Yield point behavior in extruded aluminum rod

The microstructure of aluminum, extruded under industrial conditions at 250°C has been investigated in relation to the purity of the billet. Electron microscopy was used to observe the substructure and Kikuchi diffraction techniques were used to measure boundary angles and thus distinguish between recrystallization and repolygonization for samples of two different purities (99.7 pct Al and 99.99 pct Al) extruded under identical conditions. High tensile flow stresses of about 8000 to 9000 psi (55 to 62 MN/m²) were observed in specimens taken from the first sections of the high purity extrusion. These high strength levels were attributed to the presence of fine microstructure. When small recrystallized grains (0.5 to 2.0 μm diam) were present a yield drop was observed. This phenomenon is associated with the condition where nearly all the dislocations are likely to be immobile. The absence of a yield point in the 99.7 pct purity aluminum extruded under the same conditions as the 99.99 pct purity aluminum is due to the existence of fine subgrains instead of the fine recrystallized structure. A small yield point in 99.7 pct aluminum was induced by subsequent heat treatment resulting in the formation of small recrystallized grains of similar character to those in the higher purity extrusion.