Fe-rich Intermetallic Particles Research Articles

The effect of hexafluorozirconic acid concentration on the formation and corrosion resistance of trivalent chromium conversion coatings on AlSi12Cu1(Fe) cast alloy

AlSi12Cu1(Fe) alloy was subjected to the conversion bath containing: Cr2(SO4)3, HF and 200–800 ppm Zr (as H2ZrF6). Coatings composed mostly of ZrO2 and to a smaller extent by Cr(OH)3 with granular morphology were obtained. Concentration of Zr in the conversion bath did not affect the coatings thickness, which was non-uniform and ranged from 35 to 90 nm. Cracking and detachment were observed, which are due to the presence of possible phases like: Si, Cu0.01Al0.97Si0.02, FeAl3Si2 and Al2Cu, except the α–Al phase. These Cu- and Fe-rich intermetallic particles serve as the cathodic areas that are responsible for the formation of micrometric sizes numerous spheroids in the coating due to intensive hydrogen evolution. Longer protective effect, up to 240 hrs in a neutral salt spray and the lowest corrosion currents after 5 days in 0.05 M NaCl were obtained at 600 and 800 ppm Zr in the bath, respectively. It was due to higher content of oxidized chromium and aluminum compounds in these TCC coatings.

Contribution of the Fe-rich phase particles to the high temperature mechanical behaviour of an Al-Cu-Fe alloy

The high temperature strengthening mechanisms in Al-Cu-Fe aluminium alloy was analysed basis on a comprehensive microstructural study of the as-cast alloy and mechanical testing results. An approach which extracts the specific contribution of solid solution atoms, precipitates, and Fe-rich particles to the creep strengthening was utilized. Additionally, the Shear Lag model, was employed to account for the contribution to strengthening of the Fe-rich intermetallic particles through a load transfer mechanism. For this study, an as-cast Al-Cu alloy, which does not contain Fe impurities, has been also used for comparative purposes. The findings suggest that the large and 3D interconnected Fe-rich phase particles (Al6(MnFe) and α-Fe) generate the main strengthening effect at high temperature through a load transfer mechanism, whereas the contribution of the nanometric Al2CuMg precipitates were very limited.

Multiscale modeling of particle-induced damage in AA7075 aluminum sheet at large plastic strains

Large plastic deformation and ductile damage in precipitation hardening aluminum alloy AA7075 is driven by plastic flow in the vicinity of particles as well as fracture and decohesion of second phase particles embedded in the matrix. The current work investigates the deformation and damage characteristics of Fe-rich intermetallic particles, and η and θ precipitates in AA7075-O sheet. A multiscale model simulation methodology is presented and applied to analyze large-scale plastic deformation and damage characteristics. The methodology utilizes nanoscale molecular dynamics simulation to obtain matrix-particle interface strength properties and Weibull statistics to capture experimental fracture characteristics of particles. The above sub-models are incorporated within a 2-D real particle microstructure-based finite element model to conduct a comparative study of the roles of decohesion and fracture in the development of plasticity and void damage in AA7075-O sheet. In addition, in-situ SEM uniaxial tensile tests are carried out to assess the effectiveness of the simulation methodology and to compare the experimental and numerical results. Post-test high resolution X-ray computed tomography (HR-XCT) was also carried out to qualitatively observe particle-induced voiding in the material. The numerical methodology is shown to capture well the experimental trends in damage evolution of the individual particles. It is observed that particle damage is a function of inherent particle properties, their morphological features, and matrix strain localization characteristics. Larger-size Fe-rich particles are observed to undergo damage at an earlier stage, and consequently, affect the strain localization characteristics the most. In contrast, η precipitates are found to be most resistant to particle damage, followed by θ precipitates. Higher stresses inside strain localization bands, caused increased void damage initiation and growth of η and θ precipitates. Overall, particle fracture is observed to be marginally higher compared to particle decohesion, irrespective of the particle type.

