Characterization of high-energy ball milled prealloyed copper powder containing 2.5 wt%Al

Abstract

High-energy milling in air of prealloyed copper powder has been applied successfully for production of dispersion strengthened Cu-Al2O3 alloys [1–3]. During this process aluminum oxidizes in situ leaving in the matrix a fine Al2O3 dispersed particles. The prealloyed copper powder particles change their morphology and structure due repeated deformation, fracturing and rewelding. High-energy milling results in highly refined and typical lamellar structure [4]. In the present work some of these changes have been considered including hardening. Inert gas atomized prealloyed copper powder containing 2.5 wt%Al was milled 0, 3, 5, 10 and 20 h in a planetary ball mill using 4.8 diameter hardened steel balls. The powder to steel balls weight ratio was 1 : 35. The milling was performed in air in order to enable formation of Al2O3 dispersoids by internal oxidation. After milling the powder was treated in H2 at 400 ◦C for 1 h in order to reduce copper oxide formed on powder surfaces during milling. The compacts were produced by hot pressing in argon atmosphere at 800 ◦C for 3 h under a pressure of 35 MPa. After 5 h milling, the compact was additionally treated at 800 ◦C up to 5 h with regards to its thermal stability. The as-milled and as-compacted powders were characterized by X-ray diffraction analysis, optical and scanning electron microscopy. X-ray diffraction was performed with Siemens D-500 X-ray powder diffractometer using Cu Kα Ni-filtered radiation. The average lattice distortion and crystallite size were determined from the broadening of the first four X-ray lines (111, 200, 220 and 311) applying Williamson and Hall approach [5, 6]. The relationship between Cu-2.5 wt%Al lattice parameter and milling time is shown in Fig. 1. One can see that rapid decrease in lattice parameter occurs at the beginning of the milling process. The change in lattice parameter decreases with prolonged milling. The decreasing of lattice parameter is assumed to be due to the oxidation of dissolved aluminum, which forms Al2O3 dispersoids. Aluminum is less noble than copper and oxidizes first [7, 8]. High diffusion rate of oxygen through deformed copper matrix accelerates this process during milling. Cu-2.5 wt%Al lattice parameter decreases for about 0.30% after 20 h milling. Since the calculation has shown that Cu-2.5 wt%Al lattice parameter (aCu−2.5 wt%Al = 0.36260 nm) is 0.30% greater than the elemental copper lattice parameter (aCu = 0.36150 nm), it is taken that after 20 h of milling, almost all aluminum has left the copper matrix. 2.5 wt% dissolved aluminum generates by internal oxidation 4.7 wt% alumina in the copper matrix. These alumina particles are very fine [9] and well within the range required for dispersion hardening [10]. X-ray diffraction of Cu-2.5 wt%Al powder shows a progress in line broadening (Fig. 2) with milling time, as a result of severe lattice distortion and crystallites size refinement [5, 6]. The effect of milling time on the size of crystallites and lattice distortion of examined powder is presented in Fig. 3. The crystallite size mainly represents a single-crystal region determined by dislocations and grain boundaries while the lattice distortion represents average relative deviation of the lattice parameters from their mean value [5]. As can be seen, the most intensive refinement of crystallites occurs in the early stage of milling. In the period of 5 to 20 h, the crystallite size remains practically constant ranging to 30 nm. Fig. 3 illustrates that the lattice distortion strongly increases during 5 h of milling, while after that it is less evident up to 20 h,what is rather in agreement with some earlier assumption [5],