Strengthening of Al through addition of Fe and by processing with high-pressure torsion

Iron is a common impurity element in aluminum and is expected to be used in a controlled manner. In this study, high-pressure torsion (HPT) was applied to 10-mm diameter bulk disk-type samples of Al-Fe alloys with different Fe additions: 2 and 4 wt%, and different initial states: as-cast, extruded, and annealed. Intense strain was introduced to the materials by HPT processing at room temperature under a pressure of 6 GPa for up to 75 revolutions. Tensile tests showed that a significant increase in the UTS above 500 MPa occurs with 13 % elongation in the Al-2 % Fe sample processed by HPT from the as-cast state. Microstructural analyses revealed that a close-to nanograined microstructure with a size of 125 nm and dispersion of intermetallic particles below 50 nm was attained, along with a maximum supersaturation of Fe of ~0.67 wt%. The Al-4 % Fe sample reached even higher supersaturation of Fe to ~0.99 % and similar strength but lower elongation due to insufficient fragmentation of coarse intermetallics. It is concluded that the eutectic structures in the cast state are a major contributor to the enhanced strengthening and the retained elongation. The saturated states of the microhardness at equal Fe contents were shown to be similar regardless of the initial state upon sufficient straining by HPT.