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31SHS IN A MELT OF Al-TiC(B) COMPOSITES
R. M. Nikonova*", A. V. PanteleyevaA, and V. I. Lad'yanov"
aUdmurt Federal Research Center, Ural Branch, Russian Academy of Sciences, Izhevsk, 426067 Russia
bUdmurt State University, Izhevsk, 426034 Russia *e-mail: rozamuz@udman.ru
DOI: 10.24411/9999-0014A-2019-10110
There has been much research in recent years focused on the development of aluminium matrix composites because of their high specific strength, modulus of elasticity, wear resistance, hardness, etc. Especially popular are aluminium matrix composites with such ceramic strengthening phases as SiC, AhO3, TiB2, and TiC. Titanium carbide for example attracts the researchers' attention with its high hardness and modulus of flexibility, low density and uniform distribution of TiC particles in an aluminium matrix at the expense of good wettability by liquid aluminium. Apart from being a strengthening phase TiC serves also as a center of crystallization in aluminium, thus assisting grain refinement in a composite alloy [1-5].
A promising method of composites preparation is self-propagating high-temperature synthesis (SHS) [6-8]. A SHS method in melt, which is an in situ method, is a SHS variant that combines a traditional casting practice and self-propagating high-temperature synthesis. During the process of SHS in melt strengthening phases are synthesized from initial components or their compounds directly in melt.
Apart from the benefit of one-stage preparation of composites, this provides their thermal stability, tight contact, and good adhesion between the matrix and the strengthening phase. Among the advantages of the method there are low energy consumption and manufacturing cost, high productivity, possibility of controlling structure and properties of a product synthesized [6].
The purpose of the present research is modification of aluminium alloy by titanium carbide and titanium diboride using SHS in melt.
The research objects were pure aluminium and Al-TiC and Al-TiB obtained by SHS in melt. For better wetting and hence reactivity 1 g of cryolite was added.
Quantitative distribution of introduced elements was determined on a GDA-650HR spectrometer using glow discharge method, which also provided metallographic etching of the samples. The sample microstructure was studied using a Philips SEM-515 scanning electron microscope (SEM) equipped with a Genesis 2000 XMS energy-dispersive X-ray spectroscopy (EDS). Phase composition was determined using a Dron-6 diffractometer with CoKa radiation. Microhardness was measured using a PMT-3 tester under a load of 100 g.
According to EDS results the composite Al-TiC contained up to 4% titanium and 0.9% carbon; the composite Al-TiB contained up to 12% titanium and 2.8% boron. XRD analysis (Fig.1) demonstrated the presence of titanium carbide in the Al-TiC alloy apart from pure aluminium in quantity of ~ 3.5%. This evidences the formation of TiC during SHS. In the Al-TiB sample, two strengthening phases, AbTi and TiB2, (4% of each phase) were distinguished. The XRD pattern also contains a carbon line, which can be attributed to the method of alloys preparation (graphite heater was used). As we see from the surface analysis (see Fig. 2) the grain size in aluminium matrix in case of Al-TiC (Figs. 2c, 2d) is 20-35 ^m; the size of inclusions is no more than 20 ^m. In case of Al-TiB (Figs. 2e, 2f) the grain size is 10-20 ^m. Keeping in mind the grain size of 70 ^m in initial aluminium we can state a 2-3 times decrease in grain size in Al-TiC and 3-5 times in Al-TiB.
■SHS 2019
Moscow, Russia
Fig. 1. XRD pattern of composite Al-TiC and Al-TiB.
Fig. 2. Surface of (a, b) Al, (c. d) Al-TiC, and (e, f) Al-TiB after etching. (a, b, c) x150, (d, e, f) x600.
The superposition of concentration maps obtained by SEM and EDS (Fig. 3) demonstrated (a) in case of Al-TiC the presence of titanium carbide on dark areas of aluminium; (b) in case of Al-TiB fairly uniform distribution of boron, with occasional inclusions of titanium, aluminium and boron ascribed to TiB2 of aluminium matrix. Based on the results from [9], needle-like elements represent Al3Ti-phase.
Fig. 3. Concentration maps for (a) Al-TiC and (b) Al-TiB.
Figure 4 shows that the formation of TiC in the composite Al-TiC and AbTi and TiB2 in Al-TiB results in the increased microhardness of metal. For pure aluminum it is 75 kgf/mm2, and for Al-TiC it makes up 116 kgf/mm2. In Al-TiB an increase in microhardness makes up as much as 98%. Such a difference in microhardness of the composites is attributed to a two times bigger content of the strengthening phase in Al-TiB as compared to Al-TiC.
Thus, by means of SHS in melt composites Al-TiC and Al TiB were obtained. XRD analysis, SEM, and EDS demonstrated that an "in situ" technology applied lead to the formation of TiC, AbTi, and TiB2 phases that modified aluminium. It was also shown that SHS in the presence of the additives (Ti + C) (Ti + B) made it possible to decrease grain size from 70 to 20-10 ^m and to increase microhardness by 55-98 %.
Fig. 4. Microhardness for Al, Al-TiC, and Al-TiB.
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