Al–Ti–B grain refiners via powder metallurgy processing of Al/K2TiF6/KBF4 powder blends

The response to thermal exposure of ball-milled Al/K₂TiF₆/KBF₄ powder blends was investigated to explore the potential of PM processing for the manufacture of Al–Ti–B alloys. K₂TiF₆ starts to be reduced by aluminium as early as 220 °C when ball-milled Al/K₂TiF₆/KBF₄ powder blends are heated. The reaction of KBF₄ with aluminium follows soon after. The Ti and B thus produced are both solutionized in aluminium before precipitating out as Al₃Ti and TiB₂. All these reactions take place below the melting point of aluminium. The ball-milled Al/K₂TiF₆/KBF₄ powder blends heat treated at approximately 525 °C can be compacted to produce Al–Ti–B pellets with in situ formed Al₃Ti and TiB₂ particles. These pellets are shown to be adequate grain refiners for aluminium alloys.     

Reactivity of commercial silicon and silicides towards copper(I) chloride. Effect of aluminium, calcium and iron on the formation of copper silicide

The reaction of CuCl with silicon, containing Al, Fe and Ca as impurities, or with silicides (Si₂Ca, Si₂Fe, Si₂Al₂Ca, Si₈Al₆Fe₄Ca) has been investigated in the temperature range 200–300°C. For the reaction between CuCl and commercial Si, it was found that, at 282°C, aluminium promotes the reaction between Cu₃Si and CuCl while the rate of consumption of Cu₃Si is greatly reduced by the presence of iron. The combined action of these two impurities leads to the formation of more copper–silicon alloy. In the presence of mixed silicides, the reaction with CuCl also leads to the formation of Cu₃Si. For the quaternary Al–Ca silicide containing iron the rate of formation of Cu₃Si is not increased, which allows us to attribute a stabilizing effect to iron.     

Magnetic and structural studies of mechanically alloyed (Fe50Co50)62Nb8B30 powder mixtures

Amorphous (Fe₅₀Co₅₀)₆₂Nb₈B₃₀ powder mixture was prepared by mechanical alloying from elemental Fe, Co, B and Nb powders in a planetary ball mill under argon atmosphere. Structural, thermal and magnetic properties were performed on the milled powders by means of X-ray diffraction, differential scanning calorimetry and magnetic measurements. The amorphous state is reached after 125 h of milling. The excess enthalpy due to the high density of defects is released at temperature below 300 °C. Crystallisation and growth of crystal domains are the dominating processes at high temperatures. The saturation magnetisation decreases rapidly during the first 25 h of milling to about 15.24 A m²/kg and remains nearly constant on further milling. Coercivity, H_c, value of about 160 Oe is obtained after 125 h of milling.     

Preparation of TiAl3-Al composite coating by cold spraying

TiAl₃-Al coating was deposited on orthorhombic Ti₂AlNb alloy substrate by cold spraying with the mixture of pure Al and Ti as the feedstock powder at a fixed molar ratio of 3:1 when the spraying distance, gas temperature and gas pressure for the process were 10 mm, 250 °C and 1.8 MPa, respectively. The as-sprayed coating was then subjected to heat treatment at 630 °C in argon atmosphere for 5 h at a heating rate of 3 °C/min and an argon gas flow rate of 40 mL/min. The obtained TiAl₃-Al composite coating is about 212 μm with a density of 3.16 g/cm³ and a porosity of 14.69% in general. The microhardness and bonding strength for the composite coating are HV525 and 27.12 MPa.     

Formation of Al/(Al<Subscript>13</Subscript>Fe<Subscript>4</Subscript> + Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) Nano-composites via Mechanical Alloying and Friction Stir Processing

Combination of mechanical alloying and friction stir processing was used for the fabrication of Al/(Al₁₃Fe₄ + Al₂O₃) nano-composites. Pre-milled hematite + Al powder mixture was introduced into the stir zone generated on 1050 aluminum alloy sheet by friction stir processing. Uniform and active milled powder mixture reacted with plasticized aluminum to produced Al₁₃Fe₄ + Al₂O₃ particles. Al₁₃Fe₄ intermetallic showed elliptical shape with a typical size of ~ 100 nm, while nano-sized Al₂O₃ exhibited irregular floc-shaped particles that formed clusters with the remnant of iron oxide particles in the fine recrystallized aluminum matrix. As the milling time (1-3 h) of the introduced powder mixture increased, the volume fraction of Al₁₃Fe₄ + Al₂O₃ particles increased in the fabricated composite. The hardness and ultimate tensile strength of the fabricated nano-composites varied from 54.5 to 75 HV and 139 to 159 MPa, respectively; these are much higher than those of the friction stir processed base alloy (33 HV and 97 UTS). The highest hardness and strength were achieved for the nano-composite fabricated using the 3-h milled powder mixture; hard nano-sized reaction products and fine recrystallized grains of Al matrix had major and minor roles on enhancing these properties, respectively.     

