Researcher:
Mohagheghi, Samira

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PhD Student

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Samira

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Mohagheghi

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Mohagheghi, Samira

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2017 - 20205

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    Decoupled versus coupled growth dynamics of an irregular eutectic alloy
    (2020) Bottin-Rousseau, Sabine; Akamatsu, Silvere; N/A; Department of Mechanical Engineering; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888
    We 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.
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    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; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888
    Solidification 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.
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    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; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888
    The 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.
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    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; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; 329277; 44888
    In 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.
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    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; Mohagheghi, Samira; Şerefoğlu, Melis; PhD Student; Researcher; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; N/A; 44888
    We 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.