Field Assisted Sintering Technique Research Articles

High entropy alloys obtained by field assisted powder metallurgy route: SPS and microwave heating

The aim of this work was to investigate the field assisted powder metallurgy route for producing HEAs at equimolar composition, i.e. FeCoNiCrAl, starting from metal powders. Both mixed, mechanically activated and mechanically alloyed powders have been used. The powders obtained by mechanical alloying were synthesized only by SPS, whereas the remaining ones were sintered by SPS or microwave heating. The investigated field assisted sintering techniques allowed an extremely short alloying time, high energy density on the load and negligible contamination by the surrounding environment. Both the conducted sintering-synthesis technology resulted not definitive to produce chemical homogeneity and to obtain a single stable structure. Thus a subsequently heat treatment was required. The post heat treatment, indeed, led to a single crystalline structure (FCC) and the material was fully recrystallized. After heat treatment samples are isomorphic: they exhibit two different phases with the same FCC cell, but different chemical composition, in detail Fe-Cr richer and Al-Ni richer. SPS-ed samples present a reduced porosity, while microwave processed ones are much more porous and this is reflected in the mechanical properties.

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

AbstractIn this study, calcium lanthanum sulfide (CaLa2S4, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.

Microstructural characterization of field assisted sintered bulk nanostructured V-4Cr-4Ti alloys

ABSTRACTThis article deals with synthesis of single-phase bulk nanostructured V-4Cr-4Ti alloy by mechanical alloying and field assisted sintering. Mechanically alloyed powders were sintered at 1050 and 1100°C with different sintering time. Both powder and sintered samples were subjected to structural and morphological characterization using x-ray diffraction and electron microscopy studies. x-ray diffraction and transmission electron microscopy studies of sintered samples confirm that field assisted sintering technique aids nanostructuring in V-4Cr-4Ti alloy. Microstructural parameters of sintered samples were calculated from x-ray diffraction patterns. Role of sintering variables on the microstrain evolution and subsequent domain size reduction in bulk V-4Cr-4Ti alloy are briefly discussed. Optimal sintering parameters to achieve nanostructured V-4Cr-4Ti alloy by field assisted sintering were deduced. Micro-hardness and relative density of sintered V-4Cr-4Ti alloy are found to increase with sintering temperature and sintering time.

Electric current‐controlled synthesis of BaTiO3

AbstractIn BaTiO3, flash‐sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti‐excess secondary phases because of discharging. We applied field‐assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine‐grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.

Cryomilled and spark plasma sintered titanium: the evolution of microstructure

Bulk ultra-fine grained (UFG) commercially pure Ti was prepared by cryogenic milling in liquid argon and subsequent spark plasma sintering (SPS). During cryogenic milling, individual powder particles are repetitively severely deformed by attrition forces. Powder particles were not significantly refined, but due to severe repetitive plastic deformation, ultra-fine grained microstructure emerges within each powder particle. Cryogenic milling can be therefore considered as a specific severe plastic deformation (SPD) method.Compactization of cryomilled powder by SPS technique (also referred to as field assisted sintering technique – FAST) requires significantly lower sintering temperatures and shorter sintering times for successful compaction when compared to any other sintering technique. This is crucial for maintaining the UFG microstructure due to its limited thermal stability.Several specimens were prepared by varying processing parameters, in particular the sintering temperature. The microstructure of powders and compacted samples was observed by scanning electron microscopy (SEM). Increased sintering temperature results in recrystallization and grain growth. A trade-off relationship between the density of compacted material and grain size was identified. Microhardness of the material was found to depend on residual porosity rather than grain size.This contribution presents cryogenic milling and spark plasma sintering as a viable alternative for achieving UFG microstructure in commercially pure Ti.