Processing,characterization, room temperature mechanical properties and fracture behavior of hot extruded multi-scale B4C reinforced 5083 aluminum alloy based composites

Microstructural characteristics and mechanical behavior of hot extruded Al5083/B₄C nanocomposites were studied. Al5083 and Al5083/B₄C powders were milled for 50 h under argon atmosphere in attrition mill with rotational speed of 400 r/min. For increasing the elongation, milled powders were mixed with 30% and 50% unmilled aluminum powder (mass fraction) with mean particle size of >100 μm and <100 μm and then consolidated by hot pressing and hot extrusion with 9:1 extrusion ratio. Hot extruded samples were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), tensile and hardness tests. The results showed that mechanical milling process and presence of B₄C particles increase the yield strength of Al5083 alloy from 130 to 566 MPa but strongly decrease elongation (from 11.3% to 0.49%). Adding <100 μm unmilled particles enhanced the ductility and reduced tensile strength and hardness, but using the >100 μm unmilled particles reduced the tensile strength and ductility at the same time. By increasing the content of unmilled particles failure mechanism changed from brittle to ductile.     

Scanning Auger electron spectroscopy study of the oxide film formed on dendritic and interdendritic regions of C containing Fe3Al intermetallic

The oxide films formed during early stage of oxidation at 800 °C on dendritic and interdendritic regions of the cast Fe–16Al–1C (wt.%) alloy were studied using scanning Auger electron spectroscopy. Microhardness measurement and elemental depth profiles by Auger spectroscopy reveal that the carbide, Fe₃AlC_0.69, is the major constituent of the interdendritic region, while dendrites are predominantly Fe₃Al phase. Between the two, the interdendritic region is found to be more prone to oxidation than the dendritic region, which was attributed to presence of carbides with low-Al content. In spite of the difference in oxide film thickness exhibited by both the phases, they consist of an inner aluminium oxide layer and an outer iron oxide layer.     

Consolidation of Al2O3/Al Nanocomposite Powder by Cold Spray

While the improvement in mechanical properties of nanocomposites makes them attractive materials for structural applications, their processing still presents significant challenges. In this article, cold spray was used to consolidate milled Al and Al₂O₃/Al nanocomposite powders as well as the initial unmilled and unreinforced Al powder. The microstructure and nanohardness of the feedstock powders as well as those of the resulting coatings were compared. The results show that the large increase in hardness of the Al powder after mechanical milling is preserved after cold spraying. Good quality coating with low porosity is obtained from milled Al. However, the addition of Al₂O₃ to the Al powder during milling decreases the powder and coating nanohardness. This lower hardness is attributed to non-optimized milling parameters leading to cracked particles with insufficient Al₂O₃ embedding in Al. The coating produced from the milled Al₂O₃/Al mixture also showed lower particle cohesion and higher amount of porosity.     

Effect of mechanical activation on the production of SiC from silica sand

In this study the effect of 100 and 200 h low energy ball milling on the carbothermic reduction of SiO₂ and C powder mixture was investigated. Microstructure studies of the mixture by SEM revealed that the particle size had been decreased and the SiO₂ particles had been covered by C particles due to the milling. The results of thermal analysis (TG-DTA) of milled and unmilled mixtures clearly showed that the reduction temperature decreased due to milling process. XRD pattern of 200 h activated mixture proved that β-SiC had been formed almost completely after reduction at 1500 °C.     

Nanocrystalline τ2 phase obtained by mechanical alloying of Al60Fe15Si15Ti10 powder mixture followed by consolidation

In the present work an elemental powder mixture of Al60Fe15Si15Ti10 (at.%) was mechanically alloyed in a high-energy ball mill. A part of the milling product was examined in a calorimeter, while another portion was subjected to consolidation by hot-pressing at 1000 °C for 180 s under a pressure of 7.7 GPa. The results obtained show that a nanocrystalline cubic phase with the lattice parameter a₀ = 11.645 Å, isomorphous with the τ₂ (Al₂FeTi) phase, is formed during mechanical alloying process. Heating of the milling product in the calorimeter up to 720 °C causes limited growth of grains, however the τ₂ phase remains nanocrystalline with the mean crystallite size of 28 nm. Grain growth takes place during consolidation of the milling product as well, although the τ₂ phase remains nanocrystalline with the mean crystallite size of 34 nm. The microhardness of the bulk nanocrystalline sample is 1013 HV0.2 and its open porosity is 0.3%. The results obtained show that the quality of compaction with preserving nanometric grain size of the τ₂ phase is satisfactory and its microhardness is relatively high.     

