SiC–Ti layered material prepared by binder-treated powder sintering

Ti–SiC layered material was prepared by binder-treated powder metallurgy method. Ti, SiC and C powders were ball-milled and then binder treated. Mixture of the treated powder was loaded into a stainless steel mold first and followed by loading the pure Ti powder on top of the treated powder, then compacted under a pressure of 200 MPa at 165 °C with a pressing speed of 250 mm/min. The green compacts were debinded at 500 °C for 1 h and sintered at 1500 °C for 2 h under argon atmosphere. The sintering temperature was determined by measuring the phase formation temperatures between Ti, SiC and C powders, using differential scanning calorimetric method. The reaction products after sintering were analyzed. Microstructure of the prepared Ti–SiC layered material was also studied.     

Fabrication of W–20 wt.%Cu alloys by powder injection molding

W–20 wt.% Cu balls were fabricated by powder injection molding using a binder system consisted of paraffin wax, high density polyethylene, ethylene vinyl acetate and stearic acid. By optimizing the injection molding parameters, defect-free green parts were obtained. A two-step debinding process was employed to extract the binders in the molded samples. All soluble ingredients of the binders in the green parts were extracted during solvent debinding, and the residual binders can be removed in thermal debinding. The debound W–Cu samples were sintered in H₂ atmosphere at temperatures ranging 1050–1150 °C for 2 h. It was shown that relative density of the sintered W–Cu samples increases from 87.37% of the theoretical to 95.58% as sintering temperature rises from 1050 °C to 1150 °C. Microstructures of the molded, the debound and the sintered W–Cu samples were observed by scanning electron microscope, and the sintered W–Cu balls have fine and homogeneous microstructures. Maximum compressive strength of W–Cu balls with 8.5 mm diameter reaches 58 kN.     

Stainless steel bonded NbC matrix cermets using a submicron NbC starting powder

The microstructure and mechanical properties of 316 L and 430 L stainless steel bonded NbC cermets were assessed. NbC starting powder mixtures with 15 and 30 vol% steel binder were pressureless vacuum sintered for 1 h at 1420 °C. The liquid forming temperature and shrinkage behaviour of the green powder compacts were investigated by differential scanning calorimetry and dilatometry. Microstructural and compositional analysis were conducted by electron probe microanalysis (EPMA) and XRD to investigate the effect of the steel binder on NbC grain growth and Cr-rich carbide precipitation. Rapid NbC grain growth was observed and the average NbC grain size decreased with increasing binder content. The residual Cr-rich carbide located at NbC grain boundaries can be eliminated by the addition of carbide forming metal precursors such as TiH₂ or by a thermal annealing process of the sintered NbC cermets at 1200 °C. The hardness and fracture toughness of the NbC-steel cermets was influenced by the steel binder type and content. A maximum hardness of 13.6 GPa was measured for the NbC-15 vol% 430 L cermet, combined with a modest fracture toughness of 7.3 MPa m^1/2.     

Microstructure instability in TiC-316L stainless steel cermets

Titanium carbide (TiC) based cermets are commonly used in wear and corrosion resistance applications. The microstructural evolution, and related compositional instability, of TiC-based cermets prepared with a 316-L stainless steel binder is described in the present work. Samples were fabricated using a simple vacuum melt-infiltration procedure, with 5 to 30 vol.% binder. Infiltration temperatures ranged from 1475 °C to 1550 °C, held for up to 240 min, typically resulting in sintered samples with densities in excess of 99% of theoretical. It is demonstrated that irregularly shaped grains (concave/hollow) can arise after sintering, especially at 1475 °C, which is discussed in terms of the ‘instability of the solid-liquid interface’ theory. It is demonstrated that a complex, multi-layer core-rim structure arose for the cermets, with accommodation of selected steel constituents into the rim of the TiC grains. In particular, it is shown that the Mo in the original 316-L stainless steel is essentially fully depleted from the metallic binder phase, forming a Mo-rich inner-rim layer on the TiC grain cores.     

Preparation of self-lubricating Ti(C,N)-based cermets by solid carburization and wear behavior

In this work, self-lubricating Ti(C,N)-based cermets were prepared by solid carburization. The sintered cermets were wrapped by carburizing agent and sintered again at 1440 °C with different time. The microstructure and composition of cermets were studied. The wear behavior of cermets containing graphite phase was also evaluated using a block-on-ring tribometer. The results showed that the carbon content increased gradually in binder phase with carburizing time. When the carburizing time was 3 h, the carbon got saturated in binder phase. When the carbon content exceeded the solubility in the binder, excessive carbon precipitated and formed graphite phase. Uniformly distributed graphite clusters formed in cermets after carburization for 4 h. The graphite clusters consisted of flocked graphite particles. With the carburizing time extended to 5 h, the graphite clusters became large and some of them interlaced together. Besides, the wear results indicated that the volume loss of cermets containing graphite phase was half of that without graphite due to the formation of smooth tribofilm on the worn surface of cermets.     

