Porous FeAl Research Articles

Improving oxidation resistance of porous FeAl-based intermetallics with high boron/yttrium alloying

The coalloying with high contents of chromium (Cr), boron (B) and yttrium (Y) for porous B2-structured FeAl intermetallics fabricated through reactive synthesis was conducted. The oxidation behaviors of porous FeAl-based materials were investigated by evaluating the pore-structure evolution, oxidation kinetics and oxide-scale configuration. The results show that with the coalloying of high contents of Cr, B and Y, the oxidation mass gains of porous FeAl materials at 600−800 °C are significantly reduced. The combination of B enriched on the surface of oxide scales and Y located at the scale−metal interface promotes the formation of thin protective nodular α-Al2O3 oxide scales. It is indicated that introducing relatively high contents of reactive elements such as B and Y can benefit the selective growth of α-Al2O3 scales at relatively low temperatures without pre-treatment.

Porous FeAl alloys via powder sintering: Phase transformation, microstructure and aqueous corrosion behavior

This study investigates the phase transformation and microstructure of porous FeAl parts sintered from elemental powder mixtures using in-situ neutron diffraction and in-situ thermal dilatometry. A single B2 structured FeAl phase was determined in the sintered FeAl alloy. The combined effects of the Kirkendall porosity, transient liquid phase, and phase transformations associated with powder sintering all contribute to the swelling phenomenon of the final sintered part. The aqueous corrosion test indicates that the corrosion products include iron oxides in the porous FeAl parts. The accumulation of corrosion products blocks the pore channel and decreases pore size and permeability over the soaking time.

Effects of porosity on tensile mechanical properties of porous FeAl intermetallics

Abstract Uniaxial tensile tests and scanning electron microscopy (SEM) experiments were carried out on the porous FeAl intermetallics (porosities of 41.1%, 44.2% and 49.3%, pore size of 15–30 μm) prepared by our research group to study the macroscopic mechanical properties and microscopic failure mechanism. The results show that the tensile σ–ɛ curves of the porous FeAl with different porosities can be divided into four stages: elasticity, yielding, strengthening and failure, without necking phenomenon. The elastic modulus, ultimate strength and elongation decrease with the increase of porosity and the elongation is much lower than 5%. A macroscopic brittle fracture appears, and the microscopic fracture mechanism is mainly intergranular fracture, depending on the Al content in the dense FeAl intermetallics. In addition, the stochastic porous model (SPM) with random pore structure size and distribution is established by designing a self-compiling generation program in FORTRAN language. Combined with the secondary development platform of finite element software ANSYS, the effective elastic moduli of the porous FeAl can be determined by elastic analysis of SPM and they are close to the experimental values, which can verify the validity of the established SPM for analyzing the elastic properties of the porous material.

Effects of solid loading on pore structure and properties of porous FeAl intermetallics by gel casting

Solid loading as a key parameter in gel casting influence both rheological behavior and stability of slurry. It also has strong impacts on microstructure and final properties of porous FeAl intermetallics. In this study, a designed non-aqueous gel casting has been developed for fabrication of porous FeAl intermetallics. The SEM analysis, bubble point method and compression test were used to investigate pore structure and properties of porous FeAl intermetallics. Results reveal that higher solid loading will improve the uniformity of pore structure and lead a decrease in maximum pore size. Gas permeability decreases but compressive strength increases with increasing solid loading. The quantitative relationship between maximum pore size (dm) and solid loading (φ) is dm = 10.52 − 0.15φ (35 ≤ φ ≤ 50); the relationship between gas permeability (K) and solid loading (φ) is K = 13.99 − 0.22φ (35 ≤ φ ≤ 50). Further, the distinctive phase transformation and pore formation mechanism are the main reasons for tiny volume expansion and highly open porosity of final products, respectively. Altogether, this work provides a strategy to fabricate porous FeAl intermetallics with adjustable pore structure and properties by controlling the solid loading, suggesting that the non-aqueous gel casting is a promising candidate for the preparation of porous FeAl intermetallics.