The effect of Fe2Al5 as reducing agent in intermediate steps of 'Al2O3/TiC-Fe composite production process

Al₂O₃/TiC-Fe cutting tools can be produced in-situ by using ilmenite, aluminum and graphite. The formation mechanism of the system was investigated in some researches. Most of them have diagnosed the aluminum as reducing agent of TiO₂, but in this study, it is proven that Fe₂Al₅ acts as the reducing agent for TiO₂ in the intermediate steps of the reactions. To achieve this goal, firstly, the reactions of synthesized ilmenite, aluminum and graphite system were investigated in detail. Then, pure Fe₂Al₅ was synthesized, and mixed with definite amount of TiO₂. The sample mixture was heat treated at the same temperature at the real reaction temperature, namely, 930 °C. The final products were analyzed with XRD. It was found that the product heat treatment of F₂Al₅ and TiO₂ mixture was the same as the sample of ilmenite, aluminum and graphite mixture with a molar ratio of 1:2:1 heat treated at 930 °C.     

Comparative study on the corrosion behavior of milled and unmilled magnesium by electrochemical impedance spectroscopy

The corrosion behavior of milled Mg prepared by high-energy ball milling for 10 h has been investigated in alkaline solutions by electrochemical impedance spectroscopy and compared with that of unmilled Mg. X-ray powder diffraction indicates a crystallite size of 34 nm for the milled Mg compared to >100 nm for the unmilled powder. Chemical analyses show no significant iron contamination in milled Mg powder, indicating the absence of tools erosion during the milling procedure. In contrast, significant MgO enrichment in the milled powder is observed (6.5 wt.% after 10 h milling compared to 1.0 wt.% before milling). The oxygen contamination is mainly attributed to the powder oxidation occurring during milling. From XPS analyses, no MgO enrichment is detected on milled Mg electrode surface, confirming that MgO is dispersed homogeneously in the bulk of the material rather than to segregate on its surface. Electrochemical impedance spectroscopy demonstrates clearly the better corrosion resistance of milled Mg compared to unmilled Mg in passive conditions (KOH solution, pH=14) and in more active corrosion conditions (borate solution, pH=8.4). This is illustrated by a nobler corrosion potential and by a significant increase of the interfacial resistance related to the film and charge-transfer reaction. Moreover, the variation of the different electrochemical parameters (corrosion potential, interfacial resistance and capacitance) with immersion time is less accentuated and tends more rapidly to a steady state with milled Mg, suggesting an enhancement of the Mg(OH)₂ formation kinetic. The origin of the distinctive passivation behavior of ball-milled Mg is discussed.     

Effect of temperature on mechanical alloying of Cu-Zn and Cu-Cr system

Cu-Zn and Cu-Cr powders were milled with an attritor mill at room temperature, −10, −20 and −30 °C, respectively. Phase transformation and morphology evolution of the alloyed powder were investigated by X-ray diffractometry(XRD), X-ray photoelectron spectroscopy(XPS) and scanning electron microscopy(SEM). The results show that lowering temperature can delay mechanical alloying(MA) process of Cu-Zn system with negative mixing enthalpy, and promote MA process of Cu-Cr system with positive mixing enthalpy. As for Cu-Cr and Cu-Zn powders milled at −10 °C, lamellar structures are firstly formed, while fewer lamellar particles can be found when the powder is milled at −20 °C. When the alloyed powder is annealed at 1 000 °C, Cu(Cr) solid solution is decomposed and Cr precipitates from Cu matrix, whereas Cu(Zn) solid solution keeps stable.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

Assessment of the surface state behaviour of Al71Cu10Fe9Cr10 and Al3Mg2 complex metallic alloys in sliding contacts

Electrochemical measurements and friction measurements during continuous and intermittent unidirectional sliding tests are used to monitor and to evaluate the surface characteristics of two types of metallic materials characterized by a huge unit cell, namely Al₇₁Cu₁₀Fe₉Cr₁₀ and Al₃Mg₂. The modification of the surface characteristics results from the periodic mechanical removal of a surface film during sliding, and the subsequent (electro)chemical re-growth of a surface film in-between successive sliding contacts. Al₇₁Cu₁₀Fe₉Cr₁₀ and Al₃Mg₂ materials were tested in a phosphate buffer solution pH 7 at 25 °C to compare their depassivation and subsequent repassivation behaviour. The Al₃Mg₂ material was also tested in a 0.1 M KOH solution pH 13 and 25 °C to reveal the role of constituting metallic elements on the surface film formation. The effect of film formation and removal on the coefficient of friction recorded during unidirectional sliding is discussed.     

