Dissolution of the θ′ Precipitates in an Al-1.7at.%Cu Alloy Deformed by Equal-Channel Angular Pressing+

The microstructural evolution during the equal channel angular (ECA) pressing process of an α+ θ′ two-phase Al-1.7at.%Cu alloy has been studied by transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EF-TEM). The θ′ precipitates are fragmented into fine particles after a few passes of ECA perssing. The θ′ precipitates are almost completely dissolved after 8 passes of the ECA pressing, and nearly single phase α with a fine grain size of approximately 500 nm is obtained. The dissolution process of the θ′ precipitates and the distribution of Cu atoms during the severe plastic deformation have been observed by energyfiltered TEM. Introduction Numerous investigations have been carried out to produce ultrafine-grained microstructures using severe plastic deformation (SPD). Since these materials exhibit high strength without loosing ductility, studies of ultrafine grained materials receive considerable research interest recently. One of the most attractive SPD methods is the equal-channel angular (ECA) pressing, by which intense plastic strain is imposed to rod-shape bulk specimens. Many ultrafine-grained aluminum alloys have been produced by the ECA pressing and the mechanical properties of the ECA-pressed materials have been reported. However, not many attempts have been made so far to process two-phase alloys by ECA pressing. As seen from examples of pearlitic steel wires, heavily deformed ultrafine two-phase alloys have a large potential to exhibit ultrahigh strength. In fact, Senkov et al. reported that a two-phase Al-5.6at.%Fe alloy processed by SPD in torsion on Bridgman anvils caused dissolution of the Al 13 Fe 4 phase to form a supersaturated Al solid solution and that a noticeable precipitation hardening occurred by the subsequent aging. Although SPD was successfully imposed to disk specimens by the torsion method, there are few investigations on the ECA pressing of two-phase bulk alloys. This study aimed to obtain ultrafine two-phase microstructures of an Al-Cu alloy by ECA pressing. The final goal was to apply severe plastic deformation to eutectic microstructure, but it turned out to be extremely difficult because the two phase microstructure with a high volume fraction of the secondary phase is not ductile enough to tolerate severe plastic deformation. Thus, in order to obtain basic understanding of the deformation process of the two phase microstructure, the α + θ′ two-phase fine microstructure produced by aging has been processed by ECA pressing, and the microstructural change by severe plastic deformation has been examined by energy-filtered TEM. Experimental Rod-shaped specimens of an Al-1.7at.%Cu alloy of 10 mm in diameter and 60 mm in length were prepared by casting and machining. The specimens were homogenized at 550°C for 7 days, then solution treated at 525°C for 30 min, and subsequently water quenched. The solution treated specimens were aged for 26 h at 200°C to obtain an α + θ′ two-phase microstructure. The aged specimens were subjected to ECA pressing at room temperature with MoS 2 lubricant. The angle subtended by the arc of curvature at the point of intersection was ~45° and the imposed strain after one pressing was ~1. For comparison, single phase supersaturated solid solution of an Al1.7at.%Cu alloy was also ECA-pressed. TEM specimens were sectioned perpendicular to the longitudinal axis of the rods. High resolution electron microscopy observations were carried out using JEOL JEM-2000EX, operated at 200 kV. Energy-filtered images were acquired on the CCD camera in a Gatan Imaging Filter (GIF) equipped on a Philips CM200FEG transmission microscope, operated at 200 kV and processed using EL/P (version 3.0) and DigitalMicrograph (version 2.5) software. Exposure time of 10-30 s were used to acquire most of the energy filtered images. Zero-loss images were obtained using a 10 eV window centered at 0 eV, and the Cu images were formed using the Cu L 2,3 edge at approximately 945 eV, respectively. Results and Discussion Figures 1 (a) and (b) show a zero-loss image and an Cu image of the θ′ precipitates in the nondeformed two-phase specimen, respectively. The presence of the θ′ platelet with smooth and sharp θ′/α interface is observed. The intensity profile was taken across the θ′ precipitate (Fig. 1 (c)). The intensity profile was averaged over 120 pixels parallel to the interface. When the intensities of the precipitate and matrix regions are extrapolated across the interfacial region, it is evident that the full width of half maximum (FWHM) of the intensity peak is about 9 pixels which corresponds to the thickness of the θ′ precipitates (approximately 3 nm). Figures 2 (a) and (b) show a zero-loss image, [001] selected area electron diffraction (SAED) pattern and an Cu image of the θ′ precipitates obtained from the two-phase specimen that was subject to a single passage of ECA pressing (ε~1). The θ′ precipitates are severely deformed. The 110 θ′ reflection in the SAD pattern is elongated to the circumferencial direction. This suggests that rotation of the θ′ precipitates occurs by deformation of the precipitates. The local diffraction contrast suggests that some strains exist in the matrix grain as well. However the 002 and 022 aluminum spots are not elongated, suggesting that the Al matrix stays as a single crystal within the selected area aperture. The <001> streaks observed from the Al reflections are due to the shape effect of the θ′ plates, but the streaks are much weaker than that of the undeformed specimen. This means that the aspect ratio of the θ′ precipitates become much smaller due to the fragmentaion of the θ′. This result indicates that the fracturing of θ′ phase occurs in the two-phase alloy prior to the rotation of the matrix grain by multiple shearing. The Cu image shows that unlike the undeformed specimen, the shape of the θ′ precipitates are wavy plates with the curved and diffuse θ′/α interface. In addition, the intensity profile (Fig. 2 (c)) shows that the intensity change across the precipitate is more diffuse and the FWHM of the main intensity is about 17 pixels or 4.4 nm. This (b) (a)