Structure of electrodeposited graded composite coatings of Ni–Al–Al2O3

The structure of electrodeposited composite coatings of Ni–Al–Al₂O₃, with Ni as the matrix and Al and Al₂O₃ as second-phase particles, was investigated using light microscopy and scanning electron microscopy. Ni coatings with no particles, which were used as reference samples, had progressively coarser structures with increasing current density. Co-deposition with Al resulted in refinement of the Ni matrix structure at high (>10 A dm⁻²) current densities. For single-particle baths, the co-deposition of Al₂O₃ was more strongly affected by current density and bath particle content than was the co-deposition of Al. However, for baths containing both Al and Al₂O₃ the amount of incorporated Al₂O₃ no longer depended on current density. With the choice of appropriate conditions, coatings of Ni with up to 39 vol.% Al₂O₃ were made. Similar experiments with Al yielded a maximum of 17.5 vol.% only. Uniform and graded mixed-particle coatings were also produced. When coatings containing Al were annealed, the reaction of the two elements resulted in the formation of either single-phase γ or two-phase γ–γ' alloys, in agreement with the equilibrium phase diagram.     

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)–Al₂O₃ interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al–Al₂O₃ system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal–ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8 wt% Mg alloy and 15% dense alumina foams at 915°C in a flowing N₂ atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin-film samples, specifically produced across the Al(Mg)–Al₂O₃ interface, were prepared using a dual-beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg₃N₂ formed in the foam at the molten alloy–ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal–ceramic interface and MgAl₂O₄ and MgSi₂ grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N₂ atmosphere that improve the wettability between molten Al and Al₂O₃ and induce spontaneous infiltration.     

Interfacial reaction and its influence on the mechanical properties of the ASZ short fibre-reinforced Al alloy composite

The effect of interfacial reaction on the mechanical properties of the AC8A Al alloy reinforced with ASZ short fibres (ASZ/AC8A composite) was studied. In the ASZ/AC8A composite, the interfacial reaction was observed to proceed between the SiO₂ binder layer and Mg of the matrix to form MgAl₂O₄ at the interface. Formation of MgAl₂O₄ was believed to enhance the interfacial bonding strength, resulting in the improved composite strength. However, the interfacial reaction in the ASZ/AC8A composite always took place at the expense of Mg in the matrix, resulting in the composite devoid of the Mg bearing precipitates such as Al₂CuMg and Mg₂Si. Interfacial reaction mechanisms were investigated for composites containing various Mg contents. The resultant mechanical properties of the composite were measured to determine the adequate amount of Mg within the composite. Microstructural changes of the composite were observed using transmission electron microscopy and differential scanning calorimetry to provide qualitative analyses on the experimental observations.     

Subsurface deformation of machined Al2O3 and Al2O3/5vol%SiC nanocomposite

Under machine grinding, material removal in monolithic Al₂O₃ is by intergranular fracture and grain pull-out. In comparison, under the same grinding conditions, an Al₂O₃/5%SiC nanocomposite undergoes significant surface grooving and intragranular fracture. The subsurface deformation mechanisms were investigated by cross-sectional transmission electron microscopy. For Al₂O₃, the residual deformation zone was localized very close to the surface in the first layer of grains, with dislocations occurring only within 1.5 µm of the top surface and a high density of basal twins penetrating to a depth of one single grain. Cracks were present along grain boundaries or basal twin interfaces. For Al₂O₃/SiC nanocomposites, the main residual plastic deformation is observed to be dislocations activated to a depth of about 10 µm (approx. 3–4 grains), with twinning rarely observed. Possible mechanisms by which the SiC particles influence the subsurface deformation and material removal modes are discussed.     

Active coatings for SiC particles to reduce the degradation by liquid aluminium during processing of aluminium matrix composites: study of interfacial reactions

The application of a surface coating on SiC particles is studied as an alternative means of solving problems of reactivity between SiC reinforcements and molten aluminium and problems of low wetting which limit the application of casting routes for fabrication of Al–SiC_p composites. The selected active barrier was a ceramic composed of SiO₂, which was generated by controlled oxidation of the SiC particles. The coating behaves as an active barrier, preventing a direct reaction between molten aluminium and SiC to form Al₄C₃ as the main degradation product. At the same time, the SiO₂ provokes other interfacial reactions, which are responsible for an improvement in wetting behaviour.
Composites were prepared by mixing and compacting SiC particles with Al powders followed by melting in a vacuum furnace, and varying the residence time. Transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and field emission TEM were employed as the main characterization techniques to study the interfacial reactions occurring between the barrier and the molten aluminium. These studies showed that the SiO₂ coating behaves as an active barrier which reacts with the molten Al to form a glassy phase Al–Si–O. This compound underwent partial crystallization during the composite manufacture to form mullite. The formation of an outer crystalline layer, composed mainly of Al₂O₃, was also detected. Participation of other secondary interface reactions inside the active barrier was also identified by HREM techniques.     

