CC BY
13iSHS 2019
Moscow, Russia
SYNTHESIS OF POROUS MATERIAL FROM MIXTURES OF BORON, TITANIUM, AND NICKEL-CLAD ALUMINUM COMPOSITE POWDERS
(NPA75-80)
M. A. Ponomarev*" and V. E. Loryan"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
*e-mail: map@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10128
The aim of the study was to synthesize a porous composite material in one technological stage by the SHS method in Al-Ni-Ti-B system. The initial billets were samples layer-by-layer compacted from model mixtures consisting of titanium and boron powders, and composite Al-Ni granules, in which spherical aluminum particles were clad with nickel layer. In the previously studied powder mixtures of Ti-Al, Ni-Al, Ti-B containing large particles of the components, the initial heating of the billets up to 800-1000°C was required to initiate the combustion [1, 2]. In the model system under consideration, synthesis of Al-Ni-Ti-B is possible without preheating, since the chemical reaction between titanium and boron powders, being a "chemical ovens" toward more inert clad granules [3], is able to heat the granules and provide their chemical reaction [2]. Melting and spreading of the melt of the metal components contributes to the appearance of numerous pores of different sizes and morphology in the product. The stage nature of the synthesis is characterized by rapid appearance of boride matrix with subsequent filling of its pores with the melt of intermetallic compounds formed during the reaction in the granules, which leads to the appearance of the synthesis product with the microstructure close to the composite one [3].
Model mixtures of (Al + Ni) + (Ti + 2B) were used in the experiments. They included:
- amorphous boron (99.8 wt %, particle size db = 0.2-6 |m);
- titanium powder Ti(d1), grade PTS (average particle size dd1 ~ 120 |m; the content of chemical impurities in titanium: 0.08 wt % N, 0.05 wt % C, 0.35 wt % H, 0.4 wt % Fe + Ni, 0.10 wt % Si, 0.004 wt % Cl);
- composite clad powders with a size of40-160 |m made by hydrogen reduction under pressure in autoclaves, with the following compositions: (1) (Al + Ni(75)) aluminum covered with nickel NPA75 (25 wt % Al and 75 wt % Ni); (2) (Al + Ni(80)) aluminum covered with nickel NPA80 (20 wt % Al and 80 wt % Ni). The clad granules had an average diameter of d1 = 100±60 |m, d2 = 100±60 |m and a coating thickness of 10-20 |m. The particle morphology is shown in Fig.1.
Fig. 1. Shape and morphology of clad granules of NPA75 powder (a); cross-section (Wood's alloy) (b).
A mixture of Ti(Ji) + 2B (69 wt % Ti and 31 wt % B based on the final product of TiB2) was prepared. Mixtures of a(Al + Ni(75)) + (1-a)(Ti(d1) + 2B) and a(Al + Ni(80)) + (1-a)(Ti(Ji) + 2B) were made from Ti(d1) + 2B and (Al + Ni) clad particles (a is the mass fraction of clad particle in the mixture, the variation interval a = 0-0.8). When filling the cylindrical quartz moulds with the reaction mixtures, a consecutive portion compaction was used (quartz moulds length of 43mm, internal diameter of D = 4-4.4 mm, external diameter of Do = 7.8 mm, D/d1 ~ 40, D/d2 ~ 40) [4]. The compaction of each layer of the mixture was due to the pulse obtained from the striker of m = 684 g, falling from h = 43 mm. The height of the layers was 1200-1400 ^m. The billet contained 19-23 layers.
The synthesis was carried out in argon atmosphere (at 1 atm). The quartz mould and the plugs on its ends eliminated the sample resizing. The temperature in the synthesis wave was measured with a tungsten-rhenium thermocouples (type A, diameter of the seal with a protective layer of 0.25-0.35 mm) and sent by a temperature recorder QMBox 4050-8-1 to the computer ASUS-A52J. The burning rate was determined due to the shooting of the process with a camera Sony HDR-CX130E. Electron microscopic study was performed using SEM LEO 1450 VP, Carl Zeiss.
The synthesis in a(Al + Ni(75)) + (1-a)(Ti(d1) + 2B) samples proceeded in the combustion mode at a = 0-0.5, at the initial temperature T0 = 20°C. At a = 1 combustion did not occur, that is, the clad granules themselves could not react in the combustion mode. The average combustion rate (u) was ~ 3 cm/s at a = 0.4. The temperature profile in the combustion wave Tb = ft) is shown in Fig. 2.
10,0 105 11,0 115 f s
Fig. 2. Thermogram of a(Al + Ni(75)) + (1-a)(Ti(d1) + 2B) sample combustion at a = 0.4.
