Browsing by Author "Turgay T."

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Effect of homovalent Bi/Ga substitution on propagations of flaws, dislocations and crack in Bi-2212 superconducting ceramics: Evaluation of new operable slip systems with substitution
(2019-12-01) Turkoz M.B.; Zalaoglu Y.; Turgay T.; Ozturk O.; Yildirim G.
This study defines a strong methodology between the mechanical performance behaviors and formation of possible operable slip systems in the crystal structure of Bi-2212 superconducting phase with trivalent Bi/Ga substitution with the aid of Vickers hardness tests exerted at various indentation load intervals 0.245 N–2.940 N. It is found that the mechanical performance behaviors improve regularly with the increment in the trivalent Bi/Ga partial substitution level up to the value of x = 0.05 due to the formation of new operable slip systems. Namely, the optimum gallium (Ga) impurities serve as the strain fields and associated forces for the interaction of dislocations within the different orientations with each other to impose the surface residual compressive stresses orienting favorably the superconducting grains. Thus, the propagation of dislocations, flaws and cracks divert in the crystal structure. On this basis, the presence of optimum Ga impurity in the Bi-2212 crystal system strengthens the mechanical strength, critical stress, resistance to the plastic deformation, stiffness and durability nature. Moreover, the experimental results advance in-depth understanding of fundamental links between the porosity and Young's moduli of elasticity founded on the impurity level and applied test loads. It is observed that in case of the optimum level of x = 0.05 the propagation of flaws, dislocations and cracks proceed along the transgranular regions instead of the intergranular regions as a consequence of improvement in the durable tetragonal phase. On the other hand, the excess Ga content level in the polycrystalline Bi-2212 system results in the augmentation in the stress raisers, crack surface energy and crack-initiating flaws, activating the stress-induced phase transformation.
Evaluation of key mechanical design properties and mechanical characteristic features of advanced Bi-2212 ceramic materials with homovalent Bi/Ga partial replacement: Combination of experimental and theoretical approaches
(2019-12-01) Turkoz M.; Zalaoglu Y.; Turgay T.; Ozturk O.; Akkurt B.; Yildirim G.
This study models the variations in the key mechanical design properties and mechanical characteristic features of Ga substituted Bi-site Bi-2212 ceramics prepared within the different molar ratios of x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.100 and 0.300 with the assistant of available theoretical approaches; namely, Meyer's law, proportional sample resistance, elastic/plastic deformation, modified proportional sample resistance model, Hays Kendall and indentation-induced cracking methods for the first time. The mechanical modeling parameters are gathered from the microhardness (Vickers) experimental tests performed at various applied loads interval 0.245 N–2.940 N. The results provide that the key mechanical design features improve systematically with the augmentation of trivalent Bi/Ga partial replacement level up to x = 0.05 due to the rapid decrement in the main structural problems; namely, the grain orientations, lattice strains, distortions, dislocations, grain boundary interaction/coupling problems, crack-initiating and crack-producing omnipresent flaws in the advanced Bi-2212 ceramic system. Accordingly, the optimum Ga inclusions strengthens the mechanical durability towards the applied stress due to the increased stabilization in the durable tetragonal phase. After the critical substitution amount of x = 0.05, the mechanism turns reversely, and the general mechanical characteristic features including the stiffness, mechanical durability and strength degrade remarkably. Additionally, the mechanical modeling results demonstrate that the Bi/Ga impurity leads to vary positively the quality of standard indentation size effect (ISE) feature until x = 0.05, beyond which the excess Ga additives damage seriously ISE feature of Bi-2212 inorganic compounds. Besides, the indentation-induced cracking (IIC) model is noticed as the best method to describe the true microhardness parameters of Bi/Ga substituted Bi-2212 compounds for the mechanical characterization.