Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Approach of the spark plasma sintering mechanism in Zr57Cu20Al10Ni8Ti5 metallic glass

Spark plasma sintering (SPS) was used to sinter gas-atomized Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ amorphous powder. Systematic analyses were performed to study particle size and annealing time effects on the parts structure and properties. Partial devitrification and particles welding were observed and correlated to particle size and thermal conditions. Mechanical testing, through compression and micro-hardness, reveals that the sintered parts show strength similar to a quenched bulk metallic glass and damaging before failure. However, the pulsed current input does not seem the most relevant way to sinter amorphous powders: during the sintering initial stages (when necks are small), excessive over-heating is generated in the vicinity of particles necks, and is responsible for partial devitrification; further current input at large necks leads to complete densification. Effects of the stress, the thermo- and electro-transports on the sintering are evaluated to provide a better understanding of the SPS mechanisms of densification of metallic glasses.     

Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α₂-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α₂/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.     

Densification behavior of nanocrystalline W–Ni–Fe composite powders prepared by sol-spray drying and hydrogen reduction process

This paper studied the densification behavior of nanocrystalline composite powders of 93W–4.9Ni–2.1Fe (wt.%) and 93W–4.9Ni–2.1Fe–0.03Y synthesized by sol-spray drying and hydrogen reduction process. The X-ray diffraction (XRD) analysis showed that γ-(Ni, Fe) phase was formed in the final obtained powders. Powders morphology characterized by scanning electron microscope (SEM) showed that the 93W–4.9Ni–2.1Fe nanocrystalline composite powders exhibited larger agglomeration and grain size compared with the 93W–4.9Ni–2.1Fe–0.03Y nanocrystalline composite powders. Both kinds of green compacts can obtain full density if sintered at 1410 °C for 1 h. When sintering temperature was above 1410 °C, the sintering density for both compacts decreased rapidly. In addition, the sintering density, densification rate and grain coarsening rate of 93W–4.9Ni–2.1Fe compacts were higher than those of 93W–4.9Ni–2.1Fe–0.03Y. The effect of trace yttrium on the densification behavior of nanocrystalline composite powders was also discussed.     

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.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

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.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

The mechanisms of microstructure formation in a nanostructured oxide dispersion strengthened FeAl alloy obtained by spark plasma sintering

A bulk dense nanostructured material, obtained by spark plasma sintering (SPS) of Y₂O₃ dispersion strengthened milled Fe–40Al powder, is characterized in detail using scanning (SEM) and transmission electron microscopies (TEM) in order to investigate the mechanisms of its microstructure formation. The sintered material displays a fairly heterogeneous microstructure that covers nano- and ultrafine together with large micrometric grains. The fine grains result from recovery and recrystallization, while the larger ones from grain growth or local melting. Under the present SPS conditions, large temperature differences in the range 570–670 °C, due to rapid heating–cooling and also to no holding stage applied, essentially account for such a structural heterogeneity. Controlling SPS of the milled powder thus provides a feasible processing route to get dense hetero-nanostructured material. In addition, complex oxide particles formed in the material are analyzed to be related to precipitation reaction and oxide evolution at different sintering temperatures.     

Fabrication of full density near-nanostructured cemented carbides by combination of VC/Cr3C2 addition and consolidation by SPS and HIP technologies

The aim of present work is to study the effect of VC and/or Cr₃C₂ in densification, microstructural development and mechanical behavior of nanocrystalline WC-12wt.%Co powders when they are sintered by spark plasma sintering (SPS) and hot isostatic pressing (HIP). The results were compared to those corresponding to conventional sintering in vacuum. The density, microstructure, X-ray diffraction, hardness and fracture toughness of the sintered materials were evaluated. Materials prepared by SPS exhibits full densification at lower temperature (1100 °C) and a shorter stay time (5 min), allowing the grain growth control. However, the effect of the inhibitors during SPS process is considerably lower than in conventional sintering. Materials prepared by HIP at 1100 °C and 30 min present full densification and a better control of microstructure in the presence of VC. The added amount of VC allows obtaining homogeneous microstructures with an average grain size of 120 nm. The hardness and fracture toughness values obtained were about 2100 HV₃₀ and close to 10 MPa m^1/2, respectively.     

