Transmission electron microscopy observation of interstitial clusters in neutronirradiated niobium

Abstract

Irradiation of moderately pure niobium with fast neutrons to a fluence of 8 × 10¹⁹ n/cm² (E > 0.1 MeV) at 80°C was shown to produce large defect clusters (LDC) of interstitia in a previous paper. The present report deals with the geometry and dislocation characteristics of the LDC analyzed by TEM techniques. Experimental procedures are described for the determination of the habit plane and the Burgers vector of the LDC under the circumstances that the subclusters inside the LDC cannot be resolved under dynamical imaging conditions. The LDC display an overall habit plane of {111}, and their Burgers vectors probably involve more than one a/2(111) or one a/2(110). Evidence is presented to indicate that the LDC are neither fully-developed rafts nor defect complexes of impurity atoms. A model is proposed for the LDC, and the formation mechanism is suggested to include the following four processes: (I) the formation of dünterstitials on the (110) planes, (2) the collapse of interstitial clusters to produce dislocation loops, (3) the conversion of loops from a/2(110) partial to a/2(111) perfect dislocations and (4) the rotation of a/2(111) loops from the (110) planes to the (111) planes.