CERES :: Browsing by Author "Georgarakis, Konstantinos"

Browsing by Author "Georgarakis, Konstantinos"

Now showing 1 - 20 of 44

Open Access
Advanced ultra-light multifunctional metallic-glass wave springs
(Elsevier, 2020-05-01) Panagiotopoulos, Nikolaos T.; Georgarakis, Konstantinos; Jorge Jnr, Alberto Moreira; Aljerf, M.; Botta, Walter José; Greer, A. Lindsay; Yavari, Ahmad
We show that, using thermo-elastic processing, metallic-glass foils can be shaped, without being embrittled, into linear and annular wave springs. These springs exhibit an undulatory behaviour, unique to metallic-glass foils, in which under compression the number of arcs in the spring increases, increasing the load-bearing capacity and the spring constant. We evaluate the performance limits of the metallic-glass wave springs, and consider how the undulatory behaviour can be exploited. The metallic-glass springs can operate over the same load-ranges as commercially available crystalline wave springs, but have material volumes (and therefore weights) that are one to two orders of magnitude less. Their energy storage per unit material volume is as high as 2600 kJ m−3. We suggest that the undulatory behaviour is important in rendering the springs fail-safe in case of overload. We discuss the range of applicability of thermo-elastic processing, the likely working limit of metallic-glass wave springs, and the potential for application of metallic-glass springs in MEMS devices.
Open Access
An atomistic study of the structural changes in a Zr–Cu–Ni–Al glass-forming liquid on vitrification monitored in-situ by X-ray diffraction and molecular dynamics simulation
(Elsevier, 2020-04-29) Louzguine-Luzgin, Dmitri V.; Georgarakis, Konstantinos; Andrieux, Jérôme; Hennet, Louis; Morishita, T.; Nishio, K.; Belosludov, R. V.
Structural changes in the Zr55Cu30Ni5Al10 liquid alloy on cooling from above the equilibrium liquidus temperature are studied by synchrotron radiation X-ray diffraction and compared with the results of first-principles molecular dynamics simulation. In-situ vitrification of the studied alloy is achieved using a containerless levitation technique. Subsequent analysis of the atomic and electronic structure of the alloy in liquid and glassy states reveals formation of medium-range order on cooling and its relationship with liquid fragility. The structural changes in this alloy are smaller in comparison with a more fragile one.
Open Access
The benefit of the glassy state of reinforcing particles for the densification of aluminum matrix composites
(MDPI, 2022-05-07) Kvashnin, Vyacheslav I.; Dudina, Dina V.; Ukhina, Arina V.; Koga, Guilherme Yuuki; Georgarakis, Konstantinos
In metallic glass-reinforced metal matrix composites, the glassy phase can serve a dual purpose: (i) it can behave as soft binder and porosity remover during consolidation; and (ii) it can act as the hard reinforcing phase after densification. The present work aimed to demonstrate the benefit of the glassy reinforcing particles for the densification of aluminum matrix composites. The consolidation behavior of Al–50 vol.% Fe-based alloy mixtures prepared using a glassy Fe66Cr10Nb5B19 alloy powder (Tg = 521 °C, Tx = 573 °C) or a crystalline Fe62Cr10Nb12B16 alloy powder was studied under spark plasma sintering (SPS) and hot pressing (HP) conditions. The powders were consolidated by heating above the glass transition temperature of the glassy alloy (up to 540 °C in SPS and 570 °C in HP). When the coarse aluminum powder was used, the reinforcing particles formed chains within the microstructure. In composites formed from the fine Al powder, the particles of the Fe-based alloy were separated from each other by the metallic matrix, and the tendency to form agglomerates was reduced. The glassy state of the alloy was shown to be beneficial for densification, as the metallic glass acted as a soft binder. The densification enhancement effect was more pronounced in the case of reinforcing particles forming chains. The hardness of the Al–50 vol.% glassy Fe66Cr10Nb5B19 composites obtained by SPS was twice the hardness of the unreinforced sintered aluminum (110 HV1 versus 45 HV1).
