Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.</jats:p

Effect of pre-deformation on age-hardening behaviors in an Al-Mg-Cu alloy

The effects of 3%–50% pre-deformation following solution heat treatment on the age hardening of an Al-3Mg-1Cu alloy have been investigated by Vickers microhardness measurement, tensile tests, differential scanning calorimetry, scanning electron microscopy, and transmission electron microscopy. Pre-deformation has a strong effect on subsequent age-hardening behavior. The precipitation was accelerated, hardness peaks appeared earlier, formation of clusters was inhibited, and a larger fraction of precipitates was observed along the dislocation lines. The contribution of the precipitates to the hardness was evaluated by dissolution tests. It was found that pre-deformation followed by artificial aging resulted in a good strength-elongation balance. The results are significant for the development of combined mechanical deformation and heat treatment processes.publishedVersio

Structural- and Compositional Investigations of Grain Boundaries in Y-Doped BaZrO3

Proton-conducting electrolytes are candidates for a range of electrochemical applications. The search for one which may operate in the intermediate temperature range of 400-800 °C has been ongoing for a while, and Yttrium-doped BaZrO3 (BZY) have been established as one of the most promising materials. Unfortunately, its highly refractory nature makes processing difficult, and generally results in a small grained polycrystalline material with high areas of highly resistive grain boundaries. The cause of resistive grain boundaries is attributed to the formation of space-charge layers, where negatively charged defects segregate close to the grain boundaries in order to compensate the positively charged grain boundary core. A sample prepared unconventionally using extreme temperatures of 2200 °C overcame the grain boundary problem, but an unanswered question is: what did occur during the high temperature treatment? In this thesis, conventional samples were prepared by the Solid State Reaction (SSR) method and characterised by a range of methods. X-Ray Diffraction (XRD) indicates the presence of two cubic phases with slightly different lattice parameters, and X-ray Photoelectron Spectorscopy (XPS) shows Y situated two different chemical states with a seperation of ~2.6 eV. Energy Dispersive Spectroscopy (EDS) performed in the Transmission Electron Microscope (TEM) shows an enrichment of Y and depletion of Ba at the grain boundaries. These are strong indications that Y also substitutes on Ba site in the structure. The microstructure has grains of ~600 nm and the grain boundaries mostly adapt random configurations. Conductivity measurements by impedance spectroscopy indicates the presence of highly resistive grain boundaries, as the conductivity of grain boundaries is three orders of magnitude lower than that of bulk (0.001 vs. 0.000001 S/cm at 400 °C in wet O2). In the high temperature sample the same degree of Ba depletion is not observed, but Y also segregates to the grain boundaries. The grain boundaries are also more frequently near a Coincident Site Lattice (CSL) type grain boundary; an indication of grain boundary migration during the high temperature treatment. Furthermore, two samples prepared through the Solid State Reactive Sintering (SSRS) method, with Ba non-stoichiometry (x = ±0.02, in Ba_{x}Y_{0.15}Zr_{0.75}O_{3}), have been studied by TEM. The goal was to gain more understanding on how the addition of small amounts of NiO assists in the sintering stage, and how it is distributed in the resulting sample. The results shows that Ni rich phase(s) are present at the grain boundaries and triple points in the Ba rich sample. In the Ba deficient sample no secondary phases are observed, but the Ni content is found to be twice as high at the grain boundaries than in the grain interior (bulk)

Microstructural Characterisation of Features Related to Grain Boundary Corrosion Phenomena in Aluminium Alloys

Aluminium (Al) alloys are of great interest as components for the automotive and aerospace industries. These alloys have great mechanical properties and are lighter than their alternatives. Light alloys will effectively reduce the greenhouse gas emission and allow for longer driving range in electric cars. However, many of these alloys suffer from corrosion problems which prevents them from being used to their full potential. Finding ways to improve corrosion problems, while maintaining the mechanical properties, will enable Al alloys to become even more applicable. Understanding these corrosion properties better requires detailed understanding of the microstructure at the nanoscale. A tool enabling such studies is the transmission electron microscope (TEM). Al alloys undergoes a processing route, involving exposure to different temperatures and mechanical deformation, before reaching its final shape. These stages will influence the corrosion behavior in the final product. The final stage produces large amounts of nanoscale secondary phases (precipitates) within the grains of the alloy. These provides the high alloy strength as compared to pure Al. Two Al alloys series have been investigated in this work. These are the Al-Mg-Si and Al-Zn-Mg alloys. These suffer from corrosion phenomena along the grain boundaries in the material. This is called intergranular corrosion and stress corrosion cracking. The main findings involve new understandings concerning the precipitation behavior in both these alloys series. It is shown that corrosion can be reduces by altering the processing parameters, and that the fundamental reason for this is closely related to the precipitates. Methodologies for improved investigation of grain boundaries has been presented and are applicable to other material systems. In addition, a nearly 100-year-old riddle concerning the atomic structure of clusters forming in room temperature has been solved. The combined findings will be important in producing Al alloys with excellent mechanical properties and corrosion resistance

