Surrounding Al Matrix Research Articles

A high-resolution characterisation of localised corrosion of AA5083-H111 in simulated seawater by TEM

ABSTRACT The localised corrosion of aluminium alloy (AA) 5083-H111 in 3.5 wt-% NaCl solution was studied by combing scanning electron microscope (SEM), focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. It was found that the Ti-rich intermetallic particles (IMPs) served as the cathodes and triggered the dissolution of its adjacent Al matrix; as a result, a trench was formed between the IMP and its surrounding Al matrix. Raised ‘circles’ were identified on the alloy surface at the periphery of the IMPs, which was considered to be the cathodic region on which corrosion products deposited. A sub-surface that consisted of fine Al grains and corroded Al–Mg phases were observed under the raised ‘circle’ region.

Significant reinforcement of mechanical properties in laser welding aluminum alloy with carbon nanotubes added

Laser welding 2A12 (Chinese grade) aluminum alloy with carbon nanotubes (CNTs) added was carried out, and the mechanism of CNTs strengthening joints was explored in detail. Firstly, to achieve the same penetration depth, single laser welding and laser welding with CNTs require speeds of 40 mm/s and 60 mm/s respectively. The addition of CNTs increases the welding efficiency by 50%. Furthermore, the maximum tensile strength of the joint with CNTs increased by 45.5% to 326 MPa. Fracture analysis showed that the significant improvement in the mechanical properties of the joint was partly due to the CNTs reducing the pore size by an order of magnitude. Then, phase analysis reveals that a large number of precipitated phases increase the dislocation density and provide a basis for dislocation strengthening. The failure mode of CNTs is pulled out and the HRTEM test found that the CNTs are dispersed in the aluminum matrix. Indicating that the CNTs in the weld are tightly bonded to the surrounding Al matrix and load transfer from the matrix to CNTs is improved. This work provided an idea of laser welding aluminum with high efficiency and significantly enhanced mechanical properties. Finally, the process window for laser welding 2A12 aluminum alloy has also been established.

Remarkable near-surface microstructure of nanoparticles and oxide film in laser shock peened Al-Zn-Mg-Cu alloy

The near-surface microstructure in an Al-Zn-Mg-Cu (7075-T6) alloy after Laser Shock Peening without Coating (LSPwC) was characterized by advanced electron microscopy methods. A remarkable microstructure composed of a ~2 μm wide newly solidified matrix recast surface layer embedded with O-rich Al nanoparticles (NPs) with the close-packed orientation relationship (OR) as the surrounding Al matrix has been discovered, together with a nano-scale aluminum oxide layer formed on the outermost surface. The formation mechanism is associated with high-pressure surface ablation leading to melting, vaporization and shock-assisted rapid solidification during the LSPwC process. The close-packed OR between NPs and matrix is believed to be due to surface energy minimization.

Numerical and Experimental Analysis of Material Removal and Surface Defect Mechanism in Scratch Tests of High Volume Fraction SiCp/Al Composites.

This paper addresses a comprehensive and further insight into the sensitivity of material removal and the surface defect formation mechanism to scratch depth during single-grit scratch tests of 50 vol% SiCp/Al composites. The three-dimensional (3D) finite element model with more realistic 3D micro-structure, particle-matrix interfacial behaviors, particle-particle contact behaviors, particle-matrix contact behaviors and a Johnson-Holmquist-Beissel (JHB) model of SiC was developed. The scratch simulation conducted at scratch velocity 10 mm/min and loading rate 40 N/min revealed that the scratch depth plays a crucial role in material removal and the surface forming process. Brittle fracturing of SiC particles and surface defects become more deteriorative under a large scratch depth ranging from 0.0385 to 0.0764 μm. The above phenomenon can be attributed to the influence of scratch depth on SiC particles’ transport; the increase in the amount of SiC particle transport resulting from an increase of scratch depth raises the occurrence of particle-particle collision which provides hard support and shock for the scratched particles; therefore, brittle fracturing gradually becomes the major removal mode of SiC particles as the scratch depth increases. On the deteriorative surface, various defects are observed; i.e., lateral cracks, interfacial debonding, cavies filled with residually broken particles, etc. The von Mises stress distribution shows that SiC particles bear vast majority of load, and thus present greater stress than the surrounding Al matrix. For example: their ratio of 3 to 30 under the scratch depth of 0.011 mm. Namely, SiC particles impede stress diffusion within the Al matrix. Finally, the SEM images of the scratched surface obtained from the single-grit scratch experiments verify the numerical analysis’s results.