Open Access
Caught in the act: The structural pathway of liquid metals to vitrification monitored in situ by synchrotron X-ray diffraction.
(Cranfield University, 2021-09) Stiehler, Martin E.; Georgarakis, Konstantinos; Jolly, Mark R.
When a metallic melt is undercooled fast enough below its liquidus temperature, crystallisation can be avoided and a metallic glass, i.e. a metallic solid with amorphous structure, be formed. This kind of solidification is called vitrification. The prerequisites for this phenomenon are still not clear. An extensive review of the available relevant literature was carried out. To reveal the structural changes taking place at the atomic scale during undercooling and vitrification, data obtained by ultrafast synchrotron X-ray diffraction during aerodynamic-levitation experiments of different metallic-glass forming liquids was analysed. The complete pathway from temperatures well above the liquidus temperature during undercooling and vitrification down to temperatures well below the glass-transition temperature Tg was studied. During undercooling, a non-linear evolution of structural metrics in real as well as in reciprocal space takes place. Especially the height of the first maximum in the structure factor can be described by a structural analogue to the Curie-Weiss law. This behaviour was also found in published data re-analysed here. Indications of universal behaviour among the investigated alloys below a certain temperature as well as for a liquid-liquid crossover in Ti₄₀Cu₃₄Pd₁₄Zr₁₀Sn₂ were found. Small differences in the temperature dependence of the structural behaviour among the different alloys are possibly related to their different glass-forming abilities. To facilitate the analysis of the real-space structure the novel concept of the anti-shell was introduced. Temperature affects different length scales differently. Below Tg the structural behaviour is dominated by the Debye-Waller factor as well as by normal thermal-expansion behaviour. Above Tg an apparent negative thermal expansion of the first nearest-neighbour distance can be attributed to the influence of the structure-forming processes. In addition to short- and medium-range order, a third structural range for distances beyond the third nearest-neighbour is proposed. A disordering of the atomic structure of metallic glasses by the introduction of further alloying elements, facilitated by emergent effects among the components, could be demonstrated. The importance of the influence of global electronic interactions on structure formation was shown as well as their limitation to distances beyond the third nearest-neighbour.
Open Access
Core - shell particle reinforcements - a new trend in the design and development of metal matrix composites
(MDPI, 2022-04-02) Dudina, Dina V.; Georgarakis, Konstantinos
Metal matrix composites (MMCs) are a constantly developing class of materials. Simultaneously achieving a high strength and a high ductility is a challenging task in the design of MMCs. This article aims to highlight a recent trend: the development of MMCs reinforced with particles of core–shell structure. The core–shell particles can be synthesized in situ upon a partial transformation of metal (alloy) particles introduced into a metal matrix. MMCs containing core–shell particles with cores of different compositions (metallic, intermetallic, glassy alloy, high-entropy alloy, metal-ceramic) are currently studied. For metal core–intermetallic shell particle-reinforced composites, the property gain by the core–shell approach is strengthening achieved without a loss in ductility. The propagation of cracks formed in the brittle intermetallic shell is hindered by both the metal matrix and the metal core, which constitutes a key advantage of the metal core–intermetallic shell particles over monolithic particles of intermetallic compounds for reinforcing purposes. The challenges of making a direct comparison between the core–shell particle-reinforced MMCs and MMCs of other microstructures and future research directions are discussed.
Open Access
Corrosion of high strength pipeline steel weldment using submerged jet impingement.
