The C-Hf (carbon-hafnium) system

Okamoto, H.

doi:10.1007/bf02843319

Cited by 19 publications

(8 citation statements)

References 16 publications

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“…This phase has a face-centered cubic (fcc) structure. [45] which is consistent with the atomic resolution STEM observations along various crystallographic orientations as shown in Figures 7(b) through (d). The average size of the hafnium carbide phase is estimated to be 19, 23, and 20 nm for the 1, 2, and 4 at.…”

Section: Resultssupporting

confidence: 90%

Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

Saber

et al. 2015

Metall Mater Trans A

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The low thermal stability of nanocrystalline metals severely limits their applications at high temperatures. In this study, we investigate the nanocrystalline stabilization mechanisms for Fe14Cr alloys with 1, 2, and 4 at. pct Hf addition at 1173 K (900°C). Microstructural characterizations using aberration-corrected scanning transmission electron microscopy and energydispersive X-ray spectroscopy reveal high density of HfO 2 nanoparticles with sizes of~4 nm dispersed throughout the ferritic matrix. This indicates that kinetic stabilization by HfO 2 nanoparticle pinning is primarily responsible for the observed high thermal stability. In addition, some Hf and Cr segregation on grain boundaries is observed in the Fe-14Cr-4Hf, suggesting the existence of thermodynamic stabilization at high Hf content. Second-phase precipitations such as hafnium carbide, M 23 C 6 , and Fe-Cr-Hf intermetallic phase are also found in the Fe-14Cr-4Hf, but their large sizes and inter-spacing suggest that their contribution to stabilization is minimal.

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Section: Resultssupporting

confidence: 90%

Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

Saber

et al. 2015

Metall Mater Trans A

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“…Zirconium carbide belongs to the family of ultra high‐temperature ceramic (UHTC) compounds that are known for their excellent thermomechanical properties, hardness, and wear resistance . These include the Group IVB carbides along with Ta and W, which demonstrate applications in extreme environments of thermal protection systems, field emitters, thermal coatings, and creep strengthening of refractory metals …”

Section: Introductionmentioning

confidence: 99%

“…1 These include the Group IVB carbides along with Ta and W, which demonstrate applications in extreme environments of thermal protection systems, field emitters, thermal coatings, and creep strengthening of refractory metals. [2][3][4][5][6][7][8] Conventional powder synthesis of metal carbides generally involves reduction of milled powder oxides in an environment rich in carbon that is sourced from gas phases, resins, or powdered carbon. [9][10][11][12] In all cases, high temperatures are required for carbon self-diffusion, resulting in extended isotherms and large carbide particle size.…”

Section: Introductionmentioning

confidence: 99%

Synthesis and Evolution of Zirconium Carbide via Sol–Gel Route: Features of Nanoparticle Oxide–Carbon Reactions

et al. 2013

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A sol–gel route was used to synthesize nanocrystalline zirconium carbide (ZrC). The starting materials were zirconium propoxide, with carbon introduced by furfuryl alcohol (FA). A block copolymer surfactant was used to homogenize the oxide and carbon components. ZrC was produced at a low temperature of 1250°C and complete conversion achieved at 1450°C. The powder was nanocrystalline size less than 100 nm. Phase changes were studied using X‐ray diffraction and Raman spectroscopy. Rietveld analysis followed the changes in crystallite size and lattice parameters of the phases during carbothermal reduction. Morphology changes were observed using nitrogen gas sorption. High‐resolution TEM and EDS were used to image the carbide lattice, surface oxides, and graphene‐like carbons. The results indicate that nanoparticle carbothermal synthesis involves agglomeration and necking as the most viable mode of mass transport to complete the carbothermal reduction.

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“…According to the literature [16][17][18], the melting point of NbC, TiC, and HfC are 3610, 3067, and 3950 • C, respectively, much higher than the current re-melting temperature. In addition, the carbides also possess high hardness and stiffness.…”

Section: Formation Of a Carbides Layer On The Surface Of The Alloy Rementioning

confidence: 99%

Re-Melting Nb–Si-Based Ultrahigh-Temperature Alloys in Ceramic Mold Shells

Yin

Guo

2019

Metals

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In furnaces with different heating elements, Nb-Si based ultrahigh-temperature alloy rods were re-melted in pure yttria mold shells and zirconia face-coat mold shells at 1850 • C for 30 min. The results evidenced that in the furnace with a tungsten heating element, the microstructure of the re-melted alloy became coarser, and the composition varied depending on the type of mold shell. Although the interface reaction layer between the re-melted alloy and the zirconia face-coat mold shell was much thicker, the deformability of the mold shell and the sand burning phenomenon of the alloy inside it were improved and ameliorated, respectively. However, after being re-melted in the furnace with a graphite heating element, the misrun phenomenon occurred in both specimens. Both re-melted alloys inside the mold shells were divided by a gap into an internal and an external part, with totally different microstructures and compositions. No reaction layer emerged at the interface between the re-melted alloy and the mold shells. Instead, infiltration zones arose in the mold shells adjacent to the interface.Metals 2019, 9, 721 2 of 15 coating in mold shells for investment casting of active and refractory metals. Especially, yttria has been successfully applied in the preparation of directionally solidified Nb-Si based ultrahigh-temperature alloys with a single orientation structure and high mechanical properties. As an example, Guo and Guo [11] performed integrally directional solidification of Nb-Si based ultrahigh-temperature alloys, in which yttria was employed as crucible material. Ma et al. [12] technically studied the interface interactions between Nb-Si-based ultrahigh-temperature alloys and pure yttria mold shells and found only a 5 to 15 µm-thick reaction layer composed of HfO 2 and Y 2 O 3 formed at the metal-mold shell interface. This further confirms that yttria is relatively inert to this alloy and is a qualified material for the mold shell during investment casting of this alloy. However, yttria is inclined to be hydrated with water, which leads to a significant loss of the life period of the as-prepared slurries as a result of gelatinization [13]. This ineluctably increases the overall technical costs of investment casting. On the contrary, it is not an issue for slurries filled by zirconia. However, in view of the substitution reaction between ZrO 2 and Hf, the application of zirconia on re-melting Nb-Si-based ultrahigh-temperature alloys is scarce. Nevertheless, the addition of Hf is becoming unnecessary, considering the impact of this element on the alloy, as reported recently [14]. It can be predicted that the content of Hf is likely to be reduced or replaced completely by other elements in the modified alloy. Hence, zirconia is also a potential mold shell material with practical application in confectioning this alloy.Besides the mold shell problems to be solved, the issues of the filling capability, microstructure homogeneity, element segregation, casting defects, and sand burning should all be evaluated fo...

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The C-Hf (carbon-hafnium) system

Cited by 19 publications

References 16 publications

Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

Microstructures and Stabilization Mechanisms of Nanocrystalline Iron-Chromium Alloys with Hafnium Addition

Synthesis and Evolution of Zirconium Carbide via Sol–Gel Route: Features of Nanoparticle Oxide–Carbon Reactions

Re-Melting Nb–Si-Based Ultrahigh-Temperature Alloys in Ceramic Mold Shells

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