A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy

Mixed 93W–4.9Ni–2.1Fe powders were sintered via the spark plasma sintering (SPS) and hybrid spark plasma sintering (HSPS) techniques with 30 mm and 60 mm samples in both conditions. After SPS and HSPS, the 30 mm and 60 mm alloys (except 60 mm-SPS) had a relative density (> 99.2%) close to the theoretical density. Phase, microstructure and mechanical properties evolution of W–Ni–Fe alloy during SPS and HSPS were studied. The microstructural evolution of the 60 mm alloys varied from the edge of the sample to the core of the sample. Results show that the grain size and the hardness vary considerable from the edge to the core of sintered sample of 60 mm sintered using conventional SPS compared to hybrid SPS. Similarly, the hardness also increased from the edge to the core. Furthermore, the 60 mm-HSPS alloy exhibited improved bending strength of 1115 MPa when compared to that of 60 mm-SPS, 920 MPa. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–Ni–Fe, however in the 60 mm-SPS alloy peeling of the grains was also observed which diminished the properties. The mechanical properties of SPS and HSPS 93W–4.9Ni–2.1Fe heavy alloys are dependent on the microstructural parameters such as tungsten grain size and overall homogeneity.     

Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

Microstructure and mechanical properties of a spark plasma sinteredTi–45Al–8.5Nb–0.2W–0.2B–0.1Y alloy

A high Nb containing TiAl alloy from pre-alloyed powder of Ti–45Al–8.5Nb–0.2B–0.2W–0.1Y was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The optimized conditions yield high densities and uniform microstructure. Specimens sintered at 1100 °C are characterized by fine duplex microstructure, leading to superior room temperature mechanical properties with a tensile strength of 1024 MPa and an elongation of 1.16%. Specimens sintered at 1200 °C are of fully lamellar microstructure with a tensile strength of 964 MPa and an elongation of 0.88%. The main fracture mode in the duplex microstructure was transgranular in the equiaxed γ grains and interlamellar in the lamellar colonies. For the fully lamellar structure, the fracture mode was dominated by interlamellar, translamellar and stepwise failure.     

Sub-micron binderless tungsten carbide sintering behavior under high pressure and high temperature

Pure tungsten carbide (WC) compacts of about 200 nm grain size were prepared by high pressure and high temperature (HPHT) method. The best property sample with high relative density (99.2%), high Vickers hardness (2925 kg·mm^− 2) and high fracture toughness (8.9 MPa·m^1/2) was obtained in the condition of 1500 °C temperature and 5 GPa pressure. By means of scanning electron microscopy (SEM) and transmission electron microscope (TEM) observations, a large number of twins and stacking faults appeared in sintered samples, and the grain size of sintered samples maintained in the initial range. The XRD patterns of bulk samples reveal that there is a phase transition from WC to W₂C with the increasing of temperature. Moreover, the effect of HPHT condition for sintering kinetics, microstructure evolutions, and mechanical properties of the sintered samples were also discussed.     

Self-compacting high density tungsten–bronze composites

Green compacts of W–bronze were encapsulated in shells of bronze powder, placed in a ceramic mold and sintered in alumina tube furnace at 1150 °C. Throughout the sintering cooling stage the differential coefficient of thermal expansion ΔCTE of W–bronze was employed to induce an external compressive densification action. The process included simultaneous sintering, hot isostatic pressing (HIP) and infiltration act to enhance densification. By this technique, pilot sintered compacts of different W50–80 wt.%–pre-mix bronze of 97–99% theoretical density were produced. This process resulted in compacts of higher hardness, higher sintered density and better structure homogeneity as opposed to similar compacts densified by the conventional sintering process. The results showed a gain in hardness by 10–20% and in density by 5–15%. The impact of different cooling rates of 3, 4, 8 and 30 °C min^?1 on sintered density, microstructure and densification mechanisms was examined and evaluated. Low cooling rates of 3 and 4 °C min^?1 gave the best results.     