SVET and ToF-SIMS Studies on the Galvanic Corrosion of β-phase/Aluminum Couple in Aqueous Solutions as a Function of pH

The galvanic corrosion of the β-phase (Al3Mg2)/Al couple has been studied using electrochemical methods, scanning vibrating electrode technique (SVET) and Time of flight-secondary ion spectroscopy (ToF-SIMS). Polarization tests reveal that Al3Mg2 is anodic relative to pure Al at pH ≤ 10. Visualization of the current density distribution over the Al3Mg2/Al galvanic couple by SVET reveals that at pH 2, an intense and stochastic anodic current density is evolved from Al3Mg2 while a heavy cathodic current flux is sustained on the surrounding Al matrix. At pH 6 and 10, localized both cathodic and anodic regions are seen on Al3Mg2 indicating its self-dissolution while the surrounding Al matrix remained relatively inert. However, at pH 13, a reversal of polarity occurs and Al3Mg2 becomes cathodic relative to Al. The severity of the galvanic coupling of Al3Mg2 to Al is in the order pH 13 > pH 2 > pH 6 > pH 10. ToF-SIMS analysis reveals that the dealloying of Al3Mg2 in the galvanic couple is by selective dissolution of Mg leading to the deposition of corrosion products dominated by Mg compounds in an acidic and neutral environment. In an alkaline environment, a relatively thin hydroxide rich film is formed.

The Influence of Interface Structure on the Electrical Conductivity of Graphene Embedded in Aluminum Matrix

AbstractGraphene is considered a promising reinforcement to improve the electrical conductivity of metals because of its excellent intrinsic electrical conductivity. However, graphene/Al matrix composites with enhanced electrical conductivity have not yet been reported. In this work, it is attempted to understand the factors influencing the electrical conductivity of graphene embedded in an Al matrix by adjusting the interfacial structure and composition. By sandwiching graphene (Gr) or graphene oxide (GO) with either pristine or passivated Al foils, three kinds of typical composite interfaces are successfully fabricated: pristine‐Al/Gr/pristine‐Al (Al/Gr/Al), passivated‐Al/Gr/passivated‐Al (Al‐O/Gr/O‐Al), and pristine‐Al/GO/pristine‐Al (Al/GO/Al). As revealed by electrical current mapping of the interfacial area, the Gr layer in the sandwiched Al/Gr/Al composite shows an electrical conductivity ≈70 times higher than that of the surrounding Al matrix. Gr with a high electrical conductivity is not observed in the Al‐O/Gr/O‐Al and Al/GO/Al composites. The mechanisms are understood by microstructural characterizations and first‐principle calculations. The results indicate that high‐quality Gr and metal–Gr contact are simultaneously needed to improve the electrical conductivity of Al by incorporating graphene.

Crystallographic relationships of T-/S-phase aggregates in an Al–Cu–Mg–Ag alloy

T-(Al20Cu2Mn3) phase dispersoids are important for limiting recovery and controlling grain growth in Al-Cu alloys. However, these dispersoids can also reduce precipitation hardening by acting as heterogeneous nucleation sites and may lead to increased susceptibility towards pitting corrosion when galvanically coupled with S-(Al2CuMg) phase precipitates. The interplay between T- and S-phases is therefore important for understanding their effect on the mechanical and electrochemical properties of Al-Cu-Mg alloys. Here, the crystallographic relationships between the T-phase, S-phase, and surrounding Al matrix were investigated in an Al-1.31Cu-1.14Mg-0.13Ag-0.10Fe-0.28Mn (at.%) alloy by combining scanning precession electron diffraction with misorientation analysis in 3-dimensional axis-angle space and high-resolution transmission electron microscopy. Orientation relationships are identified between all three phases, revealing S-T orientation relationships for the first time. Differences in S-Al orientation relationships for precipitates formed at T-phase interfaces compared to their non-interfacial counterparts were also identified. These insights provide a comprehensive assessment of the crystallographic relationships in T-/S-phase aggregates, which may guide future alloy design.

