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20THE PROCESSES OF STRUCTURE FORMATION IN THE MECHANICALLY ACTIVATED POWDER MIXTURE Ti + Al SUBJECTED TO y-IRRADIATION
V. Yu. Filimonov*", M. V. Loginova", S. G. Ivanov", A. A. Sitnikov", V. I. Yakovlev",
A. V. Sobachkin", A. Z. Negodyaev", A. Yu. Myasnikov", A. V. GradoboevA,
B. P. Tolochkoc, and M. R. Sharafutdinovc
aAltai State Technical University, Barnaul, 656038 Russia bTomsk Polytechnic University, Tomsk, 634050 Russia cInstitute of Solid State Chemistry and Mechanochemistry, SB, RAS, Novosibirsk, 630128 Russia *e-mail: vyfilimonov@rambler.ru
DOI: 10.24411/9999-0014A-2019-10042
It is known that the high temperature synthesis in preliminary mechanically activated powder mixtures (MASHS) has several advantages over traditional high-temperature synthesis (SHS) [1]. These advantages include: the formation of almost ideal contact of reactants in the solid phase; cleaning of reagent surfaces from oxides and impurities; formation of non-equilibrium defects, amorphous phases and disordered structures during mechanical activation (MA). All these factors greatly facilitate the processes of solid-phase diffusion and make possible the realization of true solid-phase combustion. However, the processes of structure formation in solids have a number of differences from similar processes in liquids or gases which associated primarily with low diffusion rate. The hierarchy of structural inhomogeneities (which are determined by a different scale of heterogeneity in the volume of a mechanocomposite [2]) leads to the multistep chemical transformations and to formation of a multiphase synthesis product which is distributed unevenly in the sample volume. This is is extremely undesirable from point of view of practical materials science.
It can be assumed that y-irradiation can be used to implement the "soft" structure management of activated mixtures. Indeed, during gamma irradiation, the concomitant processes of radiation-stimulated diffusion could contribute to the processes of dissolution and homogenization, changes in the boundaries of homogeneity of the phases and solid solutions, creating the favorable conditions for interdiffusion at interphase boundaries [3]
In the present investigation, the experimental study of the effect of gamma radiation on the microstructure of activated Ti + Al powder mixture and on the dynamics of the phase formation processes for irradiated mixture was carried out. The high-temperature synthesis was realized in a thermal explosion mode. The study consisted of several stages: (1) Mechanical activation of powder mixture Ti + Al. Mechanical milling was performed in the AGO-2 planetary ball mill with two vials. The volume of the vial was 160 cm3, the ball diameter was 8 mm, the powder mass in each vial was 10 g, the mass of the balls was 200 g. The centrifugal acceleration of balls was 400 ms-2 (40 g). In order to prevent oxidation, the vials were vacuum pumped and then filled with argon up to a pressure of 0.3 MPa. The milling time was 7 min. (2) y-irradiation. The irradiation processes were conducted on a certified stationary setup "Researcher" (60Co isotope, RISD, Tomsk). The intensity of y-irradiation doses was 98 Gy/s. The level of exposure to y-quanta (with energy of 1.17 MeV) was determined by absorbed doses of Dy [Gy], which were: 1103 Gy (exposure time 22 min), 5 103 Gy (exposure time 1 h 51 min), 2104 Gy (7 h 28 min), and 5 104 Gy (exposure time 18 h 41 min). (3) Microstructural and elemental analysis of irradiated samples. The microstructure of the initial mixture and synthesis products was studied on microsections using a Tescan MIRA scanning electron microscope equipped
ISHS 2019 Moscow, Russia
with an EDS X-Act microanalyzer (Oxford Instruments) with a Si-drift detector. The device allows the analysis of elements ranging from boron to uranium. (4) The synthesis in the thermal explosion mode was carried out using microwave radiation according to the method described in [4]. To study the phase formation dynamics, we used radiation from the electron storage ring of VEPP-3 station 5b "Diffraction cinema" at the Institute of Nuclear Physics, SB, RAS (Novosibirsk). A continuous frame shooting of diffraction patterns was carried out using monochromatic radiation with a wavelength X = 1.505 A, in the range of scanning angles of 36°-68°. The frame rate was varied from 0.3 to 1 frame/s.
The microstructural analysis of the samples cross sections made it possible to establish that two types of heterogeneous microstructures are formed during MA. The first structure is characterized by large spheroid inclusions (tens of microns) of titanium particles in aluminum matrix. The second structure is characterized by a submicron scale of heterogeneity. It was established that the diffusion zone with dimensions of several micrometers is formed at the boundaries of the contact of large inclusions of titanium and aluminum matrix during the activation. The increase of the radiation dose up to the maximal leads to expansion of the diffusion zone by 3-4 times. Wherein, the small titanium inclusions disappear. This confirms the stated hypothesis of increasing the degree of homogenization under the influence of radiation.
Figure 1 shows the dependence of the components content on time for the unirradiated sample (a) and the sample irradiated at the maximal dose (5-104Gy) (b) during thermal explosion. As follows from the figure, the kinetics of the primary phase formation processes for the irradiated and unirradiated sample is different. Both in the irradiated and in the non-irradiated mixture, already in the process of preliminary heating (up to 600°C), the significant amount of reaction products is formed. Along with the unreacted starting components, the compounds TiAl, TiAb are present in the mixtures. Insignificant amount of Ti3Al compound is present in the irradiated sample. In the process of rapid self-heating, the content of the TiAl compound increases in the unirradiated mixture while the titanium content decreases. The content of the TiAl3 phase varies slightly. In the irradiated mixture, the content of Ti3Al compound almost does not change but it decomposes at the certain temperature. The rapid increase in the content of the TiAl phase is accompanied by a decrease in the content of the TiAl3 compound (in contrast to the unirradiated mixture).
Figure 2 shows the X-ray diffraction patterns of the quenched samples at various high temperature annealing times (the heat source did not turn off).
76 78 80 82 84 86 88 Q8 90 92 94 96 98 100 102 104
Time, s Time, s
(a) (b)
Fig. 1. Dynamics of phase formation during thermal explosion for (a) unirradiated and (b) irradiated (5 • 104 Gy) mixtures.
Fig. 2. XRD patterns of the quenched samples at various high temperature (1200°C) annealing times: (a) unirradiated sample; (b) irradiated sample.
In both irradiated and non-irradiated samples, the single-phase compound TiAl is formed after 2 min of annealing. However, in the unirradiated sample, the decomposition and recrystallization of the TiAl phase occurs up to 7 min of annealing. This process is accompanied by formation of a-Ti and TiAb compound. In the irradiated sample, the composition of the product does not change. Moreover, with an increase in the annealing time, the structure stabilizes that is determined by increase in the intensity of the diffraction peaks and decrease in their width.
Based on the above, it can be assumed that the irradiation of mechanically activated Ti + Al mixture contributes to the processes of solid-phase dissolution of the components and the homogenization of the system. As a result, the reaction paths in the irradiated and unirradiated mixture are different. The reaction product which synthesized in the irradiated mixture is more resistant to thermal effects.
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3. A.C. Damask, Studies in radiation effects on solids (vol. 2), Gordon and Breach, 1967.
4. V.Yu. Filimonov, A.A. Sitnikov, A.V. Afanas'ev, M.V. Loginova, V.I. Yakovlev, A.Z. Negodyaev, D.V. Schreifer, V.A. Solov'ev, Microwave assisted combustion synthesis in mechanically activated 3Ti + Al powder mixtures: Structure formation, Int. J. Self-Propag. High-Temp. Synth., 2015, vol. 24, no. 4, pp. 210-214.