The development of porous titanium products using slip casting

An optimized titanium slurry was developed from 43 vol.% of titanium powder, 0.3 dw.% of dispersant, 0.8 dw.% of plasticizer and 0.8 dw.% of binder, mixed with a balance of distilled water, which produced a viscosity of 40 cP. It was then poured into a plaster mold to form compacts with a green density of 45%. Thermal debinding was carried out at 320 °C with an argon flow for 2 h, followed by vacuum sintering different samples at 1000 °C and 1200 °C for 0.5 h, respectively. The porous sintered compacts had satisfactory tensile strength with some plastic deformation. The increase in oxygen and carbon content during processing was minor. An X-ray diffraction pattern showed pure alpha titanium peaks without any indication of contamination from organic additives. The results from this investigation suggested that slip casting is a potentially low-cost, simple production route for manufacturing porous titanium products.     

Study on the formation of cubic texture in Ni–7 at.% W alloy substrates by powder metallurgy routes

One of the main challenges for coated conductor applications is to produce sharp cubic textured alloy substrates with high strength and low magnetism. In this work, the cubic textured Ni–7 at.% W substrates were prepared from different powder metallurgy ingots by rolling-assisted biaxially textured substrate processing. The fabrication processes of cubic texture in the Ni–7 at.% W tapes by two powder metallurgy routes are described in detail. Through the optimized process, full width at half maximum values of 6.7° and 5.0° were obtained, as estimated by X-ray (1 1 1) phi scan and (2 0 0) rocking curve in the textured Ni–7 at.% W tape, respectively. By electron backscattering diffraction (EBSD) analysis, the percentage of the cubic textured component in the Ni–7 at.% W tape surface was found to reach 97.0% within a tolerance angle smaller than 10°. Moreover, the formation mechanism of the cubic texture in the Ni–7 at.% W tape were also investigated by EBSD. Particular attention was focused on the difference in the texture components along the thickness of the partially recrystallized samples.     

Alkali doped poly(vinyl alcohol) for potential fuel cell applications

Optical transparent, chemically stable alkaline solid polymer electrolyte membranes were prepared by incorporation KOH in poly(vinyl alcohol) (PVA). The distributions of oxygen and potassium in the membrane were characterized by XRD and SEM–EDX. It is demonstrated that combined KOH molecules may exist in the PVA matrix, which allow it to be an ionic conductor. In particular, the chemical and thermal stabilities were investigated by measuring changes of ionic conductivities after conditioned the membrane in various alkaline concentrations at elevated temperatures for 24 h for potential use in fuel cells. The membranes were found very stable even in 10 M KOH solution up to 80 °C without losing any membrane integrity and ionic conductivity due to high dense chemical cross-linking in PVA structure. The measured ionic conductivity of the membrane by AC impedance technique ranged from 2.75 × 10^?4 S cm^?1 to 4.73 × 10^?4 S cm^?1 at room temperature, which was greatly increased to 9.77 × 10^?4 S cm^?1 after high temperature conditioning at 80 °C. Although, a relatively higher water uptake, the methanol uptake of this membrane was one-half of Nafon 115 at room temperature and 6 times lower than that of Nafion 115 after conditioned at 80 °C. The membrane electrolyte assembly (MEA) fabricated with PVA–KOH in direct methanol fuel cell (DMFC) mode showed an initial power density of 6.04 mW cm^?2 at 60 °C and increased to 10.21 mW cm^?2 at 90 °C.     

Effect of starch filler content and sintering temperature on the processing of porous 3Y–ZrO2 ceramics

A direct casting process was used to produce porous 3Y–ZrO₂ ceramics using starch as a fugitive filler and binder. The compositions with low additions of starch had higher porosity than the volume fraction of starch initially in the green body (X_st), whereas, the compositions with high amounts of starch produced lower porosity than the predicted value. The well ordered structure consisted of spherical pores of 8–10 μm diameter, retained from the original starch particles, connected by channels. The interconnection between pores was dependent on the volume fraction of starch incorporated, as well as on the sintering temperature. Pore interconnection was observed for all the compositions sintered at 1000–1300 °C. Increasing the sintering temperature to 1400–1500 °C produced a marked dependence of the open to total porosity ratio on X_st. For a high porosity material, a bimodal channel size distribution was found at 1400 and 1500 °C. The primary pore channel diameter was 0.7 μm and the secondary one was close to 4 μm. As the sintering temperature increased, the volume of the connecting channels decreased; at 1500 °C only a minor volume of the larger channels was found.     

