Researcher: Şerefoğlu, Melis
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Şerefoğlu, Melis
Şerefoğlu Kaya, Melis
Şerefoğlu Kaya, Melis
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Publication Metadata only Directional solidificaiton experiments in materials science laboratory of the international space station(Milli Savunma Üniversitesi Hava Harp Okulu, 2018) Department of Mechanical Engineering; Department of Mechanical Engineering; Şerefoğlu, Melis; Researcher; College of Engineering; 44888Solidification is taking place in industrial applications ranging from casting, welding, soldering to additive manufacturing. Even for materials that are shaped by plastic deformation or machining, the ingotor workpieceis generally produced by casting. Although application of these processesmay modifysolidification microstructure, theinitial cast structure still plays a deterministicrolein chemical and morphological distribution in the material. Material properties are highly dependent on these distributions as well as the microstructural featuresof the material. Therefore, investigatingmicrostructure evolution during solidification is crucial. on Earth, duetoconvection in the liquid during solidification of alloys,two-dimensional (2D)samples, i.e. thicknesses on the order of μm, must be used to avoid convection and obtain quantitativeresults. However, the wall thicknessesof cast componentsusedin industry are at least mm-thick. In order to understand the fundamentals of the solidification processin three-dimensional (3D)samples, and examine the direct effect of experimental parameters on microstructure selection, experiments wereperformed in microgravity conditions. Details of these microgravity experiments performed in the International Space Station as well as the ones performed on Earth in the Microstructure Evolution Laboratory are given in this article. Additionally, objectives of both μg and 1g experiments are summarized. / Katılaşma, döküm, kaynak, lehimleme ve katmanlı imalatgibi endüstriyel uygulamalarda gerçekleşmektedir. Plastik deformasyon veya işleme ile şekillendirilen malzemeler için bile, külçe veya iş parçası genellikle döküm ile üretilir. Bu işlemlerin uygulanması,katılaşma mikroyapısını değiştirebilmesine rağmendöküm sırasında elde edilen yapı,malzemedeki kimyasal ve morfolojik dağılımda belirleyici bir rol oynamaktadır. Malzeme özellikleri, malzemenin mikroyapısal niteliklerininyanı sıra bu dağılımlara oldukça bağımlıdır. Bu nedenle, katılaşma sırasında gerçekleşen mikroyapı evriminin araştırılması çok önemlidir. Yeryüzünde yapılan deneylerde, katılaştırma sırasında sıvıda oluşan konveksiyonu engellemek ve kantitatif sonuçlar elde edebilmek için, iki boyutlu (2B), yani kalınlığı μm mertebesinde olan numunelerkullanılmalıdır. Fakat, endüstride kullanılan döküm parçalarının duvar kalınlığı en az mm seviyelerindedir. Üç boyutlu (3B)numunelerde katılaşma sürecinin temellerini anlamak ve deneysel parametrelerin mikroyapı seçimi üzerindeki doğrudan etkisini incelemek için deneyler mikrogravite koşullarında gerçekleştirilmiştir. Bu makalede, Uluslararası Uzay İstasyonu’nda gerçekleştirilen bu mikrogravite deneylerinin ve yeryüzünde Mikroyapı Evrim Laboratuvarı'nda yapılan deneylerin detayları verilmiştir. Ekolarak, μg ve 1g deneylerinin amaçları özetlenmiştir.Publication Metadata only Decoupled versus coupled growth dynamics of an irregular eutectic alloy(2020) Bottin-Rousseau, Sabine; Akamatsu, Silvere; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888We present an experimental study of irregular growth patterns observed in real time during thinsample directional solidification of a faceted/nonfaceted eutectic alloy, namely, the transparent 2-amino-2-methyl-1,3-propanediol (AMPD)-succinonitrile (SCN) system. The body-centered cubic SCN crystals are nonfaceted, while monoclinic AMPD crystals grow as faceted needles. At low velocities (<0.3 mu ms(-1)), a decoupled growth regime is observed, during which the tip of the AMPD crystals grows ahead of the SCN-liquid interface. At intermediate velocities, an unsteady coupled-growth regime takes place, with intermittent pinning of triple SCN-AMPD-liquid junctions, and frequent noncrystallographic branching. At higher velocities (>1 mu ms(-1)), two-phase fingers form.Publication Metadata only On the growth dynamics of nearly-locked grain in the three-phase in-bi-sn eutectic system(Springer, 2019) N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888Solidification microstructures are significantly affected by the anisotropy of crystal/crystal interphase energy. A recent experimental work on a three-phase eutectic system by the authors suggested that two distinguishable eutectic grains, i.e., quasi-isotropic and locked, form when crystal/crystal interphase energies contain negligible and strong anisotropy, respectively (Mohagheghi and Serefoglu in Acta Mater 2018, vol. 151, pp. 432-42, 2018). In two-phase eutectic systems, in addition to these two grain types, another class of eutectic grain called nearly-locked (NL) was reported. In order to investigate the existence of the NL grain in three-phase eutectic systems, real-time directional solidification (DS) and rotating directional solidification (RDS) experiments are performed on thin samples of In-Bi-Sn eutectic alloy. It is found that NL grains also form in three-phase eutectics and they contain some characteristic features of both quasi-isotropic and locked grains. The anisotropy is strong enough to tilt the lamellar pattern with respect to the thermal gradient axis, as in the case of locked