Issue 35

A. Pawełek et alii, Frattura ed Integrità Strutturale, 35 (2016) 21-30; DOI: 10.3221/IGF-ESIS.35.03 23 chamber of vacuum induction furnace (Balzers) under low argon pressure (1000 Pa, of 99.999% purity) after previous evacuation (10 -2 Pa). The applied preparation procedure was similar to that reported in [15] and used also in our previous works [5-9]. Then, the alloys were machined into standard specimens with rectangular cross-section. The tensile tests at elevated temperatures were carried out using a special temperature chamber connected with the Zwick 1200 testing machine using a flat dog-bone samples of operating dimensions 2x6 mm. The elongation was measured with a laser extensometer. The force was recorded with load cell at 100 kN capacity. A temperature chamber was used to control the test temperature. The specimens were elongated at RT, 50°C, 100°C, 150°C and 200°C. The strain tests were performed at the same speed as the above mentioned one. Acoustic emission measurements The AE method has been applied to the investigations of poly- and single crystals of metals and alloys by the authors of the present paper for many years [5,7,10,17]. The investigations concentrated on explaining the correlations between AE descriptors and the mechanisms of deformation of the materials subjected to the tensile and compression tests. The obtained results allowed the authors to put forward the following thesis: the dominant contribution to the recorded AE signals is derived from the collective movement of many dislocations, associated with their acceleration as well as with the synchronized annihilation of many dislocations, including the annihilation at the free surface of the deformed material. The measuring system of AE signal was functionally coupled with the stress/strain testing machines and it was described in more details in [5,18]. A broad-band piezoelectric sensor (standard WD type, certified by Physical Acoustics Corporation) enabled to record the acoustic pulses in the frequency range from 100 kHz to 1 MHz. AE signal processing unit was realized with application of the 9812 ADLINK type card hosted in a PC computer. Owing to the suitable software, the analysis of the energy and the time duration of the individual events could be carried out, because the dedicated program determined the time of AE event occurrence, its maximum amplitude and the moment of a significant decline of AE signal amplitude. In the case of compression test the contact between the detector and the sample in a channel-die was maintained by means of a steel rail of a shape of rectangular prism of 100 mm length and 10x10 mm cross section which formed a natural waveguide. In order to eliminate the undesired effects of friction against the channel walls, each sample was covered with Teflon foil. Since there was no possibility to place the AE sensor directly in the sample undergoing the tension test in high-temperature chamber therefore the AE sensor was coupled with elastic spring to the steel clamps which was holding the sample and was operating at room temperature condition due to the contact with the machine framework. In other words the waveguide was attached to grips and mounted outside the temperature chamber. The AE sensor was connected to the waveguide. The amplification of the AE analyser was 86 dB and the threshold voltage of the discriminator was 1.17 V. The output signal was converted into voltage and amplified with a low-noise charge-sensitive preamplifier. A full-wave rectifier drove the integrator at the output of which an envelope of a single AE pulse was obtained. The signals from the mean-value detector were transmitted directly to the voltage discriminator; those which exceeded the threshold level being counted only once, corresponding to a single recorded AE event. The amplitude and duration of the AE event was measured via analog to the digital acquisition system what enabled the calculation of the energy E of AE events using the approximate formula, E = 0.5 v 2 max Δt , where ν max was the maximum value of AE signal in the course of the event, and Δt was its duration. R ESULTS AND DISCUSSION Mg4Li5Al alloys – tensile tests igs.2 to 5 show the results of preliminary examinations of the influence of plastic instabilities on the fracture of tensile tested Mg4Li5Al alloys at room (Fig.2a), and elevated temperatures 100ºC (Fig.3a), 150ºC (Fig.4a) and 200ºC (Fig.5a) together with the corresponding SEM images of fracture (Fig.2b to 5b). They show that the plastic instabilities, related to the PL effect, twinning and/or shear band are correlated with the generation of AE events, and affect the final form of fracture. It is necessary to note here that the maximally high AE level at the beginning of the test is related to the yield point (including microplasticity) and it is a result of the creation and operation of dislocation sources. This is a well known fact in literature (see e.g. [8]), and we discuss the AE behavior after this maximum. Similarly, the high AE at the end of test is related to the breaking of the sample (with exception, sporadically, when local contact between the sensor and the sample at 200°C was probably not completely correct, as in Fig.8). F