Crystallization kinetics of mechanically alloyed al based amorphous powders

Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY SCHOOL OF MATERIALS SCIENCE AND ENGINEERING ======== THESIS OF GRADUATION Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders Students Pham Ngoc Huyen – 20113038 Nguyen Thi Anh Nguyet – 20113882 Class Metallic Materials – K56 Supervisor Nguyen Hoang Viet, PhD Hanoi, June 2016 Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders Table of Contents Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders Forewords This thesis could not have been realized without a great deal of guidance and both mental and practical support We would like deeply thank those people who, during the several months in which this project lasted, provided us with everything we needed First of all, we would like to express the deepest appreciation to my advisor Dr Nguyen Hoang Viet for his guidance, encouragement, suggestion and very constructive criticism which have contributed immensely to the evolution of our ideas on the project Despite the length of this project, he never hesitated to answer our questions Without his thoughtful encouragement, careful supervision and persistent help this thesis would not have been possible to complete on time We would also like to acknowledge the teachers and staffs of Laboratory of Powder Metallurgy, Department of Non-ferrous Metal Materials and Composite, School of Materials Science and Engineering, HUST The time working there was definitely one of the best experience in our life In addition, a thank you to Dr Nguyen Thi Hoang Oanh, for her support, advices, valuable comments and suggestions All the things that she was willing to share with us were highly inspiring, not only for this specific project, but also for the communications practice in general To all our friends who helping us a lot from sharing the work to undertaking heavy workload throughout the difficult times A big thank you to our family, who usually expect that I can manage to the things they did not manage to yet, have taught us to appreciate knowledge in all forms Pham Ngoc Huyen Nguyen Thi Anh Nguyet Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders Abstract Al-based alloys of Al80Fe20, Al82Fe18, Al82Fe16Y2, and Al84Fe16 were prepared by mechanical alloying X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM) were applied to examine the structure evolution and particles’ morphology of these alloys Differential scanning calorimetry (DSC) analyses were applied to examine the thermal stability of these alloys The goal of this study was to investigate crystallization kinetics of Al-based amorphous powder through determinating activation energy and Avrami index based on Johnson-MehlAvrami (JMA) model The broad diffraction peaks in the XRD patterns for powder indicated that all of the samples exhibit an amorphous structure The activation energies of the alloys were calculated from differential thermal analysis data using the Kissinger, Ozawa and Augis–Bennett models The increased activation energy for Al80Fe20 and Al82Fe16Y2 samples compared to Al84Fe16 and Al82Fe18, respectively indicated that the higher thermal stability The value of the Avrami exponent indicated that the crystallization is interface - controlled growth for all of the samples Keyword: amorphous alloy, mechanical alloying, crystallization kinetics, Johnson-Mehl-Avrami model, activation energy, Avrami index Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders CHAPTER GENERAL INTRODUCTION 1.1 Overview of amorphous alloys Amorphous Alloys are a new class of metallic alloys with a unique amorphous atomic structure The atomic structure is the foremost striking characteristic of the amorphous alloys as it fundamentally differentiates from ordinary metals The atomic structure of conventional metals is a periodic structure in which the layout of atomic species shows repeating patterns over an extended range This atomic structure is called "crystalline" and is said to have long-range order By contrast, no discernable patterns exist in the atomic structure of amorphous alloys, which is called amorphous and said to have no long-range order This unique atomic structure places amorphous alloys in a new domain of properties unattainable by ordinary metals Fig 1.1 describes the crystalline structure, in which exist long – range order and grain boundaries, and the amorphous structure in materials Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders Fig 1.1.a Crystalline structure Fig 1.1.b Amorphous structure 1.1.1 Glass forming ability Glass forming ability (GFA) of an alloy is related to the ease of vitrification that reflects the physical nature of the alloy and indicates whether the alloy can be produced in bulk form GFA can be characterized by various parameters and one of the most appropriate was the critical cooling rate Inoue had proposed three empirical rules for glass formability: [1-3] 1) the multicomponent system consisting of more than three elements 2) significant difference in atomic size ratios above about 12 % among the main constituent elements 3) negative heats of mixing among their elements We can explain three empirical rules based on the point of view about thermodynamics, kinetics and structure Firstly, we investigate the reason for the high glass-forming ability from the thermodynamical point of view It is generally known that the high glassforming ability is obtained in the condition of low free energy ΔG(T) for the transformation of liquid to crystalline phase In the relation of ΔG=ΔHf - TΔSf for Gibbs free energy, the low ΔG value is obtained in the cases of low ΔHf and large ΔSf Here, the ΔHf and ΔSf are enthalpy of fusion and entropy of fusion, respectively The large ΔSf is expected to be obtained in Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders multicomponent alloy systems because ΔSf is proportional to the number of microscopic states The free energy at a constant temperature also decreases in the cases of low chemical potential caused by the low enthalpy and high reduced glass transition temperature as well as of high interface energy between liquid and solid phases Based on these thermodynamical aspects, it is concluded that the multiplication of alloy components leading to the increase in ΔSf causes an increase in the degree of dense random packing which is favorable for the decrease in ΔHf and the increase in solid/liquid interface energy This interpretation is consistent with the present result that the high glass-forming ability