grains; however, the NL grains also retain some of the characteristic features of the quasi-isotropic grains, such as lambda-diffusion, systematic eutectic spacing adjustment, and recovery mechanisms. As a result, these grains tend to form a relatively uniform ABAC-type growth pattern, similar to quasi-isotropic grains. Using the equilibrium shapes extracted from the interphase traces of RDS patterns, the gamma plot of the anisotropic interphase, which contains 2 twofold smooth and distinct minima, is determined.Publication Metadata only Quasi-isotropic and locked grain growth dynamics in a three-phase eutectic system(Pergamon-Elsevier Science Ltd, 2018) N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888The anisotropy of the interphase boundaries has a substantial impact on the eutectic pattern formation. Using rotating directional solidification (RDS) technique, two distinct eutectic grains, namely quasi-isotropic (floating) and crystallographically-locked grains, were identified in three-phase In-Bi-Sn alloy system. The growth dynamics of these grains were investigated in thin samples. The ABAC-type growth pattern, where A, B, C are eutectic phases, is preserved in the quasi-isotropic grains upon evolution of the pattern over the entire crystallographic orientations in RDS experiments. In the locked grains, however, the regularity of the ABAC pattern is disturbed due to the existence of two-fold singularities, i.e. cusps over a well-defined angular range at the surface energy plot. Nevertheless, the ABAC pattern is recovered after the sequence of the unlocked/transient/locked/transient states. The characteristics of both grains are analogous to the nearly steady-state pattern formed upon standard directional solidification experiments. The quasi-isotropic and locked grains in a three-phase ternary eutectic system essentially have the similar dynamical features as the two-phase binary eutectic systems although the microstructural details are highly complex in the three-phase system. These similarities designate that the formation of quasi-isotropic and locked patterns is merely due to the inherent characteristics of the eutectic grains.Publication Metadata only Lamellar eutectic growth with anisotropic interphase boundaries: experimental study using the rotating directional solidification method(Elsevier, 2012) Akamatsu Silvere; Bottin-Rousseau Sabine; Faivre Gabriel; Department of Mechanical Engineering; Department of Mechanical Engineering; Şerefoğlu, Melis; Researcher; College of Engineering; 44888We report on an experimental study of the effects of interphase boundary anisotropy on eutectic microstructures using a new methodology called rotating directional solidification (RDS), which consists of rotating a thin sample with respect to a fixed unidirectional thermal gradient. The systems used are thin, large eutectic grains of the CBr 4-C 2Cl 6 and In-In 2Bi lamellar eutectic alloys. The shape of the observed RDS lamellar trajectories turns out to be a reproducible eutectic-grain-dependent feature, in agreement with the theoretical prediction that these trajectories are approximately homothetic to the Wulff form of the interphase boundary in the sample plane. We show that different modes of lamellar growth, ranging from quasi-isotropic to (crystallographically) locked, exist in different eutectic grains of the two alloys studied. A detailed characterisation of these modes is given, with particular attention to the as-yet poorly understood aspects of locked lamellar growth.Publication Metadata only Dynamics of spacing adjustment and recovery mechanisms of ABAC-type growth pattern in ternary eutectic systems(Elsevier, 2017) N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888In directionally solidified 2D samples at ternary eutectic compositions, the stable three-phase pattern is established to be lamellar structure with ABAC stacking, where A, B, and C are crystalline phases. Beyond the stability limits of the ABAC pattern, the system uses various spacing adjustment mechanisms to revert to the stable regime. In this study, the dynamics of spacing adjustment and recovery mechanisms of isotropic ABAC patterns were investigated using three-phase In-Bi-Sn alloy. Unidirectional solidification experiments were performed on 23.0 and 62.7 mu m-thick samples, where solidification front was monitored in real-time from both sides of the sample using a particular microscopy system. At these thicknesses, the pattern was found to be 2D during steady-state growth, i.e. both top and bottom microstructures were the same. However, during spacing adjustment and recovery mechanisms, 3D features were observed. Dynamics of two major instabilities, lamellae branching and elimination, were quantified. After these instabilities, two key ABAC pattern recovery mechanisms, namely, phase invasion and phase exchange processes, were identified and analyzed. After elimination, ABAC pattern is recovered by either continuous eliminations of all phases or by phase exchange. After branching, the recovery mechanisms are established to be phase invasion and phase exchange.Publication Metadata only Crystal orientation relationships in ternary eutectic Al-Al2Cu-Ag2Al(Elsevier, 2018) Steinmetz, Philipp; Dennstedt, Anne; Sargın, Irmak; Genau, Amber; Hecht, Ulrike; Department of Mechanical Engineering; Department of Mechanical Engineering; Şerefoğlu, Melis; Researcher; College of Engineering; 44888The microstructure of ternary eutectic Al-Al2Cu-Ag2Al arranges in several patterns of three solid phases during directional solidification. One key question