| Among the few compounds in Ta-C system,namely cubic TaCy,rhombohedralζ-Ta4C3-x-x and hexagonal Ta2C,the rhombohedralζ-Ta4C3-x-x phase shows a high potential for ultra-high temperature applications due to high melting point,good thermal/electrical conductivities and excellent mechanical properties.Particularly for its high fracture toughness and deformation capability at high temperatures,ζ-Ta4C3-x-x has recently attracted much research.Although the earliest report onζ-Ta4C3-x-x can be dated back to its discovery in 1958,efforts in synthesizing single-phaseζ-Ta4C3-x-x ceramics have been unsuccessful yet.In this work,ζ-Ta4C3-x-x ceramics were prepared by reaction hot pressing,using TaC and Ta powders as the raw materials.Effects of nominal C:Ta ratios(TaC0.5-0.7),reaction hot pressing temperatures（1600-1900°C）and additions of Al,Fe,Co,Ni,Cu or Ag（0-30mol%）and other factors onζ-Ta4C3-x-x reaction,densification,microstructure and mechanical properties of the ceramics were invesitaged by means of Archimedes’principle,X-ray diffraction,scanning electron microscopy,transmission electron microscopy,three-point bending and single edge-notched beam method s.The conclusions of the research include:（1）Usage of Al,Fe,Co,Ni,Cu or Ag metal additives facilitated densification of theζ-Ta4C3-x-x ceramics.Relative densities increased with the metal additive contents to reach full densities at 20mol%Al,Fe,Co,Ni,Cu,Ag additions and above.（2）The metallic additives of Al,Fe,Co,Ni,Cu and Ag could belong to three groups according their effects on the TaC+Ta reaction.In detail,some MAX phase of Al2Ta3C was formed in case of Al due to Al reaction with TaC+Ta;In cases of Fe,Ni and Co additions,intermetallic compounds of Fe7Ta3,Co3Ta,Ni3Ta was formed,respectively.Neither Cu nor Ag reacted with TaC+Ta to form any Cu/Ag-Ta-C compound.（3）In the TaC+Ta+Al reaction systems,presence of Al2Ta3C promoted elongation of theζ-Ta4C3-x-x lamellae.In cases of Fe,CO and Ni additions,theζ-Ta4C3-x-x grain growth showed refinement of the average grain size but significant increase in aspect ratio with increase of the atomic number of the metallic additives.In the TaC+Ta systems with Cu/Ag additions,Cu was favorable over Ag in view of refinement of theζ-Ta4C3-x-x grain size and increase the aspect ratio of the grains.（4）Mechanical properties of theζ-Ta4C3-x-x ceramics were in close relation with densification and lamellar grain growth.In general,mechanical properties of theζ-Ta4C3-x-x ceramics increased with content of the metal additives due tobetter densification.The TaC0.6-30mol%Cu composites containing a large population of fine grainsζ-Ta4C3-x-x grains with high aspect ratios showed the best flexural strength and fracture toughness of 682 MPa and 12.9 MPa·m1/2,respectively.Presence of some Al2Ta3C lamellae in the microstructures also showed certain strengthening/toughening effects to theζ-Ta4C3-x-x composites containing some Al.（5）XRD analysis showed that additions of Cu or Ag did not change the reaction sequences TaC+Ta→ζ-Ta4C3-x,despite the beneficial effects on mater transportation to promote theζ-Ta4C3-x-x reactions.Because of the metallic liquids,ζ-Ta4C3-x-x could nucleate and grow both by dissolution-precipitation machine in the liquids and re-stacking the close-pacted Ta-atom planes in the parental cubic TaCy grains.In case of solid-state transformation of TaCy,the long-range ordering of…cchhcchh...,where c and h is cubic and hexagonal Ta-atom plan stacking,respectively,for pureζ-Ta4C3-x-x was frequently interrupted by stacking faults,to result in hybrid grains consisted of cubic TaCy,and hexagonal Ta2C and rhombohedralζ-Ta4C3-x-x in the same grain,rising a situation of difficulty in producing single-phaseζ-Ta4C3-x-x ceramics.There lacked sufficient TEM experiments on structure analysis of the systems containing other metallic additives rather than Cu and Ag,,effects of Al,Fe,Co and Ni additions onζ-Ta4C3-x-x reaction,grain growth and microstructure development were unclear yet. |
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