General Summary and Concluding Remarks
This thesis examined the mechanical properties and microstructures of Al-Fe alloys processed by High-Pressure Torsion (HPT), as a method of controlling the microstructures to create a high strength and ductility Al alloy which uses Fe -its most common impurity- as the sole alloying element.
A brief introduction to the Al-Fe binary system was presented in Chapter 1. The main characteristic to be highlighted from the phase diagram is the extremely low solid solubility of Fe in Al, which in turn leads to the formation of not one, but several intermetallic phases which are dependent on the treatment that is given to the material, such as manipulation of the cooling rate during solidification or thermomechanical treatment. Nonetheless, it was stated that proper control of the secondary phases of Fe in Al is not an easy task, and to date there is no successful processing route to fully enable a major use of Fe as the main alloying element in a commercial alloy. Processing with HPT was proposed as a method to impose high strain and achieve microstructure refinement in Al-Fe alloys by solid state processing, and avoid difficulties of more complicated extreme methods such as rapid quenching or mechanical alloying.
In Chapter 2, exploratory work was carried out by processing Al-Fe alloys using increasing Fe concentrations: 0.5%, 1%, 2%, 4% (5% for powder-consolidated) and initial states of the microstructures: conventional bulk forms prepared by combinations of casting, hot-extrusion and annealing, as well as powder mixtures consolidated by HPT.
It was shown that significant grain refinement down to the submicrometer level, well below 1 µm was achieved by HPT processing. Fragmentation and dispersion of second-phase intermetallics were observed in the bulk samples and the powder mixtures were successfully
138 consolidated at room temperature with similar grain sizes as in the bulk samples. However the hardness and tensile strength increased more prominently in the as-extruded bulk samples, followed by the extruded and annealed samples and lower in the powder-consolidated samples. The strength increased unequivocally with increasing the Fe content, but there was similarity between the Fe content of 2% and 4% in the bulk samples.
Since the microstructure was not saturated at the level of imposed strain used in Chapter 2, the numbers of revolutions by HPT were extended in Chapter 3 for bulk samples and Chapter 4 for powder-consolidated samples to study the extent of fragmentation and dissolution of Fe-containing particles through further straining by HPT.
It was shown in Chapter 3 that the evolution of microstructure and mechanical properties of bulk samples were enhanced when processing by HPT from the as-cast material in comparison to the extruded and the annealed microstructure. The intermetallic phases in eutectic structures are a major contributor to the increase in strength and the retained elongation, as observed in the superior tensile properties of the Al-2%Fe samples. The coarse intermetallic phases in the Al-4% Fe samples decreased the total elongation to failure because only partial fragmentation of this phase to an average size of ~15 μm was achieved by HPT.
The formation of a close-to nanograined microstructure in the Al matrix, with an average grain size of ~120 nm and effective dispersion of intermetallic particles of eutectic nature with sizes well below 200 nm was demonstrated. Dissolution of Fe in the Al matrix to a supersaturated state was achieved in both Al-2%Fe and Al-4%Fe samples. These properties saturate to similar levels at equal Fe concentration regardless of the initial state of the microstructure, when sufficient strain is introduced to the material.
Chapter 4 evaluated the evolution of mechanical properties and microstructure from a powder-consolidated Al-10% Fe alloy. Room temperature in situ consolidation with a relative density > 99% was successfully achieved by HPT. Upon consolidation, an ultrafine-grained
139 Al matrix with an average size of ~145 nm was obtained by HPT processing. The secondary phase having a size with a bimodal distribution of Fe particles was refined to average sizes of dp~15 µm and dp~2.5 µm for coarse and fine particles, respectively.
There was a significant increase in strength in the powder-consolidated samples after large deformation, with a complex distribution of hardness across the volume of the disk due to the grain refinement in the Al matrix and the dissolution of Fe particles achieved by the HPT process. Excellent elongation properties ranging from ~10% to ~50% were observed at intermediate levels of imposed strain, which provided the best compromise of strength and ductility.
It was demonstrated in Chapter 5 that age hardening was achieved by formation of a supersaturated solid solution of Fe in Al via the solid-state processing by HPT. Microstructure control was achieved by precipitation of dissolved Fe as fine intermetallic particles within the ultrafine-grained matrix. As a result of the HPT processing the intermetallic structures in the initial eutectics were partially dissolved up to a maximum ~1% Fe in supersaturated solution and otherwise fragmented and dispersed in the Al matrix by HPT. The high strength after HPT processing was achieved to maximum UTS of ~600 MPa as a result of a combination of the structure refinement in the matrix grains and intermetallic particles.
Artificial age hardening was achieved within 0.25 h of aging at 200 °C to maximum UTS of ~700 MPa via precipitation of dissolved Fe content as nano-sized Al6Fe and Al3Fe particles
~10 nm in size, within the ultrafine grains and near the grain and subgrain boundaries. The uniform elongation exceeded ~12% even at intermediate levels of imposed strain by HPT, but it decreased to ~6% with the aging treatment. However, the combinations of strength and elongation achieved in this study are comparable to the highest strength commercial Al alloys.
The high thermal stability of the microhardness in the ultrafine-grained structure was verified by natural aging for more than six months.