Yellow Luminescence (YL) band and blue luminescence (BL) band in a studied unintentionally doped GaN sample show a transient behaviour where the observed luminescence intensities change with the exposure time of the sample under 325 nm laser beam excitation at 10–300 K. Such an intensity variation is accompanied with a red-shift for YL peak at 10–140 K and one for BL peak at 140 K. We propose that such behaviours are related to the chemical transformations of YL-related CN and CNON defects, and BL-related CN-Hi and CNON-Hi defects during the exposure.
By using a low rotation rate of 200 rpm with additional water cooling, a friction stir welded (FSW) joint with equal fatigue strength to the base material (BM) was obtained in 5083Al-H19 rolled plates due to the significantly decreased softening and the residual stress effect. Though the hardness value in the heat affected zone (HAZ) was a little lower than that of the BM, the decreased tensile effect of the residual stress in the HAZ greatly improved the fatigue performance, which was even higher than that of the BM, so the final fracture occurred at the BM. This study provides an effective strategy to enhance the fatigue performance of the FSW joints.
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