Interaction between hydrogen and solute atoms in {101¯2} twin boundary and its impact on boundary cohesion in magnesium

Hydrogen is a common impurity or artificially added element in magnesium alloys and is generally known to assist crack propagation along grain boundaries. However, how hydrogen atoms interact with atoms of other alloying elements segregated to grain boundaries and its impact on grain boundary cohesion of Mg alloys are still unclear. In this work, interactions between hydrogen and each one of the 18 alloying elements commonly used in Mg alloys in {101¯2}twin boundary, and their impacts on interfacial cohesion of the {101¯2} twin boundary are investigated using first-principles calculations. It is found that hydrogen is unlikely to bond with Al, Sn, Bi, Zn, or Ag, while H prefers to bond with solute such as Li, Ca, Mn, Zr, Y, La, Pr, Nd, Sm, Gd, Tb, Dy, or Ho in the Mg lattice and in the twin boundary. As a result, hydrogen is unlikely to segregate to a {101¯2} twin boundary that has the presence of Al, Bi, Zn, or Ag, while H atoms will be attracted to a twin boundary to form a hydrogen-solute cluster around the solute such as Li, Ca, Mn, Zr, Y, or Nd that is already present in the twin boundary. The strengthening or weakening effects of segregated solute atoms and their interactions with hydrogen atoms on the twin boundary cohesion are found to be closely related to the electronic interactions of atoms of Mg with those of the segregated solutes. Mn, Zr, Y, or Nd can significantly enhance the twin boundary cohesion, in the absence of any hydrogen segregation in the boundary, because of the strong interactions between the d electrons of atoms of the solute element and the p electrons of Mg atoms. However, electron transfer from the solute atoms to hydrogen atoms will reduce the strengthening effect, making the boundary less resistant to fracture. This work uncovers the hydrogen-induced interfacial fracture from the perspective of the interactions of hydrogen and alloying elements, which is important for understanding hydrogen embrittlement in Mg alloys.