Yttria catalyzed microstructural modifications in oxide dispersion strengthened V–4Cr–4Ti alloys synthesized by field assisted sintering technique

The present work deals with synthesis, characterisation and elevated temperature mechanical property evaluation of V–4Cr–4Ti and oxide (yttria = 0.3, 0.6 and 0.9 at%) dispersion strengthened V–4Cr–4Ti alloy processed by mechanical alloying and field-assisted sintering, under optimal conditions. Microstructural parameters of both powder and sintered samples were deduced by X-ray diffraction (XRD) and further confirmed with high resolution transmission electron microscopy. Powder diffraction and electron microscopy study show that ball milling of starting elemental powders (V–4Cr–4Ti) with and without yttria addition has resulted in single phase α–V (V–4Cr–4Ti) alloy. Wherein, XRD and electron microscopy images of sintered samples have revealed phase separation (viz., Cr–V and Ti–V) and domain size reduction, with yttria addition. The reasons behind phase separation and domain size reduction with yttria addition during sintering are extensively discussed. Microhardness and high temperature compression tests were done on sintered samples. Yttria addition (0.3 and 0.6 at.%) increases the elevated temperature compressive strength and strain hardening exponent of α–V alloys. High temperature compression test of 0.9 at% yttria dispersed α–V alloy reveals a glassy behaviour.

Synthesis and Characterization of Bulk Nanostructured Thermoelectric Ca3Co4O9

Nanostructuring has been proposed as an effective strategy for the reduction of the phonon contribution to the thermal conductivity, resulting in an increase in the figure of merit of thermoelectric materials. However, obtaining bulk samples presenting high relative density and nanometric grain size can be quite challenging, particularly in the case of ceramic phases. Only few examples have been reported and none in the case of Ca₃Co₄O₉. In this work, we used a sol–gel synthesis coupled with ball milling to prepare powders of Ca₃Co₄O₉ presenting a grain size as small as 4 nm. These nanopowders were then sintered at different temperature and pressures using the High-Pressure Field-Assisted Sintering Technique (HP-FAST). Relative densities up to 95 vol% where obtained while maintaining a nanometric grain size. The microstructural and electrical properties of the sintered samples have been characterized. The results show that in this oxide a reduction to the nanometric grain size produces a drastic reduction in the electrical conductivity, which cannot be compensated by the reduction in the thermal conductivity. The Seebeck effect, on the other hand, appears to be affected only marginally by the grain size and porosity.

Sintering behavior and mechanical properties of Mo-TZM alloyed with nanotitanium carbide

Molybdenum alloys are used in a number of commercial, defense, nuclear, and aerospace applications that take advantage of their high melting temperature, good creep resistance, and decent mechanical properties at elevated temperature. Titanium-Zirconium-Molybdenum (TZM) alloys are the most commercially important Mo alloy as they possess substantially higher strength than pure molybdenum while still maintaining strong mechanical performance at elevated temperature. High density components of these alloys are of interest for high performance defense and aerospace applications, however obtaining high relative densities by sintering has proven difficult. Here we report nano-titanium carbide addition as an effective method of improving TZM sintered density as well as increasing alloy hardness and flexural strength. Pure TZM was sintered by Field Assisted sintering Technique (FAST) to over 98.5% density at 2000°C under 55MPa applied pressure, while density was improved to >99% using TiC addition. TiC addition was found to result in reduced grain size and improved hardness of as-sintered alloys, including over a 95% reduction in grain size and 75% increase in hardness for TZM with 10vol%TiC addition.

Powder Consolidation of Titanium and Titanium Alloys by a Powder Compact Forging Process

Powder compact forging in combination with induction sintering, a field assisted sintering technique (FAST), was used to produce commercially pure (CP) Ti and Ti-13V-11Cr-3Al parts. Green powder compacts with high relative density were manufactured by cold compaction and warm compaction, respectively. During the powder compact forging process, CP titanium powder was consolidated completely to produce a near net shaped top cover for a diving helmet with full density and good mechanical properties. Also, a Ti-13V-11Cr-3Al alloy was fully consolidated into a cylinder using blended elemental powders. As a comparison, raw titanium powder with different oxygen contents was used to make a Ti-13V-11Cr-3Al powder compact forging. Using a starting powder with low oxygen content, a forged cylinder with good mechanical properties was produced.