(2019-03) Nofrizal; Impey, Susan A.; Georgarakis, Konstantinos
The aim of this research is to evaluate submerged jet impingement (SJI) as a tool for assessing the effect of flow and the associated corrosion behaviour across an X65 high strength pipeline weldment. The focus is corrosion caused by turbulent artificial sea water at different velocities impinging onto the weldment. An SJI target consisting of 3 rings (centre, inner and outer) based on a previous design [13], was constructed from an X65 pipeline steel weldment. Parent material (PM), heat affected zone (HAZ) and weld metal (WM) are analysed together in a high shear stress environment and changes in the weldment in a range of hydrodynamic conditions evaluated. Electrochemical measurements were performed with X65 pipeline steel with stagnant and flowing artificial seawater saturated with carbon dioxide at 1 bar at 0 -10 m/s at 25˚C and pH 4. The behaviour with and without an inhibitor was also examined. Galvanic current characteristics between coupled weldment regions were recorded using a zero-resistance ammeter (ZRA), and self-corrosion analysed using linear polarisation resistance (LPR) measurements. Computational fluid dynamic (CFD) analysis was undertaken to understand the hydrodynamic effects and velocity changes across the SJI target in flowing conditions. The velocity distribution across the target varied in each weld region where the centre, outer and inner electrodes are 25, 33 and 50% of the impinging jet velocity respectively. The inner electrodes of the SJI target experience 30% of the expected wall shear stress calculated from the impinging jet velocity. Alternative SJI target configurations are proposed to represent a wide range of shear stress and fluid velocities across the target with good precision and accuracy. For low turbulence, the target centre electrode is minimised or placed at the target edge. To achieve maximum turbulence, an electrode 4 mm from the target centre is proposed. To achieve 10 m/s on the suggested target, a jet velocity of just over 15 m/s is required.
Open Access
Degradation and fatigue life methods for ceramic matrix composite and nickel superalloy materials in gas turbine combustor liner
(Springer Nature, 2024-12-08) Karadimas, Georgios; Pagone, Emanuele; Georgarakis, Konstantinos; Salonitis, Konstantinos
This paper investigates the durability of Ceramic Matrix Composite (CMC) and Nickel Superalloy materials in gas turbine combustor liners across different flight conditions. Thermo-mechanical and physical properties of CMCs are assessed and compared with selected superalloys (i.e., Inconel 625, Hastelloy X, SiC/SiC, Al2O3/B4C, and Al2O3/SiO2) using Finite Element Analysis (FEA). An integrated methodology employing Computational Fluid Dynamics (CFD) and FEA is introduced to analyze thermal stresses and fatigue life, establishing a direct connection between combustion dynamics and structural responses. The study addresses a research gap by thoroughly evaluating CMC materials in gas turbine combustor liners, offering valuable insights for material selection and design strategies in aerospace engineering. The findings enhance understanding of CMC behavior, showcasing their reliability and durability for gas turbine components and their usage for advancements in aerospace applications.
Open Access
Design optimisation of the feeding system of a novel counter-gravity casting process
(MDPI, 2018-10-11) Papanikolaou, Michail; Pagone, Emanuele; Georgarakis, Konstantinos; Rogers, Keith; Jolly, Mark R.; Salonitis, Konstantinos
The appropriate design of feeders in a rigging system is critical for ensuring efficient compensation for solidification shrinkage, thus eliminating (shrinkage-related) porosity and contributing to the production of superior quality castings. In this study, a multi-objective optimisation framework combined with Computational Fluid Dynamics (CFD) simulations has been introduced to investigate the effect of the feeders’ geometry on shrinkage porosity aiming to optimise casting quality and yield for a novel counter-gravity casting process (CRIMSON). The weighted sum technique was employed to convert this multi-objective optimisation problem to a single objective one. Moreover, an evolutionary multi-objective optimisation algorithm (NSGA-II) has been applied to estimate the trade-off between the objective functions and support decision makers on selecting the optimum solution based on the desired properties of the final casting product and the process characteristics. This study is one of the first attempts to combine CFD simulations with multi-objective optimisation techniques in counter-gravity casting. The obtained results indicate the benefits of applying multi-objective optimisation techniques to casting processes
Open Access
Devitrification of thin film Cu-Zr metallic glass via ultrashort pulsed laser annealing
(Elsevier, 2021-08-03) Antonowicz, Jerzy; Zalden, P.; Sokolowski-Tinten, K.; Georgarakis, Konstantinos; Minikayev, R.; Pietnoczka, A.; Bertram, F.; Chaika, M.; Chojnacki, M.; Dłużewski, P.; Fronc, K.; Greer, A. Lindsay; Jastrzębski, C.; Klinger, D.; Lemke, Ch.; Magnussen, O. M.; Murphy, B.; Perumal, K.; Sobierajski, R.
