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Abstract

In recent years, high entropy alloy research has experienced increased interest and it was found that some of these materials have extraordinary properties. High entropy alloys also show an increased damage resistance to high-energy particle irradiation, mainly due to effects caused by the increased configuration entropy. So far, no detailed studies have been carried out regarding the interaction with high-energy electromagnetic radiation, particularly by means of lasers. In this work, we compare results of ultrashort-pulse laser-matter interaction of the CrMnFeCoNi alloy (Cantor alloy), the most researched representative of this material group, with the conventional alloy stainless steel AISI 304. Since metals can in general be processed efficiently with ultrashort pulses, which is of particular interest for industrial applications, we performed our experiments with single infrared sub-picosecond pulses. The crater surface morphology and process energetics are discussed in detail and the validity of established ablation models is investigated. We find that the damage threshold of the CrMnFeCoNi alloy is slightly lower than that of AISI 304 and consequently CrMnFeCoNi alloy shows an increased ablation volume. Therefore, the high entropy alloy CrMnFeCoNi can be processed efficiently with ultrashort-pulse lasers.

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... Mass fluctuations of elements in HEAs lead to phonon broadening, which significantly depresses the contribution of phonon thermal conductivity [38]. On the other hand, thermodynamic properties (phase transition enthalpies or temperatures) are not substantial affected by the retarded electron mobility [37,39]. Since thermodynamic and transport properties are crucial parameters for understanding ultrafast laser ablation, HEAs provide an ideal illustration for the stress confinement hypothesis and are therefore best suited for the isolation of the effect of early mechanical motion on the ablation efficiency. ...
... Subsequent polishing with (9, 3 and 1) µm polycrystalline diamond suspensions resulted in a root mean square surface roughness of 1.2 nm. A detailed description of the manufacturing process, sample preparation and analysis of the sample quality can be found in our previous works [39,40]. ...
... The fluence and pulse duration resolved crater diameters (D 2 measurements), used to determine the depicted values, are given in sFigure 2 of the supplementary materials. At 0.5 ps pulse duration, the evaluated ablation threshold Φ thr is 0.24(1) J/cm 2 , which is consistent with our previous work [39]. Assuming that a specific energy density threshold leads to material ablation, for example, by reaching the evaporation enthalpy, which is ΔH v = 47.4 ...
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In the context of current state of the art, understanding the laser ablation efficiency decrease for pulse durations exceeding the mechanical relaxation time of a few ps remains a pending research question. A heuristic approach may be used to reveal the role of effective penetration depth on ablation efficiency. Extending familiar contributions of this quantity by a term related to the mechanical surface expansion during pulse irradiation, the relation of ablation efficiency and pulse duration is deciphered. Thus, longer pulses are coupled into an expanded surface, revealing a direct link to the violation of stress confinement. To best demonstrate this hypothesis, a material with high electron-phonon coupling as well as low thermal conductivity, i.e., strong electron localization, is required. These properties are accomplished by high-entropy alloys, and the CrMnFeCoNi alloy serves as prime candidate. We report on single-pulse ablation efficiency experiments of the CrMnFeCoNi alloy which are support by our proposed model.
... Fluence isophotes were not examined in this case. The shape of crater profiles may give an initial indication of the characteristics of the ablation process that took place [45]. Ten craters per peak fluence were examined for the determination of D 2 eff , the crater depths, and the volumes. ...
... Similar to the D 2 method, the Beer-Lambert law may be applied to describe the crater depth with threshold-like ablation behavior. A short-coming of this approach is that spallation typically gives a minimum crater depth near the ablation threshold, which is not accurately described by exponential behavior [45]. We therefore extend the Beer-Lambert approach with an additional spallation depth term d spl in Eq. (2) to give: ...
... The fit results are given in Table 1. In Fig. 1(a), a flat crater profile with a depth of about 25 nm for a peak fluence of 0.23 J/cm 2 may be observed, which gives an indication of a spallation [45]. Around 25 nm for peak fluences above 0.23 J/cm 2 , a notable bend in the crater profile may be observed where there is a transition from the steep crater profile near the edge. ...
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The interaction of ultrashort laser pulses above the ablation threshold of thin-film indium tin oxide (ITO) is examined with pump-probe microscopy. We are able to observe photomechanical spallation at delay times of hundreds of picoseconds, which plays a stronger role near the ablation threshold of 0.17 J/cm2. A phase explosion may also be observed at tens of picoseconds, playing a stronger role for increasing peak fluences. As one exceeds the material removal efficiency maximum near 0.6 J/cm2, a second spallation is observable in the center of the irradiated spot at a delay time of one nanosecond and corresponds to a crater depth of 50 nanometers. No discernable ridge formation has been observed. We recommend an industrial processing window of at least two pulses per position with a peak fluence between 0.6-1.0 J/cm2.
... In practice, the threshold fluence thr * determined from the ablated volume can differ from the threshold fluence thr gained from the ablation diameter by Equation (4) due to different interactions between the laser and the material at the surface and in the volume [14,15]. ...
... The ablation threshold thr * determined from the ablated volume with Equation (8) is lower than thr determined from the spot diameters with Equation (4) in Figure 6. Such discrepancy between the values for the ablation threshold is also observed and analyzed in the literature for other materials and explained by melt phenomena or the formation of a thin spallation layer [14,15]. ...
