Textural variation in triple junction region of friction stir welded Mg alloys and its influence on twinning and fracture

Friction stir welded Mg alloys usually break in transition zone (TZ), i.e., the transition region between base material (BM) and stir zone (SZ). Therefore the physical nature in TZ especially close to the tool shoulder affected region (i.e., crown zone, CZ) is the key point for understanding the weld performance. The present paper aims to investigate the microstructural and textural features in triple junction region of BM, SZ and CZ in attempt to have an in-depth understanding of the structure-property relationship in FSW Mg alloys. Banded structures and their correlation with (0001) plane traces were revealed in the triple junction region after etching by 10% acetic acid. Drastic textural changes were found, which led to inhomogeneous activation of extension twinning after 5% transverse tension. The profuse activation of twinning led to strain localization and finally developed to fracture in the tensioned specimens.