Microstructural analysis of Al alloys dispersed with TiB2 particulate for MMC applications

A dispersion of TiB₂ particulates in an Al alloy matrix was formed via the in-situ reaction between mixtures of K₂TiF₆ (K₂ZrF₆), KBF₄ and molten aluminium. The dispersion of the ceramic phase in the aluminium matrix was also achieved in some experiments by adding exogenous TiB₂ particles to the fluoride melt in contact with molten aluminium.
In this work, we have examined the microstructure of the as-cast metal matrix composites using analytical electron microscopy and X-ray diffraction techniques. The phases formed as a result of the reaction between the molten fluoride flux and liquid aluminium have been identified. These were (Ti, Zr, Al)B₂, Al₃Ti and possibly AlB₁₂ in the Al-matrix, and KAlF₄ and KMgF₃ in the solidified flux. The mechanism of formation of TiB₂ and Al₃Ti is explained. The role of alloying elements is also explained in the context of interfacial chemistry and dispersion.     

Scanning and transmission electron microscopy study of the microstructural changes occurring in aluminium matrix composites reinforced with SiC particles during casting and welding: interface reactions

Processing of aluminium matrix composites (AMCs), especially those constituted by a reactive system such as Al–SiC, presents great difficulties which limit their potential applications. The interface reactivity between SiC and molten Al generates an aluminium carbide which degrades the composite properties. Scanning and transmission electron microscopes equipped with energy-dispersive X-ray spectroscopes are essential tools for determining the structure and chemistry of the Al–SiC interfaces in AMCs and changes occurring during casting and arc welding. In the present work, an aluminium–copper alloy (AA2014) reinforced with three different percentages of SiC particles was subjected to controlled remelting tests, at temperatures in the range 750–900 °C for 10 and 30 min. Arc welding tests using a tungsten intert gas with power inputs in the range 850–2000 W were also carried out. The results of these studies showed that during remelting there is preferential SiC particle consumption with formation of Al₄C₃ by interface reaction between the solid SiC particle and the molten aluminium matrix. The formation of Al₄C₃ by the same mechanism has also been detected in molten pools of arc welded composites. However, in this case there was formation of an almost continuous layer of Al₄C₃, which protects the particle against further consumption, and formation of aciculate aluminium carbide on the top weld. Both are formed by fusion and dissolution of the SiC in molten aluminium followed by reaction and precipitation of the Al₄C₃ during cooling.     

Identification of intergranular phases in ceramic nanocomposites

Analytical FEG-TEM was used for nanostructural and nanochemical characterization of Al₂O₃–TiN (composite I) and Si₃N₄–TiN (composite II) ceramic composite systems. The presence of vitreous intergranular phases in pockets at multiple grain junctions and in thin films (≈ 0.8 nm thick) at grain boundaries was revealed by high resolution and Fresnel fringe imaging techniques. The existence of a Ti-rich thin intergranular film at alumina grain boundaries was revealed by EDS line-scanning across internal interfaces at the 1.5 nm lateral resolution level. Extracting interface specific information at subnanometre levels by means of quantitative spatial difference EELS allowed an identification of intergranular phases. Ti sub-oxide existed in thin films at Al₂O₃ and TiN grain boundaries, whereas a mixed Al–Ti–O–N glassy phase was observed in pockets at triple grain junctions in composite I. In composite II, residual siliceous oxide and oxynitride glass phases were identified in thin films at Si₃N₄ grain boundaries and multiple grain junctions, respectively. These observations indicated that the chemistry of the intergranular phase in thin grain boundary films is notably different from that in larger pockets at multiple grain junctions.     

Identification of weak interfaces in composites using transmission electron microscopy

A new experimental technique was developed to identify crack paths with a resolution of nanometres in fibre-reinforced composites. Cracks were introduced through Vickers indentations on one side of the sample prior to starting the thinning process. Indentations were placed close to the fibres in order to get enough cracks at the fibre/matrix interface in the electron-transparent region of the thinned sample. The technique was used in a Nicalon-fibre Al₂O₃ matrix composite prior to and after a heat treatment at 1200 °C for 1 h. The analysis of the crack paths allowed the identification of the weakest interface in each condition.     

