Chris Grovenor

© 2019 Oxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.

© 2019 Acta Materialia Inc. Displacive transformations have been widely reported in metals, alloys and ceramics, but rarely reported to be important in the aqueous corrosion of alloys. We report here our analysis of the formation of the hexagonal-ZrO suboxide during the aqueous corrosion of α-Zr alloys and propose this to be a paraequilibrium displacive transformation with the rate controlled by oxygen diffusion. Two orientation relationships were identified between α-Zr and hexagonal-ZrO, (0002)α−Zr∥(1¯011)h−ZrO and [2¯110]α−Zr∥[101¯2]h−ZrO or (0002)α−Zr∥(224¯1¯)h−ZrO and [2¯110]α−Zr∥[11¯01]h−ZrO, with the first one more commonly observed. No specific orientation relationships between either hexagonal-ZrO and monoclinic-ZrO2 or α-Zr and monoclinic-ZrO2 were identified, which suggests that the formation of often-reported bulk oxide texture during aqueous corrosion is not related directly to the texture of the metallic substrate. These results provide a guideline for understanding the mechanisms of crystallographic evolution during oxide growth on commercial zirconium alloys, and also demonstrate the capability of transmission Kikuchi diffraction to investigate orientation relationships in nano-scale materials.

© 2019 We have used (S)TEM, TKD, NanoSIMS and APT to study nano-porosity in the oxide grown on deuterated Zr-1.0Nb and Zr-2.5Nb alloys. A detailed analysis of “barrier layer” nano-porosity by TEM and TKD has revealed that the oxide grain structure is much more disorganised and the nano-porosity network better developed in the rapidly oxidising post-transition alloy. Direct observations of the trapped deuterium (D) distributions from NanoSIMS analysis also shows much more penetration of the oxide layer post-transition. APT analysis shows that there is Fe and D segregation to some of the oxide grain boundaries with occasional evidence of porosity containing trapped D and H. We conclude that interconnected porosity would offer a dominant pathway for the transport of hydrogenic species to the metal substrate during the aqueous corrosion of zirconium alloys in service.

© 2019 Published under licence by IOP Publishing Ltd. High magnetic field gradients can be used for various medical applications including magnetically targeted drug delivery, magnetic cell separation and controlled local heating for the ablation of tumours. These processes involve the use of biocompatible magnetic nanoparticles directed to the area of interest by the use of a field gradient. The force on the nanoparticle is proportional to the field gradient product, so high fields are required for effective delivery. Bulk superconductors are an attractive solution for both drug delivery and the next generation of low cost magnetic resonance imaging magnets. In particular, MgB2 is seen as an attractive material due to its low cost, simple processing and relatively high transition temperature (∼39 K). This paper describes the development of a breadboard compact delivery system suitable for medical applications. This incorporates a cryogenic stage which utilises long life space-proven technology and state of the art ex-situ processed MgB2 pellets.