Spark plasma sintering of ZrB2 powders synthesized by citrate gel method

The densification behavior of nanocrystalline zirconium diboride (ZrB₂) powders with nickel (5 vol%) is reported by spark plasma sintering (SPS) technique. SPS experiments were performed at 1600 and 1900 °C with 65 MPa pressure and 1 min holding time. A maximum relative density around 95% was obtained after SPS processing of ZrB₂ at 1900 °C while the density of ZrB₂ sample sintered at 1600 °C reached 88% of the theoretical density. Hardness and fracture toughness values are 11 GPa and 4.11 MPa m^1/2 for the sample sintered at 1600 °C and 13.7 GPa and 2.65 MPa m^1/2 for the sample sintered at 1900 °C, respectively.     

A study on the microstructure of D023 Al3Zr and L12 (Al+12.5 at.% Cu)3Zr intermetallic compounds synthesized by PBM and SPS

The spark plasma sintering (SPS) of L1₂ phase Al₃Zr and (Al+12.5 at.% Cu)₃Zr powders with a nanocrystalline microstructure has been studied to produce bulk intermetallic compounds which maintain metastable structures such as L1₂ structure and nanocrystalline microstructure. The powders were prepared by 10 h planetary ball milling (PBM). Full-density L1₂ (Al+12.5 at.% Cu)₃Zr intermetallic compounds were obtained by SPS for 0 min at 600 °C. The specimens prepared with a longer holding time than 0 min at 600 °C or a higher temperature than 600 °C had local melting areas where micro-cracks were found. They had a lower relative density than the specimen SPS sintered at 600 °C for 0 min. The smallest grain size was obtained in the specimen prepared at 600 °C for 0 min, which was 20–30 nm as confirmed by TEM observation. This was the smallest grain size ever reported in the trialuminide specimens processed by various consolidations of nanocrystalline powders. Accordingly, the highest micro-hardness, 989.5 H_V, was obtained in the specimen and this value was three times higher than those of the specimens with micro grain sizes. Full density Al₃Zr intermetallics were prepared by SPS at 700 °C for 0 min. However, their crystal structure was D0₂₃ and micro-hardness was 778.1 H_V. By using SPS, the sintering time can be reduced within 10 min. It was thought that the decrease in sintering temperature for the PBM Al₃Zr and (Al+12.5 at.% Cu)₃Zr powders by 200–300 °C compared with the conventional sintering temperature resulted in the refinement of microstructure to the nano-size level.     

Microstructure and mechanical properties of beta TiAl alloys elaborated by spark plasma sintering

The microstructure evolution and room temperature mechanical property of beta containing Ti–44Al–3Nb–1Mo–1V–0.2Y alloy consolidated by spark plasma sintering was studied. Pre-alloyed powders were sintered for 2 min in the range 900–1250 °C under 100 MPa. It was found that duplex and lamellar microstructures were obtained depending on the SPS temperature. The duplex microstructure formed at 1150 °C and 1175 °C, and the lamellar structure was achieved above 1200 °C. However, coarsening of lamellar colonies occurred with further increasing of the sintering temperature. The specimen with fine lamellar colonies exhibited a relatively high compressive strength, whereas the one with duplex microstructure showed a superior final strain.     

Microstructure and mechanical properties of the TiZrNbMoTa refractory high-entropy alloy produced by mechanical alloying and spark plasma sintering

The equiatomic refractory high entropy alloy (HEA) TiZrNbMoTa was investigated. The alloyed powders with face-centered cubic (FCC) structured solid solution phase were prepared by mechanical alloying (MA) and then sintered by spark plasma sintering (SPS) at 1300, 1400, 1500, and 1600 °C. The microstructure and mechanical properties of the bulk alloy were investigated. The body-centered cubic (BCC) structured solid solution phase and the ZrO₂ phase precipitated from the FCC structured solid solution phase during cool-down from sintering. The highest compression fracture strength (3759 MPa) and fracture strain (12.1%) were obtained in the refractory HEA sintered at 1400 °C. The grain boundary strengthening, precipitation strengthening, solid solution strengthening, transformation-induced plastic (TRIP) effect, and toughening effect of the ZrO₂ phase are the important factors for the high strength and ducitily of the refractory HEA prepared in this study.     

