Autowave Propagation of Exothermic Reactions in Ti–Al Thin ...

Doklady Physical Chemistry, Vol. 381, Nos. 1–3, 2001, pp. 283–287. Translated from Doklady Akademii Nauk, Vol. 381, No. 3, 2001, pp. 368–372.Original Russian Text Copyright © 2001 by Grigoryan, Elistratov, Kovalev, Merzhanov, Nosyrev, Ponomarev, Rogachev, Khvesyuk, Tsygankov.PHYSICALCHEMISTRYAutowave Propagation of Exothermic Reactionsin Ti–Al Thin Multilayer FilmsA. E. Grigoryan*, N. G. Elistratov**, D. Yu. Kovalev*, Academician A. G. Merzhanov*,A. N. Nosyrev**, V. I. Ponomarev*, A. S. Rogachev*, V. I. Khvesyuk**, and P. A. Tsygankov**Received August 6, 2001Multilayer nanofilms containing hundreds or thousandsof alternating layers, each being 1–1000 nmthick, are quite promising for creating new materialsand coatings on their basis. They exhibit unusual properties:for example, the hardness of a multilayer filmcan considerably exceed the hardness of each individuallayer and approach the hardness of diamond [1]. Itwas discovered relatively recently that, in some multilayernanofilms, autowave processes of the gaslesscombustion type can occur, and the velocity of suchreaction waves reaches several meters per second [2].Self-sustaining reaction waves in films containinghundreds of alternating nanolayers have already beendetected in the Al–Monel 400 [2] and Nb–Si [3] systems.The laws of the propagation of such waves and themechanisms of the formation of the reaction productsare, as yet, little studied.Previously, gasless combustion was explored inlayer systems consisting of alternating Ni and Al foils,each being 50 µm thick [4, 5]. The combustion velocitywas 0.37 cm/s, and the reaction involved diffusionthrough the product layer and dissolution of nickel inliquid aluminum. Combustion waves were alsoobserved in two-layer films obtained by electrodeposition[6] or vacuum metal vapor deposition [7], wherethe layer thickness was 0.1–200 µm. Note that layermodels of a reaction medium are widely used in thecombustion theory for analyzing autowave processes inheterogeneous media [8, 9]. Analytical expressions forthe velocity of combustion of layer systems were alsoobtained and compared with experimental data [2, 5].In this paper, we experimentally studied gaslesscombustion in Ti/Al multilayer films and the structureformation in the reaction products. We discovered apreviously unknown phenomenon of “inheritance” ofthe predominant crystallographic orientation of the initialreactants by the gasless combustion products.Alternating Ti and Al layers were deposited onto acooled (no higher than 50°ë) polished stainless-steel* Institute of Structural Macrokinetics and Problemsof Materials Science, Russian Academy of Sciences,Chernogolovka, Moscow oblast, 142432 Russia** Bauman Moscow State Technical University,ul. Vtoraya Baumanskaya 5, Moscow, 107005 Russiasupport by vacuum magnetron sputtering according toa published procedure [10]. The microstructure period(i.e., the Ti/Al bimetallic layer thickness) was 100–110 nm, and the total film thickness after deposition of150 double layers was 16 ± 2 µm. The Ti and Al depositionconditions were chosen so that these elements inthe sample were in a specified molar ratio. Samples ofthe Ti/Al and Ti/3Al compositions were studied. Metallographicanalysis showed that the initial samples ofboth compositions were almost poreless and were characterizedby a good contact between Ti and Al layers.The measured electrical resistivities of films (by thefour-electrode method parallel to the layers) were severalfoldhigher than the values that should have beenobserved for bulk samples of the same compositions.The expected electrical resistivity was calculated fortwo (Ti and Al) parallel-connected conductors, whosecross sections were proportional to the volume fractionsof the metals in a multilayer system. The expectedelectrical resistivities for the Ti/Al and Ti/3Al