Elasto-plastic deformation in Al-Cu cast alloys for engine components

High efficiency, increasing power densities, lower weight and pollution reduction require new materials for combustion engine components in order to withstand the higher thermo-mechanical loads. Cast Al-Si alloys are used for cylinder heads where they combine light weight, good castability and suitable mechanical properties. Despite casting difficulties and their vulnerability to hot tearing these Al-Cu alloys can be used as candidates to achieve even better mechanical properties at elevated temperatures. These alloys are formed by a ductile precipitation hardening α-Al matrix and brittle intermetallics, mostly Al2Cu. Tensile deformation of AlCu4 as cast and AlCu7 solutionized and aged conditions is investigated by combined application of non-destructive testing and imaging methods (using electron, neutron and synchrotron radiation) to study the stress distribution between the Al2Cu structures and the surrounding Al matrix. The external loads are transferred into the microstructure by strain control due to percolating eutectic structures in AlCu4 and AlCu7 as cast. Homogenization by solution treatment and age hardening significantly increases the strength, although stress controlled behavior becomes dominant. (Some damage tolerance is observed in the as cast as well as in the heat treated condition, as load is transferred from particles which were broken first to the remaining ones maintaining some deformation strengthening.) The stress-strain behavior of the samples is compared, conclusions on deformation mechanisms and internal architectures are drawn.

Solidification behavior of indium droplets embedded in aluminum by differential fast scanning calorimetry

Indium droplets embedded in aluminum matrix were successfully prepared by melt spinning technique. The results showed that an amount of nano-sized indium droplets were embedded inside aluminum grains, while micro-sized indium droplets were distributed along aluminum grain boundaries. The nano-sized indium droplets exhibited an orientation relationship with the aluminum matrix of $$(\bar{1}01)_{\text{In}} ||(\bar{1}1\bar{1})_{\text{Al}}$$ , $$(\bar{1}10)_{\text{In}} ||(00\bar{2})_{\text{Al}}$$ and $$[111]_{\text{In}} ||[110]_{\text{Al}}$$ . The nucleation of indium droplets was catalyzed by the surrounding Al matrix on the {111}Al or {002}Al lattice planes. Employing differential scanning calorimetry and differential fast scanning calorimetry, it was found that the solidification of indium droplets consisted of three stages. Based on the classical nucleation theory, the solidification behavior of nano-sized In droplets was studied. The results displayed that the contact angle at larger undercooling was far greater than that in smaller undercooled melt, implying that the catalyzing effect of the matrix on the nucleation in larger undercooling could be depressed.

Micromechanics-based thermal expansion characterization of SiC nanoparticle-reinforced metal matrix nanocomposites

Understanding the structure–property relations for metal matrix nanocomposites reinforced with nanoparticles is a key factor for a reliable and optimal design of such new material systems. In the present study, coefficient of thermal expansion of silicon carbide (SiC) nanoparticle-reinforced aluminum (Al) matrix nanocomposites is predicted using a three-dimensional unit cell based micromechanical approach. The model takes into account the aluminum carbide (Al4C3) interphase region formed due to the reaction between SiC nanoparticles and surrounding Al matrix. The effects of some critical parameters, including volume fraction and diameter of SiC nanoparticles, interphase features such as geometry and material properties on the coefficient of thermal expansion of Al nanocomposite are extensively investigated. The obtained results clearly reveal the high influence of the interphase region on the coefficient of thermal expansion of Al nanocomposite. Based on the simulation results, the coefficient of thermal expansion of Al nanocomposite nonlinearly decreases with the increase in the interphase thickness or decreasing SiC nanoparticles diameter. Furthermore, the role of interphase in the thermal expansion behavior of Al nanocomposite becomes more prominent with the reduction in the nanoparticle diameter. Also, the coefficient of thermal expansion of Al nanocomposite linearly decreases as SiC nanoparticle volume fraction increases.

Effect of extrusion treatment on the microstructure and mechanical behavior of SiC nanowires reinforced Al matrix composites

In the present work, microstructure and mechanical performance of extruded SiCnw/Al composites have been investigated. The extrusion treatment leads to the alignment of SiC nanowires and the densification of the composites. The average length of SiC nanowires has been shortened after extrusion. High density dislocation tangles have been observed in Al matrix after extrusion. As the “non-deformable” phases, SiC nanowires inhabited the movement and deformation of surrounding Al matrix, leading to the formation of the grain boundaries. Therefore, SiC nanowires were mainly found at the boundary of Al grains in the extruded composites. Regardless of content of SiC nanowires, the yield strength, tensile strength and elongation of SiCnw/6061Al composites have been improved after extrusion treatment. Based on generalized mixture rule considering the effect of porosity, the increase of elastic modulus of SiCnw/6061Al composites is mainly due to the densification effect. The strengthening factor and efficiency of SiC nanowires are highest among all SiC reinforcements, and the strengthening behavior of SiC reinforced Al matrix composites could be well explain by the modified shear-lag model. The ratio of strengthening efficiency of SiCnw/Al composites before and after extrusion was about 0.6, indicating that orientation optimization of SiC nanowires is an effective method to improve the mechanical properties of SiCnw/Al composites.

Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates

Abstract Lithiation-induced volume changes in Si result in fracture and fragmentation of Si anodes in Li-ion batteries. This paper reports the self-healing behaviour of cracks observed in micron-sized Si particles dispersed in a ductile Al matrix of a Si-Al electrode electrochemically cycled vs. Li/Li + using a high lithiation rate of 15.6 C. Cross-sectional high-resolution transmission electron microscopy and Raman spectroscopy revealed that an amorphous layer with a depth up to ∼100 nm was formed at the surface of Si particles. In-situ optical microscopy performed during electrochemical experiments revealed development of cracks in Si particles as the voltage decreased to 0.02 V during lithiation. Self-healing of cracks in Si particles occurred in two steps: i) arresting of the crack growth at the Si/Al interface as the surrounding Al matrix had a higher fracture toughness and thus acted as a barrier to crack propagation, and ii) closure of cracks due to compressive stresses applied to the crack faces by the amorphous zones formed on each side of the crack paths.

Local melting in Al embedded with TiNi powder induced by microarea self-propagating high-temperature synthesis

Microarea self-propagating high-temperature synthesis (microSHS) was ignited by the heat of mixing generated at the boundaries between an Al matrix and TiNi particles during plastic deformation at room temperature. The temperature of the boundaries was rapidly increased by microSHS; the temperature elevation resulted in local melting of the TiNi particle and the surrounding Al matrix, because the heat of mixing was localized in the vicinity of the TiNi particle although the amount of the heat of mixing was limited. Since the amount of the local melting region induced by microSHS is restricted, not only major elements (i.e. Al, Ti and Ni) but also impurities were involved in the solidification followed by local melting. As a result, FeNi nanoprecipitates, which have not been reported in SHS studies, were formed by inclusion of Fe, initially included as an impurity in raw materials. The formation mechanism of FeNi nanoprecipitates is discussed based on reference to the Al–Fe–Ni ternary alloy phase diagram. It is expected that local melting induced by microSHS is a key phenomonon for amorphization during severe plastic deformation of elemental sheets.

Microstructural corrosion of aluminium alloys: a predictive finite element model based on corrosion–mimicking experiments

ABSTRACTThe purpose of this study is to implement the basis of a finite element model (FEM) based on the resolution of the Nernst–Planck equation in order to progress in the predictive simulation of microstructural corrosion on aluminium alloys. Certain constituent intermetallic particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localised trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alkalinization on 200 nm thick aluminium coatings, promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alkalinization was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation. The same FEM model was then successfully applied to simulate the damage formed around a micrometric copper particle embedded in pure aluminium. Copyright © 2013 John Wiley & Sons, Ltd.

Simulation of pH-controlled dissolution of aluminium based on a modified Scanning Electrochemical Microscope experiment to mimic localized trenching on aluminium alloys

Some constituent intermetallic (IMPs) particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localized trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alcalinisation on 200 nm thick aluminium coatings promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. The local alcalinisation was induced by the oxygen reduction reaction on the tip of a SECM which mimics the surface of an isolated IMP. From a phenomenological point of view, reproducible cylindrical damage develops in front of the platinum ultramicroelectrode (UME). Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alcalinisation was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation.

A nanometre-scale fibre-to-matrix interface characterization of an ultrasonically consolidated metal matrix composite

Future ‘smart’ structures have the potential to revolutionize many engineering applications. One of the possible methods for creating smart structures is through the use of shape memory alloy (SMA) fibres embedded into metal matrices. Ultrasonic consolidation (UC) allows the embedding of SMAs into metal matrices while retaining the SMA's intrinsic recoverable deformation property. In this work, NiTi SMA fibres were successfully embedded into an Al 3003 (0) matrix via the UC layer manufacturing process. Initially the plastic flow of the Al matrix and the degree of fibre encapsulation were observed using optical microscopy. Then microstructural grain and sub-grain size variation of the Al 3003 (0) matrix at the fibre—matrix interface, and the nature of the fibre—matrix bonding mechanism, were studied via the use of focused ion beam (FIB) cross-sectioning, FIB imaging, scanning electron microscopy, and mechanical peel testing. The results show that the inclusion of the NiTi SMA fibres had a significant effect on the surrounding Al matrix microstructure during the UC process. Additionally, the fibre—matrix bonding mechanism appeared to be mechanical entrapment with the SMA surface showing signs of fatigue from the UC embedding process.

