Accepted Manuscript Synthesis of metal-intermetallic laminate (MIL) composites with modified Al3Ti structure and in situ synchrotron X-ray diffraction analysis of sintering process
D.V. Lazurenko, I.A. Bataev, V.I. Mali, A.M. Jorge, A. Stark, F. Pyczak, T.S. Ogneva, I.N. Maliutina PII: DOI: Reference:
S0264-1275(18)30308-3 doi:10.1016/j.matdes.2018.04.038 JMADE 3851
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
23 January 2018 12 April 2018 13 April 2018
Please cite this article as: D.V. Lazurenko, I.A. Bataev, V.I. Mali, A.M. Jorge, A. Stark, F. Pyczak, T.S. Ogneva, I.N. Maliutina , Synthesis of metal-intermetallic laminate (MIL) composites with modified Al3Ti structure and in situ synchrotron X-ray diffraction analysis of sintering process. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi:10.1016/ j.matdes.2018.04.038
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ACCEPTED MANUSCRIPT Synthesis of metal-intermetallic laminate (MIL) composites with modified Al3Ti structure and in situ synchrotron X-ray diffraction analysis of sintering process D.V. Lazurenko1*, I.A. Bataev1, V.I. Mali2, A.M. Jorge Jr.3,4,5, A. Stark6, F. Pyczak6, T.S. Ogneva1, I.N. Maliutina1 1
Novosibirsk State Technical University, 630073, Russia, Novosibirsk, Karl Marks str. 20 Lavrentiev Institute of Hydrodinamics, SB RAS, 630090, Russia, Novosibirsk, Lavrentiev av. 15 3 Federal University of São Carlos, Via Washington Luiz, Km 235, 13565-905, São Carlos, SP, Brazil 4 Université Grenoble Alpes, SIMAP, 621 avenue Centrale 38400 Saint-Martin-d'Hères 5 Université Grenoble Alpes, LEPMI, 621 avenue Centrale 38400 Saint-Martin-d'Hères 6 Helmholtz Zentrum Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany *corresponding author:
[email protected], +79232455243
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Abstract Al3Ti-based alloys attract exceptional attention due to their high specific mechanical properties. However, their application is still insufficient due to their low ductility and fracture toughness. Several approaches were previously proposed to address these problems. The first one is stabilization of the cubic modification of titanium trialuminide by alloying. Another approach consists in fabricating metal-intermetallic laminated composites (MIL). In this study, we combined both methods to synthesize the first MIL composite with cubic Al3Ti interlayers. Copper additions were used to stabilize the cubic modification of Al3Ti and produce a novel TiAl5CuTi2 MIL composite. First mechanical characterization by indentation tests showed that the binary Al3Ti intermetallic tended to crack at a load of 0.2 kg while the fracture was not observed in the Al5CuTi2 layers at least at a load of 1 kg. These results are an indirect evidence of a higher ductility and fracture toughness of the composite with cubic Al3Ti compared to tetragonal one. The sequence of the phase transformations in the Al-Ti-Cu system was studied using in situ synchrotron X-ray radiation diffraction. The formation of Al5CuTi2 occurred via several intermediate stages including eutectic melting of Al and Cu and the formation of binary AlCu and Al3Ti compounds.
