Search

This work investigated the possibility of the direct preparation of aluminum alloys by aluminothermic reduction of deep-sea nodules with a high excess of aluminum. The process was found to be unable to obtain aluminum alloy, but an aluminum-rich manganese-based alloy was obtained instead, being composed of intermetallics. The alloy was characterized in the as-reduced state, as well as after crushing and sintering in the temperature range of 800–950 °C. The sample sintered at 900 °C was also heat-treated by annealing at 800 °C for 3 h and rapidly cooled. It was observed that with the increasing sintering temperature, the original matrix phase Al11Mn14 was transformed into a duplex matrix with a structure corresponding to Al11Mn14 and Al4Cu9, and this mixture was further transformed to the matrix with the structure corresponding to Al4Cu9. Furthermore, the mechanical properties and wear resistance of the samples were described. The highest microhardness was reached in the sample, which was annealed after sintering. Sintered samples reached a lower wear rate because of the fragmentation of brittle intermetallics during crushing. [ABSTRACT FROM AUTHOR]

Ti–Al–Si alloys are prospective material for high-temperature applications. Due to low density, good mechanical properties, and oxidation resistance, these intermetallic alloys can be used in the aerospace and automobile industries. Ti–Al–Si alloys were prepared by powder metallurgy using reactive sintering, milling, and spark plasma sintering. One of the novel SPS techniques is high-pressure spark plasma sintering (HP SPS), which was tested in this work and applied to a Ti–10Al–20Si intermetallic alloy using a pressure of 6 GPa and temperatures ranging from 1318 K (1045 °C) to 1597 K (1324 °C). The low-porosity consolidated samples consist of Ti5Si3 silicides in an aluminide (TiAl) matrix. The hardness varied between 720 and 892 HV 5. [ABSTRACT FROM AUTHOR]

Our previous works on the synthesis of intermetallics by Self-propagating High-temperature Synthesis revealed strong dependence of microstructure of the products on heating rate. In this paper, the application of various heating regimes and sources was tested in preparation of Ni Ti shape memory alloy. It was found that high heating rate (approx. over 100 °C min −1 ) was required to obtain the material with maximized amount of NiTi shape memory phase and no unreacted metals. Heating in electric resistance furnace preheated to the process temperature or induction heating furnace seemed to be promising for this purpose. On the other hand, Spark Plasma Sintering was found to be inapplicable, because the strongest increase of the temperature occurred on the surface of the particles, producing layers of intermetallics that further acted as diffusion barriers. [ABSTRACT FROM AUTHOR]

In this work, the formation of intermetallics in Ni–Ti system by reactive sintering at 500 and 650 °C was studied. Mechanism and kinetics of the reactions leading to Ni–Ti phases were determined by DTA and the application of an experimental model consisted of nickel-plated titanium. It was found that Ti 2 Ni phase forms already at 500 °C. At 650 °C this phase undergoes a reaction with nickel to NiTi and Ni 3 Ti. The reactions were determined to be diffusion-controlled. The formation of Ni 3 Ti contributes to the porosity evolution during reactive sintering production of NiTi alloy. [ABSTRACT FROM AUTHOR]

Abstract: This work is devoted to the preparation of ultrafine material based on Al–Cu–Fe quasicrystalline phase by powder metallurgy using mechanical alloying and Spark Plasma Sintering. The dependence of microstructure and phase composition of powders on the conditions of mechanical alloying was described. It was found that the Al60Cu30Fe10 quasicrystalline phase forms directly already after 2 h of milling under optimized conditions. The stability of this quasicrystalline phase was studied at various temperatures of Spark Plasma Sintering compaction process. [Copyright &y& Elsevier]

Abstract: Alloys formed by Fe–Al intermediary phases have lower density than common metallic high-temperature alloys and good high-temperature oxidation resistance. Previously it was proved that silicon additions to these alloys enable to produce them efficiently by reactive sintering and improves the wear resistance. In this work, the oxidation resistance of the novel Fe–Al–Si alloys containing 10–30 wt. % of aluminium and 5–30 wt. % of silicon produced by the reactive sintering technology was tested. Cyclic and isothermal oxidation tests were carried out at 800 °C in air. Tested alloys exhibit excellent oxidation resistance, which increases with silicon content up to 20 wt. %. The reasons are discussed in terms of the phase composition of the oxide layer and the changes in chemical composition under the oxide layer during oxidation. [Copyright &y& Elsevier]

