High-entropy alloys (HEAs) have attracted significant interest in the materials science community over the last 15 years. At the first moment, what caught the attention was the fact that these alloys tend to form solid solutions at room temperature, despite being composed of multiple elements in equiatomic or near-equiatomic concentrations. It was initially concluded that the configurational entropy plays a key role in the stabilization of the solid solutions. Later studies revealed the importance of lattice strain enthalpies, enthalpies of mixing, structural mismatch of constituents, and kinetics in phase formation/stability.
The study presented in this thesis was branched into three major parts, all related to understanding phase formation, stability, or metastability in this class of alloys. The first part deals with developing an empirical method to predict single-phase solid solution formation in multi-principal element alloys. The second, which makes the core of this thesis, are non-equilibrium solidification studies of CrFeNi and CoCrNi medium-entropy alloys, and CoCrFeNi, Al0.3CoCrFeNi, and NbTiVZr high-entropy alloys. The last part is devoted to understanding the thermophysical properties of CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys.
An empirical approach, based on the theoretical elastic-strain energy, has been developed to predict the phase formation and its stability for complex concentrated alloys. The conclusiveness of this approach is compared with the traditional empirical rules based on the atomic-size mismatch, enthalpy of mixing, and valence-electron concentration for a database of 235 alloys. The proposed “elastic-strain energy vs. valence-electron concentration” criterion shows an improved ability to distinguish between single-phase solid solutions, mixtures of solid solutions, and intermetallic phases when compared to the available empirical rules used to date. The criterion is especially strong for alloys that precipitate the μ phase. The elastic-strain-energy parameter can be combined with other known parameters, such as those noted above, to establish new criteria which can help in designing novel complex concentrated alloys with the on-demand combination of mechanical properties.
The solidification behavior of the CoCrFeNi high-entropy alloy and the ternary CrFeNi and CoCrNi medium-entropy suballoys has been studied in situ using high-speed video-camera and synchrotron X-ray diffraction (XRD) on electromagnetically levitated samples at Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) and German Synchrotron DESY, Hamburg. In all alloys, the formation of a primary metastable body-centered cubic bcc phase was observed if the melt was sufficiently undercooled. The delay time for the onset of the nucleation of the stable face-centered cubic fcc phase, occurring within bcc crystals, is inversely proportional to the melt undercooling. The experimental findings agree with the stable and metastable phase equilibria for the (CoCrNi)-Fe section. Crystal-growth velocities for the CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys, extracted from the high-speed video sequences in the present study, are comparable to the literature data for Fe-rich Fe-Ni and Fe-Cr-Ni alloys, evidencing the same crystallization kinetics. The effect of melt undercooling on the microstructure of solidified samples is analyzed and discussed in the thesis.
To understand the effect of Al addition on the non-equilibrium solidification behavior of the equiatomic CoCrFeNi alloy, the Al0.3CoCrFeNi HEA has been studied. While the quaternary alloy melt could be significantly undercooled, this was not possible in the five-component alloy. Therefore, the investigations on phase formation, crystal growth, and microstructural evolution were confined to the low undercooling regime. In situ XRD measurements revealed that the liquid crystallized into a fcc single-phase solid solution at this undercooling level. However, ex situ XRD revealed the precipitation of the ordered L12 phase for a sample solidified with ΔT = 30 K. Crystal growth velocities are shown to be smaller than in the CoCrFeNi, CrFeNi, and CoCrNi alloys; nonetheless, they are in the same order of magnitude. Spontaneous grain refinement, without the formation of crystal twins, is observed at low undercooling of ΔT = 70 K, which could be explained by the dendrite tip radius dependence on melt undercooling.
In situ studies of the equiatomic NbTiVZr refractory high-entropy alloys revealed the effect of processing conditions on the high-temperature phase formation. When the melt was undercooled over 80 K, it crystallized as a bcc single-phase solid solution despite solute partitioning between the dendritic and interdendritic regions. When the sample was solidified from the semisolid state, it resulted in the formation of two additional bcc phases at the interdendritic regions. The crystal growth velocity, as estimated from the high-speed videos, showed pronounced sluggish kinetics: it is 1 to 2 orders of magnitude smaller compared to literature data of other medium and high-entropy alloys.
