Papers by Keyword: NbC

Abstract: In this study, microstructural and mechanical properties of a Al-5 wt.% Si/2 wt% (NbB2, NbC) composite synthesized by sequentially milling (mechanical alloying (MA) and/or cryogenic milling (CM)) were investigated. Nb2O5, B2O3 and C powder blends were milled using high energy milling for 5 h and annealed at 1400 oC for 12 h to produce NbB2-NbC hybrid powders. The NbB2-NbC hybrid powders were mixed with the matrix Al-5 wt.% Si powders to constitute the Al-5 wt.% Si/2 wt.% (NbB2-NbC) powders blends which were mechanically alloyed (MA'd) for 4 h using SpexTM Mixer/Mill, cryo-milled for 10 min in a SpexTM 6870 Freezer/Mill and finally MA’d for 1h in SpexTM Mixer/Mill again. As-blended, MA’d and cryomilled powders were compacted in a hydraulic press with a uniaxial pressure of 450 MPa. Compacted samples were sintered at 570°C for 2 h under Ar gas atmosphere. Microstructural characterizations of the as-blended/MA'd powders and the sintered composites were performed using X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Density and microhardness measurements and sliding wear tests were performed on the sintered composite samples. Sequentially milled and sintered Al-5 wt.% Si-2 wt.% (NbB2-NbC) samples had the highest mean microhardness value (2.29 ± 24.98 GPa) and the lowest wear volume loss (0.038 mm³).

Abstract: The fracture toughness and hardness of Ti (C_0.7N_0.3)-19Mo₂C-xNbC-24Ni cermets (x = 0, 5, 20) were studied. Fracture toughness of 5 NbC was the largest, and it of 20 NbC was the lowest. The microstructures of all the cermets consisted of Ti (C,N) and solid soluted Ti (C,N) hard phase, and Ni binder phase. The solid soluted Ti (C,N) surrounded Ti (C,N), namely, core-rim structures were observed in 0NbC and 5NbC. On the other hand, the isolated Ti (C,N) and solid soluted Ti (C,N) were observed in 20NbC, as a result of the phase separation between Ti (C,N) core and solid soluted Ti (C,N).

Abstract: The present work investigate the possibility of obtainment by mechanical alloying of Ni superalloys based on the Ni-Cr-Nb-C system strengthened by γ”(Ni3Nb), since γ”(Ni3Nb) as γ’ (Ni3Al) are typical coherent phase strengthening mechanisms in nickel superalloys. In order to evaluate this possibility, a composition with 71,65wt%Ni, 7,90wt%Cr, 20,00wt%Nb and 0,45wt%C was processed in a SPEX mill by 8 hours, consolidated and sintered at different temperatures (1200oC, 1250oC and 1300oC). The powder processed by MA and the sintered products were characterized by x-ray diffraction, SEM and EDS.

Abstract: Insulation is considered one of the effective solutions to achieve energy savings in buidings. Better insulation having low thermal conductivity contributes significantly to new construction and retrofitting existing buildings. The Energy Conservation Building Code and National Building Code of India define the prescriptive and mandatory requirements for the U-factor and R-values for different climates but the way to achieve these values is left to the designers. As none of the walling and roofing assemblies in buildings fulfill the criteria for overall thermal transmittance, the study deals with determining the thermal conductivity of sustainable walling materials and prefab roofing technologies as well as insulating materials using Guarded Hot-Plate Apparatus. The MATLAB program is developed for computing the U-values and for predicting the desired retrofit insulation thicknesses to make different materials and roofing assembly combinations comply the Code requirements in different climatic regions of India. The results of the study are used for computing the performance with and without insulation using DesgnBuilder software for improving energy efficinecy of the buildings in composite climate in India.

Abstract: This paper mainly studied the effect of niobium on graphite which was quantitatively analyzed by the image analysis software. The results showed that with the amount of niobium increasing the number of nodular graphite and the percent of spheroidization in ductile iron were reduced whereas the size of nodular graphite was increased.

Abstract: In this work, 10 wt.% stainless steel (AISI304, FeCrNi) was used as a binder in WC-based hardmetal. Graphite powder (2 wt.%) was added in the mixture to avoid the formation of η-phase ((MW)₆C) during sintering. NbC with 1, 1.5, 2 and 5 wt.% was used as WC grain growth inhibitor. The sintering process was done in a vacuum furnace at 1300°C for 1h. The results showed that there was no formation of η-phase or free graphite after sintering indicating the suitability of added graphite. Increasing the NbC contents resulted in decreasing of WC grains. 2 wt.% NbC was considered to be the optimal content for fine microstructure and uniform WC grain distribution. The highest hardness was achieved at 2 wt.% NbC with HV₃₀ of 1660 kg/mm². However, higher amount of added NbC (5 wt.%) caused the existence of coarse (Nb,W)C grains which decreasing the hardness.

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: Ceramic materials based on alumina are considered excellent for produce cutting tools used to machining hard metals. However, low mechanical strength and toughness presented by these materials limit their application. Traditionally particles, such as TiC, TiN and ZrO₂, are added to the alumina matrix to improve their mechanical properties, increasing the range of applications. Recent studies have shown that the addition of particles of different sizes in alumina matrix can promote simultaneous increase in mechanical strength and tenacity. In this work sintering behavior of Al₂O₃ micro-nanocomposite containing nanometric particles of NbC and micrometric particles of WC, was studied by dilatometry using heating rate of 20°C/min up to 1800°C. The addition of carbides in alumina matrix is prejudicial to sintering causing an increase in temperature of shrinkage.

Abstract: The influence of adding 6 wt% (NbC) niobium carbide on the sintering temperature and microstructure of high speed steel - AISI M₂ (0.87% C, 5.00% Mo, 6.00% W, 4,00% Cr, 2.00% V and Fe bal.) powder was studied. The starting material was obtained by vacuum melting followed by atomization in water. The samples were axially cold compacted in a cylindrical matrix and then vacuum sintered at 1250 and 1350 °C. Dilatometry and differential scanning calorimetry measurements indicated an increase in sintering temperature with addition of niobium to the AISI M₂ steel. Optical and scanning electron microscope observations revealed a microstructure with uniformly distributed niobium carbides.

Abstract: Precipitation strengthening by fine Nb-rich particles represents an important element on the design of low carbon high strength steels. This is typically obtained on steel strips by thermal holding at temperatures above 600°C following the austenite to ferrite transformation. These conditions are beneficial to obtain a large precipitation of small Nb-rich precipitates. On the other hand, precipitation at lower temperatures, in a phase already hard, such as bainite, has been scarcely studied. In this work, the precipitation phenomena occurring during isothermal treatments following the austenite to bainite transformation at 450°C are investigated. For this purpose, two Nb-alloyed low carbon steels with and without silicon are studied and the evolution of the microstructure is determined by the use of transmission electron microscopy and followed by hardness measurements. The presence of a hardness peak is not detected until long isothermal times (150h). Preliminary atom probe tomography (APT) characterization provides insight on the possible presence of fine NbC precipitates at the peak hardness treatment. A comparison with a Nb-free alloy indicates a significant hardening effect of niobium on the bainitic structure.