In this work we report on an ultrashort pulsed laser annealing-driven devitrification of thin film Cu67Zr33 metallic glass characterized by micro-beam X-ray diffraction and electron microscopy techniques. The essential feature of ultrashort pulsed laser annealing is ultrafast heating (1014 K/s) by femtosecond optical excitation followed by extremely rapid cooling (1010–12 K/s) due to heat dissipation into the film substrate. During repetitive optical excitation, we take X-ray diffraction snapshots of the intermediate, frozen-in stages of the glass-crystal transformation to study its kinetics. A quantitative analysis of the diffraction patterns supported by electron microscopy result shows that the glass-crystal transformation proceeds by a rapid formation of an energetically favourable layer of crystalline ZrO2 on the free surface of the glassy film accompanied by nucleation and growth of fcc-Cu in the residual amorphous matrix. We demonstrate that at low effective annealing temperatures the devitrification kinetics of both products is correlated, while at high temperatures they decouple and ZrO2 forms an order of magnitude faster than Cu
Open Access
The effect of Ni or Co additions on the structure of Zr60Cu30Al10 bulk metallic glass revealed by high-energy synchrotron radiation
(Elsevier, 2022-04-13) Stiehler, Martin E.; Panagiotopoulos, Nikolaos T.; Keeble, Dean S.; Ivanov, Yurii P.; Menelaou, Melita; Jolly, Mark R.; Greer, A. Lindsay; Georgarakis, Konstantinos
The effect of substituting Cu by elemental additions of Ni or Co on the atomic structure of the Zr60Cu30Al10 ternary bulk metallic glass (BMG) is studied using high-energy synchrotron radiation X-ray diffraction. Analyses of the structural features in reciprocal and real space using the structure factors S(Q) and pair-distribution functions (PDF) point to an increase in the structural disorder for the Ni- or Co-bearing quaternary alloys. This is consistent with the “confusion principle” since upon alloying the initially nearly identical atomic sizes of Cu, Ni and Co diversify due to local electronic interactions. In real space, the disordering is manifested by a reduced deviation from the average particle density visible in the nearest-neighbour (NN) atomic shell structure over the complete short- and medium-range order region. Despite their similar atomic size, enthalpies of mixing with the main alloy elements and apparent disordering of the structure, the additions of Ni or Co have different effects on thermal stability of the ternary “mother” alloy.
Open Access
An experimental and simulation screening of X-65 steel weldment corrosion in high flow rate conditions
(Elsevier, 2024-06) Nofrizal, Nofrizal; Wulandari, Meyliana; Impey, Susan; Georgarakis, Konstantinos; Papanikolaou, Michail; Raja, Pandian Bothi
Over many decades, the oil and gas industry has encountered significant challenges due to weldment corrosion. The issue of internal-pipeline local corrosion at the welded joint region has garnered significant concern, especially due to the combined impact of high shear stress and electrochemical corrosion. This combination can lead to pipeline rupture with relative ease. Hence, a new approach to screen the flow corrosion of X-65 steel via electrochemical methods, predicting fluid shear stress and velocity using computational fluid dynamic (CFD) simulation is positively tested and presented here. For that, the X-65 steel specimens were cut/designed as the inner, centre, and outer electrodes of the target to analyse the Weld Metal (WM), Heat Affected Zone (HAZ), and Parent Metal (PM). Electrochemical screening was carried out simultaneously at a flow of 10 m/s using a brine solution saturated with CO2. The PM and HAZ will corrode less than the WM, in some cases at 30–23% of the rate of the WM. Thus in an environment of uninhibited brine saturated with CO2 at 10 m/s, preferential weld corrosion (PWC) is expected to occur. In addition, the surface morphology screening (scanning electron microscope with energy dispersive x-ray analysis, X-Ray diffraction, focus ion beam, raman spectroscopy) was employed to monitor the corrosion damage on the metal surface and also to support the electrochemical measurements (linear polarization resistance, galvanic measurement, and electrochemical impedance spectroscopy).