Structural studies of pulsed-laser deposited nanocomposite metal-oxide films

Pulsed laser deposition in vacuum has been used to develop metal-oxide nanocomposite films with well controlled structural quality. Results for the copper–aluminium oxide (Cu:Al₂O₃) system are used to illustrate the main morphological and structural features of these films. High resolution transmission electron microscopy (TEM) analysis shows that the films consist of Cu nanocrystals with average dimensions that can be controlled between 2 nm and 10 nm embedded in an amorphous Al₂O₃ matrix. It is observed that the in-plane shape of the nanocrystals evolves from circular to elongated, and the number of nanocrystals per unit area decreases as their size increases. This evolution is explained in terms of nucleation at the substrate surface and coalescence during the later stages of growth. The thermal stability of the films has been studied by in situ TEM annealing and no transformation could be observed up to about 800 °C when partial crystallization of the Al₂O₃ starts.     

Transmission electron microscopy studies of squeeze cast Al-AlN composites

Aluminium-matrix composites containing ∼45 vol.% AlN particles were fabricated by melt infiltration of aluminium into an AlN preform under a pressure up to 130 MPa. Three types of aluminium alloy (2024, 6060 and 5754) were used. The as-prepared composites were studied by light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. As a result of the melt infiltration process, the composites are very dense and the microstructure shows a homogeneous distribution of the reinforcement. The interfaces are clean with very little porosity. Composites with 2024 and 6060 matrices were carefully studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) after heat treatments. Dislocation density in the matrix of the reinforced material increases due to the difference in thermal expansion coefficients of aluminium alloys and AlN. This can induce an accelerated ageing response of the coherent and semicoherent precipitations of age-hardened matrices. This behaviour has been studied in the 2024 and 6060 composites by using microhardness measurements and TEM. Reactions between the AlN reinforcement and aluminium matrices (6060 and 5754) were observed and analysed by TEM. Matrices containing some of magnesium display a MgAl₂O₄ spinel formation at the AlN/matrix interface. The spinel formation is probably due to the reaction between magnesium of the matrix and the thin Al₂O₃ layer on the AlN surfaces. This reaction can affect the mechanical behaviour of the composite infiltrated with the 5754 matrix. This has been confirmed by overageing some samples at high temperatures (300 °C and 550 °C) for 10 days in order to emphasize the interfacial reactions.     

Carbon-fibre reinforced magnesium alloys: nanostructure and chemistry of interlayers and their effect on mechanical properties

The micromechanical fracture behaviour of C/Mg–Al composites of varying interface reactivity was investigated by scanning electron microscope bending tests. Structure and chemistry of fibre/matrix interlayers were studied down to the atomic scale by imaging and spectroscopical transmission electron microscope techniques (high-resolution electron microscopy, energy dispersive X-ray spectroscopy, parallel-recording electron energy loss spectroscopy and energy-filtered transmission electron microscopy). The chemical reactions at the fibre/matrix interfaces of the C/Mg–Al composites were found to form plate-shaped carbidic precipitates, mainly Al₂MgC₂, which strongly influence the composite's mechanical properties by changing the fibre/matrix bonding strength.     

HRTEM studies of NiNbZr + Ag amorphous-nanocrystalline composites

Amorphous powder of composition corresponding to Ni60Ti20Zr20 (in at%) was obtained by ball milling in a high-energy mills starting from pure elements. Formation of the amorphous structure was observed already after 20 h of milling, although complete amorphization occurred after 40 h. The microhardness of powders increased from about 30 HV for pure elements to above 400 HV (1290 MPa) after 40 h of milling. Transmission electron microscopy (TEM) allowed to identify nanocrystalline inclusions of intermetallic phases of size 2–10 nm. Uniaxial hot pressing was performed in vacuum at temperature below the crystallization T_x it is 510°C and pressure of 600 MPa, Mixed amorphous powders and nanocrystalline silver powders were used to form a composite, in which microhardness was near 970 MPa HV and 400 HV for the amorphous phase and nanocrystalline silver, respectively. The compression strength of the composite containing 20 wt% of nanocrystalline Ag powder was equal to 600 MPa and plastic strain was 2%. Microstructure studies showed low porosity of composites of less than 1%, uniform distribution of the silver phase and a transition zone between both components, about 150 nm thick, where diffusion of nickel, niobium and zirconium into silver was observed. High-resolution TEM allowed identifying the structure of nanocrystalline inclusions in the amorphous matrix after hot pressing as either Ni₃Zr or Ni₁₇Nb₃. The identification was performed basing on measurements of angles and interatomic distances using inverse Fourier transformed images with enhanced contrast using Digital Micrograph computer program.     