Effect of pH Change on the Electrochemical Behavior of Intermetallic Particles in AA1050 Aluminum

Commercial purity aluminum alloys, such as AA1050 and AA1100, are widely used for infrastructure and battery materials for their high formability, high electrical conductivity, and excellent corrosion resistance. The commercial purity aluminum alloys are known to contain Fe-rich intermetallic particles (IMPs). Although the IMPs are needed to provide the valuable properties, some of the IMPs could turn into an initiation site for pitting under salty environments. Since the Fe-rich IMPs show higher electrode potential than that of the Al-matrix, the IMPs work as a cathodic reaction site. As oxygen reduction reaction proceeds on the IMPs, the pH around the IMPs increases locally. The dissolution of the Al-matrix (often referred as trenching) is caused by the local alkalization, and a trench is formed around the IMPs. Crystallographic pitting is likely to be initiated inside the trench. Although the IMPs have an important role in trenching, the electrochemical behavior of the IMPs during trenching is still unknown. In this study, the dissolution behavior of the IMPs and surrounding Al-matrix was observed in situ using an optical microscopy in order to analyze the morphological change during trenching. The effect of pH change on the electrochemical behavior of the IMPs was analyzed using the bulk IMPs fabricated by arc-melting.

On the intermetallic constituents in the sodium-induced edge cracking of hot-rolled AA5182 aluminum alloys

ABSTRACT The productivity and recovery of hot-rolled Al-Mg-Mn sheets often suffer from sodium-induced edge cracking. In this study, AA5182 ingots with different levels of Na were prepared by direct-chill (DC) casting, and were then subsequently subjected to homogenisation and hot-rolling processes. The edge crack occurrence during hot rolling in the pass #11 (84% reduction) and pass #18 (96% reduction) was investigated, with special attention paid to the impact of intermetallic constituents in the ingot subsurface region, where all edge cracks were initiated. Correlations between the susceptibility to edge cracking, enriched intermetallic constituents in the inverse segregation zone, Mg2Si cavitation, meniscus bands, and Na contamination were observed. The results revealed that Fe-rich intermetallic particles were not involved in crack formation, while Mg2Si particles had a strong tendency to debond from the Al matrix and form cavities at the edges of the rolled strip. The higher the Na content, the more severe the Mg2Si cavitation, and consequently, the higher the susceptibility to edge cracking. Numerous debonded Mg2Si particles contaminated with Na in the subsurface region provided favourable conditions for crack initiation and propagation. The role of the meniscus bands on the large and profound edge cracks was also elaborated.

A Study of the Localized Ceria Coating Deposition on Fe-Rich Intermetallics in an AlSiFe Cast Alloy.

Corrosion inhibiting conversion coating formation is triggered by the activity of micro-galvanic couples in the microstructure and subsequent local increase in pH at cathodic sites, which in the case of aluminium alloys are usually intermetallics. Ceria coatings are formed spontaneously upon immersion of aluminium alloys in a cerium conversion coating solution, the high pH gradient in the vicinity of intermetallics drives the local precipitation of ceria conversion compounds. Cu-rich intermetallics demonstrate a highly cathodic nature and have shown the local precipitation reaction to occur readily. Fe-rich intermetallics are, however, weaker cathodes and have shown varied extents of localized deposits and are in focus in the current work. Model cast Al-7wt.%Si alloys have been designed with 1 wt.% Fe, solidified at different cooling rates to achieve two different microstructures, with big and small intermetallics, respectively. Upon subjecting the two microstructures to the same conversion coating treatment (immersion in a 0.1 M CeCl3 solution) for a short period of 2 h, preferential heavy deposition on the boundaries of the big intermetallics and light deposition on the small intermetallics was observed. Based on these observations, a mechanism of localized coating initiation at these Fe-rich intermetallic particles (IM) is proposed.

Influence of Ageing on the Intergranular Corrosion of an Al–Mg–Si Alloy

This report investigates the influence of ageing on the susceptibility to intergranular corrosion (IGC) of an Al–Mg–Si alloy. The selected AA6063 Al-alloy has been exposed to ageing treatments at a predetermine temperature of 423 K for durations of 1 to 168 h to achieve the wide range of ageing states covering under- to peak- to over-aged. The ageing behaviour of the selected alloy has been determined based on the results of hardness and tensile properties. The susceptibility to IGC has been evaluated following the British standard (BS ISO-11846) method B and assessed based on the average and maximum corrosion depths apart from in-depth surface and sub-surface characterizations of corroded specimens. It has been observed that the depths of corrosion increase monotonically with increase in ageing time. With progress of artificial ageing, the dominant mode of corrosion gradually changes in the order of minor pitting, moderate pitting, pitting plus localized IGC, localized IGC and uniform IGC. The phenomenon of preferential dissolution of Mg from the β″ (Mg5Si6) and β (Mg2Si) precipitates as well as the existence of Fe-rich intermetallic particles at the grain boundaries cause the pitting and intergranular corrosion because of the formation of local galvanic cells between the lower corrosion potential of Mg-depleted precipitates or precipitate free zones and to the adjacent anodic grain boundary precipitates.