Cooling rate and carbon content effect on the fraction of secondary phases precipitate in as-cast microstructure of ASTM F75 alloy

Simple test castings were used to study the effect of cooling rate and carbon content in as-cast microstructure of alloy ASTM F75, Co–26 wt.% Cr–5.7 wt.% Mo. Alloys with four C content (0.45, 0.33, 0.36 and 0.25 wt.%) were poured into investment ceramic molds. In order to obtain different cooling rates, the castings were constituted of three axisymmetrical cylinders of different diameters (12, 16 and 24 mm). Cooling curves were obtained from each cylinder and the fraction of secondary phases in as-cast microstructure was measured by image analysis. Average cooling rates of 100, 60 and 20 °C/min, were obtained in the 12, 16 and 24 mm diameter cylinders respectively, at the first solidification step occurring in the temperature range from 1390 to 1350 °C. A significant effect of this cooling rate range on the fraction of secondary phase was not observed.It was observed that the fraction of blocky carbides increased proportionally as the C content increased, whereas the amount of the eutectoid constituent showed a significant increase only in the sample containing 0.45 wt.%, as compared with the samples of other carbon contents. It was also found that the initial solidification undercooling affects the temperature of sigma phase precipitation with which the solidification of the alloy is completed.     

Processing and characterization of sintered reaction bonded Si3N4 ceramics

The present contribution reports the influence of nitridation and sintering conditions on the densification, microstructure, mechanical and thermal conductivity properties of sintered reaction bonded Si₃N₄ (SRBSN) mixed with 3.5% Y₂O₃-1.5% MgO. The nitridation of samples was carried out at 1450 and 1500 °C for different time schedules (2.5, 8 and 16 h) in order to increase β Si₃N₄ phase and subsequently sintering was performed at various temperatures (1850, 1900 and 1950 °C) for 10 h to enhance densification and properties of SRBSN ceramics. It was observed that the density of the samples slightly decreased and β Si₃N₄ phase significantly increased to 87% with increasing nitridation temperature and time. The density of gas pressure sintered (GPS) samples increased with increasing sintering temperature, almost full density was measured for all the samples at the respective sintering temperature (except those samples which were given nitridation at 1500 °C for 16 h). The microstructure of SRBSN samples were characterized by bimodal microstructure with equiaxed and rod like elongated grains and average grain size of SRBSN samples varied between 1.62 and 2.43 μm and aspect ratio of grains varied from 3.78 to 6.88 with varying the sintering temperature. Depending on the sintering density and microstructure, the SRBSN samples exhibited hardness (16.69 to 19.47 GPa), fracture toughness (7.02 to 9.20 MPa·m^1/2) and thermal conductivity (77.32 to 98.52 W/m·K). The coarsening of grain size and aspect ratio negatively affected hardness and fracture toughness, on the contrary the thermal conductivity increased. Among all samples, the SRBSN (which was subjected to nitridation at 1500 °C for 16 h; GPS at 1950 °C for 10 h) measured with good combination of hardness: 17.32 GPa, fracture toughness: 8.36 MPa·m^1/2and thermal conductivity: 98.52 W/m·K.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

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.     

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.     

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.     

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.     

Fabrication of W–Cu functionally graded material by spark plasma sintering method

Three-layered (W–25Cu/W–50Cu/W–75Cu, volume fraction) W/Cu functionally graded material (FGM) was synthesized by spark plasma sintering (SPS) at different temperatures for 5 min under a load of 40 MPa. The influences of different sintering processes on relative density, hardness, thermal conductivity and microstructure at various layers of sintered samples were investigated. The experimental results indicated that the graded structure of the composite could be well densified after the SPS process. The relative density increased with the increment of sintering temperature and it was up to 96.53% as sintered at 1050 °C. In addition, the thermal conductivity reached 140 W/m·K at room temperature and 151 W/m·K at 800 °C, which could be ascribed to the specific structure that W particles enwrapped by net-like Cu. And the Vickers hardness was converted from 4.11 to 4.68 GPa.     

