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Niobium is recently considered one of the potential alloying elements for tool steels due to the formation of hard and stable carbides of MC type. Its use is limited by the fact that these carbides tend to coarsen during conventional melting metallurgy processing. This work explores the potential of additive manufacturing for processing Nb-alloyed tool steel with a high content of carbon. Directed energy deposition was used as the processing method. It was found that this method allowed us to obtain a microstructure very similar to that obtained after the use of consolidation via spark plasma sintering when subsequent heat treatment by soft annealing, austenitizing, oil quenching and triple tempering for secondary hardness was applied. Moreover, the soft annealing process could be skipped without affecting the structure and properties when machining would not be required. The hardness of the steel was even higher after additive manufacturing was used (approx. 800–830 HV 30) than after spark plasma sintering (approx. 720–750 HV 30). The wear resistance of the materials processed by both routes was almost comparable, reaching 5–7 × 10−6 mm3N−1m−1 depending on the heat treatment. [ABSTRACT FROM AUTHOR]

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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 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]

Due to the lack of mineral resources, unconventional raw materials have to be considered. In this work, the use of reduced deep-sea nodules from Clarion-Clippertion Fracture Zone of Pacific Ocean was tested as the natural mixture of alloying elements for aluminum alloys. The nodules were reduced by aluminothermic process without the extraction of individual elements. As the technology for the manufacture of the alloys, the powder metallurgy comprising mechanical alloying and spark plasma sintering was applied. The produced Al-Mn based alloys are characterized by uncommon spherical morphology of Al-Mn intermetallics, which leads to a significant ductility even in the case of high total amount of additives (up to 20 wt%). The wear resistance of the alloys strongly increases with growing amount of the reduced deep-sea nodules. • New aluminum alloys with reduced deep-sea nodules prepared by powder metallurgy. • Unique rounded morphology of intermetallics was obtained. • Improved strength compared with common Al-Mn alloys. • Reduced deep sea nodules improve tribological properties of aluminum. [ABSTRACT FROM AUTHOR]

One of the current problems concerning the re-use of aluminium alloys is the recycling of aluminium alloys with increased content of iron. One of the recent trends is the processing by powder metallurgy, producing completely new grades of aluminium alloys. The aim of the present work was to test the properties of Al–Si–Fe alloy, which can be manufactured from the high-irony scrap, as a possible material for elevated temperature applications. Since chromium and nickel accompany the iron in the scrap when stainless steels are admixed, the effect of chromium and nickel on thermal stability and mechanical properties at room and elevated temperature was also studied. Al–Si–Fe–X alloys were rapidly solidified by using melt spinning technique. The rapidly solidified ribbons were pulverized by planetary ball mill and the obtained powders were used for compaction by means of Spark Plasma Sintering. It was found that nickel improves the thermal stability as well as the mechanical properties at the elevated temperature. On the other hand, the effect of chromium on the mechanical properties at elevated temperature was almost detrimental. [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 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]

Abstract: In this work we studied the influence of the nitriding atmosphere composition on hardness, microstructure, phase composition and wear resistance of duplex treated cold work PM tool steel alloyed with niobium (2.6 wt.%). Pulsed-plasma nitriding was carried out at 500 °C for 180 min, the atmosphere composition was N2:H2 =1:3 and 1:6. After nitriding, the material was PVD coated (magnetron sputtering) with TiAlN layer. The PVD coat was also applied upon a non-nitrided material. We found out that when both nitriding conditions were applied the wear resistance was significantly higher than the PVD — only coated material. The best wear resistance was achieved when the TiAlN coating was supported by a layer applied by nitriding in a lower-nitrogen atmosphere (N2:H2 =1:6). These conditions prevented formation of an undesirable compound layer in the nitrided layer. [Copyright &y& Elsevier]

Abstract: The aim of this work was to evaluate the corrosion and abrasive-wear resistance of plasma-nitrided PM tool steel with increased content of niobium (2.6 wt.%). It was found that the wear behaviour was significantly influenced not only by surface hardness, but also by other factors, mainly by surface roughness and by the presence and properties of a compound layer. The maximum wear resistance was achieved by plasma nitriding at 500 °C for 180 min. It was also shown that nitriding led to an increase of the free corrosion potential and that the compound layer on the surface acted as a barrier against corrosion attack. The nitriding conditions that resulted in the maximum wear resistance also provided significantly increased corrosion resistance. In addition, it was observed that the conventional gas nitriding conducted at the same conditions did not produce such significant improvement of the wear resistance. [Copyright &y& Elsevier]