Microstructure and Mechanical Properties of Multiscale Zirconia Ceramics Prepared by Field Assisted Sintering Technique

Two kinds of powders of 3 mol. % yttria stabilized zirconia (3Y–TZP) with different particles sizes; one was 20 nm denoted by N whereas the other was 0.5 µm denoted by M, were mechanically mixed via ball milling machine using different amounts of N wt. % to obtain multiscale zirconia composite powder. Then the mixed powders were sintered by field assisted sintering technique (FAST). The effect of N content on the microstructure as well as on mechanical properties of zirconia is investigated. Results show that the microstructure of M completely surrounded by N emerged in zirconia composites, and tetragonal phase is presented in all the sintered samples. The obtained zirconia ceramics with 15 wt. % N own a highly dense structure (~ 99.9 % relative density) and high flexural strength of 813.59 MPa wherein a 15 % increase in flexural strength compared to zirconia ceramics without adding N, but the fracture toughness of the composites just lightly decreases. The improved flexural strength of the composites is caused by the multiscale effect.

Field Assisted Sintering of Li7-3xLa3Zr2AlxO12 Solid Electrolyte and the Influence of the Microstructure on the Electrochemical Performance

To enable large-scale applications of lithium ion batteries for transportation and storage of renewable energy high energy densities, high power, reduced costs and, most importantly, safety are required. The use of liquid organic electrolytes that exhibit high vapor pressures and flammability poses a substantial safety issue in particular for large-scale systems. Solid electrolytes offer a potential solution due to their inherent chemical stability. Recently, several material systems such as Li7-3x La3Zr2Al x O12 (LLZO) have been identified that exhibit a large electrochemical window, ionic conductivities similar to those of liquid electrolytes at moderate temperatures and are stable against lithium metal, paving the way to the integration in high energy density battery cells. In this work the synthesis and processing of fine-grained Li7-3x La3Zr2Al x O12 solid electrolyte is performed for the first time using a combination of nebulized spray pyrolysis (NSP) and field assisted sintering technique (FAST). The microstructure, i.e. grain size and microstrain, has a crucial influence on the electrochemical properties of the solid electrolyte and thereby on the all-solid-state battery performance. During sintering using FAST, the grain growth is suppressed and dense ceramics with 93 % of the theoretical density are obtained. The microstructure and the phase composition are characterized using scanning electron microscopy and X-ray diffraction, respectively. The total lithium ion conductivity of the solid electrolyte at room temperature is found to be in the order of 10-4 Scm-1, which is in very good agreement with the literature values. The total conductivity is determined in a wide temperature range from 220 K to 393 K for the determination of the activation energy. Galvanostatic cycling of symmetrical cells using lithium electrodes shows a good cycling stability over 100 hours. The interfacial resistance in contact with lithium metal is determined using alternating current impedance spectroscopy and is amongst the best reported values so far. Several techniques for the integration of Li7-3x La3Zr2Al x O12 solid electrolyte in an all-solid-state battery are presented and discussed.

Room and ultrahigh temperature structure-mechanical property relationships of tungsten alloys formed by field assisted sintering technique (FAST)

Tungsten based alloys have become of critical importance in a number of applications including plasma-facing materials in nuclear fusion reactors, rocket nozzles for aerospace applications, and in kinetic energy penetrators in the defense industry. Formation of components for these uses by powder metallurgical techniques has proven challenging, due to tungsten's relatively poor sinterability. Here we report the use of field assisted sintering technique (FAST) to produce high density, fine grain alloys with mechanical properties comparable or superior to that of components produced by conventional techniques. Alloys of pure tungsten, W-3vol%TiC, W-5vol%TiC, and W-10vol%Ta were synthesized at 2100°C, 35MPa for 25min using FAST. Microstructural characterization revealed effective reduction of grain size with TiC addition and preferential diffusion of oxygen into the center of tantalum particles in tantalum containing alloys. Tensile testing of alloys revealed TiC addition to W resulted in substantially improved ultimate tensile strength at the cost of ductility in comparison at temperatures up to 1926°C (3500°F) however this strengthening effect was lost at 2204°C (4000°F). Addition of 10vol%Ta to W resulted in reduced hardness at room temperature, but substantially increased yield strength at the cost of slightly reduced ductility at 1926°C and 2204°C.

