Role of analytical transmission EM in the study of Zr Pressure-tube alloys

Abstract

Zirconium alloys are widely used in the Candu nuclear power generating system because of their unique combination of creep and corrosion resistance properties coupled with a low neutron cross section. The Zr-2.5 wt%Nb alloy has been extensively studied because it is the key component in pressure tubes. Pressure tubes are fabricated by extrusion in the two phase (α + β) field in the binary phase diagram, followed by cold work and stress relief heat treatments. As a result of this treatment, the alloy has a complex microstructure consisting of a duplex α + βzr or α + βNb structure (depending on the particular heat treatment), superimposed on which is a dislocation structure associated with the working operation. Typical microstructures are shown in Figs.l and 2. If the alloy is quenched from the β phase field (1000° C), a Widmanstätten α-βzr microstructure is produced (Fig.la). In the extruded state (Fig.lb), the βZr phase is now elongated and linked by a substructure which contains both <a> and <c> dislocations. Cold working followed a stress relief heat treatment at 400°C for 24 hrs introduces predominantly <a> dislocations, while the βZr phase (which is metastable) has decomposed to form discrete arrays of βNb (Fig-lc). Examples of the dislocation substructure are shown in Fig.2. Boundaries containing both <a> and <c> dislocations are observed. The misorientation at the boundaries is typically a few degrees. They are often stepped or faceted. In Fig.2 the steps correspond to a <c> dislocation which has dissociated to form two <c>/2 dislocations.