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84THE STUDY OF HIGH-ENTROPY CERAMICS Hf0.2Ta0.2Ti0.2Nb0.2Mo0.2C AND Hf0.2Ta0.2Ti0.2Nb0.2Zr0.2C OBTAINED BY SHS AND SPARK PLASMA SINTERING
A. Sedegov*", S. Vorotilo", V. Tsybulin", K. Kuskov", D. Moscovskikh", S. VadchenkoA, and A. Mukasyan"'c
aNational University of Science and Technology MISiS, Moscow, 119049 Russia bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia cUniversity of Notre Dame, Notre Dame, IN 46556 USA *e-mail: a.sedegoff@gmail.com
DOI: 10.24411/9999-0014A-2019-10150
Conventionally, the majority of alloys are based on one main constituent element. For the increase of properties and performance, they are alloyed by alloying elements which enhance the alloy's strength, corrosion resistance and other properties. Thus, the families of alloys are made. However, the number of elements is limited; therefore, the number of alloys which could be developed on the base of one or two main constituent elements is finite. However, what if one is to go outside the classical definition and increase the number of constituent elements? This conception was first suggested in 1995 [1], and these materials were later branded as high-entropy alloys [2].
High-entropy materials by definition contain five or more main constituent elements with the concentration between 5 and 35% [3]. The currently available data in the areas of physical metallurgy and binary and ternary diagram suggests that such multicomponent alloys might contain dozens of phases and intermetallics, which are usually brittle and have limited applicability. However, in contrary to these expectations that the increased entropy in these alloys leads to the formation of solid solutions with a relatively simple structure, thus diminishing the content of unwanted phases or eliminating them. These peculiar features, related to high entropy, are of the utmost importance for the development and applications of the high-entropy alloys.
During the last years of investigation of high-entropy materials, more than 300 compositions with wide functionality were developed on the basis on non-ferrous and refractory metals. Currently, the hottest topic is the development of high-entropy alloys (HEA) for high-temperature applications, which require high phase stability upon heating up to 1800°C, oxidation resistance and high-temperature strength.
Due to their unique multi-component compositions, HEAs might possess a complex of outstanding properties, including high strength and hardness, unmatched high-temperature strength, good structural stability, and corrosion resistance. The close interest in the development of these materials is driven by the demand for the materials with increased operating temperature from the aerospace industry.
Equimolar composition HfTaTiZrNbMo was investigated in [4] and demonstrated the high mechanical properties at room temperature and 1200°C. Xueliang Yan et al. in their recent article [5] synthesized a high-entropy with the composition (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C from the monocarbides, which were mixed in a mill and consolidated by spark plasma sintering at 2000°C. The produced material is characterized by high thermal conductivity, thermal diffusivity, and the coefficient of thermal expansion, which was compared to the initial monocarbides. In various articles dedicated to the investigation of HEAs, the samples are usually produced by arc melting. However, this production route often leads to the formation
iSHS 2019
Moscow, Russia
of a dendritic structure, leading to the degradation of mechanical properties. To retain the fine-grain structure, which is most beneficial for mechanical properties, the powder metallurgy route is most perspective, since it ensures the homogeneity of material. Also, as a new class of high-entropy, Jian Luo et al. [6] described the ultra-high-temperature diboride ceramic based on the Hf-Ta-Ti-Nb-Mo-Zr system. In our work, a high-entropy alloy Hf-Ta-Ti-Nb-Mo-Zr and the related high-entropy carbide were produced by high-energy ball milling and spark plasma sintering.
The majority of high-entropy materials are produced by vacuum arc melting, which requires high energy expenditures. In this work, high-entropy materials were produced by various routes, which involve the mixing of metal powders and carbon, subsequent synthesis, and sintering.
For the obtain of high-entropy ceramics (HEC-1 corresponds to the composition (Hfo,2Tao,2Tio,2Zro,2Nbo,2Moo,2)C; HEC-2 corresponds to the composition (Hfo,2Tao,2Tio,2Zro,2Nbo,2Zro,2)C), the mixture of powders were treated in the planetary ball mill Activator-2S (Russia). High-energy ball milling (HEBM) was organized according by the following scheme: milling of the mixture of metals during 9o min (+ 5 min with hexane to ensure the unloading the powder from the surface of jars and balls), then milling of pre-milled metal powders with graphite for 5 min i.e mechanical activation (MA) of reaction mixtures.
After MA, self-propagating high-temperature synthesis of powders was carried out. It can be seen in Fig. 1 that after MA of metals and carbon powder, a mixture without a crystalline structure is obtained (the black line for the two mixtures is similar). And as a result of the SHS, a material with an FCC lattice is formed. First-stage HEBM produced amorphous powders, which did not react with the carbon during the second stage of HEBM. Then the high entropy carbide was synthesized in the lab reactor in an argon atmosphere. Combustion temperature and rate were measured during the synthesis. Combustion rate was 4.17 and 7.69 mm/s for HEC-1 and HEC-2, respectively.
Fig. 1. XRD patterns for powders after HEBM and combustion synthesis.
Sintering of the powders produced by combustion synthesis (9o-min milling of metals, 5-min milling of mixture of metals and graphite and subsequent combustion synthesis in lab reactor) was conducted on the hot pressing installation Dr. Fritsch DSP 515 (Germany) and spark plasma sintering installation SPS Labox 65o (Japan) in a graphite matrix in vacuum at temperatures 18oo C, pressure 5o MPa, dwelling time 5 min.
Figure 2 presents the results of the SEM of sintered powder samples. Where there is clearly visible the presence of other phases and the difference in the size of the grains during hot pressing and the SPS method. The EDS analysis (Figs. 2b, 2d, 2h, 2f) showed that the light areas are more saturated with heavy metals such as hafnium and tantalum, and the black
inclusions in the grains are iron left after processing in steel drums. In general, it is clear that all elements are evenly distributed over the entire area, which makes it possible to call this material - high entropy ceramic.
at 1800°C (c, d); HEC-2 hot-pressed at 1800°C (e, f); HEC-2 SPSed at 1800°C (g, h).
The results of hardness measurements are presented in Table 1. It is important to note that the material sintered by the SPS method has a higher hardness compared with hot-pressed.
Table 1. Hardness test result.
Material Method of sintering Hardness, GPa
HEC-1 Hot pressing 1800° C 15.95
HEC-1 SPS 1800° C 22.62
HEC-2 Hot pressing 1800° C 18.58
HEC-2 SPS 1800° C 20.53
This work was conducted with the finantial support of the Russian Science Foundation (grant
no. 18-79-10215).
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