Investigation on microstructure, mechanical properties and corrosion behavior of VPPA welded Al–Mg–Mn–Sc–Zr alloy

Variable Polarity Plasma Arc Welding (VPPAW) can be used for the excellent welding of a 6 mm thick Al–Mg–Mn–Sc–Zr alloy, promising for the aerospace applications, due to its favorable welding quality, high production efficiency and small welding deformation. At present, investigations on the corrosion behavior of this type of welded joints are still insufficient. Here, the microstructure, mechanical properties and corrosion behavior of VPPAW-based Al–Mg–Mn–Sc–Zr alloy were systematically studied. The results show that the lowest value of the hardness of the welded joint occurs in the center of the weld, the fracture position of the welded joint is located in the welded region, and the bond zone (BZ) has the poorest corrosion resistance. The ultimate tensile strength (UTS) and welding coefficient of the welded joint are 327 MPa and 74.5 %, respectively. The low strength of the welded joint is mainly due to the combustion of the Mg element, the growth of grain size and the dissolution of second phase particles, resulting in the weakening effects of solid solution strengthening, precipitation strengthening, and grain boundary strengthening. The poor corrosion resistance of the BZ is attributed to the higher density of Fe-rich intermetallic particles and the weaker passivation film in this region.

Two years pitting corrosion of AA5005-H34 aluminium alloy immersed in natural seawater: morphology characterisation

ABSTRACT Pitting corrosion of aluminium alloy AA5005-H34 immersed in natural sea water for up to 2 years is presented, together with new interpretations of the development of the pitting process over extended exposure periods. Trenching initiated at surface-located Fe-rich intermetallic (IM) particles. It occurred more extensively in sea water compared to atmospheric exposures. With increasing exposures, the pitting morphologies became more complex. Three types, i.e. hemispherical pits (Type 1), crystallographic pits (Type 2) and characteristic ‘petal’-like pits (Type 3), were observed after 12 months. It is suggested that the formation of Type 2 and Type 3 pits occurs at an advanced corrosion stage, and are associated with large Fe-rich IM particles near the metal surface. The influence of marine growth on pitting was negligible.

Modelling the damping response of biomimetic foams based on pomelo fruit

Some sorts of fruits, like pomelo (Citrus maxima or Citrus grandis), also spelled pummelo, are of special interest for researchers and engineers during the development of impact resistant structures. It is assumed that the interaction of structural features on different length-scales of such fruits enables the dissipation of large amounts of energy during their impact against a hard surface. Accordingly, the aim of this paper is to perform an analysis by using a numerical model at different hierarchical levels by means of Finite Element Methods (FEM) in order to identify different structural features that contribute to the damping performance of the pomelo fruit. The considered numerical model was created on the basis of homogeneous aluminum (AlSi7Mg0.3) foam structure, inspired by the pomelo fruit shell structure. In the present approach, the Si- or Fe-rich intermetallic particles on the struts of AlSi7Mg0.3 foam are considered additionally, this enabled more detailed investigations on the structural behavior of the foam during different loading conditions. Comparison of the results from simulation and experimental compression tests showed promising results with respect to the deformation behavior, which are offering support to the design of biomimetic metallic foams. Understanding the principles of combining the structure and material inspired by biological systems enables constructing new lightweight bio-inspired materials of high impact and puncture resistance with a combination of high-energy dissipation, high damping properties and a significant recovery from large deformations.

Film formation in trihexyl(tetradecyl)phosphonium diphenylphosphate ([P6,6,6,14][dpp]) ionic liquid on AA5083 aluminium alloy

Abstract Whilst ionic liquids (IL) have been shown to inhibit corrosion on some reactive metals and alloys by forming a surface film, e.g. Li and Mg, understanding of the interaction between ionic liquids and aluminium is lacking. This research study investigated the viability of film formation on AA5083 Aluminium Alloy by electrochemical treatments in the trihexyl(tetradecyl)phosphonium diphenylphosphate ([P 6,6,6,14 ][dpp]) IL. Two-step anodic treatments were performed on AA5083 in the IL, followed by a comparison of the corrosion behaviour of the IL-treated samples with that of a control. It has been revealed that the two-step IL-treatment led to reduced current densities on AA5083 under cyclic voltammetry scan in the IL before and after the IL-treatment. Lower corrosion rates have been shown on all samples treated in IL at room temperature. Surface characterisation showed a non-uniform porous film on the 50 °C IL-treated sample with a film thickness ranging between 37 nm and 155 nm. The IL-film enhanced the corrosion resistance of AA5083 by protecting the Al-matrix and Fe-rich intermetallic particles (IMPs). Although findings of this study suggest similar IL-film formation as that on Li and Mg, more research needs to be conducted to optimise the electrochemical treatment conditions and ultimately to develop a robust IL-film formation procedure for corrosion protection.

