effect of hardening temperature on the structural-morphological characteristics of metal cements based on mechanosynthesized copper compounds
N.Z. Lyakhov1, P.A. Vityaz2, S.A. Kovaleva2, T.F. Grigoreva1,
V.G. Lugin3, A.P. Barinova1, S.V. Tsybulya4
1 Institute of Solid State Chemistry and Mechanochemistry SB RAS,
630128 Novosibirsk, Kutateladze str., 18.
2 United Institute of Mechanical Engineering NAS, Minsk, Belarus
3 Belarussian State Technological University, Minsk, Belarus
4 G. K. Boreskov Institute of Catalysts SB RAS, Novosibirsk, Russia
Introduction
Metal cements may be used in many branches of industry due to good adhesion to the materials of different types (glass, ceramics, metals etc.) and the metal character of thermal and electric conductivity. The formation of metal cements occurs through the interaction of copper (nickel) alloys with liquid metals and alloys. Interactions of a solid metal with liquid one, in particular copper with gallium, are known [1, 2] to have diffusion character; they are substantially affected by temperature and the area of contact between the reagents.
The use of mechanically synthesized copper compounds allows one to increase the contact surface between the components and to introduce doping elements (Bi, In) that improve wettability during gluing and the strength properties of the alloys to be formed. This causes a change of the kinetics of interaction between a solid metal and a liquid one due to the acceleration of diffusion processes and due to the formation of additional phases.
The goal of the present work is investigation of the effect of hardening temperature on the structural-morphological characteristics of metal cements obtained on the basis of Cu/Bi mechanocomposites and supersaturated solid solutions Cu(In).
Methods and materials.
Copper powder PMS-1 (GOST 4960–75), granulated bismuth (TU 6-09-3616–82), indium (GOST 10297–94) were used in the work. Mechanical activation of the powders was carried out for 15 min in the high-energy ball planetary mill AGO-2 with water cooling, in argon atmosphere (cylinder volume: 250 cm3, ball diameter: 5 mm, loaded wt. 200: g, the weighed portion of the sample under treatment: 10 g, the frequency of rotation of the cylinders around the common axis: about 1000 r.p.m.). Mechanocomposites having the composition Cu 10 wt.% Bi, solid solutions Cu-12 wt.% In were obtained [3]. Diffusion-hardening alloys were prepared by mixing the mechanosynthesized copper compounds with gallium melt, followed by exposure at a temperature of 20 C during the whole process of alloy formation. To study the effect of temperature on the structure and morphology of metal cements, hardening was carried out at 90 С, 120 С and 160 С.
Surface examination was carried out with the NT-206 atomic force microscope (Microtestmachines, Gomel) using standard commercial V-type probes NSC11 (Mikromasch) in the contact mode.
The structure of the resulting samples was studied using Mikro 200 optical microscope and high-resolution scanning electron microscope (SEM) MIRA\TESCAN with an attachment for micro-X-ray spectral analysis (MXSA). The diameter of the electronic probe was 5.2 nm, excitation region was 100 nm. Images were obtained in the mode of recording secondary and backward scattered electrons, which allowed us to investigate the distribution of chemical elements over the surface and to establish its composition non-homogeneity.
The phase composition of powders after mechanical activation and the final products of their interaction with liquid gallium were determined with the help of X-ray diffraction techniques. X-ray structural analysis and semi-quantitative examination of the products were carried out with the D8 Advance Bruker diffractometer (Germany) by means of powder X-ray diffraction in the θ-2θ configuration with a step of 0.1°. Phase identification was performed using the diffraction patterns recorded in CuKα radiation (1.54051 Å).
Calorimetric measurements were carried out with Netzsch STA 409 PC/PG instrument in argon atmosphere, in a crucible made of Al2O3, within the temperature range from room temperature up to 290 C, with the heating rate of 20 °/min.
Results and discussion
It was established in the previous diffraction studies of alloy formation dynamics in Cu/Bi + Ga and Cu(In)+Ga that the formation of new phases takes place within a broad time interval. During the interaction of Cu/Bi mechanocomposite in Bi that is insoluble in copper and in gallium, the formation and crystallization of the intermetallic compound CuGa2 and bismuth take place simultaneously [4].