conductorsproved to be 5.24 and 3.56 µω cm, respectively.The measured electrical resistivities of the Ti/Al andTi/3Al multilayer films were 22 ± 1 and 10 ± 2 µω cm,respectively. Estimation of the effect of the interlayerboundaries on the electrical resistivity by the Sondheimertheory [11] for a 50-nm-thick layer of aluminum(as for the material of the Ti–Al pair with the best conductionproperties) showed that the electrical resistivityin such films is 5–10 times higher than that of “thick”samples, which agrees well with the obtained experimentaldata. Thus, the interlayer boundaries substantiallyaffect the electrical resistivity of multilayer materials.Figure 1a presents an example of the X-ray diffractionpattern of the initial film. One can see a single highpeak, which is the superposition of the diffraction linesof Al(111) and Ti(002). The character of the X-ray diffractionpatterns definitely indicated that, along the filmplane, there are layers of hexagonal close packing of Tiatoms and face-centered cubic close packing of Alatoms. As is known, such an atomic arrangement isoften observed in vapor condensation or gas-phase deposition.For determining the structural type in greaterdetail, the initial films were crushed to powder. TheX-ray diffraction patterns of such pseudo-powders0012-5016/01/0011- 00283$25.00 © 2001 åÄIä “Nauka /Interperiodica”

284GRIGORYAN et al.(a)(b)K β(111)TiAl(103)Ti 3 AlK β(100)Ti 3 Al (111)TiAl(101)Ti 3 Al(002)TiAl(202)TiAl(102)Ti 3 Al(202)TiAl(113)TiAl(311)TiAl(103)Ti 3 AlK β(111)Al, (002)TiK β(100)Ti(200)Al(101)Ti (111)Al,(002)Ti(102)Ti(220)Al(110)Ti(103)Ti(c)(d)(112), (103)TiAlK 3 ,β(111) TiAl(004) TiAl 3(002) TiAl(200) TiAl 3(e)(204) TiAl 3(220) TiAl(116) TiAl 3(112), (103)TiAl 3 ,(111) TiAlK β(004),TiAl 3 ,(002)TiAl(200)TiAl(200)TiAl 3(f)(204)TiAl 3(202)TiAl(220)TiAl 3(116)TiAl 330 40 50 60 70 80 30 40 50 60 70 802θFig. 1. Typical X-ray diffraction patterns of (a) Ti/Al multilayer film, (b) Ti/Al multilayer film crushed to powder, (c) burned Ti/Almultilayer film, (d) burned Ti/Al multilayer film crushed to powder, (e) burned Ti/3Al multilayer film, and (f) burned Ti/3Al multilayerfilm crushed to powder.exhibited peaks of all Ti and Al atomic planes (Fig. 1b).Thus, the initial film consisted of crystalline layersretaining the same short- and long-range orders as thosein bulk crystals of titanium and aluminum. The diffractionlines were somewhat broadened, which was likelydue to a high concentration of structural defects.For studying the gasless combustion, the multilayerfilm was removed from the support and cut into5 × 20-mm strips, which were fixed into mica clampsand were placed into a vacuum of 10 –3 Pa. The reactionwas initiated at one of the strip ends by a short (≈0.5 s)heat pulse produced by a thin tungsten wire. Figure 2demonstrates a typical steady-state self-sustainingwave as recorded by high-speed (500 frames per second)micro video recording. For achieving a steadystatecombustion of Ti/Al samples, they had to be ini-DOKLADY PHYSICAL CHEMISTRY Vol. 381 Nos. 1–3 2001

AUTOWAVE PROPAGATION OF EXOTHERMIC REACTIONS 2854 mmFig. 2. Combustion wave in Ti/3Al multilayer film.Glow intensity, arb. units600500400433002001 21000 5 10 15Distance, mmFig. 3. Glow intensity profiles for waves of combustion of (1) Ti/Al multilayer film at an initial temperature of T 0 = 20°ë (the wavedies out near the ignition point), (2) Ti/Al multilayer film at T 0 = 100°ë, (3) Ti/Al multilayer film at T 0 = 200°ë, and (4) Ti/3Almultilayer film at T 0 = 20°ë.tially heated. This was performed by a molybdenumfoilribbon radiator positioned parallel to a sample at adistance of 2–3 cm. The temperature was controlledwith a thermocouple. Samples at an initial temperatureof 100°ë burned at a velocity of ~5 cm/s. Heating to200°ë increased the combustion velocity to 20 cm/s.Heating to 350–400°ë caused a self-ignition: the reactionwas