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12iSHS 2019
Moscow, Russia
EVOLUTION OF THE STRUCTURAL CONDITION OF REACTION POWDER MIXTURES OF THE Ti-Ni SYSTEM UNDER MECHANICAL ACTIVATION. THEORETICAL ESTIMATES BASED ON
EXPERIMENTAL STUDIES
O. A. Shkoda and O. V. Lapshin
Tomsk Scientific Center, Siberian Branch, Russian Academy of Sciences,
Tomsk, 634055 Russia e-mail: ovlap@mail.ru, O.Shkoda@dsm.tsc.ru
DOI: 10.24411/9999-0014A-2019-10159
An experimental study of the mechanical activation process (MA) of a powder mixture of nickel and titanium was carried out. It is revealed that the MA time has a significant impact on the physico-chemical and structural transformations of the crushed mixture. The reason for this effect is a change in the composition and structure of the layered agglomerates arising during the MA from the components of the mixture. The method of the inverse problem is used to estimate the kinetic constant characterizing the growth rate of the interfacial surface in agglomerates during the mechanical treatment of the powder mixture. Theoretical calculations of the layer thickness and the size of the interfacial surface in the agglomerate are carried out. Mechanical activation is the effective method of stimulation of chemical reactions, allowing to expand the possibilities of synthesis of inorganic materials [1]. For example, with the help of MA it is possible to synthesize low-energy compositions, which in traditional conditions of the organization of synthesis practically do not burn. To date, the main part of the experimental studies is devoted to the study of combustion characteristics and phase composition in the pre-MA condensed systems [2-5]. Much less work is paid to the study of the features arising in the reaction mixture at the first stage of mechanosynthesis during the preliminary MA.
As shown in [6, 7], the mechanical activation changes the morphology and microstructure of the initial powders, brittle substances are mainly crushed, and in the presence of plastic materials, layered agglomerates of various sizes are formed. Sizes can vary from a few nanometers to millimeters, depending on the composition and time of the MA. There is a constant breaking of the initial and already agglomerated particles and simultaneous connection of the newly formed surfaces into a layered agglomerate. The process of breaking and formation of new agglomerates during MA is carried out constantly.
In the layered agglomerates formed in this way, consisting of crushed initial reagents, their contact area increases and a high concentration of non-equilibrium defects and internal stresses is created. Also, there is a destruction of oxide layers and adsorbed films on powder particles, which are a diffusion barrier to interaction. Thus, the preliminary MA of the initial powders leads to a significant change in the structure of the powder mixture to the accumulation of the energy supplied and determines the nature of the subsequent combustion at the second stage of mechanosynthesis.
In this paper, we study the regularities of the formation of the internal structure of layered agglomerates formed during MA in the Ti-Ni system. The experiments used powders of titanium PTEM 1 and Nickel grade PNE 1. Mechanical activation of powder mixture of Ti-55.06 mass % Ni (stoichiometry TiNi) was performed in a planetary mill M-3 with a power density of 45 g in the environment of argon. The weight ratio of the powder to the weight of balls was 1:5. Duration was from 1 to 9 min. The total time of mechanical activation was collected in discrete periods of 30 s, between which there was a cooling period of 5 min. The
XV International Symposium on Self-Propagating High-Temperature Synthesis
structure and composition of the studied activated powder mixtures and combustion products were investigated by scanning electron microscopy (Philips SEM515) and optical metallography (Axiovert200m). Figure 1 shows the agglomerate formed after 2 min of MA. It can be seen that the agglomerate consists of sticking smaller pieces and layers.
Fig.1 Microphotography of appearance of layered agglomerate formed during MA.
Figure 2 shows the SEM images of the cross section of agglomerates after 2 min (a) and 3 min (b) of MA, respectively. Layers of light color belong to nickel, dark layers belong to titanium. With a further increase in the time of MA agglomerates constantly change their size and structure. To study the internal structure of the agglomerate, metallographic sections of layered agglomerates were made, on which a sequence of layers of titanium and Nickel powders is clearly visible.
(a) (b)
Fig. 2. Microphotography of sections of layered agglomerates formed after 2 min (a) and 3 min (b) of MA.
The thickness of the layers in the agglomerate, as well as the size of the agglomerate itself, undergo changes throughout the MA: in general dynamics, they are thinned, which contributes to the creation of a more developed interfacial surface in the agglomerate. Moreover, the differences in the layer structure of the agglomerate in the presented images are quite noticeable, although the difference between them is only 1 min of MA time.
An important aspect in experimental studies is the knowledge of effective kinetic constants that allow using the developed mathematical models to obtain preliminary prognostic estimates. In [8] a mathematical model describing the formation of layered structure in the agglomerates in the milling conditions of a binary mixture in energozatratno mill. There is also obtained the ratio, which is performed for small machining times, which determines the dependence of the thickness of the total layer, including two adjacent layers d of dissimilar components in the structure of the agglomerate, on the time of mechanical activation.
d 2
Nit
where K = kW/V is the kinetic constant; W is the power of the energy-stressed mill; V is the volume of the crushed mixture; k is the coefficient depending on the physical and chemical
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properties of the components involved in grinding and agglomeration; t is the time; |0Ti and |0Ni are the initial volume fractions of Ti and Ni components in the mechanically activated mixture. The inverse of the parameter d determines the specific interfacial surface in the agglomerate S = 1/d.
The straightening of the method of least squares of the experimental results allowed to calculate the part of the relation (1) parameter K = 0.263 (|m min2) for |0Ti = 0.62 and |0Ni = 0.38. The experimental study of the dynamics of the thickness of the layers of agglomerates in the conditions of mechanical activation of the powder mixture of titanium and nickel. It is revealed that with the increase of MA time there is a predominant decrease in the width of the layers of titanium and nickel in the agglomerate, the developed interfacial surface is formed in it.
The kinetic constant determining the formation of a layered structure in an agglomerate at MA in the Ti-Ni system is estimated. It is shown that the theoretical calculations of the dynamics of quantities d and S in the process of MA at the qualitative level correspond to the results of experiments carried out using the kinetic constant found.
1. T.F. Grigor'eva, A.P. Barinova, N.Z. Lyakhov, Mechanical-Chemical Synthesis in Metal Systems [in Russian], Parallel', Novosibirsk, 2008.
2. O A. Shkoda, L.G. Raskolenko, Int. J. Self-Propag. High-Temp. Synth., 2010, vol. 19, no. 2, pp. 128.
3. M.A. Korchagin, T.F. Grigor'eva, B.B. Bokhonov, et al, Fiz. Gor. Vzryva, 2003, vol. 39, no. 1, pp. 51-68.
4. E.A. Levashov, V.V. Kurbatkina, K.V. Kolesnichenko, Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metall, 2000, no. 6, pp. 61-67.
5. V.K. Smolyakov, V.I. Itin, N.N. Golobokov, et al, Fiz. Gor. Vzryva, 2005, vol. 41, no. 5, pp. 92-99.
6. N.G. Kasatskii, O. A. Shkoda, Fiz. Khim. Obrab. Mater, 2012, no. 5, pp. 71.
7. E.G. Avvakumov, Mechanical Methods for Activation of Chemical Properties [in Russian], Nauka, Novosibirsk, 1986.
8. O.V. Lapshin, V.K. Smolyakov, Khim, Fiz. Mezosk, 2013, vol. 15, no. 2, pp. 272-278.
