[SY-E1] The connection between ideal strengths and deformation mechanisms in BCC Refractory Metals
An important theoretical criterion to evaluate the ductility of body-centered cubic (bcc) refractory metals is the mechanical failure mode of their perfect crystals under tension along [100] directions. When the tensile stress reaches the ideal tensile strength, a perfect crystal of a group-6 element (Mo or W) fails by a cleavage fracture along (100) plane so that it is intrinsically brittle, but a perfect crystal of a group-5 element (V, Nb or Ta) fails by a shear deformation along certain slip plane so that it is intrinsically ductile. We have applied first-principles calculations and linear elastic fracture mechanics to find the alloying strategy to change their intrinsic ductility/brittleness. However, how these ideal strength properties affect the realistic deformation and fracture mechanisms of these refractory alloys are still unclear. Thus, we construct and find different modified embedded atom method (MEAM) interatomic potentials, which can duplicate the ideal strength behavior of those bcc refractory metals under multiple deformation modes. Then we apply atomistic simulations based on these interatomic potentials to investigate the dislocation and fracture behaviors near the crack tips for refractory metals with different ideal tensile strength properties, such as Mo and Nb. The results indeed show that the competitions between dislocation activities and fracture propagations in different refractory metals indeed are controlled by their ideal strength behavior in the corresponding perfect crystals. These results bring us new physical insights on the ductility-brittle mechanisms of bcc refractory metals under extreme stress conditions.