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28STRUCTURE AND PROPERTIES OF ALLOYED COMPOSITES Al-TiC FABRICATED BY SHS METHOD
A. R. Lutz*", A. P. Amosov", E. I. Latukhin", A. D. Rybakov", and S. I. Shipilov"
aSamara State Technical University, Samara, 443100 Russia *e-mail: alya_luts@mail.ru
DOI: 10.24411/9999-0014A-2019-10088
Aluminum-matrix composites (AMCs), strengthened by the dispersed phase of titanium carbide (TiC), are now increasingly used as structural materials, and therefore, the development and implementation of methods to ensure a high level of their mechanical properties is an urgent task of modern materials science. Analysis of studies on this type of AMCs shows that the increase in strength was initially achieved primarily due to an increase in the dispersion of the reinforcing phase up to the nano-level [1-3].
Recently, however, the number of publications devoted to the alloying of aluminum-matrix composites has increased dramatically. Of the alloying elements, copper is most often used in an amount of 4-5% [4-7]. Thus, 4.5% Cu is added to the base of Al-10% TiC alloy which allows to increase the tensile strength from 118 to 147 MPa and hardness from 61 to 94 HV [5].
In a number of other studies, the hardening of complexly alloyed traditional aluminum alloys by the titanium carbide phase is analyzed [8, 9]. For example, the additive of 2, 4, 6, 8 or 10 wt % TiC to the base of 6061 alloy (the analog of alloy AD33 which contains Mg, Si, Fe, Cu, Cr, Zn, Ti, Mn) allows to increase density, hardness and strength by 7.8, 20, and 19.55%, respectively [9]. At the maximum content of TiC phase (10%), a strength value of 160 MPa and a relative elongation of 9% is achieved.
Many authors note the positive effect of alloying elements on the synthesis process and the morphology of the target phase of titanium carbide. For example, it was found that with an increase in the silicon content in the aluminum base, the TiC particle size significantly decreases, and this leads to an increase in the wear resistance of Al-TiC composites [10]. There is also a positive effect of alloying elements Mg, Zn, Sn on the microstructure and strength properties of Al-TiC composites at a high content of the ceramic phase (up to 50 wt %) [12].
Doping of the matrix alloy with 1% Mo allows to increase wettability due to the formation of a molybdenum shell around the titanium carbide particle, and as a result, there is a more uniform distribution of the particles over the body of the composite material and increased ductility [12]. On the basis of these data, it becomes obvious the positive effect of alloying elements on the structure and properties of AMCs.
A number of successful studies on the fabrication of Al-TiC composites through the use of the self-propagating high-temperature synthesis (SHS) method in the aluminum melt was also recently conducted in Samara State Technical University (SSTU). The essence of the method is to conduct an exothermic reaction of the synthesis of the TiC hardening phase particles from the mixture of titanium and carbon powders (Ti + C) directly in the aluminum melt. During the development of the technological process, it was possible to fabricate a composite with nano and ultrafine phase of titanium carbide, which allowed to increase the strength properties from 110 to 202 MPa at a relative elongation of about 4% [13].
Further direction for research in SSTU was determined to study the effect of alloying the aluminum base on the structure and properties of Al-TiC composites. On the basis of comprehensive analysis, the following elements were chosen as alloying additives:
(1) copper (5%) is a part of the solid solution, forms a phase CuAh, when cooled, contributing to the dispersion hardening of the alloy;
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(2) manganese (2%) is a part of the solid solution, forms a phase MnAl6, when cooled, contributing to grain boundary hardening;
(3) nickel (4%) forms the eutectic phase of NiAb of globular shape, positively affecting the ductility and fracture toughness of the alloy.
This paper presents the results of a study on the fabrication of alloyed composites (Al-5% Cu-2% Mn)-10% TiC and (Al-5% Cu-4% Ni)-10% TiC when using SHS method. The following starting materials were used in the research: aluminum ingot of A7 grade; titanium powder TPP-7 (purity 97.9%, initial particle size < 240 |m); carbon technical P-701 (99.7%, < 0.15 |m); copper powder PMS-1 (99.5%); manganese powder MN-95 (99.0%); nickel powder PNE-1 (99.5%).
