Etude metallographique des filaments structuraux (“structural stringers”) dans le zircaloy-2

Abstract

The new technique of localised electrolytic polishing, “Ellopol”, has been applied to the study of the stringers which have often been described in papers devoted to zircaloy-2. These defects have been detected exclusively in forged billets and rolled sheets made from ingots which had been cast from alloy melted in consumable arc furnaces in an argon atmosphere. Products made from ingots produced by vacuum melting do not contain stringers. The variation of the micrographie appearance with the polishing conditions (primarily the nature of the electrolyte), and the influence of anodic oxidation tend to show that, contrary to a widely held opinion, structural stringers are not a form of porosity inherited from the ingot. They are associated with chemical heterogeneities (an unidentified intermetallic constituent, oxides) resulting from the thermal conditions of melting and from impurities in the atmosphere. A confusion between physical discontinuities and correspondingly shaped features of the microstructure can arise from the corrosive action of the etching reagents on mechanically polished surfaces. Comparison of the structure of billets and sheets, X-ray analysis and the results of observations by others permit an interpretation of the effects of hot rolling. The appearance of macroscopic defects results from the inflation, by oxygen, of filaments at the surface. Moreover, the insolubility of the relevent oxides accounts for the known fact that a prolonged treatment at high temperature can eliminate the concentration gradients responsible for so-called corrosion-stringers, while leaving unaffected those filaments which are formed by oxide segregation.