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Keywords: sintering; titanium aluminide; phase transformation; synchrotron diffraction; fracture toughness; ternary compound 1. Introduction Over the last few decades, ordered intermetallic alloys are in the focus of considerable research interest. As a rule, these materials possess excellent strength, hardness, low density and perfect oxidation resistance at high temperatures [1], which makes them promising candidates for aerospace applications, e.g., for producing elements of jet engines. Among various groups of intermetallic alloys, the system Ti-Al is of keen interest. Ti-Al intermetallics are qualified among the most advanced materials due to their outstanding combination of properties, such as high strength, chemical durability, and reasonable cost. In the category of binary Ti-Al alloys, three types of compounds (namely, Ti3Al, TiAl, and Al3Ti) attract the most exceptional attention. While Ti3Al and TiAl-based alloys have already found application in the industry, Al3Ti-based alloys are, so far, still at the research and development stage. At the same time, among the above intermetallics, titanium trialuminide possesses the highest corrosion and oxidation resistance as
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well as the highest stiffness, hardness, and lowest specific weight. For this reason, the number of studies aimed to develop approaches to transfer these alloys to the industry is continually growing. The most significant disadvantage of titanium trialuminides, which limits their widespread application, is the poor fracture toughness at low temperatures caused by the low ductility, making them extremely notch-sensitive. For instance, the strain to failure at compression tests of pure polycrystalline Al3Ti at room temperature just slightly exceeds 0 % [2]. The low ductility of titanium trialuminide is explained by particularities of its crystal structure [3]. The crystalline structure of Al3Ti is the tetragonal D022, which is typical for many other intermetallics, including trinickelides [4, 5]. The D022 structure results from the L12 structure (which essentially is an ordered face-centered cubic lattice (fcc)) by introducing antiphase boundaries with a displacement vector of 1/2[110] on each (001) plane of the L12 unit cell. Due to its low symmetry, the tetragonal D022 structure has a limited number of independent slip systems [6], which is probably the reason for low-temperature brittleness of Al3Ti compounds. Yamaguchi et al. [2] mentioned that the principal deformation mode of titanium trialuminide at low temperatures was (111)[11-2] twinning. Only this type of twins could retain the arrangement of the D022 structure. They experimentally revealed that titanium trialuminides exhibit some ductility just at 600 °С and higher. Currently, several approaches to overcome the problem of low-temperature brittleness of Al3Ti-based alloys and to increase their fracture toughness are under development. The most successful methods probably are (1) the alloying of titanium trialuminide with either iron, chromium, manganese, copper, cobalt, nickel or zinc to stabilize the cubic L12 structure, and (2) the fabrication of metal-intermetallic composites (including laminated composites). The first approach is based on the assumption that materials possessing the L12 structure having 12 independent {111} slip systems (likewise fcc structures) will be intrinsically more ductile comparing to compounds with a D022 one, which has a limited number of slip systems [6]. This approach was previously very successfully proved for other A3B-type intermetallics [7]. For instance, cubic (Fe, Co)3V, (Fe, Co, Ni)3V, and (Fe, Ni)3V intermetallics display tensile ductility higher than 40 %, while it is less than 1% for related hexagonal materials based on Co3V and (Ni, Co)3V. Application of this method to titanium trialuminides was also very efficient. At the late 1980s and early 1990s, it was observed that Ti-Al-X intermetallics (where X = Fe, Cr, Cu, Mn, Co, Ni or Zn) possess L12 structure and satisfactory low-temperature ductility (from 3% to 20 %) [6]. The second approach was also successfully implemented in various studies. Nowadays there are many publications on Al-based [8-15], and Ti-based [16-26] composites reinforced with Al3Ti compound. An attractive method to fabricate Ti-Al3Ti composites was presented by Vecchio et al. [16, 17]. Based on reactive sintering of aluminum and titanium foils, it leads to the formation of metal-intermetallic laminated (MIL) materials. Composites of this type exhibit low density and extremely high stiffness, meanwhile their fracture toughness is adequate for applications even at room temperature. Due to their properties, MIL composites are prospective materials for use as blast energy absorbers, for manufacturing of heat-exchangers, and even for some load-bearing applications. Because of the simplicity of the reactive foil sintering technique and the efficiency of the obtained materials, this approach attracted the attention of large amounts of researchers [19, 24, 27-33]. Despite the evident success demonstrated by Ti-Al3Ti MIL composites, their fracture toughness is still low and ranges between 10-50 MPam1/2 depending on the volume fraction of titanium layers [34]. Presently, numerous studies aimed at decreasing the titanium fraction and increasing in the fracture toughness of these materials simultaneously. For instance, Vecchio and Jiang significantly increased the fracture toughness of
ACCEPTED MANUSCRIPT MIL composites by reinforcement with alumina fibers [34]. A similar effect was demonstrated by Han et al. [35] and Lin et al. [36] using Al2O3 and SiC fibers, respectively. Here, we suggest combining the two approaches above intending to increase the ductility and fracture toughness of titanium trialuminides by producing L12-titanium trialuminides in MIL composites. In this paper, we report on the first Ti-Al-based MIL composite with a cubic intermetallic phase obtained by reactive sintering of titanium and aluminum foils with the addition of copper powders.