initiated locally before application of an ignitingheat pulse, and the gasless combustion front propagatedat a velocity of about 26 cm/s. An increase in thepower and the duration of an igniting heat pulse appliedto unheated samples (at an initial temperature of 20°ë)gave rise to a combustion wave, which propagated fromthe ignition point in a pulsed mode and died out, havingtraveled a distance of 1–1.5 cm. The existence of sucha wave was apparently possible only in a foil regionheated by the igniting wire. When the combustion frontarrived at unheated regions, it died out. Ti/3Al samplesreacted without preheating, and the reaction wavevelocity was 150 cm/s. Figure 3 presents the glowintensity profiles for different combustion waves. Asone can see from Figs. 2 and 3, the width of the front(the heating and reaction zone) was 1–2 mm. In Ti/Alsamples, cooling began immediately behind the front;and in Ti/3Al samples, there was quite a wide regionwhere the matter continued to burn.All samples that burned in the steady-state moderetained their sizes and shape. In samples burned in theunsteady-state mode, folds formed. The X-ray diffractionpatterns of burned foils in Figs. 1c and 1e showedthat the sample texture (the orientation of atomic layers)persisted in Ti/Al samples and was partially disturbedin Ti/3Al samples.The X-ray diffraction patterns of crushed burnedsamples exhibit diffraction lines of other crystallographicplanes (Figs. 1d and 1f). In both cases, theproduct is two-phase: in TiAl foils, the reaction yieldsTiAl and Ti 3 Al; and in Ti/3Al foils, the reaction productsare TiAl 3 and TiAl. The characteristics of the crystalstructure of the products are listed in the table. It isinteresting that additional vacuum annealing of burnedDOKLADY PHYSICAL CHEMISTRY Vol. 381 Nos. 1–3 2001

286GRIGORYAN et al.Crystal lattice parameters of combustion productsPhaseStructural type,space groupUnit cell parameters, ÅacTi 3 Al hexagonal 5.758 4.625P6^3/mmc (5.770) (4.620)TiAl tetragonal 3.999 4.065P4/mmc (3.976) (4.049)TiAl 3 tetragonal 3.885 8.593I3/mmm (3.854) (8.584)Note: Parenthesized are ASTM standard values.foils for 1 h at 1000°ë caused no substantial changes intheir composition.The most intriguing phenomenon, which, to ourknowledge, has not been described in the literature, isthe “inheritance” of the predominant crystallographicorientation of the initial reactants by the gasless combustionproduct. This phenomenon is likely to be possiblein systems where the atomic radii of the reactantsand, consequently, the interatomic distances along aclose packing plane are similar, i.e., where there is goodcrystallographic matching between the reactants. AsFig. 4 shows, crystallographic matching takes place forthe Ti–Al system. The closely packed planes Ti(002),Al(111), TiAl(111), Ti 3 Al(002), and TiAl 3 (112) differonly in the relative fractions of Ti and Al atoms and inthe order of their arrangement. Therefore, one canassume that the formation of the phases of the productin Ti/Al samples occurs by the solid-phase cross diffusionof atoms along a normal to a close packing plane(across the layers) without disturbing the orientation ofthese atomic planes. Knowing the reaction zone width l(Fig. 3), the layer thickness d, and the combustionvelocity U from the experiment, one can approximatelyestimate the diffusion coefficient D. Assuming that thereaction occurs through atomic diffusion and equatingthe characteristic diffusion time τ D = d 2 /4D to the reactiontime τ R = l/U, we obtainD ≅ d 2 U/4l.The substitution of the experimental values d ≅ 60 nm,U = 30 cm/s, and l ≅ 1 mm gives D ≅ 3 × 10 –10 cm 2 /s,which corresponds to the solid-phase diffusion, particularlyat high concentration of structural defects. However,the front glow intensity suggests that the combustiontemperature is definitely higher than the meltingpoint of aluminum. For this fact to be consistent