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29iSHS 2019
Moscow, Russia
FABRICATION OF ALUMINUM-CERAMIC SKELETON COMPOSITES BASED ON TITANIUM ALUMINIDE CARBIDE USING SHS PROCESS
E. R. Umerov", A. P. Amosov*", E. I. Latukhin", P. E. Kichaev", and V. A. Novikov"
aSamara State Technical University, Samara, 443100 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10180
As you know, the use of ceramic materials is limited by their brittleness. A new type of ceramic materials, so-called MAX-phases, occupies an intermediate position between metals and traditional ceramic materials, and the MAX-phases are less brittle compared to traditional ceramic materials [1]. Composite materials consisting of MAX-phases and metals will be even less brittle and able to provide increased strength, so the production of ceramic-metal composites (cermets) based on MAX-phase skeletons is an urgent task [2-6]. Studies of the possibility of obtaining and properties of the composites based on such MAX-phases as titanium silicide carbide (Ti3SiC2) and titanium aluminide carbide (Ti3AlC2) are the most numerous. However, the methods of powder metallurgy for producing such composites (pressing followed by sintering. hot pressing and infiltration) are complex and energy-intensive, so it is very attractive to use the method of self-propagating high-temperature synthesis (SHS) with simple technological equipment and low energy consumption, which can contribute to the creation of economically justified production of skeleton ceramic-metal composites based on MAX phases.
The application of the SHS process by burning in air was investigated for a single-stage fabrication of Cu-Ti3SiC2 ceramic-metal composite [3]. The location of a briquette of a metal powder of copper between two adjacent charge briquettes for the synthesis of Ti3SiC2 with subsequent burning of this charge briquettes made it possible to synthesize porous skeletons of MAX-phase Ti3SiC2, and simultaneously to use a large heat effect of SHS to melt the copper and infiltrate these porous skeletons of Ti3SiC2 by the Cu melt. However, as a result of the burning of these briquettes, a relatively small amount of copper was melted. which was not enough to completely fill the porous skeletons of the MAX-phase Ti3SiC2 formed on the site of the charge briquettes. Also. it was found that the presence of the Cu melt prevents the formation of MAX-phase of titanium silicide carbide, reducing its amount or completely destroying it due to deintercalation silicon from Ti3SiC2 and dissolving it in the molten copper. It was necessary to carry out separate ignition of charge briquettes with ignition delay of the second briquette after ignition of the first briquette, so that the infiltration by the molten metal began later and less prevented the formation of the MAX-phase Ti3SiC2 from the initial reagents.
To obtain aluminum-ceramic composite, it is advisable to use a MAX-phases of titanium aluminide carbide Ti3AlC2 and Ti2AlC instead of MAX-phase of titanium silicide carbide Ti3SiC2, in order to avoid the destruction of MAX-phase at the expense of deintercalation of Al atoms from the MAX-phase. In the study of the reaction activity between the copper matrix and Ti3AlC2, it is shown that at temperatures above 950°C the rate of aluminum deintercalation from Ti3AlC2 into copper increases sharply, followed by partial filling of the liberated layers of Al in Ti3AlC2 with copper atoms [4]. Obviously, the deintercalation of aluminum atoms from the MAX-phase Ti3AlC2 into metallic aluminum will be significantly less than into metallic copper or absent altogether. The paper [5] considered a single-stage technology for the fabrication of aluminum-ceramic skeleton composites by combining the processes of SHS of a porous skeleton of MAX-phase Ti2AlC and its infiltration under pressure by aluminum melt (SHS-pressing method). The influence of infiltration pressure on the distribution of aluminum
over the MAX-phase skeleton, on the mechanism of formation of the composite composition depending on the infiltration pressure was studied. In this paper we investigate the possibility of fabrication of aluminum-ceramic composites based on titanium aluminide carbide with the use of SHS process of porous skeleton MAX-phases Ti3AlC2 and Ti2AlC with subsequent thermocapillary infiltration them with aluminum melt without application of pressure. that greatly simplifies the technology of producing such composites.