In order to activate the SHS reaction, the halide salt Na2TiF6 (99.0%) was added in a number of meltings in an amount of 5 wt % of the powder charge (Ti + C). The formation of matrix alloys of aluminum-copper-manganese and aluminum-copper-nickel was carried out according to the following scheme:
(1) heating the Al melt to a temperature of 800°C, the introduction of copper powder in aluminum foil in an amount of 5% of the melting mass into the Al melt, exposure for 30 min, active stirring;
(2) heating the Al-Cu melt to a temperature of 850°C, the introduction of the powder of manganese or nickel in an amount of 2 or 4% of the melting mass into the Al-Cu melt, holding for 30 min, active stirring;
(3) heating of the Al-Cu-Mn or Al-Cu-Ni melt to a temperature of 900°C, the introduction of batches of 6-8 g of SHS - charge (Ti + C) in aluminum foil into the melt, holding each batch of the charge under the melt level before the start of an active SHS reaction, accompanied by active spark and gas release (5-20 s), active stirring;
(4) exposure after the end of the SHS reaction from the last batch of charge for 5 min, pouring the composite melt into a steel chill to fabricate cast samples of the AMCs.
To determine the microstructure, the samples were etched with a solution of 50% HF + 50% HNO3 for 10^15 s. Metallographic analysis was performed using a JEOL JSM-6390A scanning electron microscope. Elemental chemical composition was determined by the method of micro-X-ray spectral analysis (MXRSA) on the same microscope using the attachment Jeol JED-2200. The hardness of the obtained samples was determined using a TSh-2M hardness tester according to GOST 9012-59, tensile tests were carried out on the tensile testing machine Inspekt 200 according to GOST 1497-84. The analysis of the microstructure of the fabricated AMCs samples showed that alloying additions of manganese or nickel in the presence of copper in both cases contributes to obtaining a homogeneous structure with uniformly distributed particles of titanium carbide in size of 80-500 nm. The maximum dispersion of the reinforcing phase (80-250 nm) is achieved with the addition of manganese and in the presence of Na2SiF6 salt (Figs. 1, 2). MXRSA, conducted at the sites corresponding to the grain boundaries, confirmed the presence of manganese and nickel. Values of mechanical characteristics of the alloyed AMCs samples are given in Table 1.
The obtained results can be explained by the presence of lattice distortions due to the formation of a solid solution and obstacles in the form of particles of the second phase which significantly limits the possibility of sliding the dislocations and leads to additional strengthening. Thus, the simultaneous formation of solid solutions, secondary phases and highly dispersed reinforcing phase of titanium carbide, allows to achieve an increase in strength properties while maintaining a sufficient reserve of plasticity.
(c) (d)
Fig. 1. (a, b) Microstructure of samples (Al-5% Cu-2% Mn)-10% TiC, (c, d) microstructure of samples (Al-5% Cu-2% Mn)-10% TiC (+ 5% Na2TiF6).
(c) (d)
Fig. 2. (a, b) Microstructure of samples (Al-5% Cu-4% Ni)-10% TiC,
(c, d) microstructure of samples (Al-5% Cu-4% Ni)-10% TiC (+ 5% Na2TiF6).
In general, the results of the study allow us to draw the following conclusions: (1) it is possible to obtain alloyed highly dispersed composites (Al-5% Cu-2% Mn)-10% TiC and (Al-5% Cu-4% Ni)-10% TiC when using SHS method in the melt;
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(2) alloying the Al-10% TiC composite with copper-manganese or copper-nickel systems has a positive effect on its structure and properties, which makes the further research in this direction promising.
Table 1. Mechanical properties of aluminum-matrix composite samples*.
Aluminum-matrix composite 00.2, MPa Ou, MPa 5, % % HB, MPa
(Al-5% Cu)-10% TiC 74 192 16.6 17.0 621
(Al-5% Cu-2% Mn)-10% TiC 97 188 8.6 13.3 960
(Al-5% Cu-2% Mn)-10% TiC
(+ 5% Na2TiF6) 114 213 6.6 7.3 970
(Al-5% Cu-4% Ni)-10% TiC 102 220 11.2 12 920
(Al-5% Cu-4% Ni)-10% TiC
(+ 5% Na2TiF6) 118 224 6.0 7 980
*G0.2 is the yield strength; Ou is the ultimate tensile strength; 5 is the percent elongation; y is the percent reduction in area; HB is the Brinell hardness.
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