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2. Materials and methods Fig. 1 presents a scheme of the composite synthesis procedure. Commercially pure titanium and aluminum foils were alternately stacked and interspersed by homogeneous layers of copper powder between them, and then placed in a titanium shell with an inner diameter of 26 mm. Such a set was covered with a cap with an outer diameter of 30 mm. This setup allowed performing the reactive sintering at temperatures exceeding the aluminum melting point. The thickness of metallic foils was 50 µm. Thus, weights of each foil were 0.1195 and 0.0716 g correspondently for Ti and Al, while the weight of the copper powder was 0.03 g per layer. Consequently, initial workpieces were prepared in such a way that layers of Al5CuTi2 cubic phase (44.47 wt. % Al – 34.93 wt. % Ti – 20.6 wt. % Cu) with L12 structure and unreacted titanium remained after sintering. The titanium shell was mounted in a graphite die between two graphite punches and then heated by electrical current. The sintering was carried out in vacuum using a LABOX-1575 spark plasma sintering (SPS) machine [37] at 830 С under a pressure of 40 MPa for 10 minutes. Ti-Al3Ti MIL composite without the addition of copper prepared according to the same procedure was used as a reference sample.
Fig. 1. A scheme of fabrication of MIL composites with L12 titanium trialuminide layer. The microstructure of fabricated material was investigated by scanning electron microscopy (SEM) in the back-scattered electrons mode using a Carl Zeiss EVO 50XVP microscope equipped with an X-ACT (Oxford Instruments) energy dispersive X-ray (EDX) spectrometer. Phases formed in the MIL composite were investigated by X-ray diffraction (XRD) analysis. Diffraction patterns were recorded using an ARL X’TRA diffractometer with Cu Kα radiation. The fracture toughness was estimated based on Vickers microindentation tests using a Wolpert Group 402 MVD tester. A load on the diamond indenter varied from 0.2 to 1 kg.
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In-situ synchrotron X-ray radiation diffraction was used to study the reactions and evolution of phases during heating in the Ti-Al-Cu system. The sintering process was simulated during the in-situ tests as follows. Thoroughly mixed Al, Ti and Cu powders were put in titanium containers with an outer diameter of 5 mm and the inner diameter of 4 mm. The nominal composition of the mixture was Ti45-Al46-Cu8 (at. %). This ratio of elements was the same as in the multilayered workpiece sintered by SPS. The containers were covered by titanium caps and mounted in the induction furnace of a modified DIL805A/D dilatometer [38]. They were heated with a rate of 10 °C/min up to the temperature of 830 °C. The samples were held at this temperature for 1 hour and subsequently cooled to room temperature with a rate of 50 °C/min. The temperature was measured by an S-type thermocouple welded to a container wall. The experiment was carried out in a high-purity argon atmosphere at a pressure of 0.8 mbar. For minimizing the influence of oxygen on a powder mixture, the chamber with a mounted sample was evacuated three times to the pressure of 210-4 mbar using a turbomolecular pump and flushed with argon. Diffraction patterns were obtained at Petra III synchrotron radiation source of the German Electron Synchrotron (Deutsches Elektronen-Synchrotron – DESY) in the High Energy Materials Science Beamline (P07) operated by Helmholtz-Zentrum Geesthacht [39]. The radiation energy was 100 keV which corresponded to a wavelength of 0.124 Å. The spot size was 11 mm. Diffraction rings were recorded using a Perkin Elmer XRD1621 2D detector with a resolution of 20482048 pixels and a pixel size of 200200 µm in transmission mode. The sample-to-detector distance was equal to 1837 mm. The Debye-Scherrer diffraction rings were continuously recorded during heating, holding, and cooling with a frequency of 0.1 Hz. The total exposition time was 4 seconds and was provided by summation of 40 frames exposed for 0.1 seconds each. Two-dimensional diffraction rings were azimuthally integrated and analyzed as typical Intensity - 2θ powder diffraction patterns.