Ab initio simulations of clustering and precipitation in Al–Mg–Si alloys

A class of proposed coherent precipitate structures (Guinier–Preston zones) in the Al–Mg–Si alloy are investigated using first-principles density functional theory methods. The cluster expansion method is used to extract effective interaction parameters, providing the means for large scale energy calculations of alloy structures. The Mg 1Si 1 L1 0 structure and structures related to the Mg 5Si 6 β″ phase are studied in more detail, and e.g., precipitate/matrix interface energies are presented. Using direct first-principles calculations we show that the former phase is dynamically unstable and thus must be stabilized by the surrounding Al matrix. Monte Carlo simulations and free-energy techniques are used to study the Al rich side of the phase diagram with the current CE parameters, and kinetic Monte Carlo simulations are used to study clustering in the disordered phase. The implications of our findings are discussed in the framework of classical nucleation theories, and we outline possible nucleation mechanisms.

Progress in Out-Of-Pile Study of Low Enriched UMo/AlSi Fuel

AbstractThe main problem we have to face in order to improve the in-pile behaviour of the UMo/Al dispersed fuel, is the instability of the interaction zone, that is formed between the UMo particles and their surrounding Al matrix during irradiation. As the latest irradiations (e.g. IRIS 3 or RERTR 6) tend to prove, addition of Si into the Al matrix seems to be beneficial for the interaction rate decrease. To further understand the role of silicon, a wide out-of-pile interdiffusion study has been launched in CEA-France in 2006 and is still under progress. It has given promising preliminary results by showing a real effect of Si addition upon the interaction zone. The first results related to the physico-chemical aspects of the modified UMo/Al interaction as well as the kinetics aspects of the interaction layer (IL) formation are described in this paper. On a wide range of annealed couples made of UMo7 or UMo10 and Al-Si matrix (Si content ranging from 0.11 wt % to 12 wt %), the IL thicknesses have been systematically measured. Microstructures have been fully characterized by means of optical and scanning electron microscopy and the elementary compositions have been evaluated through EDX analyses. Finally, XRD characterizations and micro-hardness tests have also been performed on a few samples. On the basis of these different results, we discuss first clues about the role played by silicon on slowing down the UMo/Al interaction process.

Structural aspects of annealing and hot deformation of Al-Nb2O5 mechanically alloyed composite

A mechanical alloying and hot extrusion method was used to manufacture an Al based composite reinforced by 10 wt-% addition of Nb2O5. Annealing experiments were performed to test the effect of chemical reaction between components on the material structure. After annealing at 873 K for 12 h the material hardness was found to have increased from HB 121 (as extruded) to HB 186 (annealed). The hardness maximum was followed by material softening at prolonged aging at 873 K. Transmission electron microscopy observations revealed a very well consolidated nanoscale structure for the as extruded composite. Annealing of the material resulted in an increase in porosity that was ascribed to Nb oxide reduction by aluminium. Due to diffusion of the released Nb element into the surrounding Al matrix, formation of new intermetallic grains of Al3Nb was accompanied by very fine needlelike Al oxide particle growth within the aluminium matrix. Chemical reaction between aluminium and Nb oxide at solid state was related to local specific volume reduction being responsible for an increase in the material porosity. Hot compression tests were performed on as extruded and samples annealed at 873 K for 12 h. The good mechanical properties of the composite were ascribed to the highly refined structure of the material. In compression tests, samples were not fractured within the experimental strain range used. In hot deformation tests at lower temperatures and a strain rate of 5 10−3 s−1, some fine microcavities were observed by TEM for samples deformed to ɛt≈0·3.

HRTEM observation of interfacial dislocations at faceted Al–Pb interfaces

Al–Pb ribbons containing 1 at.% Pb have been produced by melt-spinning and subsequently investigated by high-resolution transmission electron microscopy. It is shown that the lattice mismatch of about 22% between the nanometre-sized Pb inclusions and the surrounding Al matrix is accommodated by a periodic array of misfit dislocations at the Al–Pb interface. The closing failures of Burgers circuits drawn around misfit dislocations on {111} and {100} facets identify the corresponding Burgers vectors as (a 0/4)⟨ 211 ⟩ and (a 0/2)⟨ 110⟩, respectively. The Burgers vector of (a 0/4)⟨ 211⟩ corresponds to the projected edge part of a 60° (a 0/2)⟨ 110⟩ dislocation. The Pb inclusions themselves appear to be free of defects.