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

This paper presents the results of experimental investigation on fatigue behaviors of friction stir welded joints in AA7075-T6 with ultrasonic fatigue test system (20kHz). Two kinds of particles, Fe-rich intermetallic compounds and Mg2Si-based particles, governed the fatigue crack initiation. The plastic deformation and recrystallization during welding process led to the changes in particle size and micro crack occurrence between thermo-mechanically affected zone (TMAZ) and nugget zone (NZ). Therefore, the fatigue crack initiation sites leaned to be located at the TMAZ in short fatigue life, or at the NZ in very high cycle fatigue regime.

Fatigue Life Prediction in Aluminum Shape Castings

Increasing use of aluminum shape castings in automotive and aerospace industries has drawn great attention to fatigue properties of cast aluminum components. The fatigue resistance of aluminum castings strongly depends upon the presence of casting flaws and characteristics of microstructural constituents. The existence of casting flaws significantly reduces fatigue crack initiation life. In the absence of casting flaws, however, crack initiation occurs at the fatigue-sensitive microstructural constituents. Cracking and decohesion of large silicon and Ferich intermetallic particles and crystallographic shearing from persistent slip bands in the aluminum matrix play an important role in crack initiation. This paper reviews the latest understanding in fatigue crack initiation mechanisms in cast aluminum alloys and presents multi-scale fatigue (MSF) life models for aluminum castings that account for multi-scale casting flaws and microstructural constituents.

Investigating the effects of bulk and surface severe plastic deformation on the fatigue, corrosion behaviour and corrosion fatigue of AA5083

This is a comparative study on the effects of bulk plastic deformation and surface-induced plastic deformation on the fatigue, corrosion behaviour and corrosion fatigue of AA5083. Bulk severe plastic deformation was performed through rotary swaging, whereas surface-induced plastic deformation was conducted by applying shot peening and ball-burnishing. Surface and near-surface properties such as surface topography, microhardness and residual compressive stresses were documented. In comparison to the as-received reference condition, results of potentiodyanmic polarization referred to a better corrosion resistance of the rotary swaged material, while both shot peening and ball-burnishing led to deterioration in corrosion resistance. It was found that, pitting corrosion occurred around Fe-rich intermetallic particles. Ball-burnishing led to superior fatigue performance in air and under corrosive environment in comparison to other investigated conditions.

Effect of Si Content on the Size of Fe-Rich Intermetallic Particles in Al-xSi-0.8Fe Alloys

Al-Si-Fe plates with Si contents of 4.5, 9 and 11 mass %, unmodified and Sr-modified, were quasi-directionally solidified in sand moulds with chills at one end. The size and nature of the Fe-rich intermetallics were determined along the plates. Two forms of the intermetallic were observed, α-Al8FeSi and β-Al5FeSi, in proportions and scale dependent on the cooling rate and the Si concentration. The size of the β-phase increased with the concentration of Si at low cooling rates. At high cooling rates the tendency to form α-Al8FeSi phase increased with increasing Si content reducing the size of the β-plates. Modification generally increased the size of the pre-eutectically formed plates while reducing the post eutectically formed ones.

The Fracture Characteristics of a Near Eutectic Al-Si Based Alloy Under Compression