Open Access
(FeMnNi)84(AlTi)16 high‑entropy alloy: correlation of microstructure, strengthening mechanisms and hardness at various conditions (As‑Cast, solution treated, aged)
(Springer, 2022-04-01) Konakoglou, Kimon; Mathiou, C.; Georgatis, E.; Georgarakis, Konstantinos; Karantzalis, A. E.
A (FeMnNi)84(AlTi)16 high-entropy alloy was produced by vacuum arc melting successfully. The microstructure of the as-cast state showed the existence of two FCC phases along with potential precipitates. The solution treatment response of the alloy for 2 h at 1150 °C and the effect of aging time at 750 °C in the microstructure and microhardness were also evaluated. It was observed that the solution treatment parameters were insufficiently low to dissolve the as-cast precipitates into the matrix. The double FCC matrix identified may be correlated with a solidification range and insufficient diffusion during the solidification process. The maximum hardness at 90 min aging time can be mainly attributed to the precipitation shearing mechanism in both matrix areas. The lower hardness value reported at 160 h aging time was estimated that it is derived by the change of the main strengthening mechanism from shearing to Orowan. The island-like precipitates that depleted Ti element from the Ni-rich intergranular area may be identified as a Ni2AlTi Heusler phase.
Open Access
Formation of TiC-Cu nanocomposites by a reaction between Ti25Cu75 melt-spun alloy and carbon
(Elsevier, 2018-10-02) Dudina, Dina V.; Korchagin, Michail A.; Gavrilov, Alexander I.; Bulina, Natalia V.; Batraev, Igor S.; Esikov, Maksim A.; Georgarakis, Konstantinos; Kato, Hidemi
In this work, Ti25Cu75 melt-spun partially amorphous alloy was used as a source of Ti and Cu to synthesize in-situ TiC-Cu nanocomposites. The reaction between the alloy and carbon started during ball milling and continued during Spark Plasma Sintering. At the same time, during ball milling, the alloy experienced phase transformations: crystallization of the amorphous phase was followed by decomposition of TiCu3. Copper crystallites formed during the alloy transformations were the reason for the presence of copper regions 0.5–1 µm in size free from TiC nanoparticles in the sintered composites. The Ti-Cu intermetallics transformed into non-agglomerated TiC 10–20 nm in size distributed in the copper matrix. The hardness of the synthesized TiC-Cu nanocomposites exceeded that of composites obtained by conventional sintering of ball-milled Ti-C-Cu powders.
Open Access
Friction-induced phase transformations and evolution of microstructure of austenitic stainless steel observed by operando synchrotron X-ray diffraction
(Elsevier, 2022-05-22) Emurlaev, K.; Bataev, I.; Ivanov, I.; Lazurenko, D.; Burov, V.; Ruktuev, A.; Ivanov, D.; Rosenthal, M.; Burghammer, M.; Georgarakis, Konstantinos; Jorge Junior, A. M.
A materials’ structure and its evolution due to friction play a crucial role in understanding wear and related processes. So far, structural changes caused by friction are mostly studied using ex situ destructive characterization techniques, such as microscopy of post-mortem the prepared specimen by polishing and etching techniques. In this paper, the structural changes of AISI 321 austenitic stainless steel (ASS) during frictional loading were observed by the nondestructive operando method based on synchrotron X-ray diffraction (XRD). Although the martensitic transformation in AISI 321 steel starts at ca. -187 °C, frictional loading induces γ -(ε, α′) transformation in this alloy at room or even higher temperatures. The ε-martensite formation is observed only for a relatively short time. Subsequently, a mechanically-mixed layer (MML), composed mainly of the α′ phase, forms at the sample’s surface. Using XRD peak profile analysis, we observed the accumulation of dislocations, their ordering, and/or stress field shielding before and after phase transformations. The steady-state conditions are reached after ca. 69 friction cycles manifested in reaching the threshold values of the size of the coherent scattering regions (CSRs) and dislocation density in γ and α′ phases. For a better understanding of structural evolution, the microstructure of the sample was studied by scanning electron microscopy (SEM) after the experiment. The structure of the MML, its delamination, the formation of vortices, and carbide crushing are discussed.
