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22PRODUCTION OF ULTRA-REFRACTORY CARBIDES OF VARIOUS STOICHIOMETRIES IN THE SYSTEMS Ta-Zr-C, Ta-Hf-C BY SHS
E. I. Patsera", S. A. Vorotilo", V. V. Kurbatkina", and E. A. Levashov"
aNUST MISiS, Moscow, 119049 Russia DOI: 10.24411/9999-0014A-2019-10117
The development of aerospace technologies dictates the increased demands towards the crucial heat-loaded parts, which must effectively endure temperatures above 2500°C. The solution to this problem is possible via the creation of novel heat-resistant materials; hence the relevance of the development of materials based on the ultra-high temperature carbides with melting points above 4000°C (for example, 41250C for (Ta,Hf)C with 20% HfC) [1]. Unfortunately, the high melting points of tantalum, zirconium, and hafnium carbides impede the sintering of powder materials and parts [2, 3]. The primary goal of this work was to produce the single-phase Ta(1-x),Mex)C (Me = Zr, Hf, x = 10, 30, 50%) solid solutions by the means of self-propagating high-temperature synthesis, to sinter them and to characterize the structure and properties of produced ceramics. Mechanical activation (MA) was conducted in air using AIR -0.015 and Aktivator-4M planetary ball mills (PBM) for 5, 10, and 15 min at balls-to-mixture ratio 20:1. The following PBM schemes were used:
scheme 1: simultaneous loading of all components in jars and MA;
scheme 2: two-stage MA: first the mixture of tantalum and carbon black is activated, then either Zr or Hf is added and the activation continues for extra 5 min.
When mixtures are milled in ordinary ball mills for 4 h, no noticeable refinement of metallic particles occurs, but they are uniformly covered in carbon black. Microstructural investigation of MA mixtures Ta-Zr-C revealed the formation of layered composite powders consisting of Ta, Zr, and C layers. Size and structure of layered particles depend on the employed scheme of MA (Fig. 1). Microstructural analysis of the mixtures activated using various MA schemes demonstrated that scheme 1 leads to the intense milling of Zr, whose particles become significantly smaller than for Ta. After 10 min of MA using scheme 1, medial size of composite particles is 100-300 |im, layers of tantalum and zirconium are 25^40 and 1^2 |im wide.
Fig. 1. Microstructure of reactive composite particles in Ta-Zr-C mixture, activated according to two schemes: Scheme 1: t = 5 (a), 10 (b), 15 min (c); Scheme 2: t = 5 (d), 10 (e), 15 min (f).
ISHS 2019 Moscow, Russia
MA mixtures Ta-Hf-C, prepared in PBM AIR, were characterized by uneven mixing and insufficient energy accumulation due to the low planetary acceleration of the mill (25 g). Therefore, a more powerful Aktivator 4M with the planetary acceleration up to 120 g for used for MA of Ta-Hf-C mixtures. 10-min MA leads to the formation of 10-150 |im granules comprised of alternating 0.5-2 ^m wide Ta and Hf layers, with C being located both within the granules and on their surface.
Similarly to Ta-Zr-C, phase composition of MA mixtures Ta-Hf-C prepared according to scheme 1 for 5, 10, and 15 min in air did not change. Increase in MA duration over 10 min increased the flammability of the mixtures considerably; therefore, MA durations below 10 min were used.
Prolongation of MA decreased the coherent scattering areas (CSAs) and increased the crystal lattice microdeformation of Ta, which signifies an efficient accumulation of excess energy in form of crystal lattice defects.
Products of combustion of Ta-Zr-C MA mixtures were obtained in the form of porous sinter cake (n = 40%), and consisted of single-phase (Ta,Zr)C solution with a grains size of 1-10 ^m (Fig. 2). Low density of sinter cakes facilitates their milling into micron powders. Increase in ZrC content in solid solution led to higher value of its lattice parameter.
c d
Fig. 2. Structure of combustion products with the composition 70% TaC-30%Z rC, produced using the mixtures after (a, b) ball milling and (c, d) after MA.
In case of Ta-Zr-C system, the particles size distribution of produced powders was somewhat dependent on the Zr content in the reactive mixture. Largest particles size (up to 10 |im) was characteristic for powders produced from combustion products 90% TaC-10% ZrC, whereas the finest powders (up to 3 |im) were produced from 50% TaC-50% ZrC combustion products.
Products of combustion of MA mixtures Ta-Hf-C were characterized by high residual porosity (n = 50%) and grains size of 1-5 |im (Fig. 3). Ball milling of sinter cake yielded 1-3 |im powders.
Sinterinig of the produced powders was conducted using hot pressing or spark plasma sintering at 2100 0C. Depending on the composition of solid solution, the density of produced ceramics was 93-95%. No noticeable grain coarsening occurred. Size of residual pores did not exceed 1 |im.
Nanoindentation of sintered ceramics revealed that their hardness and elastic moduli were fairly similar regardless of sintering technique (Table 1).
Fig. 3. Microstructure of the combustion products with composition: (a) TaC-10% HfC, (b) TaC-20% HfC, (c) TaC-30% HfC, (d) TaC-50% HfC.
Table 1. Mechanical properties of solid solutions measured by nanoindentation.
_Composition_Hardness, GPa_Young modulus, GPa
TaC-20% ZrC 32.2 520
TaC-30% ZrC 30.6 451
TaC-50% ZrC 38.8 582
TaC-20% HfC 27.4 484
This work was financially supported by Russian Science Foundation in the framework of project no. 17-79-10173 «Self-propagating high-temperature synthesis of single-phase ultra-refractory carbides with various compositions in the Ta-(Zr, Hf)-C system».
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