for understanding the behavior of this system is if and how the patterns depend on crystal orientation relationships (ORs) between the solid phases. In order to study the correlation between the ORs and the evolving patterns for different process conditions, electron backscatter diffraction (EBSD) is performed on samples of directionally solidified ternary eutectic Al-Al2Cu-Ag2Al which have been processed with different solidification velocities and temperature gradients. The results show that characteristic ORs occur, influencing the type of the evolving pattern, the alignment of the phases and the degree of order. For one specific OR the pattern was observed to change in response to an imposed increase in the growth velocity even though the OR was retained. Based on the obtained EBSD results, an explanation for the observed behavior is proposed. For the other ORs, specific microstructures were observed for each of them. The outcomes demonstrate that knowledge about crystal ORs is essential to improve the understanding of the pattern formation in complex eutectic alloys.Publication Metadata only A theory of thin lamellar eutectic growth with anisotropic interphase boundaries(Elsevier, 2012) Akamatsu Silvere; Bottin-Rousseau Sabine; Faivre Gabriel; Department of Mechanical Engineering; Department of Mechanical Engineering; Şerefoğlu, Melis; Researcher; College of Engineering; 44888We present a semiempirical theory of the effects of an orientation dependence of the surface free energy of interphase boundaries (interphase boundary anisotropy) on lamellar eutectic growth in thin samples. We show that, to a good approximation, thin lamellar eutectic patterns with a strong interphase boundary anisotropy travel along the growth front at such a velocity - or, equivalently, at such an inclination angle of the lamellae left behind in the solid - that the surface tension force of the interphase boundary is nearly parallel to the applied thermal gradient. This explains, among other things, the crystallographic locking of lamellar eutectic patterns that occurs in those eutectic grains, which have CUSP singularities in the Wulff plot of the interphase boundary. Based on this theory, we show that the rotating directional solidification method, by which a thin sample is rotated with respect to a fixed unidirectional thermal gradient, must yield eutectic lamellae whose trajectories are nearly homothetic to the two-dimensional Wulff form of the interphase boundary. This opens up new possibilities for the experimental study of interphase boundary anisotropy in eutectic alloys.Publication Open Access Stability of three-phase ternary-eutectic growth patterns in thin sample(Elsevier, 2018) Bottin-Rousseau, Sabine; Faivre Gabriel; Akamatsu, Silvere; Department of Mechanical Engineering; Department of Mechanical Engineering; Şerefoğlu, Melis; Yücetürk, Sinan; Researcher; College of EngineeringNear-eutectic ternary alloys subjected to thin-sample directional solidification can exhibit stationary periodic growth patterns with an ABAC repeat unit, where A, B, and C are the three solid phases in equilibrium with the liquid at the eutectic point. We present an in-situ experimental study of the dynamical features of such patterns in a near-eutectic IneIn2BieSn alloy. We demonstrate that ABAC patterns have a wide stability range of spacing l at given growth rate. We study quantitatively the ldiffusion process that is responsible for the spacing uniformity of steady-state patterns inside the stability interval. The instability processes that determine the limits of this interval are examined. Qualitatively, we show that ternary-eutectic ABAC patterns essentially have the same dynamical features as two-phase binary-eutectic patterns. However, lamella elimination (low-l stability limit) occurs before any Eckhaus instability manifests itself. We also report observations of stationary patterns with an [AB]m[AC]n superstructure, where m and n are integers larger than unity.Publication Open Access Effects of interphase boundary anisotropy on the three-phase growth dynamics in the β(In) - In2Bi - γ(Sn) ternary-eutectic system(Institute of Physics (IOP) Publishing, 2019) Hecht, U.; Bottin-Rousseau, S.; Akamatsu, S.; Faivre, G.; Department of Mechanical Engineering; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Graduate School of Sciences and Engineering; N/A; 44888We present an experimental investigation on the effects of the interphase energy anisotropy on the formation of three-phase growth microstructures during directional solidification (DS) of the β(In)-In2Bi-γ(Sn) ternary-eutectic system. Standard DS and rotating directional solidification (RDS) experiments were performed using thin alloy samples with real-time observation. We identified two main types of eutectic grains (EGs): (i) quasi-isotropic EGs within which the solidification dynamics do not exhibit any substantial anisotropy effect, and (ii) anisotropic EGs, within which RDS microstructures exhibit an alternation of locked and unlocked microstructures. EBSD analyses revealed (i) a strong tendency to an alignment of the In2Bi and γ(Sn) crystals (both hexagonal) with respect to the thin-sample walls, and (ii) the existence of special crystal orientation relationships (ORs) between the three solid phases in both quasi-isotropic and anisotropic EGs. We initiate a discussion on the dominating locking effect of the In2Bi-β(In) interphase boundary during quasi steady-state solidification, and the existence of strong crystal selection mechanisms during early nucleation and growth stages.