Development of a Ti-based alloy: Design and experiment

We proposed the design methodology for titanium-based alloys based on a combination of literature survey, simulation, and experiment. We have selected and investigated the properties of novel Ti-Fe-Zr alloys specifically designed for densification via powder metallurgy techniques. Samples were produced by die compaction of mixed elemental powders with subsequent densification by sintering at 1,275°C in vacuum. Scanning electron microscopy and optical microscopy were used to examine the sintered microstructures to compliment hardness and tensile testing. The results show that density and mechanical properties increase with the iron and zirconium content. The best property combination was obtained with the addition of 5 wt.% iron and 5 wt.% zirconium when vacuum sintered at 1,275°C for 60 min.     

Synthesis of TiN Reinforced Aluminium Metal Matrix Composites Through Microwave Sintering

Al-TiN (10, 20, 30 wt.%) composites were fabricated by using microwave radiation. Al and TiN powders were selected as starting materials, mixed in a ball mill for ~10 min and sintered for various times. Results indicate that an optimum microwave sintering time of 2 min was essential and responsible for the improved densification and mechanical properties. The presence of TiN particles at grain boundaries plays a significant role in improving the densification and hardness values. Dry sliding wear results show the improved wear resistance of the composite (Al-TiN) due to the presence of TiN particles and the wear results are superior to the Al-TiN samples made by hot pressing technique.     

Effect of medium temperature on the dispersion of WC/ZrO2/VC composite powders

This article proposed a novel method to disperse WC/ZrO₂/VC composite powders so as to attain a perfectly uniform suspension. Besides using conventional dispersing means such as adding dispersant (PEG, polyethylene glycol), mechanical stirring, ultrasonic vibration and ball milling, the temperature adjustment of dispersing-medium distilled water had also been employed. The agglomerating and dispersing mechanisms were analyzed by means of TEM observation of WC/ZrO₂/VC composite powders dispersed under five different temperatures, with the results showing that the most uniform dispersion was obtained under the temperature of 100 °C based on the criterion for conglomeration number per unit. The dispersed WC/ZrO₂/VC composite powders were dried and consequently sintered by hot-press sintering in nitrogen atmosphere at 1580 °C with pressure of 30 MPa. The testing results of mechanical properties such as relative density, hardness, bending strength and fracture toughness show that the optimal properties are obtained by using the WC/ZrO₂/VC composite powders dispersed under 100 °C. The surface crack morphologies of sintered samples are investigated and the results show that crack extended in a more tortuous path for the sample sintered from well-dispersed composite powders.     

Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders

The densification of Ti₃SiC₂ MAX phase was performed by the Spark Plasma Sintering (SPS) technique. The SPS parameters, such as sintering temperature, pressure and soaking time, were optimized to obtain fully densified samples which were characterized to obtain the best mechanical properties. The sintering temperature was varied from 1070 to 1300 °C, the soaking time from 1 to 10 min and the applied pressure from 60 to 180 MPa. The best full densified samples were sintered at 1300 °C applying 60 MPa for 7 min. Ti_xC_y and TiSi₂ secondary phases were found in samples densified at 1200, 1250 and 1300 °C, due to decomposition of Ti₃SiC₂. These secondary phases, detected by XRD patterns, were confirmed by microhardness testing, FESEM observations and EDAX analyses.     

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.     

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.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Investigation of Properties of Powder Injection-Molded Steatites

In this study, the mechanical and micro-structural properties of injection-molded steatites were investigated experimentally. Initially, steatite powders and binders of polyethylene glycol (PEG), polypropylene (PP), and stearic aside (SA) were mixed to prepare the feedstock. The mixing powders were granulated using the extruder. The short granules in cylindrical shapes were used as the feedstock in the injection-molding operations. Solvent- and thermal-debinding processes were applied to the green samples after the molding. The samples were sintered at 1300 °C for 4 h, and a theoretical density of 98-99% was achieved. Three-point bending and tensile tests were performed on the samples which were sintered at 1200-1300 °C. The maximum three-point bending and tensile strength values were found as 154 and 47 MPa, respectively. The morphology of fractured surface was done by scanning electron microscopy whereas porosity investigations were carried out using the same microscopy. Grain growth and structure on the specimens were also investigated using transmission electron microscopy.     

