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18STRUCTURAL-PHASE TRANSFORMATIONS IN LOCALIZATION ZONES OF PLASTIC DEFORMATION OF Ti-Al COMPOSITE
T. V. Fadeev*", A. K. Abkaryan", G. M. Zeer", M. N. Volochaev", I. V. Nemtsev", and L. I. Kveglis"
aSiberian State University, Institute of Non-Ferrous Metals and Materials Science, Krasnoyarsk, 660025 Russia *e-mail: timur.fadeev.96@mail.ru
DOI: 10.24411/9999-0014A-2019-10038
The work is devoted to the study of physicochemical processes caused by the localization of plastic deformation, initiated by the dynamic load of the titanium-aluminum composite material. It is shown that a high concentration of tension during high-speed loading can cause the movement of plastic deformation waves and the solid-phase synthesis of AbTi intermetallic phases, when different types of structures can coexist within the same formula unit.
This study can contribute to the development of composite materials based on Ti-Al, which are used for the manufacture of gas turbine blades, hollow and ribbed welded structures for the aviation industry, etc. An important advantage of the materials used in aircraft, is their low density. It allows to obtain composites with a high level of specific strength. The Ti-Al system has been extensively studied. However, a number of unclear questions remain, namely: what intermetallic phases can form during plastic deformation; what are the conditions for the formation of Ti-Al intermetallic phases: AbTi2, AlnTi5, AhTi, AlTi3, AbTi, among which various types of structures (stable, metastable, virtual) can be realized within one formula unit?
Samples were obtained using standard equipment at the Lavrent'ev Institute of Hydrodynamics, SB, RAS. To study the structure of transition zones between titanium and aluminum, scanning electron microscopes with microprobe TM-3000 and JEOL 7000LV were used. Phase analysis was performed on a Bruker X-ray diffractometer in copper radiation.
Figure 1 shows (a) image of a template cut from composite Ti-Al sample after punching by a bullet and (b) SEM image of the zone of shear line (area 3). The interpretation of the diffraction spectrum showed that the differences between the experimental values of the interplanar distances d from the tabulated distances are 0.75% for Ti and 0.37% for Al. The chemical composition was controlled using a JEOL7001F scanning electron microscope with microanalysis. Figure 3 shows the SEM image of the site where the solid-phase reaction took place. The results of microanalysis are shown in Table 1.
(a) (b)
Fig. 1. (a) Image of template cut from composite Ti-Al sample after punching by bullet; (b) SEM image of the zone of shear line (area 3).
iSHS 2019
Moscow, Russia
Reflex 29, deg dexp,A a-Ti Al TiAl3
number d, A hkl d, A hkl d, A hkl
1 35.0 2.56 2.54 100 2.513 113
2 38.4 2.34 2.34 002 2.33 111 2.289 202
3 40.0 2.25 2.23 101
4 44.5 2.03 2.02 200
5 65.0 1.43 1.43 220 1.429 224
6 78.0 1.22 1.23 201 1.21 311 1.265 206
7 82.2 1.17 1.16 222 1.167 422
2-Theta - Scale
Fig. 2. X-ray diffraction pattern of the zone shown in Fig.lb.
400
300
200
100
30
40
50
30
70
80
Table 1. Elemental composition (wt %) corresponding to Fig. 3.
Spectrum_Al_Ti
(1) 0.62 99.38
(2) 0.77 99.23
(3) 1.24 98.76
(4) 14.89 85.11
(5) 30.23 69.77
(6) 76.67 23.33
(7) 64.88 35.12
(8) 52.87 47.13
(9) 88.23 11.77
(10) 98.63 1.37
(11) 100.00
(12) 100.00
(13) 100.00
The elemental composition of the solid phase reaction zone indicates the absence of other elements besides titanium and aluminum. The mixing zone is about 10 |im. According to the XRD pattern, AbTi intermetallic compounds with different crystal lattices are formed in the
mixing zone [1, 2] Such polymorphism is realized due to small atomic shifts in the bending zones of the crystal lattice [3].
In the zone of stress concentration during high-speed loading, high pressures develop. As a result, the strength properties of metals become insignificant. It is known [4] that upon welding waves with regular sinusoidal shape are formed on the contact surfaces, sometimes with vortex zones. We see such sinusoids in the zone of plastic shear in Fig. 1b.
For most solids, the shear modulus decreases beyond the limit of elasticity, and in rods of such materials, at sufficiently large deformations, not shock waves, but plastic waves arise.
According to [5], the relationship between the pressure difference P and the mass transfer rate V is the ratio obtained for plane elastic waves in the contact zone of various media: AP = pVc, where p is the density of material; V is the speed of interacting elements; c is the speed of sound in the medium.
For Ti, the speed of sound is 4140 m/s, the density is 4.500 kg/m3, and the speed of a bullet flying through it is 735 m/s.
The pressure developing in the rupture zone under the action of a bullet is 13.7 GPa. This is the minimum load that can develop in the areas of localization of plastic deformation of our composite sample. For plastic deformation waves, this value is quite acceptable. Thus, we observe solid-phase synthesis with the formation of the TiAb phase in various structural modifications precisely in the zone of the shear band crossing the entire thickness of the sample. In addition to the product of a mechanochemical reaction, we see the excitation of plastic deformation waves in the stress concentration zone.
1. The structure and phase composition of the titanium-aluminum composite in the zone of plastic strain localization after high-speed dynamic loading are investigated.
2. Considerable crystal lattice distortions were found: shifts and bends, texture formation due to grain reorientation, as well as changes in Ti lattice parameters by 0.75%, for Al by 0.37%
3. The products of solid-phase synthesis with the formation of the phase TiAb in various structural modifications were found precisely in the zone of the shear band crossing the entire thickness of the sample. The emergence of polymorphism due to the curvature of the crystal lattice
4. On the interface of dissimilar metals, plastic deformation waves were detected in stress concentration zones.
The authors thank V.I. Mali (Lavrent'ev Institute of Hydrodynamics, SB, RAS) for providing the samples.
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