ATOMIC-STRUCTURE OF THE SIGMA=5, (210)TILT AND (310)TILT, [001]TILT AXIS GRAIN-BOUNDARIES IN MO - A JOINT STUDY BY COMPUTER-SIMULATION AND HIGH-RESOLUTION ELECTRON-MICROSCOPY

We present the results of a parallel study of the atomic structure of the Sigma = 5, (210), theta = 53.13 degrees and (310), theta = 36.87 d egrees, [001] tilt axis grain boundaries in bcc Mo, by computer simula tion and high-resolution electron microscopy (HREM). Excess energy val ues of different boundary configurations are obtained via a quasidynam ical minimization scheme while cohesion is described by a new n-body c entral-force, phenomenological potential which satisfactorily reproduc es static and dynamical properties of the bulk material. HREM observat ions and numerical modelling both show the symmetric configuration of the Sigma = 5, (210), theta = 53.13 degrees boundary to be of the lowe st energy. The calculations also show that the stable Sigma = 5, (310) , theta = 36.87 degrees configuration is nearly mirror symmetric, alth ough the experimental verification in this case is still incomplete. A fter a few oscillations, the interplanar spacing near the boundaries c onverges rapidly to its bulk value while producing an overall expansio n of the bicrystal. Qualitative agreement is found between calculated and experimentally determined expansion values. The present results co ntradict the conclusions of earlier work according to which phenomenol ogical n-body central-force potentials would not be able to provide a satisfactory description of the grain-boundary structure in bcc transi tion metals.