Open Access
In-situ monitoring the structural pathway of a Ti-based alloy from metallic liquid to metallic glass
(Elsevier, 2025-04-25) Georgarakis, Konstantinos; Stiehler, Martin E.; Hennet, Louis; Guo, Yaofeng; Antonowicz, Jerzy; Louzguine-Luzgin, Dmitri V.; Jolly, Mark R.; Andrieux, Jérôme; Vaughan, Gavin B. M.; Greer, A. Lindsay
A metallic glass is formed when a molten metallic alloy is cooled rapidly enough that crystallisation is avoided. However, the way the atomic structure of the liquid converts to that of the glass is generally unknown. The main challenge is the sufficiently fast experimental acquisition of structural data in the undercooled liquid regime necessitated by the high cooling rates needed to avoid crystallisation. In the present study, using aerodynamic levitation, the Ni-free Ti-based alloy Ti40Zr10Cu34Pd14Sn2 was vitrified in-situ in a high-energy synchrotron X-ray beam while diffraction data were acquired during cooling from above the liquidus temperature Tliq to well below the glass-transition temperature Tg. The structure in the undercooled liquid regime shows an accelerated evolution. Both the local order in the short (SRO) and medium range (MRO) increases rapidly as the undercooled liquid approaches Tg, below which the amorphous structure “freezes”. Nevertheless, distinct differences between the evolution of SRO and MRO were observed. The structural rearrangements in the undercooled liquid are found to be correlated with a rapid increase in viscosity of the metallic liquid upon cooling. The new findings shed light on the evolution of the atomic structure of metallic liquids during vitrification and the structural origins of the sluggish kinetics that suppress nucleation and growth of crystalline phases.
Open Access
In-situ structural identification of Zr3Al2 type metastable phase during crystallization of a Zr-based MG
(Elsevier, 2016-08-21) Xing, Yan; Li, Yan; Wang, Xiangke; Yu, Xiangtian; Zhang, Tao; Georgarakis, Konstantinos
A metastable phase was detected using higher energy synchrotron radiation when Zr-based metallic glass (MG) was annealed under vacuum in Linkam hot stage at 848 K. The formation and transformation processes of metastable phase were recorded by synchrotron radiation method. The metastable phase during crystallization was identified as Zr3Al2 structure type according to powder diffraction and TEM analysis. The structure of Zr3Al2 type MCP was experimentally evidenced by 3D diffraction patterns and mathematically described. The identification of Zr3Al2 MCP could be helpful for the understanding of cluster structure of MG.
Open Access
In-situ TEM study of the crystallization sequence in a gold-based metallic glass
(Elsevier, 2020-06-18) Ivanov, Yu P.; Meylan, Carolina M.; Panagiotopoulos, Nikolaos T.; Georgarakis, Konstantinos; Greer, A. Lindsay
The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 (at.%) is of interest as the basis for the development of gold-based bulk metallic glasses for application in jewellery. In-situ heating in transmission electron microscopy (TEM) and differential scanning calorimetry (DSC, both conventional and fast) are used to obtain a comprehensive characterization of the decomposition on heating a melt-spun glass of this composition. Linking TEM with DSC over a range of heating rates 0.083‒2000 K s‒1, allows the sample temperature in the TEM heating stage to be calibrated. On heating up to melting, the glass decomposes in up to four stages: (1) complete transformation to single-phase nanocrystalline (Au,Cu)7Si; (2) grain growth of this phase; (3) precipitation of (Pd,Ag)Si, reducing the supersaturation of silicon in the (Au,Cu)7Si matrix; (4) with the precipitate phase remaining stable, decomposition of the matrix to a mixture of (Au,Ag)8Cu2, AuCu and Cu3Au phases. At all stages, grain diameters remain sub-micrometre; some of the stable nanocrystalline microstructures may themselves be of interest for applications. The characterization of the decomposition can assist in the optimization of the glass composition to improve tarnish-resistance, while retaining adequate glass-forming ability, formability in thermoplastic processing, and resistance to crystallization. For materials in general, the close correlation of in-situ TEM and DSC results should find wide use in characterizing complex transformation sequences.
