Nano-scale Mechanical Properties and Microstructure of Irradiated X-750 Ni-Based Superalloy
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RADIATION damage in materials is a crucial concern in advanced and conventional nuclear reactors, spallation sources, isotope production facilities, and future fusion technology applications. Mechanical properties degradation and environmental interactions are key issues during lifetime extension or performance predictions for these types of systems. Thankfully, the effect of irradiation on the mechanical properties of structural materials in nuclear reactors has been the subject of intense studies over the past few decades.[1–4] Such investigations have been inaugural to the worldwide effort to produce materials with greater resistance to the environmental effect of nuclear reactors for future designs. For this purpose, a deep understanding
P. CHANGIZIAN, A. BROOKS, Z. YAO, and M.R. DAYMOND are with the Department of Mechanical and Materials Engineering, Queen’s University, Kingston K7L3N6, ON, Canada. Contact e-mail: [email protected] Manuscript submitted August 29, 2017. Article published online December 21, 2017 498—VOLUME 49A, FEBRUARY 2018
of the failure mechanisms through investigation of post-irradiation mechanical properties and microstructural changes in structural materials is of importance. Of our particular interest is the influence of radiation damage on Ni-based superalloys, such as Inconel X-750 which is used as a critical component spacer material in the CANada Deuterium Uranium (CANDU) heavy water reactor.[5] Post-irradiation mechanical examinations on X-750 spacers removed from CANDU reactors have revealed that they exhibit a reduction in ductility, which depends on operation temperature, as well as lower load carrying capacity compared to the as-installed condition.[6] Although the underlying mechanism leading to the mechanical properties degradation is not yet fully understood, some studies have been carried out on the microstructural changes of ex-service (neutron-irradiated) spacers.[5,7,8] Furthermore, some studies have tried to emulate the effect of neutron irradiation on the microstructure of X-750 alloy by means of employing heavy ion irradiation under controlled irradiation conditions.[9–11] The results show that irradiation may affect the microstructure in different ways depending on irradiation temperature, irradiation dose, and dose rate. METALLURGICAL AND MATERIALS TRANSACTIONS A
X-750 is an age-hardened Ni-based superalloy strengthened by precipitation of the c¢-Ni3 (Ti, Al) phase with an L12-ordered structure.[12] However, the strengthening precipitate is susceptible to phase instability under irradiation environments. Zhang et al.[10] investigated the stability of the c¢-phase in X-750 via in situ TEM irradiation at different temperatures using heavy ion irradiation. Their results clearly reveal that under irradiation at lower temperatures, the ordered precipitates become disordered at very low doses (~ 0.06 dpa), and also start to dissolve into the matrix at higher doses (~ 5 dpa). However, above a critical temperature which was between 400 and 500 C at the dose rate used, the
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