Effect of Residual Strain Induced by Mg2Si on the Nucleation Density of MgH2 in As-Cast Mg-Si Alloys

During hydrogenation of Mg, MgH2 forms not only as a surface layer, MgH2(sur), but also internally within the unreacted Mg core, denoted MgH2(int). The preferential formation of MgH2(int) along grain boundaries suggests that defects act as heterogeneous nucleation sites. Since severe plastic deformation is widely used to introduce strain fields for improving hydrogenation kinetics, Mg2Si precipitates in Mg-Si alloys were employed in this study to investigate the role of thermally induced strain fields in MgH2(int) formation. Mg2Si precipitates comprising anisotropic elongated segments were formed in the Mg matrix, and an analysis based on Eshelby’s ellipsoidal inclusion theory revealed strong compressive residual strain within the elongated Mg2Si segments, particularly along their longitudinal direction. The calculated hydrostatic residual stress was −373 MPa, which exceeded the elastic limit of the Mg matrix, while a compressive residual stress of 116 MPa was experimentally measured by XRD, indicating that considerable stress relaxation occurred in the surrounding matrix during cooling. After hydrogenation, MgH2(int) nucleated preferentially at the Mg2Si interfaces, and its number density increased with the Mg2Si interface length density. These findings indicated that thermally induced strain fields promote the nucleation of MgH2 providing a mechanistic basis for contribution of strain introducing processes to hydrogenation kinetics.