Abstract: Alloys formed by Ti–Al intermediary phases have low density and very good high-temperature oxidation resistance. Further improvement of high-temperature behaviour can be achieved by a suitable addition of silicon. In this work, Ti–Al–Si alloys containing 20wt.% of aluminium and 10–20wt.% of silicon produced by a reactive sintering technology were tested. Isothermal oxidation tests were carried out at 1000°C in air. The oxidation rate was determined from weight gains caused by oxide formation. Microstructure and phase composition of the oxide layers were studied. Microstructure and hardness changes after various annealing durations were described. [ABSTRACT FROM AUTHOR]

Abstract: Formation of intermediary phases during reactive sintering of Fe–Al–Si powder mixtures at 1100°C was studied in this work. Results show significant difference from already presented results achieved at lower sintering temperatures. Phase composition is closer to the equilibrium phase diagram. Silicon additions up to 10wt.% produce binary Fe–Al phases, further increase of silicon content leads to the formation of FeSi silicide and Al2Fe3Si3 ternary phase. Binary FeSi40 alloy contains FeSi and FeSi2. Porosity is reduced by addition of silicon. The lowest porosity was achieved when 10wt.% of silicon was added. Formation of binary Fe–Al phases is controlled by aluminium diffusion through the layer of intermetallics. In Fe–Al–Si system, the controlling process is the reaction producing ternary intermediary phases. The highest wear resistance was determined in FeAl30Si10 alloy with single-phase FeAl structure. An increase of silicon content reduces the wear resistance although hardness increases. [Copyright &y& Elsevier]

Abstract: In this work, the dependence of microstructure of Fe–Al–Si alloy on conditions of reactive sintering was studied. It was shown that heating rate strongly affects the structure of this alloy. The microstructure consisting of uniformly distributed Al2Fe3Si3 particles in Al2FeSi matrix can only be obtained in this material by rapid heating (over 300Kmin−1). Sufficient reactive sintering temperature to obtain Al2Fe3Si3–Al2FeSi two-phase structure is 750°C. Further increase of the reaction temperature reduces porosity but causes a growth of Al2Fe3Si3 particles. Reactive sintering process is completed during less than 10min in the investigated material. Temperatures over 1050°C produce material composed of FeSi, FeAl and Al2Fe3Si3 phases. However, strongly exothermal reaction of FeSi formation causes melting of the sample. [Copyright &y& Elsevier]

Abstract: In this work, Ti–Al–Si alloys were prepared by reactive sintering. The contents of silicon and aluminium ranged between 0–30wt.% and 0–36wt.%, respectively. Powders of titanium, aluminum, silicon and AlSi30 master alloy prepared by mechanical machining and/or milling were used for sintering. Powders were blended and pressed at room temperature. Sintering was carried out at 900°C for 30min. It was found that it is possible to produce compact materials with the structure corresponding to the as-cast materials when silicon is in the range of 15–20wt.% and aluminium content between 8 and 30wt.%. It was shown that these materials contain two phases—Ti5Si3 and TiAl. The central porosity can be reduced by induction heating. [Copyright &y& Elsevier]

This work is devoted to the preparation of alloys based on intermetallic compounds in the Ti-Al-Si system by powder metallurgy using mechanical alloying and the spark plasma sintering (SPS) method. The aim was to describe the formation of intermetallic phases during mechanical alloying of TiAl15Si15 (wt-%) alloy and to consolidate the powder prepared by optimised conditions. Phase composition, microstructure and hardness of compacted alloy were determined. Four hours of mechanical alloying is sufficient time for preparation of pure elements free material composed only of intermetallic phases. After consolidation, the TiAl15Si15 alloy has a homogeneous structure composed of silicide (Ti5Si3) in aluminide (TiAl) matrix. The hardness of the material reaches 865 ± 42 HV 5. [ABSTRACT FROM AUTHOR]

