PSI - Issue 23

Kristián Máthis et al. / Procedia Structural Integrity 23 (2019) 51–56 K. Máthis et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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The results of the ASK analysis are presented in Fig. 2, where the evolution of the relative AE source activities are plotted. This plot indicates the share of the particular deformation mechanisms in the AE signal during the cyclic straining. At the beginning there is a dominancy of dislocation slip for both alloys. This indicates activation of the basal slip at low stresses, which corresponds to its low critical resolved shear stress (CRSS). As the deformation progresses, the twinning becomes the governing mechanism. However, its extent and dependence on the hysteresis loop is different for the particular alloys. In the case of Mg2Al alloy, twinning is significant above reaching the maximum stress from the previous cycle, but during the unloading and the subsequent reloading the dislocation slip is rather active. In contrast, in Mg9Al sample the contribution of the twinning governs to the AE spectrum virtually in each part of the loading cycle. 3.3. Microstructural evolution The observed AE response is related to the twinning-detwinning behavior. Twinning has several stages: i) nucleation; ii) length – wise propagation and iii) thickening. As mentioned above, only the first two are accompanied by detectable AE. During the detwinning, first slow reduction of thickness takes place (no AE). If the twins during this “shrinking” reach a critical width, they can collapse and completely disappear at zero stress , as it can be seen by yellow highlights in Fig. 3, where the microstructures of Mg9Al sample during loading-unloading cycle is presented. Since the collapse is caused by fast shortening of the twin length, AE can be recorded again. Based on this scheme, one can assume that in Mg9Al samples the detwinning is more frequent than that in Mg2Al alloy.

Fig. 3. Microstructure evolution of Mg9Al samples observed by in-situ SEM imaging during loading up to 115 MPa and subsequent unloading to 0 MPa cycle.

The experimental results indicate the following picture for the microstructural evolution during cyclic loading of Mg-Al alloys as a function of alloying content: the deformation begins with basal slip followed by extension twinning. The alloying content significantly influences the twin nucleation and growth. At lower concentrations (Mg2Al), the twins can easily nucleate and growth. This leads to reaching a stable size for many of them already at low applied stresses. In such a case the lateral twin stresses completely relax (Siska, et al. (2017)) and the driving force for detwinning becomes virtually zero. For Mg9Al alloy the twin nucleation stress is shifted to higher stresses and the twin growth is hindered by solutes. Therefore, there is a higher frequency of detwinning during the unloading. The above listed results answer also the question about the role of twinned volume fraction and twin boundary density in anelasticity. In Fig. 4 the dependence of the anelastic strain on the change in the twinned volume fraction during unloading is plotted. It can be seen that the data follows a master curve independently on the alloying content.