The study of the linear expansion coefficient α and heat capacity at constant pressure 𝐶𝑝 of the equiatomic CoCrFeNi and the medium-entropy CrFeNi and CoCrNi alloys revealed an anomalous behavior with S-shaped curves in the temperature range of 700 – 950 K. The anomalous behavior is shown to be reversible as it occurred during the first and second heating. However, a minimum is only observed on the first heating, while in the second heating a sudden increase of both the α and 𝐶𝑝 occurs at the temperature of the onset of the minima in the first heating. Magnetic moment measurements as a function of temperature showed that the observed anomaly is not associated with the Curie temperature. Consideration of the structural and microstructural evaluation discards a first-order phase transformation or recrystallization as probable causes, at least for the CoCrFeNi and CoCrNi alloys. Based on literature evidence, the anomalies in the temperature dependences of the linear expansion coefficient and heat capacity are believed to be caused by a chemical short-range order transition known as the K-state effect. However, to reveal the exact nature of this phenomenon, further experimental and theoretical studies are required, which is outside the frame of the present work.:Abstract ....................................................................................................................... I
Kurzfassung .............................................................................................................. IV
Chapter 1: Motivation and Fundamentals .................................................................. 1
1.1 Introduction .......................................................................................................... 1
1.2 The high-entropy alloy (HEA) design concept ...................................................... 4
1.3 Empirical rules of phase formation for HEAs ....................................................... 6
1.4 Calculation of phase diagrams of HEAs ............................................................. 18
1.5 The core effects of HEAs ................................................................................... 20
1.5.1 Lattice distortion .............................................................................................. 20
1.5.2 Sluggish diffusion ............................................................................................ 22
1.5.3 Cocktail effect................................................................................................... 23
1.6 Mechanical properties ........................................................................................ 24
1.6.1 Lightweight high-entropy alloys ....................................................................... 24
1.6.2 Overcoming the strength-ductility tradeoff ...................................................... 26
1.6.3 Cryogenic high-entropy alloys ......................................................................... 28
1.6.4 Refractory high-entropy alloys ........................................................................ 30
1.7 Functional properties .......................................................................................... 33
1.7.1 Soft magnetic properties ................................................................................. 33
1.7.2 Magnetocaloric properties ............................................................................... 35
1.7.3 Hydrogen storage ............................................................................................ 36
Chapter 2: Experimental .......................................................................................... 38
2.1 Sample preparation ............................................................................................ 38
2.2 Electromagnetic levitation .................................................................................. 40
2.3 In situ X-ray diffraction ........................................................................................ 43
2.4 Microstructural and structural analysis ............................................................... 44
2.5 Thermal analysis ................................................................................................ 45
2.6 Dilatometry ......................................................................................................... 45
2.7 Magnetic moment ............................................................................................... 46
2.8 Heat treatment ................................................................................................... 46
Chapter 3: In situ study of non-equilibrium solidification of CoCrFeNi high-entropy alloy and CrFeNi and CoCrNi ternary suballoys ...................................................... 47
3.1 Introduction ........................................................................................................ 47
3.2 Results ............................................................................................................... 48
3.2.1 In situ synchrotron X-ray diffraction ................................................................. 48
3.2.2 High-speed video imaging ............................................................................... 52
3.2.3 Microstructure of the solidified samples .......................................................... 62
3.3 Discussion .......................................................................................................... 64
3.3.1 bcc-fcc nucleation and growth competition ..................................................... 64
3.3.2. Crystal growth kinetics ................................................................................... 68
3.3.3. Microstructural evolution ................................................................................ 70
Chapter 4: The effect of Al addition to the CoCrFeNi alloy on the non-equilibrium solidification behaviour.............................................................................................. 72
4.1 Introduction ........................................................................................................ 72
4.2 Results and Discussion ...................................................................................... 73
Chapter 5: Non-equilibrium solidification of the NbTiVZr refractory high-entropy alloy ................................................................................................................................. 84
5.1 Introduction ........................................................................................................ 84
5.2 Results ............................................................................................................... 85
5.2.1 In situ synchrotron X-ray diffraction ................................................................. 85
5.2.2 Room temperature synchrotron X-ray diffraction ............................................ 88
5.2.3 High-speed video imaging ............................................................................... 89
5.2.4 Microstructure and structure analysis ............................................................. 91
5.3 Discussion .......................................................................................................... 94
5.3.1 Phase formation upon solidification ................................................................ 94
5.3.2 Crystal growth kinetics .................................................................................... 98
5.3.3 Structural and microstructural features............................................................ 99
Chapter 6: Solid-state thermophysical properties of CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys ........................................................................... 101
6.1 Introduction ...................................................................................................... 101
6.2 Results ............................................................................................................. 102
6.3 Discussion ........................................................................................................ 106
6.3.1 Thermophysical properties ............................................................................ 106
6.3.2 Short-range order in medium- and high-entropy alloys ................................. 109
Chapter 7: Summary ............................................................................................... 111
7.1 Empirical rule of phase formation of complex concentrated alloys ................... 111
7.2 Non-equilibrium solidification of medium- and high-entropy alloys ................... 111
7.3 Thermophysical properties of the medium- and high-entropy alloys ................ 113
Chapter 8: Outlook ................................................................................................. 115
Appendix 1 .............................................................................................................. 117
Appendix 2 ............................................................................................................. 123
Appendix 3 ............................................................................................................. 133
Appendix 4 ............................................................................................................. 134
References.............................................................................................................. 140
Acknowledgments .................................................................................................. 164
List of publications .................................................................................................. 166
Erklärung ......................................................................................................................... 167
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:77839 |
| Date | 07 February 2022 |
| Creators | Fernandes Andreoli, Angelo |
| Contributors | Nielsch, Kornelius, Kaban, Ivan, Guo, Sheng, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | info:eu-repo/grantAgreement/Coordenação de Aperfeiçoamento de Pessoal de Nível Superior/CAPES-DAAD-CNPq International Cooperation Program/88887.161381/2017-00/ |
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