Chang CH, Tsai CW, Yeh JW. Effect of Al addition on mechanical properties and microstructure of refractory HfNbTaTiZr alloys. J Alloys Compd. 2015;624:100–107. http://doi:10.1016/j.jallcom.2014.11.064. 5 Senkov ON, Senkova SV, Woodward C. Effect of aluminum on the microstructure and properties of two refractory high-entropy alloys. Acta Mater. 2014; 68:214–228. http://doi.org/10.1016/j.actamat. 2014.01.029. 6 Gorr B, Azim M, Christ H-J, Chen H, Szabo D V, Kauffmann A, and Heilmajer M. Microstructure Evolution in a New Refractory High-Entropy Alloy W-Mo-Cr-Ti-Al. Metall Mater Trans. 2016; 47A:961 – 970. DOI: 10.1007/s11661-015-3246-0 7 Jain P, Kumar KS. Tensile creep of Mo–Si–B alloys. Acta Mater. 2010; 58:2124–2142. doi:10.1016/j.actamat.2009.11.054 8 Tanaka R, Kasama A, Fujikura M, Iwanaga I, Tanaka H and Matsumura Y. Research and development of niobium-based superalloys for hot components of gas turbines. Proc. of the Intern. Gas Turbine Congress 2003 Tokyo, pp. 1-5. 9 Schliephake D, Azim M, von Klinski-Wetzel K, Gorr B, Christ H-Jr, H Bei, George EP , Heilmaier M. High-temperature creep and oxidation behavior of Mo-Si-B alloys with high Ti contents. Metall Mater Trans. 2014; 45A:1102—1111. DOI: 10.1007/s11661-013-1944-z 19
10 Lemberg JA, Middlemas MR, Weingärtner T, Gludovatz B, Cochran JK, Ritchie RO. On the fracture toughness of fine-grained Mo-3Si-1B (wt.%) alloys at ambient to elevated (1300oC) temperatures. Intermetallics. 2012; 20:141-154. doi:10.1016/j.intermet.2011.09.003 11 Krüger M, Jain P, Kumar KS, Heilmaier M. Correlation between microstructure and properties of fine grained Mo-Mo3Si-Mo5SiB2 alloys. Intermetallics. 2014; 48:10-18. DOI: 10.1016/j.intermet.2013.10.025 12 Mileiko ST, Metal and Ceramic Based Composites, Elsevier, Amsterdam, 1997 13 Mileiko ST. Fracture-toughness/notch-sensitivity correlation for metal- and ceramic-based fibrous composites. Composites Part B. 2017; 116:1-6. doi: 10.1016/j.compositesb.2016.12.060 14 Mileiko ST, Firstov SA, Novokhatskaya NA, Gorban’ VF, Krapivka NP. Oxide-fibre/highentropy-alloy-matrix composites. Composites Part A. 2015; 76 :131-134. DOI: 10.1016/j.compositesa.2015.05.023. 15 Mileiko ST. High temperature oxide-fibre/metal matrix composites. Mater Chem Phys. 2017; 210:353-361. http://dx.doi.org/10.1016/j.matchemphys 16 Mileiko ST, Kazmin VI. Crystallization of fibres inside a matrix: a new way of fabrication of composites. J Mater Sci. 1992; 27:2165-2172. https://doi.org/10.1007/BF01117932 17 Mileiko ST, Kazmin VI. Structure and mechanical properties of oxide fibre reinforced metal matrix composites produced by the internal crystallization method. Compos Sci Technol. 1992; 45:209-220. http://doi.10.1016/0266-3538(92)90081-D 18 Mileiko ST. Single crystalline oxide fibres for heat-resistant composites. Compos Sci Technol. 2005;65:2500-2513. DOI: 10.1016/j.compscitech. 2005.05.029
20
19 Rüscher C H, Mileiko ST, Schneider H. Mullite single cyrystal fibres produced by the Internal Crystallization Method (ICM). J Eur Ceram Soc. 2003; 23:3113–3117. https://doi.org/10.1016/S0955-2219(03)00118-3 20 Mileiko ST, Serebryakov AV, Kiiko VM, Kolchin AA, Kurlov VN, Novokhatskaya NI. Single crystalline mullite fibres obtained by the internal crystallization method: microstructure and creep resistance. J Europ Ceram Soc. 2009; 29: 337-345. DOI: 10.1016/j.jeurceramsoc.2008.06.022 21 Mileiko ST, Kurlov VN, Kolchin AA and Kiiko VM. Fabrication, properties and usage of single-crystalline YAG fibres. J Eur Ceram Soc. 2002;22:1831-1837. DOI:10.1016/S09552219(01)00505-2 22 Mileiko ST, Novokhatskaya NI. On a possibility to make heat resistant composites of high gas corrosion resistance based on refractory metal matrix. Composites and Nanostructures. 2012:4:5-14 (in Russian). 23 Mileiko ST, Novokhatskaya NI. High temperature oxide-fibre/molybdenum-matrix composites of improved oxidation resistance. J Mater Eng Perform. 2015;24:2836-2840 . DOI: 10.1007/s11665014-1305-0. 24 Mileiko ST, Novokhatskaya NI, Stolin AM, Bazhin PM. Producing silicide/molybdenum composites by using internal crystallization method. Composites and Nanostructures. 2014;6:185197 (in Russian) 25 Mileiko ST, Chumichev VA, Novokhatskaya NI, Shakhlevich OF, Stolin AM, Bazhin PM and Prokopenko VM. Silicide-molybdenum fibrous composites: fracture toughness and creep resistance, Res Rep Metals 1:2 (2017).
21
26 Mileiko ST, Kolchin AA, Chumichev VA, Novokhatskaya NI, Prokopenko NA, Shakhlevich OF. Mullite-zirconia-fibre/molybdenum-matrix composites: fabrication, microstructure and mechanical properties. Composites and Nanostructures. 2016;8:211-222 (in Russian) 27 Mileiko ST, Chumichev VA. Mullite-zirconia-fibre/molybdenum-matrix composites: Strength and damage tolerance. Composites Part A. 2018;112:365-370. https://doi.org/10.1016/j.compositesa.2018.06.023 28 Phase Equilibria Diagrams, Acer – NIST, Version 3, CD-ROM Database, 2003. 29 Vardanyan EL, Nazarov AYu, Galyshev SN, Gallyamova RF, Mileiko SТ, Yttrium molybdates coating deposition of oxide-fibre/molybdenum-matrix composites, IOP Conf. Series: Materials Science and Engineering. 2018; 387:1-5. doi:10.1088/1757-899X/387/1/012082 30 Galyshev SN, Vardanyan EL, Gallyamova R, Nazarov Al, Miktybekov B, Ramazanov K, Mileiko ST. Yttrium molybdates coating of oxide-fibre/molybdenum-matrix composites. Submitted to Composites Part B.
22
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.