Microstructure of two-phase Al–1.7 at% Cu alloy deformed by equal-channel angular pressing
Introduction
Severe plastic deformation (SPD) has become a very popular method for processing ultrafine-grained microstructures [1], [2], [3], [4], [5], [6], [7], [8], [9]. Ultrafine-grained metallic materials tend to have high strength without sacrificing toughness and ductility; thus, they have recently received considerable research interest. Equal-channel angular (ECA) pressing [10] is one of the most attractive methods to impose intense plastic strain to rod-shaped bulk samples. Many ultrafine-grained aluminum alloys have been produced by ECA pressing and attractive mechanical properties such as high strength and superplasticity have been reported from ECA-pressed aluminum alloys [4], [5], [6], [7].
Previous ECA studies are concerned mostly with producing an ultrafine-grained microstructure from single-phase alloys. As seen from the example of pearlitic steel wires and in situ metal-matrix composites, heavily deformed ultrafine two-phase alloys have potential to exhibit ultrahigh strength. Therefore, studying the microstructural changes of two-phase alloys during SPD is of potential technological importance. However, not many attempts have been made so far to process two-phase alloys by ECA pressing. Senkov et al. [7] reported that a two-phase Al–5.6 at% Fe alloy processed by severe plastic deformation in torsion on Bridgman anvils caused dissolution of the Al13Fe4 phase to form a supersaturated Al solid solution. In addition, significant precipitation hardening was observed by subsequently aging the ECA-pressed sample. Although SPD can be successfully applied to disk samples by the torsion method, there are few investigations on ECA pressing of two-phase bulk alloys.
The aim of this study was to obtain ultrafine two-phase microstructures in an Al–Cu alloy by ECA pressing. The final goal was to apply SPD to eutectic microstructure, but the two-phase microstructure with a high volume fraction of the secondary phase was too brittle to be deformed by ECA pressing. Thus, in this study, a fine α+θ′ two-phase microstructure obtained by aging was deformed by ECA pressing, and the microstructural change accompanying the severe plastic deformation has been examined by transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EF-TEM). In order to investigate the feasibility of further strengthening ECA-pressed aluminum alloys by precipitation hardening, the precipitation process in ECA-pressed Al–1.7 at% Cu alloy was also examined.
Section snippets
Experimental
Rod-shaped samples of an Al–1.7 at% Cu alloy, 10 mm diameter and 60 mm in length, were prepared by casting and machining. The samples were homogenized at 550°C for 7 days, then solution-treated at 525°C for 30 min, and subsequently water-quenched. The solution-treated samples were aged for 26 h at 200°C to obtain an α+θ′ two-phase microstructure. The aged samples were subjected to ECA pressing at room temperature using MoS2 lubricant. The ECA pressing used a die having a channel angle of 90°
Results
Figure 1 shows microhardness as a function of the number of passes of ECA pressing. Initially, the hardness of the two-phase sample aged for 24 h at 100°C is higher than that of the solution-treated single-phase alloy due to precipitation hardening. However, only one pass of ECA pressing applied to the single-phase sample causes a drastic (∼170%) increase in hardness. After two passes, the hardness decreases slightly. Thereafter, a small increase in hardness is observed after each pass. On the
The microstructural change by ECA pressing
The hardness of the solution-treated sample was increased drastically after one pass of ECA pressing. After two passes, a slight decrease in hardness is observed. This is probably because the dislocation tangles have become structurally organized, forming dislocation cell boundaries, within which dislocations are not present. Thus, the initial hardening after one pass of ECA press is due mainly to work hardening. Thereafter, hardening due to grain size refinement becomes dominant. The grain
Conclusions
Changes in the microstructure during ECA pressing of an Al–1.7 at% Cu alloy have been investigated in detail using TEM and EF-TEM. The following conclusions were obtained.
- 1.
θ′ precipitates are fragmented into small grains by ECA pressing that eventually dissolve, forming a supersaturated solid solution. Plastic deformation occurs preferentially in these particles, prior to grain refinement. Consequently, the average grain size of the two-phase sample after eight passes of ECA pressing is larger
Acknowledgements
The authors wish to thank Dr T. Furuhara at Kyoto University for his permission of use of the TEM facility at Kyoto University and Mr K. Inoke at Philips Electron Optics Japan, Ltd for his experimental support.
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