Synthesis of porous cylindrical briquettes of MAX-phases with a diameter of 23 mm and a height of about 10 mm was carried out from charge briquettes, which were the mixtures of powder materials with a ratio of components: 2Ti + 2Al + 1C for Ti2AlC and 3Ti + 2Al + 2C for Ti3AlC2. (To ensure the synthesis of the maximum amount of the MAX-phase of titanium aluminide carbide. it is necessary to feed a double amount of aluminum powder into the SHS charge [6].) Taking into account the stages of formation of MAX-phase of titanium aluminide carbide [7], a pause of 7-9 s was maintained upon completion of the combustion of charge briquettes in the air ensuring the completion of the process of formation of the MAX-phase in the sample, after which the hot sample was immersed in the aluminum melt with a temperature of 850°C. (The experiment showed that exposure of more than 8-9 s in air leads to overcooling of the sample and complication of infiltration. Supercooling dramatically worsens the wettability of the MAX-phase by aluminum melt, which reduces not only the completeness of the impregnation, but also the strength of the composite.) The relatively high heat capacity of the aluminum melt leads to rapid cooling of the submerged porous sample to a melt temperature of 850°C, when the MAX phase is no longer formed, but begins to disintegrate due to interaction with the liquid aluminum. Therefore, after 10-15 s, the impregnated sample was removed from the melt and continued to cool in the air.
The microstructure and distribution of the chemical elements of Ti3AlC2-Al composite sample are shown in Figs. 1 and 2. Multidirectional MAX-phase plates with an average length of 10-20 |im are clearly distinguishable in Fig. 1a. It follows from Figs. 1b and 2 that the dark relatively homogeneous regions are aluminum and its intermetallic, since the darker and homogeneous right side is saturated with aluminum, and the left, lighter and layered, corresponds approximately to the ratio of elements in the MAX-phase Ti3AlC2. The interfacial layer with a size of 1 |im or more has not been detected, which indicates a low activity of the chemical interaction between Ti3AlC2 and Al. Visually, other phases (carbides, oxides) are practically not observed, which indicates a successfully fabricated composite with a low content of impurities.
The samples of composites were machined to give a disc shape with parallel planes. which allows to carry out tests on the compression strength on a technical complex Instron Bluehill 3. Graphs of the compression load with a compression rate of 2 mm/min on the absolute deformation of the samples are shown in Fig. 3.
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(a) ' J (b)
Fig. 1. Characteristic microstructure of fracture surface of Ti3AlC2-Al composite.
ÏSHS2019
Moscow, Russia
Fig. 2. Distribution of chemical elements by points on line of Ti3AlC2-Al sample.
Fig. 3. The loading curves of the composite samples: 1 Ti2AlC-Al, a delay time of 7 s; 2 and 3 Ti3AlC2-Al, 8.5 s; 4 Ti3AlC2-Al, 8 s.
The delay time of the sample in the caption under Fig. 3 indicates the time from the end of the combustion of the charge briquette in air to immersion into the aluminum melt in the fabrication of the sample. Samples with corresponding numbers after the compression test are shown in Fig. 4.
The photos of samples 1 and 2 show that they are porous and have dark areas, non-impregnated with aluminum. Porosity is not visible on samples 3 and 4, these samples are completely impregnated. The results of the study of the fracture surface of sample 3 were presented in Figs. 1 and 2.
Processing of the compression test results shown in Fig. 3, shows that samples 1 and 2 have the lowest values of compression strength (219 and 180 MPa, respectively) and the lowest values of deformation before failure (6.2 and 5.8%), due to the presence of residual porosity in these samples. Samples 3 and 4 have significantly higher values of compression strength (457 and 280 MPa) and deformation before failure (11.2 and 10.0%) due to a more complete impregnation with aluminum. The type of load-displacement curves indicates that the deviation from the linear portion of the elastic deformation is associated with the destruction of the ceramic skeleton in the composite. This destruction can have both the character of brittle failure with the separation of the composite into parts with a sharp drop in the load resistance (sample 3) and ductile failure with an increase in the compression load resistance due to the metal component of the composite (sample 4). Therefore, the completeness of the impregnation with aluminum plays a crucial role in ensuring the strength of the skeleton aluminum-ceramic composite on the basis of titanium aluminide carbide, fabricated by the method of capillary impregnation.
Thus, the samples of aluminum-ceramic composites Ti3AlC2-Al and Ti2AlC-Al are fabricated by the method of capillary impregnation when immersed in aluminum melt of the hot porous skeletons of MAX-phases of titanium aluminide carbide Ti3AlC2 and Ti2AlC synthesized just before this by the SHS method in combustion of powder charges in the air. It is shown that the impregnation of the skeleton of the MAX-phases of these samples with aluminum melt can be complete even without the application of excess pressure, which greatly simplifies the technology of manufacturing skeleton aluminum-ceramic composites based on titanium aluminide carbide in the application of the SHS process.
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