1080 Ádám István Hegyi et al.


Table 1. Main Parameters of the NanoTest device. Part name


X and Y stages Coarse Z stage Fine Z stage


Force sensor (with two possible presets)


Total range Resolution Accuracy ±8mm 0.5 µm 0.01 µm


9mm 2 µm 0.5 µm 35 µm 1nm


20/50mN 1/2.5 µN 1/2.5 µN 0.1nm


samples fabricated onto its surface) was attached directly to the AE transducer using a metallic spring. In addition, the acoustic contact was improved with vacuum grease. Load as a function of time and the AE signal recorded


Weiss et al., 2007; Dobroň et al., 2009; Kovacs et al., 2014). The collective motion of several tens or hundreds of dislocations is necessary to obtain a detectable AE signal (Scruby et al., 1981). Thus, in terms of AE, the motion of a single dislocation is typically “silent” and a detectable AE signal (if caused by dis- location activity) reflects cooperative dislocation motion. Crackling or avalanche-like plasticity is not only char-


acteristic for micron-scale objects but also for bulk samples (Weiss et al., 2007). Due to the enormous number of moving dislocations in macroscopic samples, averaging occurs, which results in a smooth and seemingly continuous stress–strain dependence. Conversely, the AE technique may provide valuable information about underlying dynamic processes that cannot be derived from deformation curves. To our knowledge, this study is the first attempt to record AE signals during mechanical tests on microsamples. AE signal measurements were performed with a Physical


Acoustics PCI-2 acquisition board based on the continuous storage of AE signals with a 2MHz sampling rate. The full scale of the A/D converter was ±10 V. The AE signal was preamplified by 60dB for frequencies from 100−1,200 kHz. The background noise did not exceed 24dB and the detecting threshold level was 26 dB. AE was recorded simultaneously with load–strain data during the uniaxial compression of the micropillar. A rectangular piece of material (with micropillar


during the test of the Al-5%Mg micropillar at a constant compressive strain rate are plotted in Figure 4. As expected, the sample exhibits the well-known PLC effect. We speculate that the stress drops at the very beginning of deformation and just before plastic yielding [enlarged in inset (a)] correspond to the break-out of dislocations from the surrounding solute atoms. The stress drops between these two drops are much smaller, but definitely above the noise level of the instrument. Therefore, they may correspond to dislocation avalanches. Further investigations are required to assess how PLC-type stress drops differ from dislocation avalanches, for example, in non-PLC pure Al micropillars. As seen in inset (a), a large AE signal is detected at the onset of the stress drop. For the micropillar dimensions used, 4×4×12 µm3, the PLC effect can compete with intrinsic intermittent dislocation motion (dislocation avalanches). The combined effect conceals the well-known periodic stress drop structure of the deformation curve, and randomly dis- tributed avalanches are observed. In inset (b), the waveform of the acoustic signal displays several large peaks.


SUMMARY


To understand in detail the deformation properties of micron- sized samples, experiments performed on large ensembles of specimens are needed. The micropillar fabrication method presentedisconsiderablyfasterthan previousmethods.This introduces the possibility of investigations that can reveal the statistical properties of micron-scale plasticity. The results further indicate that the detection of an AE signal related to the cooperative motion of dislocations is feasible even for microscopic samples with small volumes of ~100µm2.


ACKNOWLEDGMENTS


P.D.I. and I.G. acknowledge financial supports from the National Research, Development and Innovation Found of Hungary under the project Nos. NKFIH-K-119561 and NKFIH-PD-105256. P.D.I. is also supported by the Janos Bolyai Scholarship of the Hungarian Academy of Sciences and by the European Commission under grant agreement No. CIG-321842. P.D.I., M.K., K.M., and F.C. are grateful for the financial support from the Czech Science Foundation under contract 15-10821S.


REFERENCES


Figure 4. Load versus time curve obtained from a micropillar compression test at a constant strain rate. In inset (a), a stress drop is enlarged. Inset (b) shows the waveform of an individual acoustic emission peak.


ARZT, E. (1998). Size effects in materials due to microstructural and dimensional constraints: A comparative review. Acta Mater 46(16), 5611–5626.


BOHLEN,J., CHMELÍK, F.,DOBROŇ,P., LETZIG,D., LUKÁČ,P.& KAINER,K. (2004). Acoustic emission during tensile testing of magnesium AZ alloys. JAlloysCompd 378(1), 214–219.


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