The fracture of eutectic Si particles dictates the fracture characteristics of Al-Si based cast alloys. The morphology of these particles is found to play an important role in fracture initiation. In the current study, the effects of strain rate, temperature, strain, and heat treatment on Si particle fracture under compression were investigated. Strain rates ranging from 3 × 10−4/s to 102/s and three temperatures RT, 373 K, and 473 K (100 °C and 200 °C) are considered in this study. It is found that the Si particle fracture shows a small increase with increase in strain rate and decreases with increase in temperature at 10 pct strain. The flow stress at 10 pct strain exhibits the trend similar to particle fracture with strain rate and temperature. Particle fracture also increases with increase in strain. Large and elongated particles show a greater tendency for cracking. Most fracture occurs on particles oriented nearly perpendicular to the loading axis, and the cracks are found to occur almost parallel to the loading axis. At any strain rate, temperature, and strain, the Si particle fracture is greater for the heat-treated condition than for the non-heat-treated condition because of higher flow stress in the heat-treated condition. In addition to Si particle fracture, elongated Fe-rich intermetallic particles are also seen to fracture. These particles have specific crystallographic orientations and fracture along their major axis with the cleavage planes for their fracture being (100). Fracture of these particles might also play a role in the overall fracture behavior of this alloy since these particles cleave along their major axis leading to cracks longer than 200 μm.

Effect of Fe-Rich Particles on the Formation of Die Streaks on Anodized Aluminum Extrusions

Fe-rich intermetallic particles are one of the important constituents in the microstructure of 6xxx series aluminum extrusions. The size, amount, and distribution of these particles can be different in different locations of the extrudate due to inhomogeneous deformation during the extrusion process. Furthermore, these intermetallic particles change the dissolution kinetics of grain interiors and grain boundaries during etching, influencing the formation and severity of grain boundary grooves. An inhomogeneous distribution of Fe-rich particles results in a nonuniform distribution of grain boundary grooves and occurrence of die streaks.

Effects of Fe addition and addition sequence on carbon inoculation of Mg–3%Al alloy

The Mg–3%Al melt was treated by carbon inoculation and/or Fe addition. The effects of Fe addition and addition sequence on the carbon inoculation of Mg–3%Al alloy were investigated in the present study. The role of Fe in the grain refinement of Mg–3%Al alloy treated by carbon inoculation was closely associated with the operating sequence of carbon inoculation and Fe addition. Fe has no obvious effect on the grain refinement of Mg–3%Al alloy by carbon inoculation under the condition that Fe pre-existed in the Mg–3%Al melt before carbon inoculation. However, Fe played an inhibiting role under the condition that the Mg–3%Al melt had been inoculated by carbon before Fe addition. The Al–C–O particles were observed in the sample treated only by carbon inoculation. In addition to Al–C–O particles, Al–C–O–Fe particles could be observed in the sample treated by Fe addition and then carbon inoculation. These Al–C–O and Al–C–O–Fe particles, actually being Al–C and Al–C–Fe phases, should be the potent nucleating substrates for Mg grains, resulting in the grain refinement. However, the Al–C–O–Fe-rich intermetallic particles were mainly observed in the samples treated by carbon inoculation and then Fe addition. The Al–C–O–Fe particles, actually being Al–C–Fe phases, formed under this condition should not be the potent nucleating substrates for Mg grains, resulting in the grain coarsening.

Microstructural evolution of Fe-rich particles in an Al–Zn–Mg–Cu alloy during equal-channel angular pressing

The microstructures of a severely deformed Al–Zn–Mg–Cu (AA7136) alloy have been characterized carefully using transmission electron microscopy and three-dimensional atom probe analysis. The Fe-rich intermetallic particles are predominantly Al13Fe4 type in the as-extruded alloy. Significantly, equal-channel angular pressing (ECAP) at 200°C refines Fe-rich particles from ∼1 to 2μm to as small as ∼50nm after 4 passes processing, and effectively narrow down their size distribution with the increase of number of ECAP passes. In addition, small Fe-rich particles evolve into spherical morphology and are in a more uniform distribution. The formations of Fe-rich phases in AA7136, the kinetic and thermodynamic effects in relation to the refinement of Fe-rich particles and their morphology evolution during ECAP processing are discussed.

Effect of heat treatment on microstructure and corrosion behavior of Al6061 alloy weldment

In this research, the corrosion behavior and microstructure of Al6061 alloy welded by GTAW (Gas tungsten arc welding) and followed by various heat treatments have been investigated. The microstructure of both weld metal (WM) and base metal (BM) was studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Corrosion behavior was investigated in 3.5 wt.% NaCl aqueous solution. Results indicated that BM consists of Fe-rich coarse intermetallic particles. These particles were found to act as cathodic regions with respect to surrounding matrix. Potentiodynamic polarization curves confirmed that WM is cathode rather than BM in different heat treatment cycles. Moreover, it was found that heat treatment led to displacement of the BM corrosion potential to noble direction. Observations of corroded surfaces confirmed localized corrosion.