The influence of powder preparation condition on densification and microstructural properties of WC-Co- Al2O3 cermets

This study investigated how powder preparation during WC-10Co production with the addition of 10 wt% Al₂O₃ influenced its microstructural and mechanical properties. Powders were mixed with a mechanical shaker for 10 min and high energy milling for 2, 6, 10, 20, 30, and 50 h. The powders were then compacted at 200 MPa and sintered in a resistive dilatometric furnace for one hour, under an argon atmosphere, at a heating rate of 10 °C / min, and two sintering temperatures (1400 °C and 1550 °C). XRD and SEM/EDS analyses were carried out for both powders, which were sintered in order to examine their composition and morphology. The sintered powders were also characterized in terms of mechanical properties, densification, and dilatometric shrinkage. The results show that samples milled for 50 h and sintered at 1550 °C exhibited microstructures with denser phases than those of samples mixed in the shaker. The properties measured were around 68%, 45%, −0.30, and 280 HV for relative density, densification, dilatometric shrinkage, and hardness, respectively.     

Influence of spark plasma sintering on microstructure and wear behaviour of Ti-6Al-4V reinforced with nanosized TiN

Ti-6Al-4V/TiN composites were successfully consolidated by spark plasma sintering (SPS). TiN addition to Ti-6Al-4V was varied from 1% to 5% (volume fraction). The effect of TiN addition on the densification, microstructure, microhardness and wear behaviour of Ti-6Al-4V was studied. Experimental results showed reduction in sintered density of the compacts from 99% to 97% with increase in TiN content. However, an increase in microhardness value was recorded from HV_0.1 389 to HV_0.1 488. X-ray diffraction (XRD) analysis showed that the intensity of diffraction peaks of TiN phase in the composites increased also with formation of small amount of secondary Ti₂N phase. SEM analysis of SPS sintered nanocomposites possessed a refinement of α/β phase microstructure in Ti-6Al-4V with the presence of uniformly dispersed TiN particles. The worn surface of the composite showed improved abrasive wear resistance with non-continuous grooves as compared to the sintered Ti-6Al-4V without TiN addition.     

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

Zirconium oxycarbide powders with controlled composition ZrC_0.94O_0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (E_a) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n ? 2) governs the densification process, while a dislocation motion mechanism (n ? 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687–774 kJ mol^?1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification.     

Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering

Al₂O₃–10TiC composite was synthesized by high energy ball milling followed by spark plasma sintering (SPS) process. Microstructure of the sintered composite samples reveals homogeneous distribution of the TiC particles in Al₂O₃ matrix. Effect of sintering temperature on the microstructure and mechanical properties was studied. The sample sintered at 1500 °C shows a measured density of 99.97% of their theoretical density and hardness of 1892 Hv with very high scratch resistance. These results demonstrate that powder metallurgy combined with spark plasma sintering is a suitable method for the production of Al₂O₃–10TiC composites.     

Synthesis and consolidation of W–Cu composite powders with silver addition

Homogeneous and nanostructured W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag composite powders were prepared via a chemical precipitation method, with the aim of surveying the effect of silver on the properties of tungsten–copper composites. For this purpose, ammonium metatungstate, copper nitrate and silver nitrate with predetermined weight proportion were separately dissolved in distilled water. Furthermore, W–20 wt.%Cu composite powders were provided for comparison. The initial precipitates were obtained by reacting a mixture of the mentioned solutions under certain pH and temperature. The precursor precipitates were then washed, dried, and calcined in air to form oxide powders. In the next step, the reduction was carried out in hydrogen atmosphere to convert them into the final nanocomposite powders. The resulting powders were evaluated using X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscopy (SEM) techniques. The effect of sintering temperature was investigated on densification and hardness of the powders compacts. The results showed that at all sintering temperatures, by increasing in the amount of silver, powders showed better sinterability compared to W–20 wt.%Cu powders. Maximum relative densities of 97.7%, 98.2% and 99.6% were achieved for W–20 wt.%Cu, W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag compacts sintered at 1200 °C, respectively. Moreover, maximum hardness of 359, 349 and 255 Vickers were resulted for W–20 wt.%Cu, W–19 wt.%Cu–1 wt.%Ag and W–10 wt.%Cu–10 wt.%Ag compacts sintered at 1200 °C, respectively.     

Preparation and mechanical properties of Al2O3–15wt.%ZrO2 composites

Low-temperature-sinterable high purity α-alumina powder was mixed with Zr(OH)₄ gel synthesized by a precipitation method. The resulting gel mixture was calcined at 600 °C for 2 h. The Al₂O₃–15wt.%ZrO₂ composites were sintered for 2 h in air in the temperature range between 1350 and 1500 °C. Nearly full densification and the maximum bending strength of 932 MPa were achieved for the Al₂O₃–15wt.%ZrO₂ composites sintered at 1425 °C, whereas the highest fracture toughness of 8.5 MPa m^1/2 was obtained after sintering at 1475 °C.