• Ti-Al-Si-X (X˭Co, Cr, Fe, Mo, Nb, Ni) alloys prepared using mechanical alloying and Spark Plasma Sintering. • Alloys have fine-grained and homogeneous microstructure with low porosity. • Alloyed elements increased hardness and compressive strength values. Intermetallic alloys based on Ti-Al-Si system are significant for their excellent high-temperature properties, especially for resistance against oxidation and for achieving good mechanical properties at elevated temperatures. The main problem of these materials is high brittleness at room temperature. In the previous work, selected alloying elements were added into the Ti-Al-Si alloys prepared by reactive sintering, but the properties did not meet the requirements for subsequent use in the automotive or aerospace industry, since the materials had a very porous microstructure. Here, the addition of cobalt, chromium, iron, molybdenum, niobium and nickel into the TiAl15Si15 alloy prepared by mechanical alloying and Spark Plasma Sintering is tested and it is expected to improve the mechanical and high-temperature properties. In this paper, the Ti-Al-Si alloys have been assessed on basis of microstructure, phase composition, mechanical and tribological properties, such as hardness, fracture toughness and compressive strength. Cyclic oxidation tests were performed for 400 h at the temperatures of 800 and 1000 °C. The cyclic oxidation tests simulate industrial processes, where the material is alternately exposed to elevated temperature and subsequent cooling (eg, engine components). From the viewpoint of most of the tested properties, alloying by niobium seems to be the most promising. [ABSTRACT FROM AUTHOR]

The study is devoted to the intermetallic alloy FeAl20Si20 (wt.%) with the potential applications in high temperature aggressive environments. The samples of the same chemical composition were prepared by spark plasma sintering from the different mechanically alloyed powders (pure elements and pre-alloyed powders). Differences in mechanical properties were characterized. Whereas no significant differences were found in hardness and Young's modulus, fracture resistance was higher for the samples from pre-alloyed powders in which Palmqvist and lateral cracks were observed (contrary to the sample made of pure elements where only Palmqvist cracks were identified). [ABSTRACT FROM AUTHOR]

This work aims to describe the mechanism of intermediary phases formation in TiAl20 (wt. %) alloy composition during reactive sintering. The reaction between titanium and aluminum powders was studied by in situ diffraction and the results were confirmed by annealing at various temperatures. It was found that the Ti2Al5 phase formed preferentially and its formation was detected at 400 °C. So far, this phase has never been found in this alloy composition during reactive sintering processes. Subsequently, the Ti2Al5 phase reacted with the titanium, and the formation of the major phase, Ti3Al, was accompanied by the minor phase, TiAl. Equations of the proposed reactions are presented in this paper and their thermodynamic and kinetic feasibility are supported by Gibbs energies of reaction and reaction enthalpies. [ABSTRACT FROM AUTHOR]

Reactive sintering is currently considered as a promising production route for titanium aluminides in many research works. However, the published descriptions of the reaction mechanism are contradictory or lacking, especially at the temperatures below the melting point of aluminium. This work aims to fill this gap, providing the description of the reactive sintering process at the temperatures between 400 and 900 °C. The phases' formation sequence and reaction kinetics were studied and explained using experimental model (Ti/Al diffusion couple) and real reactively sintered samples of equiatomic Ti-Al compressed powder blend. Moreover, phase formation was thermodynamically assessed. It was revealed that Ti 2 Al 5 phase formed preferentially. This phase has not been reported previously as a starting phase in reactive sintering. According to results obtained by experimental model, its formation is controlled by diffusion at 700 °C. This phase reacted with aluminium forming pure TiAl 3 phase or with titanium, resulting in TiAl phase. Subsequently, TiAl phase reacted with titanium, leading to the Ti 3 Al phase, or with already present Ti 2 Al 5 phase yielding TiAl 2 intermetallic compound. Titanium-rich Ti 3 Al phase could form only at the temperature of 600 °C or above. Image 10885 • Phase formation is controlled by diffusion. • Ti 2 Al 5 phase forms preferentially. • Ti reacts above 600 °C, Al starts to react under its melting point. [ABSTRACT FROM AUTHOR]

