Twinning pathway in BCC molybdenum

The(2 (11) over bar <111) twinning energy landscape of BCC Mo is determined using the density functional theory for embryos containing 2 to 7 layers. The 2- layer embryo is metastable, whereas the 3- and 4- layer ones are unstable. Layer- by- layer growth starts at 5 layers. The twin boundary formation and migration energies are found to be 607mJ/m(2) and 40mJ/m(2), respectively, indicating that twin partial dislocations have wide cores and high mobilities. The stress to homogeneously nucleate an additional partial loop on the boundary of a sufficiently thick twin is only 1.4 GPa; this implies that once a deformation twin reaches critical thickness, which we estimate to be 6 layers, subsequent growth in thickness is easy.