Fabrication of porous FeAl-based intermetallics via thermal explosion

Abstract Porous FeAl-based intermetallics were fabricated by thermal explosion (TE) from Fe and Al powders. The effects of sintering temperature on phase constitution, pore structure and oxidation resistance of porous Fe-Al intermetallics were systematically investigated. Porous Fe-Al materials with high open porosity (65%) are synthesized via a low-energy consumption method of TE at a temperature of 636 °C and FeAl intermetallic is evolved as dominant phase in sintered materials at 1000 °C. The porous materials are composed of interconnected skeleton, large pores among skeleton and small pores in the interior of skeleton. The interstitial pores in green powder compacts are the important source of large pores of porous Fe-Al intermetallics, and the in-situ pores from the melting and flowing of aluminum powders are also significant to the formation of large pores. Small pores are from the precipitation of Fe-Al intermetallics particles. In addition, the porous specimens exhibit high resistance to oxidation at 650 °C in air.

Oxidation Resistance of Highly Porous Fe-Al Foams Prepared by Thermal Explosion

Open-cell Fe-Al intermetallic foams were successfully prepared by a simple and energy-saving thermal explosion (TE) process. The effects of the Fe/Al molar ratio (Fe-(40–50) at. pct Al) and thermal ...

Modification of the reactive synthesis of porous FeAl with addition of Si

Porous Fe25Al–xSi intermetallics with various contents of Si were fabricated via reactive synthesis using Fe + Al + Si powder mixtures, in which the elemental Si was added to control the self-propagating High-temperature Synthesis (SHS) occurred during sintering process. The Si element added in Fe25Al could make the exothermic reactions in the synthesis process become mild and the total heat release from the system decrease. With increasing Si content from 0 to 5 wt-%, the open porosity, the maximum pore size and permeability increased from 38·14 to 50·12%, 27·9 to 35·7 μm and 5·13 × 10−12 m2 to 11·9 × 10−12 m2, respectively. Si element could affect the microstructure and phase evolution during the middle temperature sintering. Scattered τ1 ((AlSi)5Fe3) phase surrounded Fe2Al5 layer transformed into loop-shape with increasing Si content. Fe(Al1−xSix)3 and Al3FeSi phases substitute Fe2Al5 phase as the main intermediary phase when the Si content is beyond 3 wt-%.

Analysis of the formation of FeAl with a high open porosity during electric current-assisted sintering of loosely packed Fe-Al powder mixtures

A Fe-40 at.%Al loosely packed powder mixture was subjected to pressureless treatment in a vacuum chamber of a Spark Plasma Sintering facility and sintered in a hot press in an argon atmosphere with and without applying mechanical pressure. The treatment conditions were varied to determine whether the electric current and the heating rate play any roles in the formation of FeAl compacts with a high open porosity (41–51%). Rapid heating of a loosely packed Fe-Al mixture in the SPS chamber with or without an electric current passing directly through the sample ensures a fast transformation of the reaction mixture into a highly porous FeAl product, the passage of electric current through the sample not being responsible for the preservation of high open porosities. The transverse rupture strength of porous FeAl did not depend on the sintering method for compacts of the same total porosity, but increased when the total porosity decreased.

Amino Acids Aided Sintering for the Formation of Highly Porous FeAl Intermetallic Alloys.

Fabrication of metallic foams by sintering metal powders mixed with thermally degradable compounds is of interest for numerous applications. Compounds releasing gaseous nitrogen, minimizing interactions between the formed gases and metallic foam by diluting other combustion products, were applied. Cysteine and phenylalanine, were used as gas releasing agents during the sintering of elemental Fe and Al powders in order to obtain metallic foams. Characterization was carried out by optical microscopy with image analysis, scanning electron microscopy with energy dispersive spectroscopy, and gas permeability tests. Porosity of the foams was up to 42 ± 3% and 46 ± 2% for sintering conducted with 5 wt % cysteine and phenylalanine, respectively. Chemical analyses of the formed foams revealed that the oxygen content was below 0.14 wt % and the carbon content was below 0.3 wt %. Therefore, no brittle phases could be formed that would spoil the mechanical stability of the FeAl intermetallic foams. The gas permeability tests revealed that only the foams formed in the presence of cysteine have enough interconnections between the pores, thanks to the improved air flow through the porous materials. The foams formed with cysteine can be applied as filters and industrial catalysts.

Fast synthesis and consolidation of porous FeAl by pressureless Spark Plasma Sintering

We report one-step fast synthesis and consolidation of iron aluminide FeAl of high open porosity by pressureless reactive Spark Plasma Sintering (SPS). The starting material of the Fe-40at.%Al composition was a mixture of an iron powder with an average particle diameter of 4 μm and an aluminum powder with an average particle diameter of 6 μm. The rationale behind the choice of the SPS as a processing technique and fine and comparable sizes of the two reactants for the synthesis of high-open porosity FeAl was realization of fast full chemical conversion of Fe and Al into single-phase FeAl reducing the time available for the compact shrinkage. According to the XRD phase analysis, single-phase FeAl compacts formed after SPS at 800 and 900°C. These compacts had open porosities of 41 and 46%, respectively. The transverse rupture strength of the compacts sintered at 700-900°C was found to change little with the sintering temperature in the selected range.

