Abstract
Hypereutectic Al–Fe and Al–Ni alloys offer a potentially attractive combination of properties, e.g. high-temperature strength and stability, high elastic modulus and low coefficient of thermal expansion. This potential, however, cannot be reached unless the structure of these alloys is refined so that their processing becomes possible. In this study, we for the first time apply ultrasonic melt processing for refining the structure of hypereutectic Al-4% Fe and Al-8% Ni alloys with 0.3 wt% Zr addition. Both primary Al3Fe and Al3Ni particles as well as aluminum/eutectic grains are significantly refined. It is suggested that cavitation-induced fragmentation of primary Al3Zr crystals plays a significant role in the nucleation of intermetallics as well as aluminum. Furthermore, the hardness and tensile properties of the alloys substantially increase after ultrasonic treatment due to the refined structure, which also contributes to the considerably enhanced ductility of the alloys. As a result, the fracture mode changes from brittle fracture to ductile fracture. The increase in ductility makes the alloys suitable for hot deformation, which is demonstrated by lab-scale hot rolling. In addition, precipitation hardening of the alloys can be achieved by high-temperature annealing at 450 °C due to retained Zr in the Al solid solution upon solidification. The results are supported by the analysis of the composition of a supersaturated solid solution of Zr in Al and scanning and transmission electron microscopy that confirms the precipitation of coherent Al3Zr nanoparticles. It is demonstrated that a combination of ultrasonic melt processing and alloying with Zr makes it feasible to develop new class of hypereutectic casting and wrought alloys based on the Al–Fe and Al–Ni systems.
Highlights
Considerable efforts have been devoted over decades to the devel opment of aluminum alloys of various systems because the industry requires a range of superior mechanical properties for different appli cations, including high-temperature and structural applications of cast and wrought products
The structure refinement in hypereutectic Al–Fe and Al–Ni alloys with addition of Zr can be achieved by ultrasonic melt processing in the range of primary solidification of Al3Zr through fragmentation of primary Al3Zr particles, which facilitates nucleation processes for other structure constituents
The nucleation of primary Al3Fe and Al3Ni phases and dendrites/eutectic grains on the refined Al3Zr crystals is suggested as a possible mechanism, though this requires further detailed studies
Summary
Considerable efforts have been devoted over decades to the devel opment of aluminum alloys of various systems because the industry requires a range of superior mechanical properties for different appli cations, including high-temperature and structural applications of cast and wrought products. Hypereutectic Al–Si alloys is an example of highly heterogeneous materials that are widely used as replacement of steel or cast iron due to a good combination of properties, such as low weight and thermal-expansion coefficient, high elastic modulus and wear resistance, and good castability [1]. There is a need of increasing the working temperatures of aluminum alloys, along with improving some other important properties such as rigidity and thermal expansion. Hypereutectic Al–Fe alloys were studied by Cubero-Sesin and Horita [2] and hypereutectic Al–Ni alloys by Kilicaslan and Karakose [3] as candidate materials to replace hypereutectic Al–Si alloys. These studies were, very limited in scope and scale (severe plastic deformation, nanoindentation). Belov et al [4,6] demonstrated that eutectic Al–Ni alloys with an addition of up to 0.8% Zr showed at least two-fold increase in the 100-h strength at 350 C and 3-times improved creep resistance at 400 C as compared to a piston Al–Si alloy of AA4032 type
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