For the case of Cu(In) solid solution in which the doping element is soluble in gallium, the formation of the phase of solid solution of indium has an incubation period of about 210 minutes which is determined by its concentration in the system with gallium [5].
The interaction processes are described with the following chemical reactions:
Cu/Bi + 2 Ga → CuGa2 + Bi
Cu(In) + 2 Ga → CuGa2 + In(Ga)
1. Effect of the temperature of interaction of Cu/Bi mechanocomposites with liquid gallium on the structure and morphology of the formed metal cements
It is known that the resulting mechanocomposites are nanosized copper surrounded by a thin bismuth layer [6]. Bismuth is mainly composed of the particles less than 5 nm in size.
According to the data of AFM topography, the size of mechanocomposite particles is 150÷250 nm (Fig. 1).
Fig. 1. Mechanocomposite Cu + 10 wt.% Bi after activation for 15 min:
a – SEM image, b – AFM, c – TEM.
At first, we studied the interaction of Cu/Bi with liquid gallium at room temperature.
The X-ray structural analysis of the resulting cement, carried out after the interaction for 4 and 48 hours, showed that the size of the crystallites of the intermetallic compound increases from ~ 200 nm to ~ 550 nm. The size of bismuth crystallites increases up to 100 nm. It should be noted that this is accompanied by a decrease in the size of copper crystallites down to ~ 10 nm. The final phase composition is determined as CuGa2, Bi and unreacted copper (Fig. 2).
Fig. 2. Diffraction patterns of the product of interaction Cu 10 % Bi + Ga
Figure 3 shows the high-resolution SEM images of the microstructure of the surface of the final interaction product. The SEM image of sample surface after hardening without the mechanical treatment of the surface is shown in Fig. 3a. The image of the surface obtained in the backward scattered electrons after sample polishing is shown in Fig. 3b. Because bismuth is the heaviest element in this system, it will be distinguished by the maximal brightness in the SEM image.
The data obtained by means of microscopy show that the structure of the surface of final product is facetted tetragonal crystals СuGa2 with the size up to 4 μm. Bismuth is localized at the faces of crystals and at the boundaries of CuGa2 grains as disperse formations 70-250 nm in size, and also forms separate grains with a size up to 10 μm.
a b
Fig. 3. Topography of the surface of CuGa2 +Bi alloy after the interaction for 48 hours: a – SEM image of non-polished sample in direct electrons; b – SEM image of the polished sample in backward-scattered electrons.
The use of AFM allowed us to study the microstructure of facetted tetragonal CuGa2 crystals. The presence of screw dislocations in them may be stressed; as a result, the crystalline layer grows by winding continuously on itself, so the step takes the shape of a spiral (Fig. 4). The layer-by-layer growth of crystallographic facets should also be mentioned. The edges of incomplete layers, or steps, move along the facet while they grow. The step height, that is, the thickness of the depositing layer, varies within the range 4 to 200 nm. The appearance of high growth steps may cause trapping of the melt drops and precipitation of insoluble bismuth admixture on the surface of steps of the growing crystals, which is indeed observed in Fig. 4 b. Bismuth is adsorbed on facets, steps and along the grain boundaries.
It should be stressed that the growth of faceted crystals requires special conditions: supersaturation or supercooling of the mother medium, small number of appearing nuclei. We suppose that the local thermal supercooling arises as a consequence of the chemical interaction of copper with gallium melt on the interface between the solid phase and the liquid one, with the formation of chemical compound CuGa2 with crystallization temperature higher than the temperature of the melt. The conditions of substantial supercooling are created for this compound, so its crystallization starts. In this process, bismuth particles get released into the melt. Thee particles are insoluble in liquid gallium and may act as the centres of crystallization, and also they may brake down the growth of intermetallide particles by getting adsorbed on their surface. The latent heat of melting released during crystallization raises the temperature of the melt (so, gallium remains in the liquid state during reaction at 20 C) and decreases the degree of overcooling thus creating the conditions for the growth of larger facetted intermetallide crystals from the melt.