In thermograms, there are two temperature maxima (at a = 0.4), which are associated with the staging of the synthesis process. Melting in the warm-up zone of the most low-melting components of this mixture - first aluminum, and then nickel leads, first of all, to the beginning of a chemical reaction inside the clad granules. Upon reaching the melting point of titanium, a fast and strongly exothermic reaction begins between titanium and boron. Between the clad particles (Al + Ni), the interlayers of the Ti(d1) + 2B mixture have a thickness of 100-200 p,m, which is sufficient for the synthesis in them with a high combustion rate [4]. The first maximum of temperature (at Tb = 2580°C) in the thermogram is associated with this reaction. The second temperature maximum (at Tb = 2220°C) is associated with the reaction in the clad granules. In this case, the melt of nickel and its aluminides penetrates into the boride layer that has arisen around the granules. The product of synthesis has a developed porous structure and the open porosity of ~ 45-48%, the density of 2.6-2.8 g/cm3 (Fig. 3). Three types of the pore size are distinguished (Fig. 3):
- pores of round-shape of medium size (~ 100-160 |m in diameter), connected with each other by capillaries (diameter of ~ 10-30 |m), arising at the site of the clad granules after melt spread into the pores of the boride matrix, and irregularly shaped pores (~ 10-120 |m), which appeared
ISHS 2019 Moscow, Russia
at the site of Ti(d1) particles after melting and spreading of the melt into the surrounding boron [5, 6];
- small pores of round shape with a size of ~ 1-5 |m, arising from the interaction of the fine fraction of titanium powder Ti(d1) with boron; small pores are partially or completely filled with nickel and nickel aluminide at the stage of their spreading;
- pores of submicron size (< 1 |m), which appeared during phase formation.
Fig. 3. Macrostructure of synthesis product (a = 0.4): (a) cleavage, (b) section.
The microstructure of the product of synthesis around rounded pores is mainly fine-grained (Figs. 4b, 4c). On the inner surface of the pores, the matrix structure of titanium diboride crystallites is well distinguishable (Fig. 4a).
^t^.fy CIS* V m
llltv. Mag = 30.02 KX WD = 11mm SignalA = SE1 Date 12 Feb 2019 I I Urn | EHT = 16.00 kV Time :10:50:17 2um Mas= 8.04 K X WD = 16mm SignalA=QBSD Date :12 Feb 2019 | H I EHT = 15.00 kV Time :15:13:24
(a) (b)
20ym
Fig. 4. (a) Image of crystals on the surface of the pores, (b) microstructure (polished section) and (c) composition in points (wt %) in the boride matrix and intermetallic interlayers (cleavage structure) (a = 0.4).
The main phases found in the synthesis product are TiB2, Ni3Al, NiAl, and a small amount of Ni2B and Ni2B phases. The phase composition and distribution of elements in the synthesized product reflect the stages of the processes occurring in the combustion wave. Areas with the dark gray phase correspond to the increased content of boron and titanium (Fig. 4c, points S3 and £4) and TiB2 phase (according to XRD data). In the lighter areas (0.5-1.5 p,m) around the grains of the dark gray phase, the increased nickel and aluminum content (points S1 and S2) is recorded, which corresponds to nickel aluminides (according to XRD data, Ni3Al and NiAl).
Thus, the material having a developed porous structure and the composite structure of the solid phase according to the type of interpenetrating frameworks - ceramic (fine-grained) and intermetallic is synthesized into one technological stage.
1. M.A. Ponomarev, V.E. Loryan, N.A. Kochetov, A.G. Merzhanov, SHS in preliminary structured compacts: I. Ni-Al blends, Int. J. Self-Propag. High-Temp. Synth., 2013, vol. 22, no. 4, pp. 193-201.
2. M.A. Ponomarev, V.E. Loryan, A.S. Shchukin, A.G. Merzhanov, SHS in preliminary structured compacts: II. Ti-2B and Ti-Al blends, Int. J. Self-Propag. High-Temp. Synth., 2013, vol. 22, no. 4, pp. 202-209.
3. A.G. Merzhanov, Thermally coupled SHS reactions, Int. J. Self-Propag. High-Temp. Synth., 2011, vol. 20, no. 1, pp. 61-63.
4. M.A. Ponomarev, V.A. Shcherbakov, A.S. Shteinberg, Zakonomernosti goreniya tonkikh sloev poroshkovoi smesi titan-bor [Combustion patterns of thin layers of Ti-B powder mixture], Dokl. Akad. Nauk: Proceed. Russ. Acad. Sci., 1995, vol. 340, no. 5, pp. 642-645.
5. M.A. Ponomarev, V.E. Loryan, Synthesis of porous composite materials via combustion of a mixture of titanium, VT6 alloy, and amorphous boron powders, Inorg. Mater., 2018, vol. 54, no. 8, pp. 772-778.
6. M.A. Ponomarev, V.E. Loryan, Sintez kompozitsionnogo materiala v sisteme Al-Ti-B pri gorenii poroshkov titana, bora i plakirovannykh alyuminiem granul splava VT6, [Synthesis of composite material in Al-Ti-B system during combustion of titanium and boron powders and aluminum-clad granules of VT6 alloy], Perspek. Mater., 2019, no. 3, pp. 62-73.
