The influence of chemical composition optimisation of advanced high-strength steel on liquid metal embrittlement phenomenon

This study focuses on the element effects of Al and Si on the liquid metal embrittlement (LME) phenomenon. Four series of chemical compositions have been chosen for this research, which are 20% higher Al DH780, 20% higher Si DH780, 20% higher Al DH980, and 20% higher Si DH980. The total LME crack length of the welding spot of each state sample has been statistically analysed. The high Al DH780 sample shows the lowest total crack length, whereas the high Si DH980 sample shows the highest total crack length. Then the semi-in situ tensile tests have also been employed to evaluate the mechanical properties of these four different chemical composition samples. The high Al sample shows less LME cracks and higher elongation compared with the high Si sample. The shoulder position transmission electron microscopy (TEM) sample has been cut by the focused ion beam. It could be seen from the TEM observation that the distribution of Si element is close to that of Zn. The Si and Al solute atoms would diffuse towards grain boundaries to form solute-rich clusters in the manner of solute-vacancies loops. The stronger Zn-Si adhesion bonding attracts Zn infiltrating deeper into substrate. The Zn-rich intermetallic phase nucleation on grain boundaries would further worsen the LME sensitivity. The enrichment of Al content would repel Zn on grain boundaries and also stabilise the Fe-Zn intermetallic phase. High Al could also reduce the austenite amount in steel. Thus, it mitigates the LME response of steel. The LME phenomenon of Zn would be severe for the higher Si sample.