The beneficial use of composite samples in the energy-dispersive X-ray analysis of highly radioactive Nimonic PE16

Abstract

A new technique for preparing 3 mm diameter disc samples for TEM is presented. It permits quantitative energy- dispersive X-ray (EDX) analysis of radioactive materials where otherwise high sample activity causes saturation of the EDX detector. The new composite sample geometry, comprising a small disc of radioactive material within an annulus of compatible unirradiated material, has up to ∼ 50 × reduced activity. Similar sample geometries may be used to minimize the mass of magnetic material in high-resolution TEM examinations or when samples from small diameter wires are required. Composite samples have been prepared from a Commercial Nimonic PE16 alloy containing 0.1 wt% Co after irradiation at 450° C under thermal reactor conditions. High-resolution EDX analysis in a field-emission gun STEM reveals significant irradiation-induced intergranular enrichment of nickel, silicon and phosphorus and depletion of chromium and iron. The observed changes are asymmetric with respect to the grain-boundary interface and extend over distances of 10 to 20 nm. The asymmetry may be explained by the presence of intergranular γ′ phase of mixed thermal and irradiation-induced origin. A coincident irradiation-induced deterioration in stress-rupture properties may be associated with intergranular γ′ and phosphorus. Future investigations should allow irradiation-induced changes in the mechanical properties of PE16 to be described in terms of coincident changes in grain-boundary microchemistry.