A short review on welding and joining of high entropy alloys

Fundacao para a Ciencia e a Tecnologia (FCT -MCTES) via the project UIDB/00667/2020 (UNIDEMI).High entropy alloys are one of the most exciting developments conceived in the materials science field in the last years. These novel advanced engineering alloys exhibit a unique set of properties, which include, among others, good mechanical performance under severe conditions in a wide temperature range and high microstructural stability over long time periods. Owing to the remarkable properties of these alloys, they can become expedite solutions for multiple structural and functional applications. Nevertheless, like any other key engineering alloy, their capacity to be welded, and thus become a permanent feature of a component or structure, is a fundamental issue that needs to be addressed to further expand these alloys’ potential applications. In fact, welding of high entropy alloys has attracted some interest recently. Therefore, it is important to compile the available knowledge on the current state of the art on this topic in order to establish a starting point for the further development of these alloys. In this article, an effort is made to acquire a comprehensive knowledge on the overall progress on welding of different high entropy alloy systems through a systematic review of both fusion-based and solid-state welding techniques. From the current literature review, it can be perceived that welding of high entropy alloys is currently gaining more interest. Several high entropy alloy systems have already been successfully welded. However, most research works focus on the well-known CoCrFeMnNi. For this specific system, both fusion and solid-state welding have been used, with no significant degradation of the joints’ mechanical properties. Among the different welding techniques already employed, laser welding is predominant, potentially due to the small size of its heat source. Overall, welding of high entropy alloys is still in its infancy, though good perspectives are foreseen for the use of welded joints based on these materials in structural applications.publishersversionpublishe

Complex Concentrated Alloys (CCAs)

This book is a collection of several unique articles on the current state of research on complex concentrated alloys, as well as their compelling future opportunities in wide ranging applications. Complex concentrated alloys consist of multiple principal elements and represent a new paradigm in structural alloy design. They show a range of exceptional properties that are unachievable in conventional alloys, including high strength–ductility combination, resistance to oxidation, corrosion/wear resistance, and excellent high-temperature properties. The research articles, reviews, and perspectives are intended to provide a wholistic view of this multidisciplinary subject of interest to scientists and engineers

Static and dynamic recrystallization behaviour of high entropy alloys

This thesis studied the optimum processing conditions for static and dynamic recrystallization behaviour of AlxCoCrFeNi High Entropy Alloys. As a result of recrystallization, refined microstructure was obtained and the mechanical properties were improved in AlxCoCrFeNi HEA. These findings suggest that, HEAs can be used in high temperature structural applications.<br /

Processing, Microstructures, and Mechanical Behavior of High-Entropy Alloys

Recently, high-entropy alloys (HEAs) have attracted increasing attentions because of their unique compositions, microstructures, and adjustable properties. In this work, microstructure and phase composition of the AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast and heat-treated conditions. Using a combination of Electron Backscatter Diffraction (EBSD), X-ray energy spectroscopy analysis, and synchrotron X-ray diffraction techniques, two nonequilibrium phases were identified in the as-cast condition: BCC_A2 and B2. Long-term heat treatment transformed these nonequilibrium phases into four equilibrium phases: FCC_A1, BCC_A2, B2, and σ [sigma] phase. The electron microscopic results were consistent with thermodynamic simulations of the equilibrium and nonequilibrium conditions. Tensile properties of AlCoCrFeNi high entropy alloy (HEA) in two conditions, (i) as-cast and (ii) heat-treated, were also reported. The heat treatment consisted of hot isostatic pressing at 1,373 K, 207 MPa for 1 hours followed by annealing at 1,423 K for 50 hours. A noticeable increase in the tensile ductility occurred after heat treatment. During deformation at 973 K, elongation of the heat-treated alloy was 11 %, while the as-cast alloy showed elongation of only ~ 0.9 %. The ultimate tensile strength was almost unaffected by heat treatment, and it was 407 ± 5 MPa at 973 K. The properties of the alloy were correlated to its as-cast and heat-treated microstructures. The fracture mechanisms were discussed in detail by analyzing the features on and below the fracture surface. The formation of FCC_A1 phase, less casting pores, and residual stresses removed, may contribute to the significant improvement of tensile behavior

Recent Developments in Non-conventional Welding of Materials

Welding is a technological field that has some of the greatest impact on many industries, such as automotive, aerospace, energy production, electronics, the health sector, etc. Welding technologies are currently used to connect the most diverse materials, from metallic alloys to polymers, composites, or even biological tissues. Despite the relevance and wide application of traditional welding technologies, these processes do not meet the demanding requirements of some industries. This has driven strong research efforts in the field of non-conventional welding processes. This Special Issue presents a sample of the most recent developments in the non-conventional welding of materials, which will drive the design of future industrial solutions with increased efficiency and sustainability

High-entropy alloys: a critical assessment of their founding principles and future prospects

High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.One of the authors (NGJ) would like to acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding under EP/M005607/1

Non-equilibrium solidification of high-entropy alloys monitored in situ by X-ray diffraction and high-speed video

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 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 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 ......................................................................................................................... 16

Development of an additive manufacturing processing route for high entropy alloys using powder bed fusion

