Lower B Field
Lower B field is a type of grain boundary microstructure that can be observed in alloys. It is generated when an alloy solidifies in a directional cooling process and the grains are stretched and parallel or nearly parallel. The lower B field has a linear boundary pattern consisting of low angle and random boundaries, including cell boundaries formed by cellular orientation.
The lower B field is a result of solidification processes in which the solidification rate is very high relative to the diffusion rate. In solidifying alloys, the diffusion rate is far slower than the solidification rate, resulting in the formation of cells with rapidly varying grain sizes. The rapid solidification process causes the grains to be stretched, tilted, distorted, and parallel. This parallel grain orientation creates the linear boundary pattern.
Cellular orientation is a type of boundary pattern in which cells of different grain sizes form in a linear orientation. This occurs when the solidification fronts move quickly into a material with a uniform temperature gradient. In cellular orientation, the solidification fronts form low angle boundaries at random. Cellular orientation is different from equiaxed grain orientation, in which the grain boundaries form at right angles.
The lower B field is a characteristic of aluminum-based alloys, such as aluminum-magnesium, aluminum-silicon and aluminum-zinc. The initial grain size is determined by the starting casting characteristics, such as velocity, temperature, thermal shrinkage and the cooling rate. In the aluminum-silicon system, the grains are usually elongated, randomly distributed, and have high symmetry.
In aluminum alloys, the lower B field not only contributes to the mechanical properties of the material, but also to its corrosion resistance. It contributes to improved mechanical properties by making the grain boundaries stronger and more resistant to deformation. It also has an impact on corrosion resistance because the presence of low angle grain boundaries encourages the formation of a protective oxide film on the surface of the material.
Lower B field microstructures are generally beneficial for mechanical properties, as they improve fatigue resistance and tensile strength. They are also beneficial in terms of machinability, as they make the material easier to machine and form. In addition, they can contribute to improved fracture toughness and reduced stress cracking.
Lower B field microstructures are also advantageous in terms of corrosion resistance. Low angle grain boundaries reduce porosity, which limits the penetration of corrosive media. In addition, they increase the surface area of the material, which enhances the formation of corrosion-resistant oxide layers. This increases corrosion resistance.
Overall, lower B field microstructures offer a range of advantages for mechanical and corrosion resistance in aluminum alloys. They form linear boundaries with randomly distributed low angle boundaries, enhancing the material’s mechanical strength, fracture toughness, machinability and corrosion resistance. In addition, they can also contribute to improved fatigue resistance and tensile strength. As such, lower B field microstructures are invaluable for the design and manufacture of high-performance aluminum alloys.
