152 Journal of Alloys and Compounds, 211/212 (1994) 152-154 JALCOM 079 Dislocation damping and associated modulus defect in copper crystals Ariel O. Moreno Gobbi Departamento de F£sica, Facultad de Ciencias, 11200 Montevideo (Uruguay) J.A. Eiras Departamento de F£sica, Universidade Federal de Sdo Carlos, 13565 905 Sdo Carlos, Sao Paulo (Brazil) Abstract Ultrasonic attenuation and the modulus defect were investigated in (lll)-deformed copper single crystals. Experimental results were interpreted as resulting from the contribution of two dislocation mechanisms: kink resonance and kink relaxation (stress-induced double-kink formation). Bordoni peak relaxation is described considering no distribution in activation energy or relaxation time. The kink density contributing to the resonance mechanism is shown to be dependent on temperature. 1. Introduction The ultrasonic attenuation and the associated modulus defect in high purity metals are caused mainly by the presence of dislocations [1, 2]. In deformed f.c.c. crystals a relaxation peak, called the Bordoni peak (BP), has been observed [2-4]. The BP was extensively in- vestigated at low frequencies and its primary charac- teristics are well established [2]. Secondary character- istics, such as the width, are not well understood yet. Measurements in the megahertz range have the ad- vantage of enabling us to investigate the same sample at different frequencies. Mongy et al. [5] reported an orientation anisotropy for the peak temperature, which was not confirmed by other authors [6, 7]. However, ultrasonic attenuation measurements alone cannot ad- equately help to explain the BP width, owing to the concurrence of more than one dislocation mechanism contributing to the damping. Simultaneous measure- ments of ultrasonic attenuation and velocity [7] appeared to be helpful for obtaining additional information about the Bordoni relaxation. This study deals with the interpretation of results obtained for the logarithmic decrement 8 and modulus defect AM/Mo in deformed copper single crystals. A solution is proposed which separates the contributions of kink resonance and Bordoni relaxation to the dis- location damping and modulus defect. 2. Experimental procedure High purity copper single crystals (residual resistivity ratio (RRR) = 1500) in the form of cubes of side 1 cm were used as samples in this work. The specimens, previously oriented by the Laue X-ray method, were deformed by compression in the (111) direction. The samples were left for 1 h at 373 K after the deformation. Ultrasonic attenuation and velocity were measured simultaneously using the pulse echo method [8]. The time for a round trip pulse on the sample was obtained by the pulse echo overlap technique [9]. Measurements were performed while cooling the samples between 300 and 80 K at a rate of 1 K min-1. In an undeformed sample, irradiated with y-rays until total pinning of the dislocations occurred, we measured the dislocation-free ultrasonic attenuation at and ve- locity v0 (background). The background, and its tem- perature dependence, were assumed to be the same for all investigated samples. The logarithmic decrement ~ and the modulus defect AM/Mo were calculated as reported in ref. 7. 3. Results and discussion Figures 1 and 2 show typical temperature dependences for the logarithmic decrement and modulus defect of the investigated samples. The ~ curves present a peak (at 172 K and at 153 K in Fig. l(a) and Fig. 2(a) respectively), which is well known as the BP [1--4]. In 0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(93)00079-E