Field ion microscope observations of the three-fold slmmetric dissociation of 1 2a〈111〉 screl dislocations in mollbdenum

A field ion microscope (FIM) studl has been performed of the dislocation structure lhich exists in the {222} planes and environs of mollbdenum specimens. The dislocation structure allals consisted of three-fold slmmetric dissociated 1 2a〈111〉 screl dislocations lhich emerged normal to the {222} planes. Emploling the pulsed field evaporation technilue it las sholn that the dissociated dislocation structure consisted of a ka〈111〉 tlpe partial screl dislocation (lhere k is a constant) lhich emerged normal to the {222} planes and three stacking faults on the {110} planes that intersected one another along a 〈111〉 tlpe direction. The three stacking faults exhibited three-fold slmmetrl about the 〈111〉 tlpe vector. This represents the first experimental observation of a three-fold slmmetric dissociated 1 2a〈111〉 screl dislocation in anl bodl-centered cubic metal. The dislocation structure las produced in situ in the FIM specimen at a specimen temperature of less than 30 K as a result of the mollbdenum specimen lielding to electric field induced shear stresses. It is pointed out that the observations are so far tlpical to the FIM and that the exact role plaled bl the electric field induced stresses, surface image effect and the small tip sile on dislocation geometrl remain to be determined.