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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twinning system is usually activated in order to accommodate the strain in the crystallographic c- direction (Yoo (1981)). The twinning mechanism consist of three essential steps: nucleation (appearance of twin nucleus); propagation (growth in length) and thickening (growth in width). The first two are influenced particularly by the internal stress distribution and crystallographic orientation of the parent- and surrounding grains (Beyerlein, et al. (2010)). In the case of the growth, a key role of solutes and/or precipitates was observed (Nie (2012)). Consequently, there is a high demand for investigation of the influence of the alloying elements on the twin boundary mobility. Magnesium-aluminum alloys are the most widely used magnesium-based materials in the industry. The aluminum is used for its favorable effect on strength and hardness (Avedesian and Baker (1999)). There are several numerical and experimental studies, how the aluminum content alter the twinning behavior in magnesium alloys (Drozdenko, et al. (2016), Lou, et al. (2007), Vinogradov, et al. (2016)). Nevertheless, the high spatiotemporal resolution of the experimental techniques used in this works enables a unique view into the microstructural details of twinning detwinning process. Randomly textured binary Mg-2 wt.% Al and Mg-9 wt.% Al alloys (further referred as Mg2Al and Mg9Al) were used for the experiments. Both samples were solution heat treated before the testing for 24 hours at 413 °C . The grain sizes were (110 ± 10) µm for both a lloys. The cylindrical samples (  9 mm, length 20 mm) underwent a repeated loading-unloading cycle at room temperature: compression to predefined strain values controlled by extensometer (0.05%, 0.1%, 0.5%, 1%, 2%, 3% and 6%) and a subsequent unloading to 0 MPa at a strain rate of 1 x 10 -3 s -1 . The in-situ neutron diffraction (ND) measurements during cyclic loading were carried out at the SMARTS engineering instrument in the Lujan Neutron Scattering Center. The detailed description of the experimental setup is given in Máthis, et al. (2015a) . The acoustic emission (AE) response was recorded concurrently by ND using a Physical Acoustics (PAC) PCI-2 acquisition board and a broadband MicroS50 (PAC) AE sensor. The AE signal was amplified by 60 dB. Visual confirmation of the outcomes of the ND and AE measurements was done by in-situ imaging in a Zeiss Auriga scanning electron microscope (SEM). The specimens were polished using diamond pa ste down to ¼ µm particle size, followed by electrolytic polishing at -40 ° C and 18 V using a Struers AC2 electrolyte. MTEST Quattro deformation stage installed inside SEM chamber was used for providing the same deformation cycle, as during ND measurements. The compression deformation curves for the Mg2Al and Mg9Al alloys are shown in Fig. 1. It is obvious that the flow stress is higher for the Mg9Al alloy. This result is in good agreement with the assumption that the solute atoms has a significant impact on the deformation mechanisms in the Mg-Al binary alloys. The Al solute atoms interact with the dislocation in the basal plane ( Cáceres and Rovera (2001), Máthis, et al. (2015b) ). Owing to the dominancy of basal slip in the plasticity of Mg alloys, this hardening term is prevailing. However, the hindering of the prismatic < a >-type slip by solutes also should not be omitted. ( Akhtar and Teghtsoonian (1969), Cáceres and Rovera (2001) ). The alloying elements further play an important role in extension twinning. Both the twin nucleation- and twin growth stresses are influenced by solutes higher concentrations lead to increment of these values (Cui, et al. (2017), Ghazisaeidi, et al. (2014)). The variation of twinned volume can be derived from the intensity changes of particular ND peaks. In this case the twinned volume fraction was calculated from the variation of the axial distribution function of the (0002) peak (further details see in Clausen, et al. (2008)). The twinned volume fraction is obviously larger for Mg2Al alloy, which is in good agreement with the theoretical calculations. Hysteresis loops forms during the loading-unloading cycle as a consequence of the anelastic behavior. Calculation of the anelastic strain , defined as the width of the hysteresis loop of the true stress – true plastic strain curve (not presented here) indicates that above 0.5% strain the Mg9Al sample shows the largest anelasticity (see green symbols in Fig. 1). 3. Results and discussion 3.1. Anelastic behavior and in-situ neutron diffraction measurements 2. Experimental