The microstructure, phase composition, and microhardness of both gas-atomized and mechanically milled powders of the Al7075 + 1 wt % Zr alloy were investigated. The gas-atomized powder exhibited a cellular microstructure (grain size of a few µm) with layers of intermetallic phases along the cell boundaries. Mechanical milling (400 revolutions per minute (RPM)/8 h) resulted in a grain size reduction to the nanocrystalline range (20 to 100 nm) along with the dissolution of the intermetallic phases. Milling led to an increase in the powder's microhardness from 97 to 343 HV. Compacts prepared by spark plasma sintering (SPS) exhibited negligible porosity. The grain size of the originally gas-atomized material was retained, but the continuous layers of intermetallic phases were replaced by individual particles. Recrystallization led to a grain size increase to 365 nm in the SPS compact prepared from the originally milled powder. Small precipitates of the Al3Zr phase were observed in the SPS compacts, and they are believed to be responsible for the retainment of the sub-microcrystalline microstructure during SPS. A more intensive precipitation in this SPS compact can be attributed to a faster diffusion due to a high density of dislocations and grain boundaries in the milled powder. [ABSTRACT FROM AUTHOR]

Electric field-assisted sintering has ubiquitous merits over conventional sintering technology for the fabrication of difficult-to-deform materials. To investigate the effect of sintering pressure and temperature on the densification of Inconel 718 superalloy, a numerical simulation model was established based on the Fleck-Kuhn-McMeeking (FKM) and Gurson-Tvergaard-Needleman (GTN) models, which covers a wide range of porosity. At a sintering pressure below 50 MPa or a sintering temperature below 950 °C, the average porosity of the sintered superalloy is over 0.17 with low densification. Under a pressure above 110 MPa and a temperature above 1250 °C, the sintered superalloy quickly completes densification and enters the plastic yield stage, making it difficult to control the sintering process. When the pressure is above 70 MPa while the temperature exceeds 1150 °C, the average porosity is 0.11, with little fall when the pressure or temperature rises. The experimental results indicated that the relative density of the sintered superalloy under 70 MPa and 1150 °C is 94.46%, and the proportion of the grain size below 10 μm is 73%. In addition, the yield strength of the sintered sample is 512 MPa, the compressive strength comes to 1260 MPa when the strain is over 0.8, and the microhardness is 395 Hv, demonstrating a better mechanical property than the conventional superalloy. [ABSTRACT FROM AUTHOR]

Ni-Ti alloys are considered to be very important shape memory alloys with a wide application area including, e.g., biomaterials, actuators, couplings, and components in automotive, aerospace, and robotics industries. In this study, the NiTi46 (wt.%) alloy was prepared by a combination of self-propagating high-temperature synthesis, milling, and spark plasma sintering consolidation at three various temperatures. The compacted samples were subsequently heat-treated at temperatures between 400 °C and 900 °C with the following quenching in water or slow cooling in a closed furnace. The influence of the consolidation temperature and regime of heat treatment on the microstructure, mechanical properties, and temperatures of phase transformation was evaluated. The results demonstrate the brittle behaviour of the samples directly after spark plasma sintering at all temperatures by the compressive test and no transformation temperatures at differential scanning calorimetry curves. The biggest improvement of mechanical properties, which was mainly a ductility enhancement, was achieved by heat treatment at 700 °C. Slow cooling has to be recommended in order to obtain the shape memory properties. [ABSTRACT FROM AUTHOR]

This work aims to describe the formation of intermetallics in the Ni-Ti-Al system in dependence on the heating rate, which has been determined previously as the crucial factor of thermal explosion self-propagating synthesis (TE-SHS). The tested alloys contained 1–7 wt % aluminum. Thermal analysis has been realized by the optical pyrometer under the conditions of high heating rates up to 110 °C·min−1. TE-SHS process in Ni-Ti-Al system is initiated by exothermic reaction of nickel aluminides Ni2Al3 and NiAl3 at the temperature of 535–610 °C. The next reactions occur in dependence on the heating rate. Samples containing 1–3 wt % of aluminum exhibit the similar initiation temperature as Ni-Ti binary mixture. The samples containing 5 wt % and more of aluminum were fully reacted after sintering at 800 °C with the heating rate of 300 °C·min−1 and the initiation temperature of the TE-SHS was observed close to Al-Al3Ni eutectic temperature (between 630–640 °C). [ABSTRACT FROM AUTHOR]

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)2 phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C. [ABSTRACT FROM AUTHOR]