PSI - Issue 23

Yoshikazu Nakai et al. / Procedia Structural Integrity 23 (2019) 83–88 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

87

5

Fig.7. Reconstructed grain shapes by DCT.

Fig. 8. DCT measuring point.

condition of cyclic loading, monotonic tension and compression tests were conducted. Then the minimum and maximum stress for DCT measurement was set to 85 MPa to yield for compression and small plastic deformation in tension side. During the cycle, DCT measurements were conducted at several stress levels those are shown in Fig. 8, where compression yielding by twinning occurred at Point 5, and the stress level at other points were lower than the yield stress shown in Fig. 3. Nakai et al. (2017) reported for a stainless steel that number of diffraction spots decreased with plastic deformation because the brightness of each diffraction spot decreased with increasing total misorientation of grain, and the spot diminished for grains with large plastic deformation. The change of the observable diffraction spot for the present magnesium alloy is shown in Fig. 9, where the measuring points are indicated in Fig. 8. The numbers of diffraction spot are not monotonically decreasing with loading, and it is increased in loading period from Points 5 to 8. It indicates that this change is not explained by the slip mechanism accompanying dislocation multiplication, but it is considered to be responsible for detwinning. As shown in Fig. 10, the change of the total misorientation, β , for each crystallographic plane in tension compression loading process was observed to conduct for more detailed discussion, where {0001} is basal slip plane, {1011} and {1012} are twinning planes. For plastic deformation under compression stress, the value of β decreases in the basal slip plane {0001}, and it is increases in the twinning planes {1011} and {1012} . By considering the EBSD result indicating twinning occurred under compression plastic deformation, the increase of β for twinning planes under compression stress shows that this increasing is related to the twinning deformation. Under tensile stress from Points 6 to 9, the values of β for twinning planes are almost constant while it increased with loading for basal slip

Fig. 9 Number of observed diffraction spots.

Fig. 10 Change of total misorientation of each crystallographic plane.