Magnetic monitoring approach to kinetics of phase transformations in multicomponent alloy systems

Duman, Nagehan
It is of great importance for a materials scientist both from fundamental and applicability aspects to have better understanding of solid-state phase transformations and its kinetics responsible for micro-/nano-structure development in alloys and corresponding physical and mechanical properties. Transformation kinetics can be analyzed by various experimental techniques such as thermal analysis, laborious electron microscopy combined with extensive image analysis or by measuring changes in electrical resistivity, specimen volume and relative intensities of diffraction lines caused by the phase transformation. Beyond these conventional techniques, this dissertation provides a novel magnetic monitoring approach to study the isothermal kinetics of phase transformations in multicomponent alloy systems involving measurable changes in overall magnetic moment as the transformation proceeds. This dissertation focuses on understanding the microstructural evolution, macro- and micro-alloying behavior, magnetic properties, thermal characteristics, mechanical properties and kinetics of solid-state transformations, i.e. nanoscale precipitation and nanocrystallization, in nickel aluminides and Fe-based bulk amorphous alloys. Microstructural characterization of alloys was done by X-ray diffraction, scanning electron microscopy and transmission electron microcopy techniques. Magnetic properties were analyzed by vibrating sample magnetometry whereas thermal characteristics were evaluated by differential scanning calorimetry. Mechanical properties of alloys were determined by microhardness measurements and compression tests. The influence of Fe macroalloying and 3d transition metal microalloying on the microstructure and properties of Ni-Al-Fe alloys were studied for as-cast and annealed states and it is shown that desired microstructure and related properties can be obtained by proper selection of the type and concentration of macro- or micro-alloying elements together with an appropriate annealing procedure. Thermomagnetic characterization reveals the nanoscale precipitation of a ferromagnetic second phase with annealing. In conjunction with saturation magnetization dependence on annealing, an optimum temperature is identified where nanoscale precipitates impart the highest extent of precipitation strengthening. The isothermal kinetics of ferromagnetic second phase precipitation reveals invariant Avrami exponents close to unity, indicating that nanoscale precipitation is governed by a diffusion-controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. Appropriate annealing of the Fe-based bulk amorphous alloy precursor produced by suction casting demonstrated extremely fine microstructures containing uniformly distributed and densely dispersed nanocrystals inside a residual amorphous matrix. In order to have better understanding of nanocrystallization mechanisms, kinetic parameters were determined via isothermal magnetic monitoring and non-isothermal differential scanning calorimetry where excellent agreement was obtained in Avrami exponent and activation energy. Analyzing the local kinetics, the nanocrystalline phase was found to evolve through distinct transformation regimes during annealing which were discussed on the basis of transformation kinetics theory and microscopical investigations on each characteristic transformation regime.


Atomic size effect of alloying elements on the formation, evolution and strengthening of gamma'-Ni3Al precipitates in Ni-based superalloys
Eriş, Rasim; Akdeniz, Mahmut Vedat; Mehrabov, Amdulla (2019-06-01)
Mechanical properties of Ni-based superalloys strongly depend on phase and site preferences of alloying elements which influence bonding strength within gamma'-Ni3Al precipitates and microstructural characteristics of these unique class of materials. In the current work, therefore, besides disclosure of the phase partitioning behaviours of alloying X elements (i.e. X = Co, Cr, Nb, Ta or Ti) among gamma' and gamma phases, their site occupancy tendencies in gamma' precipitates (determined via first-principles...
First order and second order phase transitions in quartz and related materials
Lider, Mustafa Cem; Yurtseven, Hasan Hamit; Department of Physics (2017)
The mechanism of the phase transitions in quartz and in the related materials is investigated. First order and second order transitions are explained on the basis of various models using experimental data from literature. Inparticular, the Landau phenomenological theory is employed in terms of the order parameters and the temperature- pressure (T-P) phase diagram is constructed for those materials of interest. The sym- metry change of crystal structure is also analyzed by using group theory. The thermodynam...
Magnetic monitoring approach to nanocrystallization kinetics in Fe-based bulk amorphous alloy
Duman, Nagehan; Akdeniz, Mahmut Vedat; Mehrabov, Amdulla (2013-12-01)
Much of the recent metallic glass research is devoted to controlling the crystallization of amorphous precursors with the purpose of obtaining amorphous matrix nanocrystalline alloys which combine unmatched soft magnetic properties with good mechanical properties. Therefore, it is crucial to have better understanding of crystallization mechanisms and thermal dependence of nanocrystals that are formed by annealing. This study deals with the nanociystallization kinetics of Cu-modified Fe-Co-Ni-B-Si-Nb bulk am...
Nano-scale phase separation in amorphous Fe-B alloys: Atomic and cluster ordering
AYKOL, Muratahan; Mehrabov, Amdulla; Akdeniz, Mahmut Vedat (Elsevier BV, 2009-01-01)
Nano-scale phase separation encountered in metallic glasses is investigated for amorphous Fe80B20 and Fe83B17 alloys in an interaction field calculated via electronic theory of alloys in pseudopotential approximation combined with MC equilibration and reverse MC simulation. The phenomenon is identified regarding three topological aspects: (1) pure Fe-clusters as large as similar to 0.9 nm and pure Fe-contours similar to 0.72 nm thick are found to exist; (2) Fe-rich highly deformed-bcc regions are observed; ...
Molecular dynamics study of random and ordered metals and metal alloys
Kart, Hasan Hüseyin; Tomak, Mehmet; Department of Physics (2004)
The solid, liquid, and solidification properties of Pd, Ag pure metals and especially PdxAg1-x alloys are studied by using the molecular dynamics simulation. The effects of temperature and concentration on the physical properties of PdxÞAg1-x are analyzed. Sutton-Chen (SC) and Quantum Sutton-Chen (Q-SC) many-body potentials are used as interatomic interactions which enable one to investigate the thermodynamic, static, and dynamical properties of transition metals. The simulation results such as cohesive ene...
Citation Formats
N. Duman, “Magnetic monitoring approach to kinetics of phase transformations in multicomponent alloy systems,” Ph.D. - Doctoral Program, Middle East Technical University, 2012.