Field Assisted Sintering of Nanostructured Zirconia-Alumina Ceramics for Demanding Applications

ABSTRACTFlash and field assisted sintering techniques (FAST) have been demonstrated in many singular ceramic systems. However, little has been done to thoroughly investigate this phenomenon in binary systems. Zirconia-alumina composites are of interest because of their widespread use in demanding situations for health care, petrochemical and energy applications. The prospect of minimizing grain growth associated with FAST whilst achieving maximum densification is vital for the above applications to improve performance. Flash sintering behaviour of several zirconia-alumina formulations was investigated under a range of DC electric fields up to 700V.cm–1. At low alumina contents (<10 wt%) the flash behaviour was controlled by the zirconia phase; intermediate compositions (10-70 wt%) show a composite effect; and at high alumina contents (>75 wt%) the effectiveness of field assisted sintering drops off. These pointers allowed to gain further insight into the mechanisms of flash sintering and will help to develop pathways for the adaptation of the technique to process complex ceramic systems.

Structural properties of MgO–ZrO2 ceramics obtained by conventional sintering, arc melting and field assisted sintering technique

MgO–ZrO2 ceramics were obtained by three different methods: conventional sintering, arc melting and field assisted sintering. The present study was undertaken in order to determine the influence of the different sintering techniques on the structural properties of MgO–ZrO2 ceramics. The phase compositions of the obtained samples consisted of MgO and ZrO2cub. ss (doped by MgO). The amount of all phases in the final product and their average crystallite sizes are strongly influenced by the sintering method. The smallest crystallites were realized for ceramics consolidated by the field assisted sintering method (20.1±2.5nm for MgO and 27.0±2.7nm for ZrO2cub. ss). About ten times larger crystallites resulted by conventional sintering. The average crystallite size in the material obtained by arc melting technique was around 116±20.4nm. It was observed that the microstructures of the samples prepared by arc melting and field assisted sintering are similar.

Joining of carbon fiber reinforced carbon composites with Ti3SiC2 tape film by electric field assisted sintering technique

Carbon fiber reinforced carbon composites (Cf/C) were first joined by combining electric field-assisted sintering technology and using a Ti3SiC2 (TSC) tape film as interlayer. A joint with shear strength of 26.3 ± 1.7 MPa was obtained within 12 min at a joining temperature of 1200 °C. The joint morphology, interface reaction, shear fracture behavior, and joining mechanism were investigated in detail. To achieve reliable joining of a Cf/C composite to itself via a TSC interlayer, the current work show the crucial controlling factor of interface reaction on the shear strength and the shear fracture behavior. Optimized bonding could be achieved without sacrificing the high strength of the carbon fibers and also taking advantage of the pseudo-plasticity feature of the TSC interlayer. A possible high-toughness joining structure was also proposed based on these results. The current-aid joining technique shortened significantly the bonding process of the Cf/C composites at moderate temperatures, and simplifies the manufacture of components with complex shapes.

Field assisted sintering of fine-grained Li7−3xLa3Zr2AlxO12 solid electrolyte and the influence of the microstructure on the electrochemical performance

The synthesis and processing of fine-grained Li7−3xLa3Zr2AlxO12 (x = 0.15, 0.17, 0.20) solid electrolyte (LLZO) is performed for the first time using a combination of nebulized spray pyrolysis (NSP) and field assisted sintering technique (FAST). Using FAST, the grain growth is suppressed and highly dense ceramics with 93% of the theoretical density are obtained. A tetragonal lattice distortion is observed after the sintering process. Although this structural modification has been reported to have lower Li-ion mobility compared to the cubic modification, the total conductivity of the sample at room temperature is found to be 0.33 mS cm−1, i.e. comparable to phase-pure cubic LLZO. The activation energy of 0.38 eV is also comparable to the literature values. Galvanostatic cycling of a symmetrical cell Li|LLZO|Li shows a good cycling stability over 100 h. The interfacial resistance in contact with Li-metal is determined using alternating current impedance spectroscopy to be 76 Ω cm2 and 69 Ω cm2 before and after cycling at different current densities, respectively.