Effects of Al2O3 addition on the microstructure and properties of Ni activated sintered W matrix composites

In the present investigation, fabrication of high dense (> 97.8%) W matrix composites with increased microhardness values were investigated. W and W–1 wt.% Ni powders were mechanical alloyed for 18 h and sintered at 1400 °C for 1 h under Ar, H₂ gas flowing conditions in order to investigate the effects of 1 wt.% Ni addition on the densification and properties of W. The effects of Al₂O₃ particles additions on the microstructural and physical properties of the sintered W–1 wt.% Ni sample were investigated. A 92.59% relative density value of the sintered W sample increased to 99.47% with the addition of 1 wt.% Ni. Moreover, despite the observed grain growth, microhardness values significantly increased from 2.81 ± 0.34 GPa to 4.07 ± 0.16 GPa with the addition of 1 wt.% Ni. The relative density values of the sintered W–1 wt.% Ni sample slightly decreased with increasing Al₂O₃ additions, a relative density value of 97.81% was measured for the W–1 wt.% Ni sample reinforced with 2 wt.% Al₂O₃ particles. As the average grain size of W in the sintered W–1 wt.% Ni sample decreased from 4.41 ± 1.71 μm to 1.29 ± 0.39 μm with the addition of 2 wt.% Al₂O₃ particles, the microhardness of the sample increased to 5.98 ± 0.31 GPa.     

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.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Microstructure and mechanical properties of bulk nanocrystalline Al–Fe alloy processed by mechanical alloying and spark plasma sintering

Nanocrystalline Al–5 at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000 MPa with a large plastic strain of 15% at room temperature and 500 MPa at 350 °C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al₆Fe nanocrystalline regions with 90 nm in diameter and a minor fraction of Al₁₃Fe₄ phase and coarsened 0.5–1 μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000 MPa at room temperature.     

Morphological evolution of semi-solid Mg2Si/AM60 magnesium matrix composite produced by ultrasonic vibration process

The application of ultrasonic vibration treatment (UVT) produced a nearly non-dendritic and refined semi-solid microstructure of Mg₂Si/AM60 composite. The effects of UVT temperature and time on microstructure of the semi-solid slurry were studied. A good semi-solid slurry with average grain size of 75 μm and shape coefficient of 0.53 could be obtained by applying UVT at 620 °C for 60 s, which were decreased by a factor of 17/20 and increased by a factor of 3 respectively as compared to the sample without treatment. The Mg₂Si and Mg₁₇Al₁₂ intermetallics are mainly located along the grain boundaries or dispersed uniformly in the metallic liquid matrix with network morphology. Mechanisms involved in the development of microstructure are discussed.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

Effects of sintering processes on the mechanical properties and microstructure of Ti(C,N)-based cermet cutting tool materials

In this study, two types of Ti(C_0.7,N_0.3)-based cermet cutting tool materials (Ti(C,N)–Mo–Ni–Co, named as TMNC, and Ti(C,N)–WC–Mo–Ni–Co–TaC–HfC, named as TWMNCTH) were fabricated by the hot pressed sintering process at different temperatures (from 1380 °C to 1500 °C) for different holding times (from 30 min to 60 min) in a vacuum atmosphere and at a compressive stress of 32 MPa. The polished surface and the fracture surface of the two types of cermets were observed by a scanning electron microscope (BSE/SEM) and energy dispersive spectrometry (EDS), and the relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that the sintering temperature and holding time both had a great influence on the flexural strength and a small effect on the hardness and the fracture toughness of the two types of cermets. The two cermets both had the optimal comprehensive mechanical properties when they were sintered at 1400 °C for 30 min. The sintering temperature and holding time also had a great influence on the microstructure of the two cermets, and the grain sizes increased when the sintering temperature varied from 1400 °C to 1500 °C and the holding time varied from 30 min to 60 min. The properties and microstructure of the two cermets were also compared. The results indicated that the cermet TWMNCTH had a lower flexural strength, a similar value of fracture toughness, a higher hardness and a thicker rim in the microstructure.