Abstract: The aim of this work is to describe the processes during plasma nitriding of Nb-containing, powder metallurgy prepared (PM) tool steel. PM steel was studied containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W, produced by melt atomization and hot isostatic pressing. The hardened and tempered steel was plasma nitrided at temperatures ranging from 470 to 530°C. The nitriding time was 30–180min. Optical microscopy, electron microscopy, XRD, EDS, WDS and hardness measurements were used to study the steel''s properties, microstructure, phases and chemical composition. The nitriding temperature of 470°C was found to be unsuitable for practical use because the layers prepared at this temperature were non-uniform. The compound layer formed by M2,3(C,N) and M4N was found after nitriding at temperatures of 500 and 530°C for at least 60min. The effect of niobium on the formation of this layer was studied. [Copyright &y& Elsevier]

A new powder production method has been developed to speed up the search for novel alloys for additive manufacturing. The technique involves an ultrasonically agitated cold crucible installed at the top of a 20 kHz ultrasonic sonotrode. The material is melted with an electric arc and undergoes pulverization with standing wave vibrations. Several different alloys in various forms, including noble and metallic glass alloys, were chosen to test the process. The atomized particles showed exceptional sphericity, while powder output suitable for additive manufacturing reached up to 60%. The AMZ4 metallic glass powder remained amorphous below the 50 μm fraction, while tungsten addition led to crystallization in each fraction. Minor contamination and high Mn and Zn evaporation, especially in the finest particles, was observed in atomized powders. The innovative ultrasonic atomization method appears as a promising tool for material scientists to develop powders with tailored chemical composition, size and structure. [ABSTRACT FROM AUTHOR]

A low-cost titanium alloy (Ti–5Al–2Fe–3Mo wt.%) was designed and fabricated by blended elemental powder metallurgy (BEPM) process. The high-temperature deformation behavior of the powder metallurgical Ti–5Al–2Fe–3Mo wt.% (PM-TiAlFeMo) alloy was investigated by hot compression tests at temperatures ranging from 700 to 1000 °C and strain rates ranging from 0.001 to 10 s−1. The flow curves were employed to develop the Arrhenius-type constitutive model in consideration of effects of deformation temperature, strain rate, and flow stress. The value of activation energy (Q) was determined as 413.25 kJ/mol. In order to describe the workability and predict the optimum hot processing parameters of the PM-TiAlFeMo alloy, the processing map has been established based on the true stress–true strain curves and power dissipation efficiency map. Moreover, microstructure observations match well with the analyses about deformation mechanisms, revealing that dynamic recovery and dynamic recrystallization are dominant softening mechanisms at relatively high temperatures. However, the kinking and breaking of microstructure prefer to occur at relatively low temperatures. [ABSTRACT FROM AUTHOR]

Powder metallurgy is a group of advanced processes for the synthesis, processing, and shaping of various kinds of materials. Initially inspired by ceramics processing, the methodology comprising of the production of a powder and its transformation to a compact solid product has attracted great attention since the end of World War II. At present, there are many technologies for powder production (e.g., gas atomization of the melt, chemical reduction, milling, and mechanical alloying) and its consolidation (e.g., pressing and sintering, hot isostatic pressing, and spark plasma sintering). The most promising ones can achieve an ultra-fine or nano-grained structure of the powder, and preserve it during consolidation. Among these methods, mechanical alloying and spark plasma sintering play a key role. This Special Issue gives special focus to the advancement of mechanical alloying, spark plasma sintering and self-propagating high-temperature synthesis methods, as well as to the role of these processes in the development of new materials. [ABSTRACT FROM AUTHOR]

Composite material prepared by spark plasma sintering (SPS) from a powder mixture of AlCrFeSi rapidly solidified alloy and 5 wt. % of Ni particles was studied in this work. It was proven that during SPS compaction at 500 °C, no intermetallic phases formed on the surface of Ni particles. The material exhibited sufficient mechanical properties obtained by tensile testing (ultimate tensile stress of 203 ± 4 MPa, ductility of 0.8% and 0.2% offset yield strength of 156 ± 2 MPa). Tensile samples were pre-stressed to 180 MPa and annealed at 450 and 550 °C for 1 h. Annealing at 450 °C did not lead to any recovery of the material. Annealing at 550 °C caused the full recovery of 0.2% offset yield strength, while the ductility was decreased. The self-healing behaviour originates from the growth of intermetallic phases between the Ni particle and the Al matrix. The sequence of NiAl, Ni2Al3 and NiAl3 intermetallic phases formation was observed. In particular, the morphology of the NiAl3 phase, growing in thin dendrites into the Al matrix, is suitable for the closing of cracks, which pass through the material. [ABSTRACT FROM AUTHOR]

