ATOMISTIC SIMULATION OF IDEAL SHEAR-STRENGTH, POINT-DEFECTS, AND SCREW DISLOCATIONS IN BCC TRANSITION-METALS - MO AS A PROTOTYPE

Using multi-ion interatomic potentials derived from first-principles g eneralized pseudopotential theory, we have studied ideal shear strengt h, point defects, and screw dislocations in the prototype bcc transiti on metal molybdenum (Mo). Many-body angular forces, which are importan t to the structural and mechanical properties of such central transiti on metals with partially filled d bands, are accounted for in the pres ent theory through explicit three- and four-ion potentials. For the id eal shear strength of Mo, our computed results agree well with those p redicted by full electronic-structure calculations. For point defects in Mo, our calculated vacancy-formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations have also been investigated. The [110 ] split dumbbell interstitial is found to have the lowest formation en ergy, in agreement with the configuration found by x-ray diffuse scatt ering measurements. In ascending order, the sequence of energetically stable interstitials is predicted to be [110] split dumbbell, crowdion , [111] split dumbbell, tetrahedral site, [001] split dumbbell, and oc tahedral site. In addition, the migration paths for the [110] dumbbell self-interstitial have been studied. The migration energies are found to be 3-15 times higher than previous theoretical estimates obtained using simple radial-force Finnis-Sinclair potentials. Finally, the ato mic structure and energetics of [lll] screw dislocations in Mo have be en investigated. We have found that the so-called ''easy'' core config uration has a lower formation energy than the ''hard'' one, consistent with previous theoretical studies. The former has a distinctive three fold symmetry with a spread out of the dislocation core along the [112 ] directions, an effect which is driven by the strong angular forces p resent in these metals.