Micromechanics of plasticity of Zr-2.5%Nb alloy
Abstract
Zirconium alloys are used in many thermal neutron fission reactors because of their very low neutron interaction cross-sections along with their excellent mechanical strength and corrosion resistance. Numerous experiments have been conducted on these alloys at the bulk scale to determine their mechanical properties. Recent advancements in micro-mechanical testing make it easier to study site-specific mechanical properties for micron or submicron-sized samples. Understanding the mechanical response of such small volumes of material is extremely important for developing accurate models to predict the ductile fracture toughness of these in-reactor materials. Pressure tubes made of Zr-2.5%Nb alloys are an integral part of the CANDU reactors as they carry the fissile uranium fuel bundles and also transport the heavy water coolant to the core. During services in the reactor, the insides surface of these tubes can develop micron-sized scratches due to repeated sliding motion of the fuel bundles. In order to properly assess the impact of these flaws, in the long run, a good understanding of localized plastic deformation in very small regions near these surface scratches is important. Uniaxial compression tests were performed on 1μm diameter micro-pillars fabricated from the axial normal (AN) and transverse normal (TN) planes of an extruded and 22% cold-drawn Zr-2.5%Nb CANDU pressure tube to assess the effect of crystallographic orientations, α/β interfaces, irradiation temperature, and deformation temperature on the mechanical anisotropy and active plastic deformation mechanisms. Some of the micro-pillars were implanted with 8.5MeV Zr+ ions at room temperature and 300°C to simulate the effect of neutron irradiation. For the non-implanted αZr pillars compressed at 25°C the flow stress at 10% strain, was significantly higher than that reported for larger diameter polycrystalline Zr-2.5%Nb pillars demonstrating the length-scale dependence of the mechanical strength of this material. The increasing of the Zr+ implantation temperature to 300°C results in reduced strength and onset of brittle cracking of the micropillars. This is attributed to the effect of concurrent thermal recovery of the implantation-induced crystal damage at 300°C on facilitating strain localization. The mechanical anisotropy of the micro-pillars was reduced as a result of increased implantation and test temperature. Strain hardening exponent n varied between 0.2-0.3 for all test conditions. Increased serrations were observed in the stress-strain response of micropillars that were implanted at a higher temperature (300°C). Resolved shear stress (RSS) for pillars oriented with the loading direction at various angles of misorientation relative to the basal plane normal was plotted and it was observed that the values follow a trend predicted by a mathematical/computational model involving deformation by simultaneous dislocation glide and twinning. Presence of α/β interface resulted in lowering of compressive strength for high temperature implanted and test samples. These observations and the acquired test data reported in this thesis are of significant benefit to the nuclear industry in performing accurate, ductile fracture toughness based life-time assessments of Zr-2.5% Nb pressure tubes currently operating in CANDU reactors.