Characterization of nanophase Al-oxide/Al powders by electron energy-loss spectroscopy

Al nanoparticles were prepared by the inert gas condensation method. After passivation with oxygen and air exposure we obtained a powdered sample of an Al-oxide/Al nanocomposite material. In the present paper we describe the use of the electron energy-loss spectroscopy (EELS) technique in a transmission electron microscope to characterize such nanostructured powders compared with a microcrystalline commercial aluminium foil. Energy-filtered images showed the presence of an alumina overlayer of ≈ 4 nm covering the aluminium nanoparticles (23 nm in diameter). EELS analysis enabled us to determine the total amount of Al₂O₃ and metallic Al and the structure of the alumina passivation overlayer in the sample. In particular, the extended energy-loss fine structure analysis of the data showed a major presence of Al tetrahedrally coordinated with oxygen in the alumina passivation layer of Al nanoparticles instead of the octahedral coordination found for a conventional Al foil. This surprising effect has been attributed to the nanoscopic character of the grains. The analysis of the electron-loss near-edge structure also determines the presence of a certain degree of aggregation in this kind of powdered sample as result of the coalescence of the nanocrystalline grains. The procedure presented here may have the potential to solve other problems during characterization of nanostructured materials.     

Characterization and microanalysis of interfacial reactions in metal–matrix composite systems

Understanding the solid-state reactions involved in metal/ceramic systems is important when combining the two types of materials into a composite. In this investigation, the solid-state reaction between Al₂O₃ (alumina) and a β-Ti alloy has been characterized by transmission electron microscopy (TEM), scanning electron microscopy, parallel-acquisition electron energy-loss spectroscopy and X-ray energy-dispersive spectroscopy. Two different systems were used to investigate this reaction. The first system utilizes a controlled reaction geometry and involved diffusion bonding single-crystal α-alumina and a β-Ti alloy. Here, three interfacial regions were found to form: a region of intermetallics (Ti₃Al and TiAl) located near the alumina interface, an α-Ti region, and a β-Ti region (rich in Mo, the β-phase stabilzer). Analysis of cross-section TEM samples of this reaction revealed the presence of both Ti₃Al and TiAl at the alumina interface. Orientation relationships between the intermetallics and the alumina are discussed. In the second, system, interfacial reactions inside metal–matrix composites that contain alumina and a β-Ti alloy were investigated. Here, different coatings used in the MMCs are investigated for their ability to prevent the reaction between the matrix and fibres. Reaction products inside the MMCs are compared with those found in the model reaction geometry.     

Characterization of cast Al86Mn3Be11 alloy

An Al₈₆Mn₃Be₁₁ alloy cast into copper mould was subjected to metallographic investigation. The as-cast microstructure consisted of a quasicrystalline icosahedral phase (i-phase), Be₄AlMn phase and, occasionally, a hexagonal phase. Al-rich solid solution represented the dominant phase. The chemical compositions of phases were determined using AES. The composition of the Be₄AlMn slightly deviated from the stoichiometric composition, whereas the composition of the i-phase was approximately Al₅₂Mn₁₈Be₃₀, containing an appreciable amount of Be. The average composition of the hexagonal phase was Al₆₆Mn₂₁Be₁₃. Deep etching and particle extraction provided a deep insight into the three-dimensional morphology of the i-phase and the hexagonal phase, whereas Be₄AlMn was slightly attacked by the etchant. The i-phase was present predominantly in the form of dendrites and a rodlike eutectic phase. The hexagonal phase was primarily in the form of hexagonal platelets, whereas Be₄AlMn was rather irregular in shape. The morphology of the i-phase can be explained by predominant growth in 3-fold directions and the lowest energy of the 5-fold planes, leading to the faceting and adopting a pentagonal dodecahedron shape. The brightnesses of phases in the backscattered electron images were rationalized by determining their backscattering coefficients. TEM investigation showed considerable phason strain in the i-phase, and the polycrystalline nature of the Be₄AlMn phase.     