withthe preservation of the orientation of closely packedatomic planes, one has to assume that either the atomicordering is retained in a thin melt layer or the productforms on the basis of the solid titanium structure. In thelatter case, the diffusion of aluminum into titaniumand/or the growth of solid product layers at the interlayerboundaries should leave the titanium dissolutionin liquid aluminum behind. The comparatively weakeffect of the dissolution is also explained by the lowsolubility of titanium in liquid aluminum: at 1000°C,the saturation concentration does not exceed 3–4 wt %.In Ti/3Al samples, the volume content of the meltis higher, which apparently enables the nucleation andgrowth of product grains, whose crystal structure isoriented differently than the initial layers. The velocityof combustion of samples of this composition ishigh (1.5 m/s), and the reaction zone width reaches5 mm; therefore, the diffusion coefficient is estimatedat D ~ 3 × 10 –9 cm 2 /s. The diffusion coefficients of aluminumatoms through titanium up to a temperature of 920 Kare available in the literature [12]. Extrapolating thesedata to higher temperatures, one can approximatelyTi (002) Al (111)TiAl (111) TiAl 3 (112) Ti 3 Al (002)Fig. 4. Closely packed atomic planes in the phases of the titanium (black circles)–aluminum (gray circles) system.DOKLADY PHYSICAL CHEMISTRY Vol. 381 Nos. 1–3 2001

AUTOWAVE PROPAGATION OF EXOTHERMIC REACTIONS 287estimate the diffusion coefficients at the adiabatic combustiontemperature (1520 K). Estimates showed that, inthe case of the formation of the TiAl 3 phase at the boundary,the diffusion coefficients of aluminum atomsthrough titanium at this temperature is 1.2 × 10 –18 cm 2 /s;and in the case of the formation of a solid solution, it is3 × 10 –15 cm 2 /s. The diffusion rate in our experimentsproved to be several orders of magnitude higher thanthe diffusion rate in bulk samples. This phenomenon islikely to be due to a large number of vacancies andother structural defects.Thus, in this paper, we experimentally studied somefeatures of the gasless combustion in multilayer filmsand discovered the phenomenon of “inheritance” in thecrystallographic orientation of the initial nanolayers bythe combustion product.ACKNOWLEDGMENTSWe thank A. E. Sychev for his help in the organizationof joint investigations. This work was supported bythe Russian Foundation for Basic Research (projectnos. 01–03–33017 and 00–15–97370) and the Scientificand Technical Program of the Ministry of Educationof the Russian Federation (no. 001/06.01.16).REFERENCES1. Yashar, P.C. and Sproul, W.D., Vacuum, 1999, vol. 55,pp. 179–190.2. Barbee, T.W. and Weihs, T., US Patent 5538795, 1996.3. Reiss, M.E., Esber, C.M., Van Heerden, D., et al., Mater.Sci. Eng., 1999, vol. 261, pp. 217–222.4. Shcherbakov, V.A., Shteinberg, A.S., and Munir, Z.A.,Dokl. Akad. Nauk, 1999, vol. 364, no. 5, pp. 647–652[Dokl. Chem. (Engl. Transl.), vol. 364, nos. 4–6,pp. 34–38].5. Shcherbakov, V.A., Shteinberg, A.S., and Munir, Z.A.,Combust. Sci. Tech. 2001. (http://xxx.lanl.gov/abs/physics/0107026).6. Vadchenko, S.G., Bulaev, A.M., Gal’chenko, Yu.A., andMerzhanov, A.G., Fiz. Gorenia Vzryva, 1987, vol. 23,no. 6, pp. 46–56.7. Myagkov, V.G. and Bykova, L.E., Dokl. Akad. Nauk,1997, vol. 354, no. 6, pp. 777–779 [Dokl. Phys. Chem.(Engl. Transl.), vol. 354, nos. 4–6, pp.188–190].8. Aldushin, A.P. and Khaikin, B.I., Fiz. Gorenia Vzryva,1974, vol. 10, no. 3, pp. 313–323.9. Hard, A.P. and Phung, P.V., Combust. Flame, 1973,vol. 21, no. 1, pp. 77–89.10. Elistratov, N.G., Nosyrev, A.N., Khvesyuk, V.I., andTsygankov, P.A., Prikl. Fiz., 2001, no. 3, pp. 8–12.11. Komnik, Yu.F., Fizika metallicheskikh plenok (Physicsof Metal Films), Moscow: Atomizdat, 1979.12. Pouliquen, J., Offret, S., and Fouquet, J., C. R. Acad.Sci., 1972, vol. 274, pp. 1760–1763.DOKLADY PHYSICAL CHEMISTRY Vol. 381 Nos. 1–3 2001