Open Access
Interaction between Fe66Cr10Nb5B19 metallic glass and aluminum during spark plasma sintering
(Elsevier, 2020-08-28) Dudina, Dina V.; Bokhonov, Boris B.; Batraev, Igor S.; Amirastanov, Yusif N.; Ukhina, Arina V.; Kuchumova, Ivanna D.; Legan, Mikhail A.; Novoselov, Aleksey N.; Gerasimov, Konstantin B.; Bataev, Ivan A.; Georgarakis, Konstantinos; Koga, Guilherme Yuuki; Guo, Yaofeng; Botta, Walter José; Jorge, Alberto Moreira
In the area of metal matrix composites, novel reinforcing options are currently being evaluated. Particles of amorphous alloys present an interesting possibility to reinforce soft metals. In the present work, the interaction between Fe66Cr10Nb5B19 metallic glass and aluminum during spark plasma sintering (SPS) was studied for the first time. In order to trace the phase and microstructural changes upon sintering, mixtures containing 20 vol% and 50 vol% of metallic glass were subjected to SPS at 500–570 °C. After SPS at 500 °C, no reaction layer between the metallic glass particles and aluminum was observed. After SPS at 570 °C, a reaction layer containing Fe2Al5 and FeAl3 formed around the Fe-based cores. The Vickers hardness of composites obtained from mixtures containing 20 vol% Fe66Cr10Nb5B19 at 540 °C was 75 HV and increased to 280 HV after sintering at 570 °C due to the formation of thicker reaction layers at the interface. The hardness of the composite sintered from the mixture containing 20 vol% Fe66Cr10Nb5B19 at 570 °C was between the values predicted by Reuss and Voigt models. Comparison of results of SPS of the powder mixtures with those of SPS of a pre-compacted pellet and electric current-free annealing suggests that local heating at the interface caused by interfacial resistance may be an important factor influencing the reaction advancement at the interface and the formation of Al-containing intermetallics
Open Access
Investigation on the mechanically-induced nanocrystallization in metallic glasses
(Elsevier, 2020-11-10) Ammari, C.; Yousfi, M. A.; Hajlaoui, K.; Georgarakis, Konstantinos
Shear-induced nanocrystallization in bent ribbons of Pd40Cu30Ni10P20 metallic glass has been quantitatively investigated via synchrotron radiation. The formed nanocrystals volume fraction during deformation has been directly estimated from X-ray diffraction spectra using peaks area integration. The nanocrystallization process during deformation was found to be strongly linked with the microstructure configuration of shear bands in amorphous alloys. A constitutive model based on free volume approach has been introduced to describe the kinetic of mechanically induced nanocrystallization. The solution of the coupled constitutive equations of the model, fitted to experimental data, permits to determine the physical and mechanical parameters governing the phenomena of shear-induced crystallization in metallic glasses.
Open Access
Mechanically induced waves in metallic glass foils
(Elsevier, 2015-10-29) Panagiotopoulos, N. T.; Yousfi, M. A.; Georgarakis, Konstantinos; Yavari, A. R.
The response of vitrified metallic arc foils under normal load is studied. Application of normal load on an initial arc shaped vitrified metallic foil is followed by multiplication of the initial arc. A sinusoidal mathematical expression can be used for the description of the produced harmonic undulations. The number of the formed waves increases as the displacement increases. Therefore, this undulatory behavior of the vitrified foils can be exploited as a flat spring with multiple spring constants. For comparison crystalline foils were tested. The enormous elastic region of vitrified alloys allows this undulatory response to occur extensively while plastic deformation is unavoidable when crystalline foils are used. Exploiting the metallic glass characteristics, the predefined extrem a positions of the formed undulations and the mechanical characteristics of the vitrified foils a new type of electromechanical switch is suggested. (c) 2015 Elsevier Ltd. All rights