Papers by Keyword: Niobium Carbide

Abstract: Composite materials are increasingly being used in several areas, especially in the automotive and aerospace industries, however, during regular operation their wear is one of the main causes of failure. Consequently, developing and researching new composite materials is essential to increase and improve service life. In addition, thixoforming is claimed to exhibit superior properties by reducing typical defects in casting like shrinkage and porosity. Therefore, the main objective of this study is to produce and analyze abrasion wear properties of the thixoformed aluminum matrix composite reinforced with NbC, obtained by the stir-casting method. Three different composites with 5 wt.%, 10 wt.%, and 15 wt.% of NbC were manufactured with the stir-casting method, compared with A380 alloy. The procedure involves an A380 aluminum alloy that was molten at 750 °C. In sequence, niobium carbide powder was added by mechanical stirring for 10 min; Mg was added to improve the wettability between the reinforcement and matrix. Chemical grain refinement by Al-5Ti-1B master alloy was used for non-dendritic feedstock production. Hence, the induction furnace was used for the thixoforming process, to achieve a mushy of 60 % solid fraction at 562 °C, determined by Differential Scanning Calorimetry (DSC) analysis. The holding time applied was 90s. Optical microscopy (OM) and scanning electron microscope (SEM) analyses allowed the microstructural characterization. Abrasive wear tests, according to the ASTM G65 standard, showed an improvement of the composites’ abrasion wear resistance after the thixoforming process, with a higher amount of NbC, potentially increasing the range of use of this technology and materials.

Abstract: Titanium and niobium were applied to stable the carbon or nitrogen which dissolved ferritic stainless steel for improving the anti-corrosion performance. The titanium nitride and niobium carbide had been formed during solidification processing. For understanding those precipitates how to influence the casting macrostructure, three steels that had different content of niobium and the fixed content of titanium had been designed. The result showed the casting macrostructure of ingot and the grain size of the centre-equiaxed crystal zones had different tendency. And the titanium nitride and niobium carbide had interacted.

Abstract: NbC-xTi (C_0.7N_0.3)-10Ni-7.5VC (vol%) based cermets with 0, 5, 10, 15 or 25 vol% Ti (C_0.7N_0.3) were prepared by conventional pressureles liquid phase sintering at 1420°C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. Sintering results indicated that the partial replacement of NbC by Ti (C_0.7N_0.3) had a significant effect on the carbide grain growth, microstructure, hardness as well as fracture toughness of the fully densified NbC-based cermets. The Ti (C_0.7N_0.3)-free NbC cermet was composed of homogeneous cubic (Nb,V)C solid solution grains, whereas core-rim structured NbC grains were observed in cermets with Ti (C_0.7N_0.3) addition. All sintered cermets with  15 vol% Ti (C_0.7N_0.3) were composed of a fcc solid solution Ni binder and a cubic core-rim solid solution (Nb,V,Ti)C phase with a Nb-rich core and a Ti-rich rim. 3.8 vol% of residual pristine Ti (C_0.7N_0.3) was present in the cermets with 25 vol% Ti (C_0.7N_0.3) addition. The 15 vol% Ti (C_0.7N_0.3) starting powder based cermet exhibited the finest average NbC grain size of 1.48 μm, with a core-rim structure and an interesting combination of hardness (1486 kg/mm²) and fracture toughness (8.7 MPa.m^1/2).

Abstract: Powder mixtures of WC–(0–2 mol%) NbC and WC–1.5 mol% NbC–(0–5 mol%) C were sintered at 1800°C using a resistance-heated hot-pressing machine; dense WC–NbC and WC–1.5 mol% NbC–C ceramics were obtained. The relative X-ray diffraction (XRD) peak intensity of W₂C_ss decreased with increasing C amount and disappeared at 5 mol% C. Small amounts of C remained after sintering at 5 mol% C. The WC–1.5 mol% NbC ceramics with 0–3 mol% of added C were composed of equiaxed small granular grains. Large WC grains formed in WC–1.5 mol% NbC ceramics above 4 mol% C. The hardness of WC–NbC ceramics decreased from 25.7 GPa for WC to 23.6 GPa for 2 mol% NbC obtained by NbC addition. The hardness change for WC–1.5 mol% NbC ceramics with up to 3 mol% of added C was small, around 24 GPa. The Vickers hardness of WC–1.5 mol% NbC ceramics above 4 mol% C decreased markedly from 23 to 13 GPa with increasing added C, due to extensive WC grain growth.

Abstract: Niobium carbide films was deposited by direct current reactive magnetron sputtering on Si (001) substrates in discharging a mixture of CH₄/Ar gas. The effects of growth temperature (T_s) and methane flow rate (F_CH4) on the phase structure, composition, mechanical and tribological properties for NbC_x films were explored. For the film grown at F_CH4=6 sccm, a phase transition from cubic-NbC phase to hexagonal-Nb₂C phases occurred with increasing the T_s; In contrast, when the film deposited at F_CH4=16 sccm, only the cubic-NbC phase was observed at different T_s. The surface of all the films became rough with increasing the T_s. In addition, when the T_s increased from RT to 600 °C, the films exhibited the compressive stress and kept rising. While as the T_s > 600 °C, the stress partially relaxed both at F_CH4=6 sccm and F_CH4=16 sccm. The hardness (H) for sample grown at F_CH4=6 sccm first increased up to a maximum value, and then decreased with increasing the T_s. And the films grown at F_CH4=16 sccm kept decreasing with the maximum super-hard value of the filmsof 40.5 GPa at F_CH4=6 sccm and 600 °C. The friction coefficient for the film obtained at F_CH4=16 sccm was lower than that at F_CH4=6 sccm, which might be due to the presence more carbon in the film grown at F_CH4=16 sccm.

