CC BY
89iSHS 2019
Moscow, Russia
THERMAL EXPLOSION IN THE 2Co-Ti-Al SYSTEM: COMBUSTION, PHASE FORMATION AND PROPERTIES
M. L. Busurina*", A. E. Sytschev", D. Yu. Kovalev", N. V. Sachkova", A. V. Karpov",
A. N. Gryadunov", and V. A. Shcherbakov"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: busurina@ism.ac.ru
Investigation of structural materials in Ni-Al and Ti-Al binary and Ti-Al-Me (Me = Ni, Fe, Co, Cu) ternary systems and their functional dependence on the chemical composition, properties and structure is a promising task for modern electronics [1, 2]. Ternary intermetallic compounds X2YZ where X and Y are the transition metals, Z is the element of III-IV groups belong to the alloys of Heusler. These alloys have very attractive properties (shape memory, super elasticity, magneto-optical and magnetocaloric properties) and can be controlled by a magnetic field. One of the promising compounds based on the ternary intermetallic system Co-Ti-Al is the Heusler alloy Co2TiAl.
Compounds based on Co-Ti-Al system are produced in various ways: arc melting, shock wave synthesis, spark plasma sintering. In the practice of creation of intermetallic materials by the method of self-propagating high-temperature synthesis (SHS), the question of obtaining intermetallic compounds of both binary and ternary systems is widely investigated. The purpose of this work is to study the features of the structure and phase formation in intermetallic material based on the Co2AlTi (Heusler phase) synthesized by SHS. Synthesis was carried out both in vacuum under a pressure of 13.3 • 10-2 Pa and in argon (~ 105 Pa). The synthesis of 2Co-Ti-Al samples occurs in the thermal explosion mode. The reaction simultaneously takes place in the entire volume of the sample with the maximum rate of temperature rise reached 3 500°C/s. Initial temperature of SHS reaction in vacuum (Fig. 1) is 565°C, which is almost 100°C below the Al melting point and indicates the beginning of a solid-phase reaction. The maximum temperature of SHS reaction in a vacuum is 1470°C. The ignition temperature in the argon is 690°C. The cooling rate of the samples in the argon is also higher than in vacuum, which is explained by a higher heat sink. The maximum combustion temperature in the argon is 1476°C and is very close to the melting temperature of Co (1494°C).
DOI: 10.24411/9999-0014A-2019-10026
1000-
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o
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Fig. 1. Temperature profiles of SHS reaction for 2Co-Ti-Al system in vacuum and argon. M. L. Busurina et al. 69
XV International Symposium on Self-Propagating High-Temperature Synthesis
X-ray phase analysis (Fig. 2) showed the formation of Co2AlTi phase (Heusler phase), the mass content of which is equal to 99 mass %. The total content of secondary phases (Co3Ti, Co2Ti) does not exceed 1 wt %. The unit cell parameter of Co2TiAl is a = 5.8433±0.0002 Â. The compound has a face-centered cubic lattice (Fm3m) and is a structural prototype of AlCu2Mn.
Combustion product has a homogeneous microstructure (Fig. 3) consisting of rounded Co2TiAl-phase grains with an average size of 20 ^m. The EDA results confirmed the chemical composition of the main Co2TiAl phase. However, there are areas where the Co2AlTi grains are separated by the layer based on intermetallic compound of variable composition TiCox with a thickness of 10 ^m. The density of the material was 5.9 g/cm3, which is lower than the theoretical density of 6.4 g/cm3. This is due to the porosity of the sample. The pore size reaches 50-100 ^m.
»
PC*
Spectrum Al Ti Co
1 2.31 18.00 79.69
2 1.54 24.58 73.88
3 15.86 22.74 61.40
4 15.23 23.63 61.13
Fig. 2. XRD pattern of combustion Fig. 3. SEM of combustion product in 2Co-Ti-Al product in 2Co-Ti-Al system. system.
Time-resolving X-ray diffraction data [3] showed that the phase formation goes through several successive stages (Fig. 4). Only lines of initial reagents Ti, Al and Co of cubic and hexagonal modification are observed on the diffraction field when heated from room temperature to the ignition temperature. In an exothermic reaction, which was initiated at a temperature close to the melting point of Al, the lines of the initial reagents disappear in a time not exceeding 1 s. Lines 200 and 220 corresponding to the Co2TiAl phase appeared after the disappearance of lines of initial components. There is a sharp shift of these lines in the region of large angles due to the cooling of Fig. 4. Time-resolved X-ray analysis °f the sample after the reaction and intense heat phase transformations in the 2Co-Ti-Al losses. In 9 seconds after ignition, the system during SHS. weak diffraction lines appear on the diffraction field. They identified as lines 111 and 114 of the intermetallic compounds Co3Ti and Co2Ti, respectively. XRD of the surface of the synthesized product showed that the main phase is the Co2TiAl phase (72 mass %). The contents of AbTi, Co2Ti, and AhO3 secondary phases were 2, 10, and 16 mass %, respectively.
For electromagnetic studies, a non-porous dense samples were produced by SHS densification. The synthesized material showed the presence of ferromagnetic hysteresis loops (Fig. 5a). The maximum value of specific saturation magnetization at room temperature was 1.64 am2/kg. Electrical resistivity (at room temperature) is 1.35 |iQm (Fig.5 b). Resistivity p increases monotonically with increasing temperature.
¿SHS 2019
Moscow, Russia
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a = 1.64 emu/g
o = 0.53 emu/g
H - 335 Oe
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-5000 0 5000
Applied magnetic field (Oe)
(a) (b)
Fig. 5. Magnetic and electrical properties of synthesized product Co2TiAl.
Resistivity p shows the metallic character of conduction being p = 1.35 |iQm at T = 293 K and increases with increasing temperature. Electrical conductivity depends mainly on the relative densification of matrix and conductive phases.
At the beginning of the SHS reaction the nucleation of the TiAb grains by diffusion of Al atoms from the melt into the Ti lattice takes place. Taking into account that the diffusion coefficient of Al in a-Ti is about 5 10-18 m2/s, and in P-Ti is 3 • 10-14 m2/s, it can be concluded that the leading mechanism of SHS reaction is diffusion of Al in P-Ti. Cobalt decreases the beta transus temperature (a-Ti ^ P-Ti) from 885°C to 758-806°C. The solubility of Co into Al at eutectic temperature does not exceed 0.009 at %. Solubility of Al in e-Co is also practically zero. The solubility of Co into AlTix intermetallic is quite high (2.3-9.6 at %). The solubility of Al in TiCos intermetallics, as well as in Co and Ti is 6.8-13.7 at %. The solubility of Co into Ti is 10.3 at % and of Ti in Co is 8.5 at %. Therefore, it can be assumed that the formation of Co2AlTi occurs due to the dissolution of Co atoms in the melt Ti-Al.
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