Comparing intergranular corrosion in Al‐Mg‐Si‐Cu alloys with and without α‐Al(Fe,Mn,Cu)Si particles

In this study, a model alloy without Fe and Mn additions, is compared with a commercial AA6005 alloy to further understand how the α‐Al(Fe,Mn,Cu)Si particles affect intergranular corrosion (IGC) behaviour. Both alloys were subjected to an accelerated IGC test for durations ranging from 1 to 120 h. Microstructures were studied using scanning and transmission electron microscopy, and electron backscatter diffraction. The presence of α‐Al(Fe,Mn,Cu)Si particles yields significantly more uniform IGC attacks and higher corrosion rates. However, the maximum depth of IGC attacks reaches similar values after ~24 h of exposure. This is attributed to the formation of Cu‐rich particles along the grain boundaries during the corrosion process, which further catalyses the cathodic reactions.publishedVersio

Grain boundary structures and their correlation with intergranular corrosion in an extruded Al-Mg-Si-Cu alloy

A detailed analysis of grain boundaries in a highly textured Al-Mg-Si-Cu alloy is presented in this work. Electron backscatter diffraction demonstrates the presence of three main categories of grain boundaries in addition to sub-grain boundaries. These grain boundaries have been systematically analysed using high–resolution scanning transmission electron microscopy. Intergranular corrosion (IGC) susceptibility was statistically correlated with the same defined grain boundaries. A high density of metastable Q′-phase grain boundary particles correlates with a reduction in Cu segregation at grain boundaries and increased IGC resistance. Results herein are relevant in further understanding grain boundary structures in Al-Mg-Si-Cu alloys and their susceptibility to IGC, and can be implemented into modelling frameworks.publishedVersio

The correlation between intergranular corrosion resistance and copper content in the precipitate microstructure in an AA6005A alloy

A positive correlation is observed between the amount of Cu incorporated in hardening precipitates and intergranular corrosion resistance in an artificially aged Cu-containing 6005A alloy. Three mechanisms have been identified to increase Cu absorption in hardening precipitates: by increasing aging temperature, by pre-deformation, and by slow cooling from solution heat treatment. These findings demonstrate the possibility for development of new processing routes to produce Cu-containing Al-Mg-Si alloys with improved corrosion resistance.acceptedVersio

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.publishedVersio

Comparing intergranular corrosion in Al‐Mg‐Si‐Cu alloys with and without α‐Al(Fe,Mn,Cu)Si particles

In this study, a model alloy without Fe and Mn additions, is compared with a commercial AA6005 alloy to further understand how the α‐Al(Fe,Mn,Cu)Si particles affect intergranular corrosion (IGC) behaviour. Both alloys were subjected to an accelerated IGC test for durations ranging from 1 to 120 h. Microstructures were studied using scanning and transmission electron microscopy, and electron backscatter diffraction. The presence of α‐Al(Fe,Mn,Cu)Si particles yields significantly more uniform IGC attacks and higher corrosion rates. However, the maximum depth of IGC attacks reaches similar values after ~24 h of exposure. This is attributed to the formation of Cu‐rich particles along the grain boundaries during the corrosion process, which further catalyses the cathodic reactions

Grain boundary structures and their correlation with intergranular corrosion in an extruded Al-Mg-Si-Cu alloy

A detailed analysis of grain boundaries in a highly textured Al-Mg-Si-Cu alloy is presented in this work. Electron backscatter diffraction demonstrates the presence of three main categories of grain boundaries in addition to sub-grain boundaries. These grain boundaries have been systematically analysed using high–resolution scanning transmission electron microscopy. Intergranular corrosion (IGC) susceptibility was statistically correlated with the same defined grain boundaries. A high density of metastable Q′-phase grain boundary particles correlates with a reduction in Cu segregation at grain boundaries and increased IGC resistance. Results herein are relevant in further understanding grain boundary structures in Al-Mg-Si-Cu alloys and their susceptibility to IGC, and can be implemented into modelling frameworks