Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Abstract

A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al₁₃Cr₂ intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al₁₃Cr₂ primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10 μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine <1 μm) particles, where the solidification front velocity exceeds the absolute stability velocity.

MST/1247a     

Effect of silicon,magnesium, cobalt and iron additions on the microstructure of Al-Li centrifugally atomized powders

The effect of the addition of silicon (up to 4 wt%), magnesium (up to 7.25 wt%), cobalt (up to 0.8 wt%) and iron (up to 1.5 wt%) on the microstructure of as-solidified Al-3 wt% Li powders, produced by centrifugal atomization in a helium atmosphere, has been studied by means of optical, scanning electron and transmission electron microscopy. The results show that the microstructure changes from dendritic in the case of Al-Li and Al-Li-Mg alloys to a eutectic (Al + AlLiSi) mixture in the Al-Li-Si alloy to a cellular structure for the Al-Li-Co alloy, and results in the direct nucleation of coarse intermetallic Al₆ Fe from the melt followed by subsequent growth in the case of Al-Li-Fe alloy.     

Interfaces in nickel aluminide/alumina fibre composites

Metal matrix composites based on the intermetallic alloy Ni₃Al and fibres of Al₂O₃ were fabricated by hot-pressing nickel aluminide powders and alumina fibres. Two matrix alloys were used in this investigation: Ni₃Al microalloyed with boron and Ni₃Al alloyed with 8 at% chromium and smaller amounts of zirconium and boron. The materials were studied using optical and transmission electron microscopy with particular emphasis placed on the characteristics of the matrix-fibre interface. The base Ni₃Al/Al₃O₃ composite displayed no evidence of chemical reaction at the interface, an intimate bond between matrix and fibre was observed, and the material exhibited 10% ductility at room temperature. Composites with the more complex matrix alloy were brittle, a phenomenon attributed to the formation of zirconia particles at the interface.     

Microstructure and thermal stability of Al–Cr–X (X=Ni,Mo, Si) powders obtained by centrifugal atomisation

Abstract

The microstructure and the thermal stability of rapid solidified centrifugally atomised AI–3Cr–X (X=1Ni, 3Ni, 0·3Mo, 1Si, or 3Si, at.-%) powders were studied. Three main types of microstructure were observed in the powders: cellular, globular, and rosettelike. Some powders exhibited a mixture of these. In the atomised state the alloys usually had two phases, intermetallic Al₁₃Cr₂ and α-Al solid solution. Thermal stability was studied for a range of temperatures from 20 to 500°C. The phase Al₃Ni appeared in the nickel containing alloys and grew upon heat treatment. The molybdenum containing alloy did not show any noticeable change upon heat treatment. With respect to the silicon containing alloys, the intermetallic Al₁₃Cr₂ transformed into Al₁₃Cr₄Si₄ at high temperatures. On the basis of bibliographic information a nucleation map was calculated relating the prevalence of the intermetallic Al₁₃Cr₂ phase and the α-Al phase to the particle diameter and the chromium concentration of powder obtained by centrifugal atomisation.

MST/3273     

Deposition of nickel nanoparticles onto aluminum powders using a modified polyol process

Aluminum is commonly used as fuel additive for propellants. The main limitation to its use lies in comparatively slow ignition and oxidation/combustion kinetics. Combustion performance of aluminized propellants can be improved through the use of Ni-coated Al particles. Nanoparticles, with its increased reactivity, also improve combustion performance. Hence, in this work, nano-sized Ni particles coated onto commercially available micron-sized Al powders using a modified polyol process were synthesized and evaluated. Ni-coated Al powders of various compositions produced by this method showed significant improvement in the oxidation kinetics as compared to untreated Al powders. The onset oxidation temperatures for the Ni-coated Al powders were found to be significantly reduced as compared to pure untreated Al. Other than the oxides, the intermetallic compound, Al₃Ni was also detected in the 10 wt% Ni-Al powders that were heated to temperature above 1050 °C.     

Intermetallic-reinforced aluminum matrix composites produced in situ by friction stir processing

Friction stir processing (FSP) is applied to produce intermetallic-reinforced aluminum matrix composites from elemental powder mixtures of Al-Cu and Al-Ti. The intermetallic phases are identified as Al₂Cu and Al₃Ti, which are formed in situ during FSP. The volume fraction of the intermetallic phases in the in situ composites may reach as high as ∼ 0.5. The composites produced by FSP are fully dense with high strength, and the composite strength increases with the reinforcement content.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Suppression method of industrial Al alloy powder explosion in dry and humid environment: A low-cost intrinsic safety technology