Microstructure and mechanical properties of B4C-TiB2 composite ceramic fabricated by reactive spark plasma sintering

B₄C-TiB₂ composite ceramic was prepared by reactive spark plasma sintering, using amorphous B, Ti, and graphite as the raw materials. The reaction process and the phase composition in the process of sintering were studied. The effects of the ratio of raw materials and sintering process on the microstructure and mechanical properties of B₄C-TiB₂ composite ceramic were investigated. The composition of the sintered sample was B₄C, TiB₂, and bits of residual unreacted graphite. B and Ti preferentially reacted to form TiB₂ at 800 °C, and then B and graphite reacted to form B₄C at 1250 °C. The 75 vol% B₄C-25 vol% TiB₂ composite ceramic synthesized with 60.6 wt% B, 25.8 wt% Ti, and 13.6 wt% graphite and sintered at 1900 °C for 15 min resulted in nearly full densification and optimal mechanical properties. The relative density, Vickers hardness, fracture toughness, and flexural strength were 98.6 ± 0.01%, 26.6 ± 0.01 GPa, 5.9 ± 0.13 MPa·m^1/2, and 605 MPa, respectively.     

Microstructure and optical properties of transparent alumina

The microstructure and optical properties are evaluated for alumina sintered by spark plasma sintering at temperatures between 1100 and 1550 °C. With increasing sintering temperature, grain growth and densification occur up to 1250 °C, and above 1300 °C, rapid grain growth and pore growth occur. Light transmission increases with the densification and decreases with the grain/pore growth. It is found that the total forward transmission and the reflection of light are related to the porosity and the pore growth, whereas the in-line transmission and the light absorption are related to the grain size and the defects, respectively. The relationships are explained by using the Mie scattering theory, model prediction and observed microstructural characteristics.     

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.     

Development of high speed steel sintered using concentrated solar energy

The steel powders were sintered under N₂–H₂ atmosphere in a solar furnace and in a Fresnel lens, after compaction of the green parts, using much higher heating and/or cooling rates as compared to conventional tubular furnace. The effects of processing parameters and the use of concentrated solar energy on densification and mechanical properties were analyzed. Experimental results demonstrated the activating effect of concentrated solar energy on the sintering process showing that an optimum densification is achieved at 1150 °C on both solar installations in just 90 min for the solar furnace and in 30 min in the case of Fresnel lens installation compared with an optimum temperature of 1290 °C in ∼10 h of total cycle in the conventional tubular furnace. Better mechanical properties were obtained using concentrated solar energy with microhardness measurements ranging between 800 and 900 HV. Microstructural analyses by scanning and transmission electron microscopy reveal the presence of submicron sized vanadium nitrides and other nanometer sized particles in the samples that could be responsible for the high hardness values obtained.     

Effect of tungsten content on microstructure and mechanical properties of PCBN synthesized in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) compacts were prepared using the mixture of cubic BN and Ti-Al-W powders at 5.5 GPa and 1550 °C for 10 min. The influence of different Tungsten (W) content on composition, microstructure, porosity, mechanical property and cutting performance of the PCBN is investigated. The results show that, with the addition of tungsten, the cubic boron nitride (cBN) crystals are connected with each other by the new product phases TiB_2, TiN, Al₃Ti and W₂B under the pressure of 5.5 GPa and the temperature of 1550 °C. The rod-shaped crystals in the PCBN are expanded from the surface portion of the cBN. As the W content increases, the amount of rod-shaped crystals and the length-diameter ratios decrease in the system. When the tungsten content is 6 wt%, PCBN presents the best comprehensive performance and cutting performance, the porosity, the hardness, the flexural strength and the flank wear are 0.55%, 30.71 GPa, 972.3 MPa and 292 μm, respectively.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Effect of Ni addition on the densification of TiC: A comparative study of conventional and microwave sintering

