Dislocations near elastic instability in high-pressure body-centered-cubic magnesium

At high pressure, Mg is expected to transform to the body-centered-cubic (BCC) phase. We use density functional theory to explore the structure of $\ensuremath{\langle}111\ensuremath{\rangle}$-type dislocation cores in BCC Mg as a function of pressure. As the pressure is reduced from the region of absolute stability for the BCC phase, the dislocation cores spread. When dislocation cores overlap the displacements of columns of atoms resemble the nanodisturbances observed in TiNb alloys known as gum metal. As the pressure is lowered further, these regions transform into the hexagonal close-packed phase. The ideal tensile strength of BCC Mg is also computed as a function of pressure. Despite its low shear modulus, BCC Mg is predicted to be intrinsically brittle at absolute zero.