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    Effect of equal-channel angular pressing on microstructure, aging kinetics and impact behavior in a 7075 aluminum alloy
    (Elsevier, 2024) Elibol, Cagatay; Sagir, Kadir; Dogan, Mert
    Among the precipitation hardenable 7xxx series aluminum alloys, AA7075 alloys (one of the most important engineering alloys) due to their low density (lightweight), high toughness and strength, and enhanced fatigue behavior have been used over the years in aerospace, aircraft, automotive, construction and marine applications. In the present study, the influence of the artificial aging through conventional heat treatment (i.e., precipitation hardening) and the thermomechanical treatment (equal-channel angular pressing (ECAP) + post-ECAP aging) on the quasi-static tensile, impact toughness and precipitation behavior as well as on the microstructure of an AA7075 alloy is investigated systematically. The results indicate that the ECAP process as well as post-ECAP aging result in considerably increased strength and hardness of the AA7075 alloys because of the superimposed effects of grain boundary strengthening (Hall-Petch relation), strain hardening (by means of the increased dislocation density) and precipitation hardening (due to the fine precipitates formed in Al matrix). Multi-pass ECAP processing has been found to result in a considerable decrease of the impact toughness (from 15.3 Jcm(-2) to 7.6 Jcm(-2)) associated with the failure mode (i.e., ductile-to-brittle transition) of the AA7075 alloys, whereas the artificial peak aging leads to a marked improvement (similar to 67 %) in the impact toughness in ECAPed conditions. Moreover, the ECAP process noticeably accelerates the precipitation kinetics of AA7075 alloys: The peak aging time in the ECAPed alloy is reduced significantly - from 30 h to 4 h. The results of the present study provide a deeper understanding of the combination of ECAP and heat treatment, and their effect on the fracture mode and toughness under impact loading, the aging kinetics and the microstructural evolution of an AA7075 alloy.
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    Processing and characterization of polymeric biomaterials
    (Elsevier, 2023) Elibol, Cagatay; Güner, Mehmet Buğra; Sagir, Kadir
    Polymeric biomaterials feature a wide range of medical applications due to their high biological suitability (noncytotoxicity, biodegradability, and biocompatibility), enhanced mechanical behavior, and good processability. Besides these characteristics, chemical, electrical, and physical properties can be improved by regulating the structural and morphological parameters during processing of polymeric biomaterials. Because of the high clinical expectations, scientists focus on advanced processing techniques. Furthermore, due to the extremely hard-working conditions of polymeric biomaterials, many studies deal with numerous precise characterization techniques, in order to determine how properly polymeric biomaterials were processed, before being used in clinical applications. Therefore, this chapter is focused on gaining a deeper understanding of processing methods and characterization techniques of polymeric biomaterials for providing an extensive overview and a guide for developing polymeric biomaterials within the scope of clinical framework. © 2023 Elsevier Ltd. All rights reserved.
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    Remineralization potential of a novel varnish: an in vitro evaluation
    (Mre Press, 2024) Gokce, Ayse Nur Parlakyildiz; Kelesoglu, Ergun; Sagir, Kadir; Kargul, Betul
    Despite fluoride's widespread use in preventing dental caries, it remains a significant oral disease with some drawbacks. Consequently, new preventative agents have emerged that can function independently of fluoride. Our aim is to demonstrate the efficacy of newly developed varnishes, 3% Rennou (theobromine calcium and phosphate) and 1% Rennou, in remineralizing initial caries. In our experiment, 40 human enamel samples were randomly allocated into four groups of 10 samples each as: Group 1 (G1): 5% NaF (Colgate PreviDent (R)), Group 2 (G2): 1% Rennou (R), Group 3 (G3): 3% Rennou (R) and Group 4 (G4): Casein Phosphopeptide-Amorphous Calcium Phosphate + Fluoride (MI Varnish (TM) GC). To produce an artificial carious lesion in the enamel, the samples were kept in a demineralizing solution for 72 hours. Samples underwent pH cycling for 6 days in order to induce remineralization. The means of the three measurements were compared, and the percentage of Surface Microhardness Recovery in (SMHR%) was calculated. Scanning Electron Microscopy (SEM) was used for qualitative assessment of surface changes. G1 had the highest SMHR% value, followed by G3, G2 and G4. The One-way ANOVA (Analysis of Variance) showed significant differences in the SMHR% values among the groups after six days of cycling (p < 0.001). In pairwise comparisons, groups did not show differences in means of SMHR% except for G1 and G4 (p = 0.006). In the SEM Images, after treatment within the NaF group, many flaky sediments were found on the enamel surface. Similarly, the maximum mineral gain was seen in the NaF and Rennou groups. SEM images of both varnish surfaces revealed a uniform layer interspersed with shapeless precipitates. All varnishes treated artificial enamel lesions to varying degrees. However, both concentrations of Rennou showed remineralization potential comparable to 5% NaF in acceptable statistical measurements. Thus, it could be used as a potentially effective preventive measure for pediatric patients.