has been obtained for the above-described multicomponent systems containing more than three elements Secondly, we investigate the reason for the high glass-forming ability from the kinetic point of view The homogeneous nucleation (I) and growth (U) rates of a crystalline phase with a spherical morphology from supercooled liquid are known to be expressed by relations (1.1) and (1.2), respectively [cm-3s-1] (1.1) [cms-1] (1.2) Here, Tr is the reduced temperature (T/Tm), ΔTr is the difference in temperature from Tm, b is a shape factor and 16π/3 for a spherical nucleus, η is viscosity and f is the fraction of nucleus sites at the growth interface α and β are dimensionless parameters related to the liquid/solid interface energy (σ), ΔHf and ΔSf can be expressed as α=(NoV)1/3σ/ΔHf and β=ΔSf/R Here, No, V and R are Avogadro number, atomic volume and gas constant, respectively In these relations, the important parameters are η, α and β The increase in the three parameters decreases the I and U values, leading to an improvement of glass-forming ability The increase in α and β also implies an increase in σ and ΔSf and a decrease in ΔHf, being consistent with the interpretation on the achievement of high glass-forming ability derived from the thermodynamical point of view Furthermore, one can notice that η is closely related to the Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 Crystallization Kinetics of Mechanically Alloyed Al-based Amorphous Powders reduced glass transition temperature and α3β reflects the thermal stability of the supercooled liquid The importance of the α 3β parameter can be understood from the following two examples; (1) when the value of αβ 1/3 exceeds 0.9, unseeded liquid would not in practice crystallize at any cooling rates, and (2) when αβ1/3 is below 0.25, the liquid would be impossible to suppress crystallization The glass-forming ability of a material is determined by the kinetics of the nucleation process and of the early stages of crystal growth This is illustrated by a TTT (time – temperature – transformation) diagram, as shown in Fig 1.2 If the liquid is quenched from above Tm (i.e., the absolute melting temperature) to well below T' (i.e., the temperature at which the crystallization rate is a maximum) in a time tmin (i.e., the minimum time for crystallization), the undercooled liquid is retained, and at Tg, the configuration is frozen to form a glass At room temperature, the time for crystallization of a glass is very long If the glass is heated to a temperature T [...]... as of intermetallic Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 17 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders compounds), in which material transfer occurs, is named mechanical alloying (MA), while ball milling of single composition powders, such as single-phase compounds, where material transfer is not required, has been termed mechanical milling (MM) Mechanical alloying:... kinetics have been reported Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 27 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders 1.2.2.3 Mechanism of Mechanical Alloying Fig 1.13 Schematic view of motion of the balls and powder mixture Fig 1.13 shows the motions of the balls and the powder Since the directions of rotation of the bowl and turn disc are opposing, the centrifugal... 19 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders It was realized that mechanical alloying can also be used to synthesize a variety of both equilibrium and nonequilibrium materials at room temperature and starting from blended elemental powders This technique has attracted the attention of a large number of researchers during the past 15–20 years or so This technique is also... materials of metal carbides and metal nitrides, using very simple techniques More recently, the MA method has been used for reducing several metal oxides at room temperature Futhermore, Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 18 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders it can be use for the fabrication of many amorphous materials at room temperature Mechanical alloying... Nguyet, K56 35 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders Fig 1.14 Strength and the elastic limit values for different types of materials Due to the different structure, these alloys have exceptional properties compared to common materials This diagram shows us the mechanical properties of glassy alloys It can be seen that the glassy alloys combine high strength of the strongest... residual resistivity near 0K for the amorphous Fe–P–C alloy is 4 times the values of the crystalline phase Fig 1.5 Variation in relative resistivity (ρ/ρrm) with temperature for the Fe75P15C10 glass alloy Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 13 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders 1.1.2.4 Corrosion resistance Although the amorphous structure in glassy alloys... values between the amorphous alloys and crystalline alloys was as large as 60%.The significant difference in the mechanical properties was thought to be a reflection of the difference in the deformation and fracture mechanisms between amorphous alloys and crystalline alloys Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 11 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders Fig 1.4... (AmBn)crystalline → (AmBn )amorphous Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 30 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders In some instances it has been reported that a solid solution forms in the initial stages of mixing of blended elemental powders, which on continued milling, becomes amorphous In some other instances it has been reported that the sequence of phase... Additionally, synthesis of nanocrystalline materials is much easier by MA than by RSP Amorphization in ordered alloys seems to follow the sequence [26]: Ordered phase → disordered phase (loss of long-range order) → fine grained (nanocrystalline) phase → amorphous phase Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 32 Crystallization Kinetics of Mechanically Alloyed Al- based Amorphous Powders The formation of. .. one of the few commercial uses of this class of material [5-6] As a result of the random positions of atoms, the ferromagnetic magnetization vector should not have any special anisotropy This idea led to the strong motivation for the development of soft magnetic materials based on iron, cobalt, and nickel The Pham Ngoc Huyen – Nguyen Thi Anh Nguyet, K56 12 Crystallization Kinetics of Mechanically Alloyed