Papers by Keyword: Reactive Sintering

Abstract: The utilization of nickel aluminide compounds in high-temperature structural applications is advantageous due to their desirable properties. One efficient method for producing nickel aluminide samples with desirable chemical reactions using minimal energy is reactive sintering. In this study, Ni-Al (20 wt.%-80 wt.%) compounds were fabricated by initially cold pressing them, followed by reactive sintering. The reactive sintering process resulted in the formation of NiAl₃ and Ni₂Al₃ phases within the Ni-Al compounds. The microstructure, porosity, and hardness of the samples were thoroughly examined and analyzed. Generally, the compounds produced through reactive sintering exhibited significant porosity attributed to shrinkage and the Kirkendall effect. Microstructural analysis confirmed the presence of porosity, NiAl₃, and Ni₂Al₃ phases. The sintered sample processed at 400 °C demonstrated higher density and hardness. Additionally, the wear test indicated a low wear rate and friction coefficient for the sintered sample processed at 400 °C.

Abstract: Reactive sintering of cemented carbides involves mechanical and thermal activation of precursor elemental powders, followed by in-situ synthesis of tungsten carbide. This approach promotes formation of ultrafine microstructure favored in many cemented carbide applications. Our study focuses on the effect of mechanical activation (high-energy milling) on the properties of powder and following thermal activation (sintering) on the microstructure characteristics and phase composition. Reactive sintering proved effective – an ultrafine grained microstructure of cemented carbides with Co and Fe binders was achieved. Formation of tungsten carbide grains was complete at low temperature during reactive spark plasma sintering, resulting in textured microstructure with anisotropic grain formation and growth.

Abstract: Ti-Al-Si alloys are materials for high-temperature applications. They are characterized by low density, good mechanical properties and excellent resistance against oxidation in comparison with other commonly used alloys, for example nickel alloys or stainless steels. The preparation of Ti-Al-Si is very problematic due to high melting points of the intermediary phases, the high reactivity of melt with the melting crucibles and with the atmosphere in the furnace or formation of the cracks and pores during the process. Powder metallurgy seems to be a promising method for preparation of Ti-Al-Si alloys but there are still many complications. In this work, Ti-Al-Si alloys were prepared by unconventional powder metallurgy techniques and the aim of this work was to describe the problems during the sintering of these materials and their solution.

Abstract: Reactive sintering is a process where synthesis reaction of the ceramic phases is combined with sintering (densification) of the composite. Dense lightweight titanium oxycarbide-aluminium oxide ceramic-ceramic composites were produced from titanium dioxide, carbon black as graphite source and aluminium precursors by high energy attritor milling, followed by reactive sintering. Titanium oxycarbide and aluminium oxide phases were synthesized during reactive sintering in situ. To investigate the microstructure evolution and phase formation, the specimens were sintered at different temperatures (600-1725 °C) in vacuum. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and phase formation. Mechanical performance (hardness and fracture toughness) was evaluated.

Abstract: NiTi alloy is usually prepared by casting and forming. As an innovative process, reactive sintering powder metallurgy is tested worldwide, aiming to prepare pure NiTi alloy easily from nickel and titanium powders. This process enables to prepare both porous and low-porosity alloy, depnding on the process conditions. However, the formation of NiTi phase in this process is always accompanied by the Ti₂Ni phase, which is hard, brittle, less corrosion resistant and does not have the shape memory. In this work, various alloying elements (Al, Si, Mg, Fe, Nb, V) were added to Ni-Ti alloy in order to lower the amount of Ti₂Ni phase or at least to minimize its undesirable effect on the alloy properties. The reactive sintering behaviour, phase composition and mechanical properties of Ni-Ti-X alloys were described.

Abstract: In this work the influence of sintering method on the surface fatigue of carbide composite was studied. The research focuses on WC-15wt%Co hardmetals prepared using different powders and different sintering techniques and as a result different microstructure: conventional WC+Co powder and novel reactive powder type W+C+Co are sintered using vacuum furnace, compression sintering (sinterhipping) and spark plasma sintering (SPS method). As tungsten carbide is a common material for cold forming punches and surface degradation causes punch failure [1], the tool life can be significantly extended by material surface fatigue life improvement. It is expected that SPS production route of WC-15wt%Co hardmetal will conclude in better microstructure, more even average grain size distribution and smaller residual porosity, and respectively better mechanical and surface fatigue properties compared to conventional production routes. There are some expectations related to the reactive sintering production routes, as this technique promotes the fine microstructure and better mechanical properties.

Abstract: Transparent aluminum oxynitride (AlON) ceramics have been prepared through a method based on direct reaction sintering of alumina and aluminum nitride powders using MgO and Y₂O₃ as co-additives. The sintering additives could cause the formation of liquid phase during sintering, which would greatly promote the densification and eliminate pores. The grain size of AlON is about 50-100μm. The influence of different component of Al₂O₃ and AlN as well as sintering temperature on microstructure and optical properties of AlON have been studied. High transparent AlON ceramics with the in-line transmittance of 80.3% at 2000 nm wavelength have been prepared when the concentration of sintering additives was 0.16wt% Y₂O₃ and 0.02wt% MgO.

Abstract: Fe/Fe₂O₃ composite powders were obtained by mechanical milling of iron and hematite up to 120 minutes in a high energy planetary ball mill. The particles size decreases by mechanical milling upon the formation of the Fe/Fe₂O₃ composite particles. After 120 minutes of milling the median particles size is at 7.2 μm. The Fe/Fe₃O₄ type composite were obtained by reactive sintering in argon atmosphere at 1100 °C of the Fe/Fe₂O₃ composite powders milled for 60 and 120 minutes. After sintering a FeO-wüstite residual phase is formed and this phase is eliminated by applying a subsequent annealing at a temperature of 550 °C. The sintered compact before and after annealing is composed by a quasi-continuous iron matrix in which are embedded iron oxides clusters (Fe₃O₄ and FeO before annealing and Fe₃O₄ after annealing). The iron oxide clusters are analogous with the Widmanstatten structure observed in steels before and after annealing. The materials have been investigated using laser particle size analysis, optical microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction.

Abstract: The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.

Abstract: Indium-doped Cu₂SnSe₃ bulk materials with the Cu₂(Sn_1-xIn_x)Se₃ (In-x-CTSe) formula at x = 0, 0.05, 0.1, 0.15, and 0.2 were prepared at 550 °C for 2 h with soluble sintering aids of Sb₂S₃ and Te. Defect chemistry was studied by measuring structural and electrical properties of In-x-Cu₂SnSe₃ as a function of dopant concentration. In-x-CTSe pellets show p-type at x = 0, 0.05 and 0.1 and n-type at x = 0.15 and 0.2. The low hole concentration of 4.56×10¹⁷ cm^-3 and high mobility of 410 cm² V^-1 s^-1 were obtained for Cu₂(Sn_1-xIn_x)Se₃ bulks at x= 0.1 (10% In). The explanation based upon the In-to-Cu antisite defect for the changes in electrical property was declared.