This study deals with the effect of conventional sintering and microwave sintering on the densification kinetics of Titanium Carbide (TiC) in the presence of Ni (1, 1.5, 2 wt%). TiC compacts were obtained after uniaxial pressing of powders synthesised by ball milling of Titanium and Carbon and sintering was done in the presence of Nickel. The samples prepared were subjected to conventional as well as microwave sintering. The XRD and SEM analysis were used for a study of the reaction of Ti and C powders upon addition of Ni, which reduced the sintering temperature to 1200 °C. The densification of TiC powders was due to the Ti-Ni eutectic system, the liquid phase formed at this temperature assisting the sintering process. The SEM images revealed the flake like structure of TiC in which the carbon diffused into Ti upon the addition of Ni, thereby supporting enhanced mass transfer. The XRD pattern showed the presence of Titanium Oxide (TiO₂) along with TiC which resulted in non-uniform distribution of hardness. Maximum hardness was achieved in the conventional sintered compacts which gradually increased with increase in Ni addition. The presence of the oxide phase and the formation of micro cracks resulted in non-uniform hardness for microwave sintered compacts. The maximum hardness of conventional sintered compact (375 HLD) was nearly 1.5 times more than the maximum hardness of the microwave sintered compact (250 HLD). The density of the microwave sintered compact was found to be higher by 8% than with the conventionally sintered compact.     

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.     

Synthesis of NiCrAl/MgO–ZrO2 cermet powders by chemical method for functionally graded coatings

Functionally graded materials (FGMs) are receiving great attention as they provide optimum thermal and mechanical properties without a discrete interface between two materials. In order to control the chemical composition and microstructure of FGMs, NiCrAl/MgO–ZrO₂ cermet powders were successfully developed in the present work. The NiCrAl/MgO–ZrO₂ powders were synthesized from a solution of NiCrAl, ZrO₂ and Mg hydroxide carbonate precursors using chemical synthesis. The powders were dried at 125 °C for 3 h and then pellet samples were sintered at 1381 °C for 30 min under N₂–5%H₂ atmosphere. The powders were characterized by scanning electron microscope, energy dispersive spectroscopy, X-ray mapping and X-ray diffraction. The obtained results showed that the optimum particle sizes of NiCrAl/MgO–ZrO₂ powders were between 45 and 90 μm. Microstructural studies have shown a uniform mixing in the cermet powders. It was also found that ZrO₂, MgO, MgZr₇O₁₄, Ni, Cr and Ni₅Al₃ phases were present in the cermet powder.     

Improved mechanical properties of ZrB2-30 vol% SiC using zirconium carbide additive

Influence the different amount of ZrC as well as fabrication parameters on the mechanical properties such as flexural strength and densification behavior of spark plasma sintered ZrB₂-30 vol% SiC composites were investigated. The composites contained 4, 8 and 12 vol% ZrC were consolidated at 1650, 1725 and 1800 °C for 4, 9 and 14 min under 20, 30 and 40 MPa pressures. Relative density and the ratio of open porosities were measured and used to appraise the densification behavior. Three-point bending instrument applied for flexural strength measurement. Microstructural investigations were carried out using scanning electron microscopy. The results showed that the ZrC till to 4 vol%, have positive effect on grain growth and acts as grain growth inhibitor while more addition caused to grain growth happening. Also, ZrC ascent from 4 up to 12 vol%, accompanied by shrinkage temperature rising (from 1263 °C to 1389 °C and 1392 °C). Relative density reduction occurred from 94.4% to 92.2% by increasing ZrC amount from 4 up to 12 vol%. Flexural strength reached to its maximum value, 460 MPa, in the presence of 8 vol% ZrC for composite which was consolidated at temperature of 1800 °C, time of 9 min under 30 MPa pressure.     

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.