During the production and processing of Al alloy, a large number of small particle size powders will be produced. If the concentration of these Al alloy dust is too high under dry conditions, it will lead to dry powder explosion, and if hydrogen is produced in contact with water under wet conditions, it will lead to hydrogen explosion. This work is intended to develop a simple, cost effective technology that inerts waste Al alloy dust and minimizes its explosion hazards in different environments. Dihydrogen phosphates (KH₂PO₄, NH₄H₂PO₄, Ca(H₂PO₄)₂) are used to test the explosion flame propagation, explosion pressure, and hydrogen evolution, of waste Al alloy dust. The results show that dihydrogen phosphate can significantly reduce the flame propagation distance, speed, explosion pressure and hydrogen evolution performance of Al alloy dust, and inhibit the explosion of Al alloy dust in dry and humid environment to varying degrees. In different experiments, Ca(H₂PO₄)₂ has the best inerting effect, low cost and simple operation, which realizes the intrinsic safety of metal dust processing process. The phosphating film on the surface of Al alloy particles blocks the generation of Al(OH)₃ and retains the metal body to the greatest extent, which provides a possibility for the recycling of waste metal dust.     

Comparative Evaluation on the In Vitro Biological Performance of Ti45Al8.5Nb Intermetallic with Ti6Al4V and Ti6Al7Nb Alloys

The corrosion resistance of Ti45Al8.5Nb intermetallic alloy in artificial saliva and its cytocompatibility was studied via electrochemical tests, scanning electron microscopy, ion release measurement, and MTT assay, with contemporary biomedical Ti6Al4V and Ti6Al7Nb alloys as comparison. The results demonstrate that the corrosion potential (E_corr) and the corrosion current density (i_corr) of the three experimental alloy samples are similar and there is no statistically significant difference among them (p > 0.05). The Al³⁺ ion releasing concentration for Ti45Al8.5Nb intermetallic and Ti6Al7Nb alloy after anodic polarization are close. The relative cell proliferation rates of the three experimental alloy extract groups are all over 90% at various cultivation periods (1, 3, and 5 d), and there is no obvious difference for the MG63 cell morphologies comparing with that of the negative group, reaching confluence after 5 d culture and showing well stretched, which indicates that Ti45Al8.5Nb intermetallic alloy has a good cytocompatibility with the Grade 1 RGR value (no toxicity) according to ISO 10993‐5: 1999.     

A novel method for the preparation of Al-Mg intermetallic coating

A novel method has been suggested to obtain an Al-Mg intermetallic compound coating on the ZL102 Al alloy by Al-Mg twin-wire arc spraying and gas tungsten arc melting (GTAM) post treatment. The composition and the morphology of the coating are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) and the corrosion behavior is investigated by potentiodynamic electrochemical tests respectively. The results show that the components of the coating after GTAM are Mg₁₇Al₁₂, Al₃Mg₂ and Al. According to polarization curves and micro-hardness tests, the Al-Mg intermetallic coating exhibits good corrosion resistance and high micro-hardness.     

The Effect of Excess Aluminum on the Composition and Microstructure of Nb-Al Alloys Produced by Aluminothermic Reduction of Nb2O5

Intermetallic aluminides including those phases of the Nb-Al system are of interest for high-temperature structural applications. Through aluminothermic reduction (ATR) of Nb₂O₅ different alloys of the Nb-Al system can be produced by varying the amount of aluminum (excess aluminum) in the thermit charge. In this work, various Nb-Al alloys were produced by varying Nb₂O₅ and Al powder blends. The resulting alloys were characterized by chemical analysis (Al, O, and C), X-ray diffraction and scanning electron microscopy. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60% over the stoichiometric amount to reduce the Nb₂O₅. The carbon content was lower than 300 wt-ppm. The oxygen content decreases with increasing excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al₂O₃) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nb_ss (niobium solid solution) and Nb₃Al (alloy produced with 10% excess Al); Nb₃Al (alloys produced with 15 and 20% excess Al); Nb₃Al, Nb₂Al, and NbAl₃ (alloy produced with 30% excess Al); and Nb₂Al and NbAl₃ (alloys produced with 40, 50, and 60% excess Al).     

Influence of neodymium on microstructure and mechanical properties of AZ31B wrought magnesium alloy

Abstract

The influences of rare earth neodymium on microstructure and mechanical properties of as cast and hot rolled AZ31B wrought magnesium alloy were investigated. The results show that the mechanical properties of both as cast and hot rolled AZ31B alloys decrease due to Nd addition. Nd reacts with Al to form Al₂Nd phase when Nd is added. Bulky and brittle Al₂Nd intermetallic degrades the mechanical properties. Moreover, the addition of Nd weakens the grain refining effect of Al on as cast AZ31B alloy, resulting in grain coarsening. Coarse grains also cause the decline of the mechanical properties of as cast AZ31B–Nd alloy. The negative influence of the bulky and brittle intermetallics on mechanical properties of AZ31B alloy can be relieved by large deformation because the intermetallics can be sufficiently broken up during the deformation process.     