Powder Metallurgy and Precious Metals: State of the Art and Future Developments

Precious metals are widely used in a variety of different industry mainly for their inertness and their intrinsic value. This latter, is also the reason to pursue time and material saving technologies, reducing scraps and metal losses. Powder Metallurgy offers some interesting and effective alternative to traditional precious metals forming, aiming either to a performance improvement (in terms of cost, time, complexity) or to a cost reduction. The ever increasing availability of metal based alloys with advanced features, supplied in powder form, opens the doors to the use of alternative sintering techniques, borrowed from smelting sectors. Powder Metallurgy applied to precious metals processing makes it possible to create both unconventional shapes and so-far unexplored alloys, using techniques already introduced in other ferrous and non ferrous metals processing. In this paper Field Assisted Sintering Techniques (FAST), Metal Injection Moulding (MIM) and Additive Manufacturing of precious powders are examined as avant-garde processes to obtain near net shape metal parts.

Rapid Synthesis of V-4Cr-4Ti Alloy/Composite by Field Assisted Sintering Technique

The present investigation deals with synthesis and characterization of V-4Cr-4Ti composite/alloy from unmilled and mechanically alloyed powders. Starting powders were sintered by field assisted sintering technique with varying process parameters viz., temperature, holding time, etc. Both powder and sintered samples were subjected to microstructural and morphological characterization by using scanning electron microscopy and phase analysis was done with the help of X-ray diffraction. Field assisted sintering of unmilled and milled powders has resulted in V-4Cr-4Ti composite and single phase V-4Cr-4Ti alloy. Differential thermal analysis was performed in powder samples to identify the recovery and recrystallization regimes of starting powders. Activation energy of the starting powder was calculated by Kissinger analysis. Field assisted sintering of mechanically alloyed powders resulted in nanostructured single phase V-4Cr-4Ti alloy whereas sintering of unmilled powders resulted in V-4Cr-4Ti composite. X-ray diffraction analysis confirms nanostructured single phase V-4Cr-4Ti alloy after field assisted sintering of milled powders. For identical sintering condition, unmilled V-4Cr-4Ti powders have consumed 75% more current than milled powders. Role of particle size, shape and their distribution on the densification behavior of V-4Cr-4Ti powders are also discussed. Interstitial contamination in the sintered samples were studied using electron probe micro-analysis. Microhardness experiments were done on the sintered samples and their corresponding Hall-Pitch plots were deduced.

Physical Properties of Polycrystalline CuGeO3 Prepared by Field-assisted Sintering Technique

The Peierls transition of polycrystalline CuGeO3 samples was investigated by thermal and magnetic investigations. We find that spin disorder induced by chain breaking and grain boundaries has similar effects to doping but without suppressing the spin-Peierls transition temperature or the spin-Peierls gap.

On the application of Electro-sinter-forging to the sintering of high-karatage gold powders

Precious metals are widely used for a variety of applications, among which are luxury goods, biomedical devices, and electronic components. Requiring precise components, those sectors are usually looking into lost-wax casting, machining, and, more recently, metal injection moulding. Material or (thermal) energy consumption and demand for the aforementioned processes are mainly suitable for stable crystalline structures. The applicability of a field-assisted sintering technique (FAST)—Electro-Sinter-Forging (ESF)—to a high-karatage gold powder is explored in the present research work. FAST is characterized by extremely rapid processing times, also enabling the production of metastable phases. Different powders with different features (as atomized, ball-milled, and admixed with nanodiamonds) were taken into account and fast-sintered; and the resulting samples were analyzed in terms of final density, microstructure, and hardness.