Surface structure - changing surface properties
Aluminium alloys are used in many market sectors, including aerospace, architecture, automotive, electronics, lithography and packaging. Fabrication processes, such as rolling, machining and mechanical grinding, produce aluminium products of the required gauge thickness and shape to ensure fitness for purpose.
The performance of surface critical products, such as aluminium façades, is mainly dependent on the alloy’s surface properties. Furthermore, the need for fuel efficiency and increased performance in transportation systems places new demands on aluminium alloys. These demands are addressed by advances in fabrication techniques that achieve the required internal microstructure, texture, surface finish and, therefore, in-service and cosmetic properties. However, only recently has attention been paid to the criticalsurface/near-surface regions, which could be significantly modified.
The Bielby layer
In 1921, using the approaches available, which lacked there solution of sophisticated electron microscopes, industrial chemist Sir George Beilby suggested that an amorphous surface layer is generated by grinding themetallic surface, because of the local melting of asperities, followed by flow and rapid solidification. This layer has been called the disturbed or white layer, the tribolayer or the flow zone in research studies examining materials subjected to different processing routes, but linked by enhanced surface shear deformation. This implies a different structure in the near-surface region compared with the underlying bulk material.
It is now established that severe strain processing, such as by equal channel angular extrusion (ECAE), can transform conventional microstructures into ultrafine grained versions. Although established fabrication processes, such as those mentioned above, do not strain the bulk material to the extent induced by ECAE, the surface and near-surface regions of alloy sheet are subjected to enhanced shear deformation. These area sare promising, with the detailed microstructures of the surface or near-surface regions in commercially rolled aluminium alloy sheet products now being tested at appropriate magnifications and resolution. For example, the combined use of high resolution scanning and transmission electron microscopes, with appropriate specimen preparation enabling the precise location of surface regions, provides a means to examine microstructure evolution in the surface and near-surface regions during fabrication and its impact on alloy performance under service conditions.
Cutting, machining, rolling, grinding and polishing of aluminium alloys result in the formation of near-surface deformed layers, with thicknesses of up to two or three micrometres, with finer microstructures from that of the bulk alloy. Typically, the outer most deformed layer is characterised by ultrafine, equiaxed grains, with grain boundaries decorated by nanosized oxide particles. A transition region, characterised by microbands of elongated grains, aligned parallel to the working surface, may be sandwiched between the surface regions and the bulk alloy. This results from severe plastic strain and increased temperatures in the surface/near-surface region during processing that are of sufficient magnitude to cause geometric dynamic recrystallisation.
The outcome is significant microstructure refinement and near-surface deformed layer formation. Mechanical alloying due to interaction between the tool and work-piece also contributes to the near-surface layer formation. Consequently, microstructure alteration with depth from the surface is a reflection of the strain and temperature distributions.
At a general level, a deformed layer, characterised by fine grains of approximately 100nm in diameter, has been generated on carbon steel by mechanical grinding. For other metallic materials, similar deformed layers may be generated at surfaces that are associated with enhanced surface shear deformation. This is to be considered in ongoing studies.
The deformed layers are stable at ambient temperature, associated with the local presence of a large fraction of high angle grain boundaries. The structure is also stabilised through pinning the grain boundaries by oxide particles and precipitates. Deformed layers are observed with grain boundaries decorated by oxide, which survive typical annealing and solution heat treatment processes. However, the presence of fine grains alone in the deformed layer, with grain boundaries free of oxide particles, is insufficient to hinder grain coarsening during typical annealing treatments.
Importantly, the heavily deformed near-surface layer has significant influence on material performance, with electrochemical and corrosion behaviour, as well as mechanical properties, material joining and optical properties affected. The high population of grain boundaries and severe deformation in the deformed layer promotes intermetallic particle precipitation during subsequent heat treatment. The increased population density of intermetallic particles and their preferred locations, compared with the bulk alloy, render the heat-treated alloys susceptible to corrosion attack. For example, a near-surface deformed layer on AA6111 automotive closure sheet alloy can be generated by mechanical grinding during rectification. Subsequent paint baking, such as thermal exposure at 180ºC for 30 minutes, promotes the Q phase particle precipitation, at ~20nm diameter andwith preferred grain boundaries within the deformed layer, but with no precipitates forming in the underlying bulk alloy. The presence of Q phase precipitation in the near-surface deformed layer dramatically increases thealloy’s susceptibility to cosmetic corrosion that propagates intergranularly, with the driving force provided by micro-galvaniccoupling between the Q phase precipitates and the adjacent aluminium matrix.
Remedial action is readily performed by alkaline etching or acid pickling of the alloy surface, restoring the corrosion resistance associated with the bulk alloy. These treatments need to be carefully controlled to ensure the deformed layer is completely removed and there is no preferential attack at susceptible grain boundaries.Such surface treatment enriches selected alloying elements in a thin layer, typically two to three nanometres thick, in the alloy immediately beneath the residual alumina film over the macroscopic alloy surface. For example, caustic etching of AA6111 aluminium alloy results in enrichment of copper from a bulk level of 0.74wt% to 40wt% in a two nanometre-thick layer, ofcomposition and structure that is grain orientation dependent.
The evolution of near-surfacelayer microstructures and their simulation, together with their impact on the appearance and performance of practical aluminium alloys, are beingexplored in collaborative studies supported by the EPSRC-funded LATESTPortfolio Partnership at The University of Manchester and IMMPETUS at the University of Sheffield, with input from Innoval Technology in Banbury, all in the UK. This work is being facilitated by 3D characterisation using low voltagescanning electron microscopy with novel specimen generation procedures, which overcome the drawbacks of conventional 2D characterisation techniques in providing an accurate description of the microstructure
Further information: The University of Manchester