PSI - Issue 23

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

3

Kristián Máthis et al. / Procedia Structural Integrity 23 (2019) 51–56

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Fig. 1. The loading-unloading deformation cycle (solid line), the corresponding development of twinned volume fractions (solid symbols) and the anelasticity (empty symbols) for (a) Mg2Al; (b) Mg9Al samples.

3.2. In-situ acoustic emission testing

There is a general agreement that the twinning-detwinning phenomenon is the main contributor to the observed strain hysteresis (Nagarajan, et al. (2012)). However, there is no consensus on the share of twin boundary density and twinned volume on the anelastic strain (Nagarajan, et al. (2012)). In order to answer this question, a combination of neutron diffraction (ND) and acoustic emission (AE) was used. As mentioned above, the variation of twinned volume can be derived from the ND data. The AE spectrum includes signal exclusively from the twin nucleation – the energy of the AE emitted by twin growth is too low for proper detection. Thus, the AE is proportional to the number of the nucleated twins. The AE response was monitored in so-called data streaming mode, when the raw signal is recorded without using threshold level. The reason for this choice was given by the concurrent activity of various AE sources such as dislocation slip, twinning etc. Commonly used threshold level detection does not allow the proper separation of signals based simply on the parameters of particular AE events (amplitude, duration, risetime etc.). On the raw AE recording the adaptive sequential k -means (ASK) analysis was applied. In this method, described in detail in (Pomponi and Vinogradov (2013)). the signals are sectioned into consecutive time windows (in our case 2 ms) with subsequent frequency-based analysis of each one. Finally, the ASK, saying by simple way, determines, which mechanisms is dominant in the AE spectrum in the given time window.

Fig. 2. The time evolution of the relative acoustic emission source activity for (a) Mg2Al; (b) Mg9Al samples as it was determined by adaptive sequential k- means analysis and the corresponding deformation curves.