The acceleration of climb-controlled creep by neutron irradiation

Abstract

This study reports two main mechanisms on the YBa2Cu3O7 − x bulk superconductor for the researchers: (I) determination of the diffusion coefficient and activation energy of Zr nanoparticles in the YBa2Cu3O7 − x matrix over the annealing temperature range of 500–945 °C by means of the energy dispersive X-ray fluorescence (EDXRF) technique, and (II) change of the mechanical performances belonging to the YBa2Cu3O7 − x bulk superconducting materials by Zr inclusions with the aid of Vickers microhardness (Hv) measurements performed at various indentation loads (0.245 N ≤ F ≤ 2.940 N) for the first time. The results obtained from the first part show that the zirconium diffusion coefficient increases dramatically with the enhancement of the diffusion annealing temperature, presenting that at higher temperatures more Zr nanoparticles begin to penetrate into the Y123 matrix. Thus, two different diffusion coefficients (in grains and over grain boundaries) are determined for the Zr individuals in the Y123 materials. The thickness dependences of the Zr diffusion coefficients are defined by the equations D1 = 1.47 × 10− 4 exp (− 1.05 eV/kBT) and D2 = 2.23 × 10− 3 exp (− 1.03 eV/kBT) in grains and over grain boundaries, respectively. The related activation energies (− 1.05 eV and − 1.03 eV) may be attributed to the relatively slow migration in the former region and rapid migration over the latter region, respectively. In other words, the physical and mechanical properties of the Y123 superconducting samples improve with the Zr additives owing to the elimination of the defects. The second part also indicates that the Zr nanoparticles lead to change in the microhardness characteristic of the Y123 bulk superconductors. Whereas the microhardness parameters such as Vickers microhardness (Hv), elastic modulus and yield strength of the undiffused sample tend to increase systematically, those of the diffused samples decrease with the enhancement of the applied load, meaning that the former sample shows the reverse indentation size effect (RISE) behavior while the latter superconductors exhibit Indentation Size Effect (ISE) feature. Additionally, in the second part, the load independent (true) microhardness values are analyzed by Meyer's law, elastic/plastic deformation (EPD), proportional sample resistance (PSR), indentation-induced cracking (IIC) and Hays–Kendall (HK) approach. It is found that the three models (Meyer's law, EPD and PSR) fail to explain the evaluation of the microhardness with the applied load. However, the IIC model is found to be superior to other models for the pure sample exhibiting the RISE behavior, whereas the HK approach is observed to be the most successful model for the description of the mechanical properties of the Zr diffused Y123 bulk superconductors obeying ISE feature. To sum up, it is not wrong to generalize that the IIC model is the best approach for the Y123 superconductor presenting the RISE feature, while the HK approach defines perfectly the mechanical properties of the Y123 materials obtaining the ISE behavior.