Synthesis of α-Mo-Mo5SiB2-Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment

A two-step mechanical alloying process followed by heat treatment was developed as a novel approach for fabrication of Mo-12.5 mol%Si-25 mol%B nanocomposite powders. In this regard, a Si-43.62 wt.% B powder mixture was milled for 20 h. Then, Mo was added to the mechanically alloyed Si-B powders in order to achieve Mo-12.5 mol%Si-25 mol%B powder. This powder mixture was further milled for 2,5,10 and 20 h. All of the milled powders were annealed at 1100 °C for 1 h. After first step of milling, a nanocomposite structure composed of boron particles embedded in Si matrix was formed. On the other hand, an α-Mo/MoSi₂ nanocomposite was produced after second step while no ternary phases between Mo, Si and B were formed. At this stage, the subsequent annealing led to formation of α-Mo and Mo₅SiB₂ as major phases. The phase evolutions during heat treatment of powders can be affected by milling conditions. It should be mentioned that the desirable intermetallic phases were not formed during heat treatment of unmilled powders. On the other hand, α-Mo-Mo₅SiB₂-Mo₃Si nanocomposites were formed after annealing of powders milled for 22 h. With increasing milling time (at the second step), the formation of Mo₃Si during subsequent heat treatment was disturbed. Here, an α-Mo-Mo₅SiB₂-MoSi₂ nanocomposite was formed after annealing of 30 and 40 h milled powders.     

Bulk amorphous Al85Fe15 alloy and Al85Fe15-B composites with amorphous or nanocrystalline-matrix produced by consolidation of mechanically alloyed powders

An Al80Fe14B6 powder mixture was subjected to mechanical alloying. Presence of an amorphous structure in the milling product was revealed by XRD investigations. The calorimetric study showed that the amorphous phase crystallised above 370 °C. The milled Al₈₀Fe₁₄B₆ powder was consolidated under a pressure of 7.7 GPa in different conditions: at 350 °C and at 1000 °C. Besides, the mechanically alloyed amorphous Al₈₅Fe₁₅ powder was consolidated at 360 °C. The amorphous structure was retained after consolidation applied at 350 °C and 360 °C. Compaction at 1000 °C caused crystallisation of the amorphous phase and appearance of metastable nanocrystalline phases. Structural investigations revealed that both bulk Al₈₀Fe₁₄B₆ samples are composites with boron particles embedded in amorphous or nanocrystalline matrix. The hardness of the nanocrystalline-matrix composite and of the amorphous-matrix one is equal to 707 HV1 and 641 HV1 respectively, whereas that of bulk amorphous Al₈₅Fe₁₅ alloy is 504 HV1. The specific yield strength of amorphous-matrix and nanocrystalline-matrix composites, estimated using the Tabor relationship, is 625 and 650 kNm/kg respectively, while that of amorphous Al₈₅Fe₁₅ alloy is 492 kNm/kg. We also suppose that application of high pressure affected crystallisation of amorphous phase, influencing the phase composition of the products of this process.     

Phase identification of Al–B4C ceramic composites synthesized by reaction hot-press sintering

A series of boron carbide (B₄C) matrix composites with different contents of Al, were synthesized by reaction hot-press sintering with milled B₄C and pure metallic Al powder at 1600 °C for 1 h. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to identify the phase constituent of the milled powders and the composites. The results have shown that parts of B₄C and Al particles were oxidized to boron oxide (B₂O₃) and alumina (Al₂O₃) during the milling. Thermit reaction occurred and B₂O₃ was reverted during hot-press sintering. A ternary phase of Al boron carbide (Al₈B₄C₇) was found in the composites, and the B₄C transformed to a rich boron phase (B_6.5C) because of the superfluous boron in the system.     

Reaction in Al-Ti-C powders and its relation to the formation and stability of TiC in Al at high temperatures

By heating Al, Ti and C powders in a DSC and by observing and analysing the microstructures produced, it has been found that, for Ti and C concentrations, equivalent to those observed in metal matrix composites, TiC forms from Al₃Ti and Al₄C₃ above 890°C. Microstructural evidence suggests that the formation of TiC occurs by reaction between Ti dissolved in Al and Al₄C₃ through the dissolution of Al₃Ti. It is, therefore, reasonable to assume that TiC particles react in Al below 890°C to form Al₃Ti and Al₄C₃.     

Formation of titanium nitride produced from nanocrystalline titanium powder under nitrogen atmosphere

The initially globular-shaped Ti powder particles were flattened to ‘pan-cake’ like shape after 12, 16, and thin flakes after 20 h of mechanical milling. Although no change peak positions of HCP Ti crystal structure, the increase in peak intensity with milling time was evident. It is found that the greater surface to volume ratio of the milled Ti powders accelerated the N₂ uptake and subsequent formation of TiN at lower temperatures (884, 856 and 833 °C for 12, 16 and 20 h, respectively) than in the unmilled powder (∼ 1100 °C). Higher nitrogen content of 41–44 at.% by EDS analysis confirmed the high rate of dissolution on the milled powders.