Direct separation of arsenic and antimony oxides by high-temperature filtration with porous FeAl intermetallic

A temperature-controlled selective filtration technology for synchronous removal of arsenic and recovery of antimony from the fume produced from reduction smelting process of lead anode slimes was proposed. The chromium (Cr) alloyed FeAl intermetallic with an asymmetric pore structure was developed as the high-temperature filter material after evaluating its corrosive resistance, structural stability and mechanical properties. The results showed that porous FeAl alloyed with 20wt.% Cr had a long term stability in a high-temperature sulfide-bearing environment. The separation of arsenic and antimony trioxides was realized principally based on their disparate saturated vapor pressures at specific temperature ranges and the asymmetric membrane of FeAl filter elements with a mean pore size of 1.8μm. Pilot-scale filtration tests showed that the direct separation of arsenic and antimony can be achieved by a one-step or two-step filtration process. A higher removal percentage of arsenic can reach 92.24% at the expense of 6∼7% loss of antimony in the two-step filtration process at 500∼550°C and 300∼400°C. The FeAl filters had still good permeable and mechanical properties with 1041h of uninterrupted service, which indicates the feasibility of this high-temperature filtration technology.

Structural and mechanical characterization of porous iron aluminide FeAl obtained by pressureless Spark Plasma Sintering

In this article, structural and mechanical characterization of porous iron aluminide FeAl with a high open porosity obtained by one-step fast synthesis is presented. FeAl compacts produced by pressureless Spark Plasma Sintering of the Fe-40at%Al powder mixtures at 800 and 900°C for 3min had open porosities of 41% and 46%, respectively. An important result of this work is high values of the open porosity of the synthesized FeAl achieved without any pore forming additives by inducing a rapid chemical reaction between iron and aluminum in an electric current-assisted sintering process. Transverse rupture strength (TRS) of the porous FeAl was determined from the experiments on indentation of a steel ball into the disk-shaped sintered specimens. Finite element modeling of the specimen rupture during indentation was carried out in ANSYS 17.0 software. The TRS values of the FeAl compacts obtained analytically were close to those obtained numerically. The TRS of the porous FeAl sintered at 700–900°C ranged between 53 and 72MPa and did not change significantly with the sintering temperature.

Fabrication of a graded pore-sized porous FeAl intermetallic membrane

Micro-porous FeAl membranes were prepared directly onto a macro-porous FeAl support through brushing and reactive synthesis using Fe and Al elemental powders with different particle sizes. X-ray diffraction, scanning electron microscope and pore structure testing were used to study the phase composition, interfacial microstructure and pore structure of the resulting graded pore-sized FeAl membrane. The effects of membrane thickness on permeability and maximum pore size were also investigated. Results showed that a metallurgically strong joint is formed between the membrane and support and that no cracks and defects may be found on the interface due to the elemental reactions, penetrating of coated powders into the pores of the support and the match of the thermal expansion coefficients between membrane and support. Membrane permeability decreased from 84.2 to 38.5 m3 m−2 kPa−1 h−1 and the maximum pore size of about 2.5 μm showed a slight change as the membrane thickness increased from 135.5 to 299.1 μm.

Effect of pressure on pore structure of porous FeAl intermetallics

A series of porous FeAl intermetallics with nominal composition of Fe–25wt.% Al was fabricated under pressure ranging from 60MPa to 360MPa by Fe and Al elemental powder reactive synthesis. The effect of pressure on the pore structure of the resultant porous FeAl intermetallics was systematically studied. Results show that open porosity, maximum pore size and permeability can be manipulated by varying the pressure. Permeability decreases with increasing pressure. Permeability (K), open porosity (θ) and maximum pore size (dm) were verified to meet the Hagen–Poiseuille equation. A universal equation of K=Adm2θ, with A=0.2379, was established to reflect the relationship between the permeability and pore structure parameters for porous FeAl intermetallics fabricated under different pressure.