Melting metallurgy is still the most frequently used and simplest method for the processing of metallic materials. Some of the materials (especially intermetallics) are very difficult to prepare by this method due to the high melting points, poor fluidity, or formation of cracks and pores after casting. This article describes the processing of Ti-Al-Si alloys by arc melting, and shows the microstructure, phase composition, hardness, fracture toughness, and compression tests of these alloys. These results are compared with the same alloys prepared by powder metallurgy by the means of a combination of mechanical alloying and spark plasma sintering. Ti-Al-Si alloys processed by melting metallurgy are characterized by a very coarse structure with central porosity. The phase composition is formed by titanium aluminides and titanium silicides, which are full of cracks. Ti-Al-Si alloys processed by the powder metallurgy route have a relatively homogeneous fine-grained structure with higher hardness. However, these alloys are very brittle. On the other hand, the fracture toughness of arc-melted samples is immeasurable using Palmqvist's method because the crack is stopped by a large area of titanium aluminide matrix. [ABSTRACT FROM AUTHOR]

Fe–Al–Si alloys have been previously reported as an interesting alternative to common high-temperature materials. This work aimed to improve the properties of FeAl20Si20 alloy (in wt.%) by the application of powder metallurgy process consisting of ultrahigh-energy mechanical alloying and spark plasma sintering. The material consisted of Fe3Si, FeSi, and Fe3Al2Si3 phases. It was found that the alloy exhibits an anomalous behaviour of yield strength and ultimate compressive strength around 500 °C, reaching approximately 1100 and 1500 MPa, respectively. The results also demonstrated exceptional wear resistance, oxidation resistance, and corrosion resistance in water-based electrolytes. The tested manufacturing process enabled the fracture toughness to be increased ca. 10 times compared to the cast alloy of the same composition. Due to its unique properties, the material could be applicable in the automotive industry for the manufacture of exhaust valves, for wear parts, and probably as a material for selected aggressive chemical environments. [ABSTRACT FROM AUTHOR]

Tungsten-based materials are the prime candidate plasma-facing materials for future fusion reactors, such as DEMO. Spark plasma sintering is a prospective fabrication technology with several advantageous features. The concurrent application of electric current, temperature and pressure enhances the sintering process, allowing for lower temperatures and shorter sintering times than traditional powder metallurgy processes. This in turn helps to avoid excessive grain growth and phase segregation in W-alloys. This study is focused on several factors that may influence the homogeneity of the sintered compacts—namely the diffusion of carbon from the graphite die, purity of the powder and sintering conditions. The following characteristics of spark plasma-sintered tungsten compacts were studied: composition (especially carbon and oxygen content), porosity, mechanical properties (hardness and fracture strength), and thermal diffusivity. The effects of the abovementioned processing factors were quantified, and local variations of selected properties were assessed. [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]

Fe-Al-Si alloys have been recently developed in order to obtain excellent high-temperature mechanical properties and oxidation resistance. However, their production by conventional metallurgical processes is problematic. In this work, an innovative processing method, based on ultra-high energy mechanical alloying, has been tested for the preparation of these alloys. It has been found that the powders of low-silicon alloys (up to 10 wt. %) consist of FeAl phase supersaturated by Si after mechanical alloying. Fe2Al5 phase forms as a transient phase at the initial stage of mechanical alloying. The alloy containing 20 wt. % of Si and 20 wt. % of Al is composed mostly of iron silicides (Fe3Si and FeSi) and FeAl ordered phase. Thermal stability of the mechanically alloyed powders was studied in order to predict the sintering behavior during possible compaction via spark plasma sintering or other methods. The formation of Fe2Al5 phase and Fe3Si or Fe2Al3Si3 phases was detected after annealing depending on the alloy composition. It implies that the powders after mechanical alloying are in a metastable state; therefore, chemical reactions can be expected in the powders during sintering. [ABSTRACT FROM AUTHOR]

The structure of Ni 8 Ti 5 was solved and refined from precession electron diffraction tomography data using dynamical refinement approach. It has trigonal symmetry R -3 m ( a = 12.24(5) Å, c = 15.33(5) Å) with a structure related to the structure of Ni 4 Ti 3 . The greatest challenge of the structure determination was the assignment of atomic types due to the proximity of scattering powers of Ti and Ni. Several approaches to determination of atom type distribution over atomic positions were used; (1) analysis of the Wyckoff position multiplicities, (2) refinement of displacement parameters, and (3) geometrical analysis of the structure and comparison with known structures. These approaches consistently pointed to one distribution, which was confirmed by a significant improvement of the refinement figures of merit. [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]