Effect of the Cr content on the sintering behaviour of TiCN–WC–Ni–Cr3C2 powder mixtures

In TiCN–W–Cr–Ni cermets produced by liquid phase sintering melting occurs at lower temperatures as their Cr content increases. For low Cr additions (up to 4 wt.%) eutectic temperatures are close to those found in the TiC–WC–Ni system. For 8 wt.% Cr and above, temperatures are similar to those found in the Cr–Ni–C system. The precipitation of M₇C₃ carbides is observed to start at 8 wt.% Cr in samples sintered at 1425 °C for 1 h. This sets a limit for the Cr solubility in the binder phase of these cermets around 18 wt.%. The dissolution of WC and Cr₃C₂ particles starts at temperatures as low as 1150 °C, but that the homogenization of the binder phase is only achieved after melting. The carbonitride phase exhibits the typical precipitation of inner and outer rims onto Ti(C,N) cores. However, a fine precipitation of Ni-rich particles is found inside Ti(C,N) cores, likely related to coalescence phenomena.     

Synthesis of nano-sized Fe–Ni powder by chemical process for magnetic applications

Synthesis of nano-sized Fe–Ni permalloy of the composition 20 wt% Fe and 80 wt% Ni took place by electroless chemical reduction method in alkaline tartarate bath using hypophosphite as a reducing agent. The powder was cold compacted at 600 MPa and then sintered at 1050 °C. Metallographic investigations were performed by optical microscope and SEM with EDAX analysis. Hot-stage XRD was performed for the investigated materials to follow the phase transformations in the material. Physical, magnetic and electrical properties were studied for the prepared powder and its related sintered compacts. FeNi powder prepared from the experiments has a 200 nm particle size with 2.4 wt% phosphorus content. The prepared powder has amorphous structure with a low saturation induction (B_s) but by raising the temperature to 500 °C, the FeNi₃ intermetallic appears first and then the cubic FeNi solid solution is formed at 1050 °C, which has the highest saturation induction value.After cold compaction and sintering, the electrical conductivity and the saturation induction increased by increasing the time of sintering but the coercive force decreased and the material becomes softer after sintering. Measurements of the magnetic permeability indicate that the optimum applying field for the investigated sintered material is between 40 and 100 Oe which gives the highest range of the magnetic permeability. From the magneto-resistance measurements, it is shown that the sintered material has a positive magneto-resistance in the field direction but a negative one in the direction perpendicular to the current and the field.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

Warm die compaction and sintering of titanium and titanium alloy powders

A study has been made of the effect of non-lubricated warm die (200 °C) compaction on the densification of hydride–dehydride (HDH) Ti powder, pre-alloyed (PA) Ti-6Al-4V and Ti-10V-2Fe-3Al powders, and HDH Ti and V-Fe-Al master alloy powder blends, compared to cold die compaction. Depending on the compaction pressure, which was varied from 200 to 1000 MPa, non-lubricated warm die (200 °C) compaction was very effective for −100 mesh HDH Ti powder, increasing the green density by 5.0–9.4% theoretical density (TD). Die wall lubrication with stearic acid showed no influence on the green density when compacted at 800 MPa. With warm die (200 °C) compaction, achieving a green density of greater than 90%TD was straightforward for HDH Ti powder when compacted at ≥750 MPa. Accordingly, near pore-free (≥99.5%TD) Ti microstructures were obtained after sintering at 1300 °C for 120 min in vacuum when compacted at 1000 MPa. The resulting increment in the sintered density was between 2.0%TD and 4.4%TD. Warm die (200 °C) compaction showed no effect on PA Ti-10V-2Fe-3Al powder and only a small effect on PA Ti-6Al-4V powder when compacted at 1000 MPa. However, it was still virtually effective for Ti-10V-2Fe-3Al powder blends made of HDH Ti powder and V-Fe-Al master alloy powder. The observations were compared with literature data and discussed in accordance with the yield strength of Ti, Ti-6Al-4V, Ti-10V-2Fe-3Al and Al₃V as a function of temperature.     

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2 GPa, 1450 °C for 6 min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co₃W₃C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3 GPa and cutting test were performed when sintered by using cBN grain size of 10–14 μm.     