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3. Results and discussion 3.1. Investigations of a sample obtained by SPS Apart from the well-known binary intermetallics, three types of ternary compounds can be formed in the Al-Cu-Ti system according to its ternary phase diagram [40]: τ1, τ2, and τ3. τ1 is a copper-rich phase possessing L21 cubic structure, which is typically represented by the chemical formula TiCu2Al. The hexagonal τ2 compound with С14 structure corresponds to a composition range which is close to equiatomic TiCuAl. τ3 corresponds to Al5CuTi2 with L12 structure and contains similar proportions of Ti and Al as in Al3Ti. Fig. 2 presents a cross-section view of the synthesized composite and elemental composition of separate layers measured by EDX analysis. Three kinds of layers predominated in the composite, one of titanium and two other formed by different intermetallics. The average chemical composition of thicker intermetallic layers was 61 at. % Al, 25.5 at. % Ti, 13.5 at. % Cu, corresponding to the τ3-phase with L12 structure. One of the main issues of this study was to achieve the synthesis of this particular phase in the MIL composites. Such synthesis is herewith successfully demonstrated. The other type of intermetallic layers was significantly thinner (about 10 µm) and had the average chemical composition corresponding to τ2-phase (40.3 at.% Al, 33.7 at.% Ti, 26.0 at.% Cu).
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Fig. 2. The structure of MIL «Ti – Al3Ticubic» composite: (a) a cross-section of material at lower magnifications; (b) τ2 interlayer between titanium alloy and cubic τ3-phase.
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X-ray diffraction patterns presented in Fig. 3 are in good agreement with data of EDX analyzes, confirming the presence of the three above mentioned phases, i.e., α-Ti and the intermetallics τ2 and τ3. It is worth noting that τ3 (Al5CuTi2) peaks shifted in the direction of higher angles when compared to the literature [41]. Such shifting may have been caused by the decrease of the lattice parameter induced by the increase of copper content in the composition of the τ3 layer (13.5 at. %). The copper percentage found in [41] was equal to 7.5 at.%. By analyzing the ternary phase diagram, one may observe that the content of Ti in the τ3-phase is constant (about 25 at. %). However, copper and aluminum concentrations may vary. In fact, Cu and Al atoms can substitute each other in a wide range of concentrations. Since the atomic radius of copper is smaller than the one of aluminum (128 and 143 pm, respectively), increased amounts of copper may lead to a decrease in the lattice parameter. Precise measurements of the τ3-phase lattice parameter, accomplished using the Nelson-Reilly function [42], indicated that, in the present study, the lattice parameter of τ3-phase was equal to 3.918 Å. This result is in good agreement with data presented in [41, 43, 44], which showed that the increase of copper content from 7.5 to 12.5 at.% leads to a decrease in the lattice parameter from 3.945 to 3.927 Å.
Fig. 3. XRD pattern of the material after the sintering, showing the presence of three different phases: α-Ti, cubic L12 titanium trialuminide (τ3-phase) with a composition close to Al5CuTi2, and τ2-phase (close to AlCuTi).
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For nearly all engineering structural materials fundamental requirements are that they must be both strong and tough. However, in most of them, such properties are generally exclusive. Considering that strength (or hardness) is always stress expressing the capacity of a material to resist an applied load without failure or plastic deformation. Conversely, toughness is the material's capability of absorbing energy and plastically deform without fracturing. Hence, it is estimated as the energy required to begin a fracture, and, of course, can also be measured using fracture-mechanics methods to assess the critical value of a crack-driving force needed to open the crack and propagate it or to propagate the pre-existing ones. Measuring mechanical properties of obtained bulk composites is a significant challenge. Such difficulty is mainly because our samples were fabricated in a laboratory spark plasma sintering machine that produces samples whose dimensions (