Abstract: High speed steels processed by Powder Metallurgy (PM) techniques present better mechanical properties when compared with similar steels obtained by the conventional process of cast to ingot and hot working. PM techniques produce improved microstructures with smaller and better distribution of carbides. Liquid phase sintering high speed steel seems to be a cheaper processing route in the manufacturing of tool steels if compared to the well-known and expansive hot isostatic pressing high speed steels. The introduction of niobium as alloying element began with the object of replacing elements like vanadium (V) and tungsten (W). Phase liquid sintering consists in a manufacturing technique to process high speed steels by powder metallurgy. The aim of this work of research is to process and obtain AISI M2 and M3:2 with and without the addition of niobium carbide by high energy milling, cold uniaxial compaction and vacuum sintering in the presence of a liquid phase. The powders of the AISI M2 and M3:2 were processed by high energy milling adding a small quantity of niobium carbide (6% in mass), then the powders were characterized by means of X-ray diffraction (XRD) and scanning electron Microscopy (SEM) plus energy dispersion spectroscopy (EDS) in order to evaluate the milling process. The powders of the AISI M2 and M3:2 with the addition of niobium carbide (NbC) were uniaxially cold compacted and then submitted to vacuum sintering. The sintered samples had their microstructure, porosity and carbide distribution observed and evaluated by means of Scanning Electron Microscopy (SEM) and the mechanical property of hardness was investigated by means of Vickers hardness tests. At least five samples of each steel were investigated.

Abstract: This paper presents a study on the synthesis of Niobium Carbide (NbC) and Vanadium Carbide (VC) in Copper (Cu) matrix by mechanical alloying (MA) technique. The elemental powders of Cu, Niobium (Nb), Vanadium (V) and synthetic graphite powder were mechanically alloyed for 30 hours at 400 rpm in a planetary ball mill Fritcsh “Pulverisette 6” according to the stoichiometric ratio of Cu-(10-x) vol%NbC-(0+x) vol%VC (x=1,3,5,7,9). The milling was performed under Argon atmosphere. The as-milled powder were compacted at 400 MPa and sintered using a microwave sintering furnace at 900°C with 1 hour soaking time. The phase identification was performed by using the X-ray Diffraction (XRD) analysis on the as-milled powders and sintered pellets. From the result, the NbC and VC phases were successfully formed after milling, and were precipitated after sintering. The average crystallite size and lattice strain of Cu, before and after sintering were 42.302 nm, 0.013%, and 71.294 nm, 0.004%, respectively.

Abstract: We study the structural and electronic properties of scandium carbide ScC and niobium carbide NbC in both the sodium chloride rock salt (NaCl) and wurtzite structures by means of accurate first principles total energy calculations. The calculations were performed employing the full-potential linearized plane wave method (FP-LAPW). We used the generalized gradient approximation (GGA) of Perdew Burke and Ernzerhof for the exchange and correlation potential. Volume optimization and density of states including spin (DOS) of the systems are presented.

Abstract: Several kinds of alloys Ni-based, Fe-based and Al-based oxide dispersion strengthened (ODS) and carbide dispersion strengthened (CDS) have been produced through mechanical alloying. Precipitation-strengthened or dispersion-strengthened steels are a kind of high strengthened steel using fine precipitation or dispersion of carbides. This work present a study of EUROFER97 steel powder reinforced with 3%wt of niobium carbide. The starting materials were wet-milled in a high-energy planetary ball mill for several times up to 5 hours. The ratio of ball to powder weight was 15:1. The milled powders were characterized by SEM, XRD, EDX and laser scattering. The results are presented on base of a microstructure analysis of composite particles of steel-carbide as a function of milling time.

Abstract: The precipitation of NbC in austenite is an important mechanism for improving the strength of steel because NbC-precipitates are known to decrease the ferrite grain size during the subsequent phase transformations upon cooling. The effect of the interaction between niobium (Nb) in solid solution and NbC-precipitates on the austenite-to-ferrite phase-transformation kinetics is not entirely clear. We study a high-purity Fe-C-Mn-Nb alloy cooled at different rates. Different annealing times at 850°C were applied to create different number densities and sizes of the NbC-precipitates in order to study the effect of NbC precipitation on the transformation kinetics. The alloy that is used in this study has an atomic ratio of Nb:C=1.3:1. The fraction of ferrite is measured as a function of temperature during cooling by means of dilatometry. The ferrite grain size is measured by means of optical microscopy. The results are interpreted with thermodynamic and kinetic models.