Metallurgical assessment of a hypereutectic aluminum–silicon P/M alloy

The objective of this study was to perform a comprehensive assessment on the press-and-sinter P/M processing of a novel hypereutectic aluminum–silicon alloy known as Alumix-231 (Al–15Si–2.5Cu–0.5Mg). As this patented commercial product is relatively new, the amount of scientific data available in the literature is highly limited. To address this issue sintered products were evaluated by analyzing the density before and after sintering, apparent hardness, microstructural features, tensile properties, and finally the response to heat treatment. It was determined that the optimum processing route for Alumix-231 involved compaction at a pressure of 600 MPa followed by de-lubrication at 400 °C for 20 min before being sintered at 560 °C for a period of 60 min. The optimum heat treatment involved solutionizing the samples at 520 °C for 1 h, followed by water quenching, and artificially aging the samples at 160 °C for 8 h. The system proved highly responsive to this manner of processing attaining a sintered density on the order of 98% of theoretical and a UTS of 330 MPa. Based on a combination of DSC, XRD, and EPMA analyses it was concluded that θ-type phases were the dominate precipitates formed during heat treatment.     

Absence of the core–rim microstructure in TixTa1 − xCyN1 − y-based cermets developed from a pre-sintered carbonitride master alloy

(Ti,Ta)(C,N) solid solution-based cermets with cobalt as the binder phase were synthesised by a two-step milling process. The titanium–tantalum carbonitride solid solution (the ceramic phase) was obtained via a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental Ti, Ta, and graphite powder blends in a nitrogen atmosphere. Elemental Co (the binder phase) was added to the ceramic phase, and the mixture was homogenised by mechanical milling (MM). The powdered cermet was then sintered in a tubular furnace at temperatures ranging from 1400 °C to 1600 °C in an inert atmosphere. The chemical composition and microstructure of the sintered cermets were characterised as ceramic particles grown via a coalescence process and embedded in a complex (Ti,Ta)–Co intermetallic matrix. The absence of the typical core–rim microstructure was confirmed.     

Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo₂C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo₂C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV₃₀) of 1077 ± 22 kg/mm² and an indentation toughness of 9.1 ± 0.5 MPa·m^1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm², whereas the toughness increased to 11.3 ± 0.1 MPa·m^1/2. Addition of 5 and 10 vol% Mo₂C into a NbC-12 vol% Ni mixtures generated the same values in HV₃₀ and K_IC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC + 4 vol% Mo₂C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m^1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.     

Ductilisation of tungsten (W): On the increase of strength AND room-temperature tensile ductility through cold-rolling

Here we show that cold-rolling is a method to achieve room-temperature ductility in commercial purity, monolithic tungsten (W). Furthermore, we show that a decrease in rolling temperature concomitantly increases the strength and ductility of tungsten. So cold-rolling is a way to overcome the strength–ductility trade-off.In this work, we assess three different cold-rolled microstructures obtained from rolling at (i) 1000 °C (1273 K), (ii) 800 °C (1073 K), and (iii) 600 °C (873 K). Benchmark experiments were performed on a sintered ingot as well as on a hot-rolled plate. From these plates tensile test specimens were cut by spark erosion and tested at room temperature. The results show an increase of total uniform elongation, A_ut, ranging from 1.38% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 1.47% (cold-rolled at 600 °C (873 K)) and an increase of the total elongation to fracture, A_t, ranging from approximately 3% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 4.19% (cold-rolled at 600 °C (873 K)) with decreasing rolling temperature.The microstructure of the plates is analysed by means of scanning electron microscopy (SEM) (grain size, subgrains, crystallographic texture) and transmission electron microscopy (TEM) (bright field imaging, scanning TEM). Furthermore, strain-rate jump tests have been performed at 400 °C (673 K) to determine the strain-rate sensitivity, m, (sintered ingot m = 0.088, cold-rolled at 600 °C (873 K) m = 0.011) and the activation volume, V, (hot-rolled W plate V = 191 b³, cold-rolled at 600 °C (873 K) V = 111 b³) of the tungsten sheets.The question of why cold-rolling increases both strength and ductility is discussed against the background of cold-rolling-induced lattice defects. We speculate that the increase of ductility is caused by the ordered glide of screw dislocations, that move with low deformation incompatibility along the high-angle grain boundary (HAGB) channels (confined plastic slip).     

Ni–NiO composites obtained by controlled oxidation of green compacts

Ni–NiO composites have been obtained by the thermally induced oxidation of metallic green compacts at temperatures between 300 and 450 °C and further sintering. Thermogravimetric studies showed that oxidation process in air follows a quadratic dependence with time for temperatures between 300 and 400 °C allowing the control in the metal to oxide ratio. Microstructural analyses of compacts sintered in inert atmosphere reveal a homogeneous distribution of phases. In the mechanical tests the metal to ceramic ratio variation is evident in the ductile to brittle transition of the fracture, making this method suitable to fabricate compacts with tailored mechanical properties.