Browsing by Author "Subutay, Halit"

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Cyclic Oxidation Behavior and Protective Oxide Scale Formation in Stainless-Steel Alloys for High-Temperature Exhaust Valve Applications
(MDPI, 2025) Cetinkal, Salih Bilal; Atas, Mehmet Sahin; Salur, Emin; Savkliyildiz, Ilyas; Subutay, Halit; Arici, Gokhan; Alhazaa, Abdulaziz
As internal combustion engine (ICE) systems are increasingly exposed to severe thermal and oxidative environments, the oxidation resistance and structural integrity of exhaust valve materials have become critical for maintaining long-term engine reliability and efficiency. This study presents a comparative evaluation of the cyclic oxidation behavior of two candidate valve steels, 1.4718 (ferritic stainless steel) and 1.4871 (austenitic stainless steel), under service-temperature conditions. The specimens were exposed to repeated oxidation at 550 degrees C, 650 degrees C and 750 degrees C for 25 cycles in ambient air. The surface and cross-sectional morphologies of the oxide layers were analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to investigate oxide scale composition, thickness, and growth characteristics. The oxidation behavior of both alloys proceeded in two distinct stages: an initial phase marked by accelerated oxidation, followed by a slower, more stable growth period. The extent of oxidation intensified with increasing temperature. The 1.4718 alloy developed relatively porous but compositionally stable oxide layers consisting primarily of Fe- and Cr-based spinels such as FeCr2O4 and Cr2SiO4. In contrast, the 1.4871 alloy formed a dense, adherent, dual-layered oxide scale composed of an outer Mn2O3-rich layer and an inner Cr2O3-rich layer, attributable to its high Mn and Cr content. The results underscore the critical influence of elemental composition, particularly Cr, Mn and Si, on oxide scale stability and spallation resistance, demonstrating the superior cyclic oxidation resistance of the 1.4871 alloy and its potential suitability for exhaust valve applications in thermally aggressive environments.
Citation - WoS: 8
Citation - Scopus: 8
Effect of High-Energy Ball Milling in Ternary Material System of (mg-Sn
(MDPI, 2023) Subutay, Halit; Şavklıyıldız, İlyas
In this study, the nature of the ball-milling mechanism in a ternary materials system (Mg-6Sn-1Na) is investigated for proper mechanical alloying. An identical powder mixture for this material system is exposed to different milling durations for a suitable mixture. First, the platelet structure formation is observed on particles with increasing milling duration, mainly formed in <200> direction of the hexagonal crystal structure of the Mg matrix. Then, the flake structure with texture formation is broken into smaller spherical particles with further ball milling up to 12 h. According to EDS analysis, the secondary phases in the Mg matrix are homogenously distributed with a 12-h milling duration which advises a proper mixture in this material system. The solid solution formation is triggered with an 8-h milling duration according to XRD analysis on 101 reflections. Conventional sintering is performed at 350 ? in 2 h for each sample. In bulk samples, XRD data reveal that secondary phases (Mg2Sn) with island-like structures are formed on the Mg matrix for a milling duration of up to 8 h. These bigger secondary phases are mainly constituted as Mg2Sn intermetallic forms, which have a negative effect on physical and mechanical properties due to a mismatch in the grain boundary formation. However, the homogenous distribution of secondary phases with a smaller particle size distribution, acquired with 12 h milling time, provides the highest density, modulus of elasticity, and hardness values for this ternary materials system. The ternary materials produced with the 12-h ball-milling process provide an improvement of about 117% in hardness value compared with the cast form.
Ultrahigh Antibacterial Response and Biochemical Activity in Mg-Sn-HA Material Systems
(Elsevier Sci Ltd, 2025) Subutay, Halit; Gunes, Eda; Erci, Fatih; Acarer, Mustafa; Salur, Emin; Arici, Gokhan; Savkliyildiz, Ilyas
The effects of metallic tin(Sn) and hydroxyapatite(HA) ceramic particles on metallic magnesium's mechanical properties and antibacterial (S.aureus) response along with bioactivity (toxicity) against D.melanogaster larvae, including the impact on survival, development, sex, longevity, were studied. Three different variations of self-assembled Mg-based materials were synthesized by mechanical alloying and densification of the samples was accomplished by hot press sintering. SEM analysis reveals that the smallest particle morphology was obtained in the Mg-3Sn alloy wherein excessive plastic deformation resulted in a monodisperse particle distribution, enabling uniform distribution of reinforcing elements. The most significant gain in mechanical properties was observed in the Mg-Sn system in which the formation of the Mg2Sn intermetallic results Brinell hardness of 184, corresponding similar to 500 % increase relative to pure Mg. The intermetallic Mg2Sn phase and its high-volume fraction in the Mg matrix leads hardening by the Orowan mechanism. In all treatment groups, the overall survival rate is >= 80 %, which shows that the produced alloys and composites are not neither toxic nor lethal to the model organisms. According to the survival rate and development time data, it was determined that the sex of the organisms shifted in favor of the first group (males) and the second group (females), while the oxidative stress (OSI) increased in organisms in contact with the first group (males). The Mg-HA and Mg-Sn-HA systems exhibit superb antibacterial properties, showing complete inhibition of S.aureus after 24-h incubation period. Overall, such alloys could have a significant impact on a range of clinical and biomedical applications because of their outstanding antibacterial properties as demonstrated in this study.