Effect of Process Parameters on Deformation of Zr-2.5wt%Nb Alloy
Cochrane, Christopher James
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Zirconium and its alloys are used extensively in the nuclear industry. In the Canadian Deuterium Uranium reactor, the primary containment in the primary coolant system is composed of Zr-2.5wt%Nb in the form of a pressure tube. The permissible chemical composition of Zr-2.5wt%Nb for use in the pressure tube in nuclear reactors is dictated by ASTMB353. Oxygen and iron are the highest content controlled elements in the standard alloy, after zirconium and niobium. Oxygen is an alpha-stabilizer, and diffuses preferentially to the alpha phase, leading to a well established increase in the yield strength of the alpha phase. Iron is a beta-stabilizer, and is concentrated in the beta phase, as well as near alpha-beta grain boundaries. While the mechanical properties of standard Zr-2.5wt%Nb alloy are well understood, there is a dearth of knowledge on the individual effect of these alloying additions, especially at non-standard concentrations. Additionally, the experimental evidence that does exist does not directly take into account the two-phase nature of the alloy, or the effect of impurities on specific deformation modes. Notably absent is experimental evidence on the effect of interstitial impurities on twinning in the hexagonal close-packed alpha phase. This work seeks to complement the present understanding of these phenomena. Mechanical tests have been performed on three specially prepared Zr-2.5wt%Nb alloys to clarify the contributions of oxygen and iron to Zr-2.5wt%Nb deformation properties. Traditional mechanical measurements were complemented by in situ and ex situ diffraction measurements. Tests were performed at a range of temperatures (77K - 673K) and strain rates (quasi-static to 10^-2/s). Increasing oxygen content from 1176wppm to 3300wppm increases the macroscopic yield stress at room temperature, and results in a transition in work hardening behaviour at low strain rates. Increasing iron content from 547wppm to 1080wppm has no effect on the macroscopic yield stress, but increases the work hardening rate at room temperature.