Micromechanical testing of oxidised grain boundaries in Ni Alloy 600
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Micromechanical testing of oxidised grain boundaries in Ni Alloy 600 Alisa Stratulat1 and Steve G. Roberts1 1 Department of Materials, University of Oxford, UK ABSTRACT Micromechanical testing of focused ion beam (FIB) machined cantilevers was used to study oxidised grain boundaries in Ni-alloy 600. The Ni-alloy 600 samples were exposed in simulated PWR primary water at 325°C for 4500h with a hydrogen partial pressure of 30kPa. The FIB was used to machine small cantilever beams at the selected sites in the Ni alloy 600, cut so that the beam contained a selected grain boundary close to the built-in end. The FIB was also used to make a pre-crack, 700 nm deep, on the grain boundary. Cantilevers were loaded at the free end using a nanoindenter. Cantilevers milled in the un-oxidised sample yielded, and did not fracture. The specimens containing oxidised grain boundaries fractured at the boundary after small amounts of plasticity. Load vs. displacement data were used to calculate the fracture toughness of the oxidised grain boundaries. The fracture toughness associated with fracture of grain boundary oxide for these cantilevers was in the range 0.73-1.82MPa (m)1/2, with an average value of 1.3MPa (m)1/2. We believe this to be the first time the fracture toughness of an oxidised grain boundary has been determined. INTRODUCTION Stress Corrosion Cracking (SCC) involves a highly complex interplay of diffusional, chemical and mechanical factors in a series of related mechanisms, and affects material performance in a wide range of materials systems and environments. Within the nuclear industry where safety and structural integrity are crucial concerns SCC is of great significance; in particular in some Ni based alloys, and in austenitic stainless steels. In such systems SCC is predominantly along grain boundary paths. Alloy 600 is a solution strengthening alloy of composition 75% Ni, 15% Cr and 7%Fe [1]. Ni Alloy 600 mainly fails intergranularly in the pressurized water reactor (PWR) environment [2] in the steam generator tubes [3-4]. One of the causes of intergranular cracking is the degradation of grain boundaries by oxidation. This study will describe a method recently developed to determine the fracture toughness of individual oxidised grain boundaries in Ni Alloy 600, following the work of Di Maio and Roberts on microcantilever fracture of silicon and WC coatings [5], and that of Armstrong, Wilkinson and Roberts on fracture of bismuth embrittled grain boundaries in copper [6]. EXPERIMENTAL DETAILS Ni Alloy 600 samples provided by AREVA were first polished down to a 1μm diamond finish and then exposed in PWR primary water at 325o C for 4500h with a hydrogen partial pressure of 30kPa. Lastly, the samples were polished with colloidal silica, which also revealed the intergranular oxide. Oxidised grain boundaries were examined by cutting FIB trenches and the ones perpendicular to the surface were selected for cantilever production. FIB was used to machine small, pentagonal cross-section cantilevers (5μm wide by 25μm long), cut
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