а / bFig. 4. AFM image of the surface of resulting alloy CuGa2 + Bi:
а - Torsion-image of bismuth on facets and growth steps of CuGa2 (the contrast is formed due to the difference in tribological characteristics of the phases of intermetallide and bismuth); b – layered spiral growth of CuGa2 crystals along the screw dislocation (marked with arrows). The upper part shows a scheme of crystal growth along the screw dislocation and the shape of the step formed in spiral growth [7].
At room temperature, the final product of the interaction of Cu/Bi mechanocomposite with liquid gallium is a matrix composed of CuGa2 intermetallide particles 1–4 μm in size, with bismuth particles distributed in it (from 70 to 250 nm), which form local agglomerations up to 10 μm in size.
X-ray studies of the alloys obtained at hardening temperature of 90 and 120 C showed that an increase in temperature to 120 C does not affect the phase composition. Similarly to the case of room temperature, the product is composed of intermetallide CuGa2 (PDF-2 No. 25-0275), bismuth (PDF-2 No. 44-1246) and residual copper (PDF-2 No. 04-0836) (Fig. 5).
Fig. 5. Diffraction patterns of CuGa2 + Bi samples obtained at temperature 40 (a), 90 (b) and 120 (c) C. Unmarked peaks relate to CuGa2 intermetallide.
With an increase in the interaction temperature, the lattice parameters of copper and CuGa2 phases remain almost unchanged. The size of copper crystallites is about 35 nm. Bismuth undergoes temperature-caused changes. An increase in the size of bismuth crystallites from 100 nm at 20 C to 180 nm at 90 C and to more than 500 nm at 120 C.
Alloys obtained by mixing the Cu/Bi mechanocomposite with liquid gallium have a composite structure after hardening. Their structure may be described as an intermetallic shell with the unreacted part of copper in its centre. The СuGa2 intermetallide has a shape of faceted tetragonal crystals up to 4 μm in size. With an increase in reaction temperature to 90 C the size of het particles of intermetallic compund increases to 6-8 μm and remains almost the same at a temperature of 120 C. In the lateral contrast mode, the facets of crystals obtained at 90 and 120 C exhibit local accumulations of bismuth, as well as substantial deformation distortions of crystals, due to the arising stretching strain in the crystal in the direction <001> (Fig. 6). Intermetallide crystal starts to have layered structure. The facets of the intermetallide obtained at elevated temperatures also exhibit deformation distortions that are likely connected with bismuth adsorption on the facets. The appearance of these lines is due to the development of "local" fluidity. They arise in the cases when the material possesses a distinct yield point; even insignificant concentration of strain promotes the appearance and development of these figures [8]. Change of the straight character of the glide lines is likely to be connected with the effect of boundary volumes, intra-grain structural strain caused by differences in the volumes of the intermetallide and bismuth, as well as by glide in different systems and with the transition from one system to the other.
/ а/ b
Fig. 6. AFM images of CuGa2 + Bi alloys obtained at a temperature of 90 (a) and 120 (b) С
аb
c
Fig. 7. Optical images of the structure of CuGa2 + Bi alloys obtained at 20 (a), 90 (b) and 120 (c) С.
Metallographic investigation of the alloy surface after polishing (Fig. 7) showed that the number of macrodefects, such as pores and discontinuity flaws, decreases with an increase in crystallization temperature. Microhardness of the intermetallide increases from HV 70 to 125.
Investigation of the distribution of chemical elements over the sample by means of SEM involving X-ray spectral analysis revealed nonuniformity of the distribution of insoluble bismuth.
Bismuth is not observed in the regions with the intermetallic compound, which may be connected with the fine distribution of disperse particles over the boundaries of the intermetallide. Local accumulations of bismuth up to 10 μm in size are observed mainly in the sites where macrodefects (pores, grain boundaries) get accumulated. With an increase in the temperature of interaction up to 120 °С, the number of local bismuth accumulations decreases but their size increases to 20 μm (Fig. 8).
а / bFig. 8. SEM images (in backward scattered electrons) of CuGa2 + Bi alloy. Hardening temperature: а – 20 С, b – 120 C.