Quasicrystal-Induced Nucleation Mechanism in Undercooled Liquids
by Güven Kurtuldu
(Laboratory of Metal Physics and Technology, Department of Materials, ETH Zürich, Switzerland)
DATE : July 18, 2019 (Thursday)
TIME : 14:00-15:00
ROOM : VYKM-2
A model alloy Mg69Zn27Yb4 has recently been discovered, which concurrently forms bulk metallic glass (BMG), metastable icosahedral quasicrystals (iQCs), and two crystalline approximant phases from the melt . The following phases were observed (at room temperature) at increasing cooling rate via a recently developed technique, fast differential scanning calorimetry (FDSC): (i) stable Mg and approximant Mg29Zn60Yb11 phase mixture; (ii) metastable Mg and iQC phase mixture; (iii) approximant Mg51Zn20-type metastable particles; and (iv) a glassy phase. Deploying a new experimental strategy, i.e. heating the previously solidified microstructure at ultrafast rates via FDSC, the equilibrium Mg29Zn60Yb11 phase has never been observed to form directly from the melt; instead the metastable iQC phase nucleated first in the liquid and then transformed into a stable approximant phase. Such a transition path (undercooled liquid → metastable QCs → stable equilibrium phase) has also recently been suggested to occur in Al–Zn:Cr  and Au–Cu–Ag:Ir  alloys. The observations made have significant effects on control over solidification microstructures via grain refinement. The phase transition path minimizes the free energy barrier for nucleation through an intermediate metastable quasicrystal phase due to the low solid–liquid interfacial energy of quasicrystals. The experimental results shed new light on the competition between metastable and stable crystal formation, and glass formation via system frustration associated with the presence of several free energy minima. The rapid heating strategy using FDSC may yield the discovery of hidden transient phases that are key to understanding crystallization pathways in metallic systems, as well as polymers, biological solutions and pharmaceutical substances.
 Kurtuldu G, Shamlaye K F, Löffler J F. Metastable-quasicrystal-induced nucleation in a bulk glass-forming liquid, PNAS, 115 (24), 6123-6128 (2018).
 Kurtuldu G, Jarry P, Rappaz M. Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al-Zn-Cr alloys: Can icosahedral solid clusters play a role? Acta Mater 61(19):7098–7108 (2013).
 Kurtuldu G, Sicco A, Rappaz M. Icosahedral quasicrystal-enhanced nucleation of the fcc phase in liquid gold alloys. Acta Mater 70:240–248 (2014).
Güven Kurtuldu received his Bachelor of Science degree in 2007 and Master of Science degree in 2009 from the Department of Mechanical Engineering under the guidance of Prof. Sabri Altıntaş working on properties of boron nitride nanotube/epoxy composites at Boğaziçi University. He continued his PhD study in the group of Prof. Michel Rappaz at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland. His thesis focused on effect of minute additions of solute elements on solidification microstructures in metallic alloys. He discovered a nucleation mechanism for which formation of metastable quasicrystals from the liquid is the key to control the as-cast microstructures. He was awarded his PhD degree in 2014. After staying one year as a postdoc in the same group, he joined the group of Prof. Jörg Löffler at ETH Zürich, Switzerland. He is currently working on several projects related to quasicrystal and bulk metallic glass formation, structure of metallic liquids, development of novel experimental strategies via fast calorimetry and microstructure formation in additively manufactured alloys.