Background
Gradient nanostructures (GNS) consist of a surface structure of nanocrystalline grains whose magnitude in size gradually increases with distance from the surface. GNS are a class of heterostructure materials that have the ability to achieve a combination of high strength and ductility without altering the overall alloy composition. GNS materials have shown improvements in surface-sensitive properties including fatigue, wear, corrosion-fatigue, and corrosion behavior of materials. However, GNS materials have limitations with processing bulk samples with controlled microstructures for mechanical properties.
Surface nanocrystallization (SNC) with the nanograins at the surface of GNS can be achieved through various plastic deformation techniques including surface mechanical attrition treatment (SMAT), ultrasonic shot peening, sandblasting, laser shocking peening (LSP), and fast multiple rotations rolling. SMAT has been proven to be the most effective at producing the smallest possible grain size at the free surface and a substantial gradient several hundred microns deep into the bulk of the sample.
SMAT has been found previously to increase corrosion resistance in nanostructured alloys made of harder materials, including stainless steel and titanium-based alloys, where contamination is minimized. However, aluminum and magnesium alloys suffer from impurity contaminations and degradation in corrosion resistance with steel-medium-based SMAT processing.
Invention Description
Researchers at Arizona State University and the Army Research Laboratory have developed a novel method of surface mechanical attrition treatment (SMAT) that increases corrosion resistance of aluminum alloys for structural applications. This method involves performing SMAT at room temperature and liquid nitrogen flow conditions to generate two distinctly different initial gradient microstructures. This method contributes to thicker oxide films with copper (Cu) and silicon oxide (SiO2) enrichment, as revealed through surface film characterization. In initial tests, the SMAT processed samples were found to have significant microstructural changes including the formation of precipitates and dissolution of inherent phases. The samples also showed a reduced anodic dissolution rate.
Potential Applications
- Transportation (e.g., automotive, aerospace, ships)
- Building and construction (e.g., bridges, storage tanks, chemical vessels)
Benefits & Advantages
- Reduces anodic and cathodic kinetics (low anodic dissolution rate)
- High polarization resistance
- Stable surface film
- Thicker oxide film with Cu and SiO2 enrichment
- Increases corrosion resistance
- Dissolution of inherent phases close to SMAT surface
Related Publication: Role of gradient nanograined surface layer on corrosion behavior of aluminum 7075 alloy