Microstructure and mechanical properties of Al-Si-Ni-Ce alloys prepared by gas-atomization spark plasma sintering and hot-extrusion

Al-Si-Ni-Ce alloys with the composition of Al_78.5Si₁₉Ni₂Ce_0.5, Al₇₆Si₁₉Ni₄Ce₁ and Al₇₃Si₁₉Ni₇Ce₁ were atomized and then sintered by using spark plasma method. The microstructure of the as-atomized powders, sintered and hot-extruded samples was analyzed. The influences of granularity and sintering parameters including time and temperature on the density of sintered alloy were also discussed. It is shown that the atomized powders are composed of Si, Al₁₁Ce₃, Al₃Ni and alpha Al. Tiny Al₃Ni particles precipitate from supersaturated matrix near the powder boundaries during SPS. Hot-extrusion process leads to the layer structure and more homogeneous distribution of precipitates. These alloys exhibit high comprehensive mechanical properties with combination of high Vicker's micro-hardness, moderate tensile properties and elongation, which provide a novel kind of promising engineering materials.     

Extraction and X-ray analysis of phases in aluminium alloys

The extraction of precipitates and second phase insoluble intermetallic or metalloid compounds from three commerical aluminium alloys was achieved by anodic dissolution of the aluminium alloy matrix in a methanolic electrolyte containing benzoic acid, oxine and chloroform. Investigated were Aluminium Association alloy 7075 containing principally Al-Zn-Mg-Cu, alloy 6061 containing principally Al-Si, and alloy 2011 containing principally Al-Cu. X-ray diffraction of the extracted residues using the Debye-Scherrer method, identified the compounds MnAl₆, MgO and Mg₂Si in the 7075 alloy in the aged condition, the insoluble phase Mn₁₂Si₇Al₅ in the 6061 alloy in both the solution heat-treated and aged conditions. In the 2011 alloy the primary precipitate, CuAl₂, was extracted in the aged condition and the lattice parameters for this tetragonal compound were determined to bea=6.064 Å andc=4.874 Å: Metallic bismuth also was identified in the extraction from this alloy. Several lines, believed to represent an unknown ternary or higher compound, could not be identified.     

Effects of liquid quenching and subsequent heating on the structure of Co-Al Alloys

The phase composition and structure of Co-Al alloys after liquid-quenching (LQ) and subsequent heating were determined by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry. After LQ, the Co-19.5 at % Al alloy consisted of two phases: CoAl (B2 structure, a=0.2852 nm) and an fcc Co_{1 – x} Al_x solid solution (a=0.3573 nm). The phase composition of the LQ alloy and the lattice parameters of the intermetallic phase CoAl and solid solution corresponded to a solidification temperature of 1100°, rather than to the eutectic temperature (1400°C). Heating to 1000°C brought the alloy to the equilibrium state. In the range 25–28 at % Al, the LQ alloys were single-phase and consisted of Co-enriched CoAl (B2). Decomposition of this intermetallic phase during heating in the calorimeter gave rise to an exothermic peak at 680°C and led to precipitation of hcp Co, which converted to an fcc Co_{1 – x} Al_x solid solution on heating to 1000°C. The activation energy of the decomposition process, evaluated by the Kissinger method, was E _a=125 ± 10 kJ/mol. The LQ Co-71.4 at % Al alloy consisted of three phases: metastable quasicrystalline phase (similar in structure to Al₁₄Co₃Ni₃), Co₂Al₅, and CoAl. Heating brought the alloy to the equilibrium state (Co₂Al₅), with an exotherm near 690°C. The activation energy of this phase transformation was E _a=195 ± 15 kJ/mol. The LQ alloys containing 76.5 and 82 at % Al were single-phase and consisted of monoclinic Co₄Al₁₃ or Co₂Al₉ with reduced lattice parameters.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 420–426.Original Russian Text Copyright © 2005 by Portnoi, Tretyakov, Filipova, Latuch.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Characterization and thermal properties of carbon-coated aluminum nanopowders prepared by laser-induction complex heating in methane

Carbon-coated aluminum (Al) nanopowders were successfully synthesized by laser-induction complex heating in methane. The characterization of the samples was carried out using X-ray diffraction (XRD), transmission electronic microscopy (TEM) and high resolution transmission electronic microscopy (HRTEM). It was found that the nanoparticles show a spherical morphology with the size ranging from 28 to 50 nm and are covered with 3–4 graphitic layers. Thermal analysis using differential scanning calorimeter (DSC) and thermal gravimeter (TG) revealed that the as-prepared powders exhibit a lower oxidation onset and peak temperature, and a higher enthalpy change, as compared with those of Al₂O₃-passivated Al nanopowders. It was also found that the mass gain for the first oxidation stage of carbon-coated Al nanopowders is 1.5 times higher than that of Al₂O₃-passivated Al nanopowders. These demonstrate that the carbon coating is an effective route to increase the reactivity of Al.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.