Preparation and Pore Structure Stability at High Temperature of Porous Fe-Al Intermetallics

Porous Fe-Al intermetallics with different nominal compositions (from Fe-8 wt.% Al to Fe-50 wt.% Al) were fabricated by Fe and Al elemental powders through reaction synthesis. The effects of the Al content on the pore structure properties, and the comparison of pore structure stabilities at high-temperatures among the porous Fe-Al intermetallics and porous Ti, Ni, 316L stainless steel samples, were systematically studied. Results showed that the open porosity, maximum pore size, and permeability vary with the Al content. Porous Fe-(25-30 wt.%) Al intermetallics show good shape controllability and excellent pore structure stability at 1073 K in air, which suggests that these porous Fe-Al intermetallics could be used for filtration at high temperatures.

Properties of Porous FeAl Manufactured from Ball Milled Fe/Al Elemental Powders by Two-Step Sintering

Porous FeAl was manufactured from ball milled Fe/Al elemental powders followed by medium temperature solid diffusion and high temperature sintering. Phase composition and porous structure were analyzed by XRD, SEM, Mercury Porosimeter and permeability test system. High temperature oxidation in air and high temperature sulfidation in SO2(3v%)+N2 at 600°C were carried out to investigate the behaviors of the porous FeAl, and the results were compared to 316L porous materials. The result showed that high sintering temperature hastened the transform of Fe2Al5 to FeAl intermatellic. The permeability of the porous FeAl increased and the most probable size decreased with sintering temperature. The porous FeAl had mass gains of 0.06% for air oxidation and 0.13% for sulphidation after 50 h at 600°C, compared with mass gains of 0.15% and 5.3% respectively of porous 316L stainless steel.

Pore structure control for porous FeAl intermetallics

Pore structure control methods for porous FeAl intermetallics were studied in this paper based on pore formation mechanism. A large range of pore structure parameters can be obtained through fabrication parameter adjustment in porous FeAl preparation procedure. Open porosity and maximum pore size of porous FeAl have a direct proportional relationship to the powder size of raw materials and an inversely proportional relationship to the pressing pressure. The relationship between maximum pore size (dm) and raw material powder size (dp) can be determined as dm=0.40dP. Open porosity and maximum pore size decrease with increasing holding time at solid diffusion reactive procedure. The quantitative relationship between open porosity (θ) and holding time (t) at 600 °C is θ=49.7−0.1t(60≤t≤240); the quantitative relationship between maximum pore size (dm) and holding time (t) at 600 °C is dm=20.3−0.065t(60≤t≤240).

Tortuosity factor for porous FeAl intermetallics fabricated by reactive synthesis

The tortuosity factor is the most critical parameter for the pore characteristic of porous materials. The tortuosity factor for porous FeAl intermetallics was studied based on the Darcy law and Hagen–Poiseuille equation. Porous stainless steel with the same pore structure parameter as porous FeAl was fabricated by powder metallurgy method for comparison. The results show that the tortuosity factor of porous FeAl intermetallics is smaller than that of porous stainless steel when their pore structure parameters are the same. The average tortuosity factor is 2.26 for the porous FeAl material and 2.92 for the porous stainless steel, calculated by Hagen–Poiseuille equation. The reason of the different tortuosity factors for porous FeAl and porous stainless steel was also explored through studying the pore formation mechanisms of the two types of porous materials.

Manufacture of Porous FeAl Intermetallics Produced by Mechanical Alloying and Sintering

Porous FeAl intermetallics were produced by high-energy ball milled Fe-Al binary composite powders followed by sintering. The microstructure evolution of milled Fe-Al composite powders during high-energy milling processing were observed and analyzed using SEM and XRD. FeAl intermetallics was synthesized by sintering at 1200°C for 4h with a pre-sintering at 620°C. The microstructure of the sinters was also investigated. XRD results and SEM images showed that milling time and sintering temperature accelerated the formation of FeAl intermetallics. The microstructure of the milled sinter apparently improved the properties of FeAl intermetallics.

Effect of aluminium evaporation loss on pore characteristics of porous FeAl alloys produced by vacuum sintering

This study investigated the effect of aluminium evaporation loss on the pore characteristics of a porous Fe–Al alloy. The porous Fe–Al alloy samples were produced through pressureless reaction synthesis from powder compacts of elemental Fe and Al powder mixture under vacuum. Cellular characteristics were analysed by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The maximum open-cell pore size, open-cell porosity and permeability increased with increasing sintering temperature. Based on the Miedema model and Langmuir equation, the evaporation weight losses were calculated and in good agreement with the experimental measurements. The relationship between the evaporation weight losses and the cellular morphology and physical properties of the porous Fe–Al samples is also discussed.