Laser powder bed fusion (LPBF) is an additive manufacturing (AM) technique that can produce components from digital models in a layer-by-layer fashion using metallic powders. The customisation of pre alloyed powders used by LPBF is expensive and time-consuming, making LPBF not ideal for alloy development. In-situ alloying approaches using blended powders as raw materials are therefore carried out to shorten the alloy development process. Recently, high entropy alloys (HEAs) have drawn growing scientific attention. The HEA concept is of great compositional flexibility, allowing vast composition spaces and advanced properties to be explored. The CoCrFeMnNi HEA has been widely studied as a representative face-centred-cubic (FCC) HEA. The feasibility of the LPBF in-situ fabrication of the CoCrFeMnNi HEA and its body-centred-cubic (BCC) variation, AlCoCrFeNi, is the subject of this study. This study aims to develop an AM processing route for HEAs through LPBF in-situ alloying. Elemental Mn and Al powders were blended with pre-alloyed CoCrFeNi powder for quasi-equiatomic composition, respectively. The in situ alloying printability was evaluated via the parametric study based on densification and defect assessments. The chemical homogenisation and phase formation in the as-built samples was examined and correlated to the laser heat input. The results showed that Mn could be in-situ alloyed into the FCC CoCrFeNi matrix with homogeneity, indicating good printability of the CoCrFeMnNi HEA. However, the attempt to in-situ fabricate the AlCoCrFeNi HEA failed to produce samples free of cracking/porosity, despite the investigation of a wide range of parameters. The resultant defects and Al segregations suggested that the BCC HEA cannot be realised using this approach. The tensile and compression properties of the in-situ alloyed CoCrFeMnNi HEA were compared with the LPBFed CoCrFeMnNi HEA fabricated using pre-alloyed powder. The tensile strength was reinforced by the oxide-dispersion-strengthened (ODS) effect, because Mn oxides were introduced during the process. Submicron voids were observed around the oxides in the deformed samples, which were responsible for the early failure during the tensile deformation. The Mn oxides were identified as MnO and Mn2O3 and their forming mechanisms were analysed. To understand the underlying mechanisms of the LPBF in-situ alloying processes, elemental homogenisation and grain development were further investigated through single-track, single-layer and three-layer experiments. The experimental meltpool dimensions were compared with the predicted ones, showing that the processing window for in-situ alloying was operated in the keyhole mode. Remelting was found to be the main mechanism of elemental homogenisation. Crystalline characteristics were found to be inherited during the accumulation of tracks, reflecting parameter structure correlations. In conclusion, the results of this study have shown that LPBF in-situ alloying had the potential for HEA development, and raised topics for further research. A comprehensive understanding of the process will help to shorten the period from alloy design to microstructural tailoring

Effect of Cooling Rate on Aluminium-Containing High-Entropy Alloys

In this work, a HEA system is devised based on the outcome of neural network models. The most successful neural network in this work achieves a testing accuracy of 92%. This neural network operates solely on the compositional data of alloys, as opposed to the orthodox approach of using Hume-Rothery (HR) data. Considering that an alloy’s composition is always known for certain (unlike HR features that are dependent on estimates), this approach is expected to enable the average researcher to rapidly screen potential HEA compositions. The outcome of the neural network model led to the study of the AlxCrCuFeNi system, whereby x = 1.4 was predicted to be the limit of the alloy’s solid-solution window. The x = 1.0, x = 1.3, x = 1.5 and x = 2.0 compositions were manufactured using an arc-melter to confirm the prediction, whereby noticeable microstructural complexities are observed in the x = 1.5 system that are not observed in the x = 1.0 and x = 1.3 systems. ‘Chinese Script’ and ‘Sunflower’ structures are observed in the x = 1.5 system, whereas the x = 2.0 system displayed a microstructure dominated by intermetallics and very brittle mechanical behaviour. The x = 1.0 and x = 1.3 alloys showed Al-Ni intermetallic needles in their interdendritic regions which adhere closely to dendrite peripheries. The x = 1.0 alloy was processed for rapid solidification using a 6.5 m long drop-tube facility. This is in order to explore the possibility of suppressing intermetallic growth and achieving a single-phase simple solid solution. The sizes of the retrieved powders ranged from ˃ 850 µm to 38 µm, with a corresponding range of cooling rates from 112 K/s to 1.13×106 K/s. With higher cooling rates, simpler microstructures are obtained and at the highest cooling rate of around 1.13 ×〖10〗^6 K/s, a microstructure free of intermetallics is observed in powders of the 38 – 53 μm size fraction. The effect of rapid cooling is also studied in the eutectic HEA (EHEA) that is AlCoCrFeNi2.1. In equilibrium conditions, AlCoCrFeNi2.1 is dual-phase L12/B2. By processing AlCoCrFeNi2.1 using the drop-tube facility, rapidly-solidified powders were achieved with sizes from 850 µm ≤ d < 1000–38 µm ≤ d < 53 µm with corresponding estimated cooling rates of 114 K/s to 1.75×106 K/s respectively. Average interlamellar spacing was found to decrease from 2.10 µm in the as-cast alloy to 348 nm in the powders of the 38 µm < d < 53 µm size fraction. Although decreased interlamellar spacing is expected to enhance microhardness, such a relation was surprisingly not as strong as expected, as microhardness of the powders was found to vary only slightly from an average value of 340 Hv0.03. This unexpected result is explained via the observation of disorder trapping and increased FCC volume fraction. With increasing cooling rate, the microstructure of AlCoCrFeNi2.1 was found to evolve gradually from regular eutectic to colony eutectic, followed by dendritic with eutectic the interdendritic regions. In some instances at the highest cooling rate, dendritic structures may be observed with no eutectic observed in the interdendritic region. In particles of size d < 212 µm BCC dendrites were observed, either dominating the structure or coexisting with FCC dendrites