Subsurface nanoindentation deformation of Cu-Al multilayers mapped in 3D by focused ion beam microscopy

A new technique for the three-dimensional analysis of subsurface damage of nanocomposites is presented. Cu–Al multilayers, grown epitaxially on (0001)Al₂O₃ single crystals by ultra high vacuum molecular beam epitaxy, have been deformed by nanoindentation. Systematic slicing and imaging of the deformed region by focused ion beam microscopy enables a 3D data set of the damaged region to be collected. From this 3D data set, profiles of the deformed sub-surface interfaces can be extracted. This enables the deformation of the individual layers, substrate and overall film thickness to be determined around the damage site. These 3D deformation maps have exciting implications for the analysis of mechanical deformation of nanocomposites on a sub-micrometre scale.     

Effect of SiC reinforcement on the deformation and fracture micromechanisms of Al–Li alloys

The effect of SiC reinforcement on the microstructure of a naturally aged 8090 Al alloy as well as on the deformation and fracture micromechanisms was investigated. To this end, the microstructural characteristics (grain and reinforcement morphology, precipitate structure) were determined in the unreinforced alloy and in the composite reinforced with 15 vol.% SiC particles. The materials were tested under monotonic tension and fully reversed cyclic deformation and then carefully analysed through scanning and transmission electron microscopy to find the dominant deformation and failure processes for each material and loading condition. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li. As a result, the plastic deformation in the composite was homogeneous, while strain localization in slip bands was observed in the unreinforced alloy specimens tested in tension and in fatigue. The unreinforced alloy failed by transgranular shear along the slip bands during monotonic deformation, whereas fracture was initiated by grain boundary delamination, promoted by the stress concentrations induced by the slip bands, during cyclic deformation. The fracture of the composite was precipitated by the progressive fracture of the SiC reinforcements during monotonic and cyclic deformation.     

Microscopy of interfaces in model oxide composites

Solid-state reactions are known to occur in composite materials during fabrication, processing or service. In the present study, a model approach for studying such reactions is illustrated. The goal of this approach is to understand the effects of, for example, temperature, time and other driving forces on the earliest stages of such reactions. Three different materials systems were used in order to investigate some of the fundamental processes occurring. Two of the systems involve spinel formation while the other is more complicated, since three different compounds can form between the end-members. For all of these systems a thin-film reaction geometry was utilized. High-quality thin films of the various oxides were deposited on bulk substrates by pulsed-laser deposition.
NiO was deposited on four orientations of α-Al₂O₃ in order to study the effect of crystal orientation (and therefore interfacial structure) on the growth kinetics of the nickel–aluminate spinel. The effect of an external electric field at elevated temperatures on the spinel-forming reaction between Fe oxide and monocrystalline (001) MgO has been analysed. Thin films of Y₂O₃ were reacted with monocrystalline substrates of α-Al₂O₃ at elevated temperatures to determine the reaction sequence: up to three different yttrium aluminates can form.     

Energy-filtering TEM and electron energy-loss spectroscopy of double structure tabular microcrystals of silver halide emulsions

Composite Ag(Br,I) tabular microcrystals of photographic emulsions were studied by the combination of energy-filtering electron microscopy (EFTEM) and electron energy-loss spectroscopy (EELS) in conjunction with energy-dispersive X-ray (EDX) microanalysis. The contrast tuning under the energy-filtering in the low-loss region was used to observe more clearly edge and random dislocations, {11⁻1} stacking faults in the grain shells parallel to {11⁻2} edges and bend and edge contours. Electron spectroscopic diffraction patterns revealed numerous extra reflections at commensurate positions in between the Bragg reflections and diffuse honeycomb contours; these were assigned to the number of defects in the shell region parallel to the grain edges and polyhedral clusters of interstitial silver cations, respectively. Inner-shell excitation bands of silver halide were detected and confirmed by EDX analyses, i.e. the Ag N_2,3 edge at 62 eV (probably overlapped with the weak I N_4,5 edge at 52 eV and the Br M_4,5 edge at 70 eV), the I M_4,5 edge at about 620 eV, and the Br L_2,3 edge at about 1550 eV energy losses. Energy-loss near-edge structure of the Ag M_4,5 edge at about 367 eV energy losses and low-loss fine structure arisen as a result of interband transitions and excitons, possibly superimposed with many electron effects, have been revealed. The crystal thickness was determined by a modified EELS log-ratio technique in satisfactory agreement with measurements on grain replicas.