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Nanofillers are added to polymeric insulating materials in order to modify the electrical, mechanical and thermal properties. This study investigates silicone rubber filled with hydrophilic and hydrophobic silica nanoparticles with respect to their resistance to laser ablation and to arc discharges, as well as their mechanical properties. The laser ablation is performed with single femtosecond laser pulses at wavelengths of 520 nm and 1040 nm. This way, the influence of the nanoparticles on the ablation by laser pulses with defined energy is investigated. The energy absorbed in the ablated volume is determined by taking into account the absorption for various fluences above the ablation threshold. For the silica nanocomposites, the energy specific ablation volume (ESAV), i.e., the ablated volume per absorbed energy, is significantly reduced compared to the unfilled polymer. The reduction is more pronounced for hydrophilic nanoparticles than for hydrophobic nanoparticles. Furthermore, the resistance to arc discharges and the mechanical strength of the nanocomposites are higher compared to the unfilled polymer. In particular, for nanocomposites with different types of silica filler particles the resistance to laser ablation, expressed by the ESAV, the resistance to arc discharges and the mechanical strength, expressed by Shore A hardness and the Young's modulus, show a strong correlation. From this, it is concluded that the modifications of the material properties are caused by interactions between the polymer and the nanoparticle surface, i.e., an interphase. The different extent of this interphase becomes obvious in the different degrees of impact on various electrical and mechanical properties.
... Thus, in addition to hydrodynamic mechanisms and plasma dynamics, the transient reflectance directly affects the ablation efficiency during laser processing. In recent experimental studies, the optimum ablation efficiency on stainless steel in terms of volume removed per pulse energy has been reported at about H 0 ≈ 1 J/cm 2 for laser irradiation with both single 36 and multipulses. 37,38 However, at higher fluences H 0 > 1 J/cm 2 , a steady decrease of the ablation efficiency was determined. ...
... Such remaining crater rims were previously attributed to the spallation ablation process during single-pulsed irradiation. 36 Contrary to the sharp edge at the spallation threshold, the threshold for phase explosion ( Figure 2, right C) is less clearly defined than in the image detected by light microscopy. Contrary to B, area C features many droplets possibly resulting from resolidification of the remaining liquid material. ...
Article
The ablation efficiency during laser processing strongly depends on the initial and transient reflectance of the irradiated material surface. This article reports on the transient relative change of the reflectance ΔR/R of stainless steel during and after ultrashort pulsed laser excitation (800 nm, 40 fs) by spatially resolved pump–probe reflectometry. The spatial resolution of the setup in combination with the spatial Gaussian intensity distribution of the pump radiation enables a fluence-resolved detection of ΔR/R. Within the first picosecond after irradiation with a peak fluence of 2 J/cm2, the spatially resolved ΔR/R of stainless steel evolves into an annular shape, in which the center almost remains at its initial reflectance, whereas the outer region features a decreased reflectance. The decreasing trend of ΔR/R is qualitatively supported by applying a two-temperature model, considering the transient optical properties of stainless steel from the literature. At larger fluences and thus higher electron temperatures, the experimental data deviate from the transient reflectance given in the literature. A drastically decreased occupation of the states below the Fermi energy and the subsequent excitation of electrons into these new vacant states by the probe radiation are considered being the most probable origin for this behavior at high fluences.
... Both effects depend strongly on the pulse duration [14][15][16]. In order to determine pulse duration dependence without these effects, single-pulse ablation has to be studied [17][18][19]. In all these previous studies, laser parameters such as wavelength, laser spot diameter, pulse-overlap and repetition rate have been changed from study to study and were even inconsistent within an individual study. ...
... We observe that the ESAV maximum for single-pulse ablation lies between 3-4.5 times the corresponding ablation threshold fluence ( Fig. 7 and Sec. 3.5) and is consistent with previous studies [18,19]. This differs from previous multi-pulse processing literate findings, which predicted and measured the ESAV maximum at a fluence of e 2 ·F thr ≈ 7.4·F thr [10,57,58]. ...
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In this work, we investigate single-pulse laser ablation of bulk stainless steel (AISI304), aluminium (Al) and copper (Cu) and its dependence on the pulse duration. We measured the reflectivity, ablation thresholds and volumes under the variation of pulse duration and fluence. The known drop of efficiency with increasing pulse duration is confirmed for single-pulse ablation in all three metals. We attribute the efficiency drop to a weakened photomechanically driven ablation process and a stronger contribution of photothermal phase explosion. The highest energetic efficiency and precision is achieved for pulse durations below the mechanical expansion time of 3-5 ps, where the stress confinement condition is fulfilled.
... But some researchers stated that the materials with four principal elements were also HEM [37]. For the entropy-based definition, the HEA has a configurational entropy larger than 1.5R (some researchers define that the entropy of HEA is higher than 1.61R), which is calculated as following equation [38]. ...
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High entropy materials (HEMs) are promising candidates as efficient hydrogen evolution reaction (HER) electrocatalyst and however, the facile fabrication of nanostructured HEMs over conventional catalyst supports remains huge challenges. Herein,...
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High entropy alloys (HEAs) are barely 12 years old. The field has stimulated new ideas and has inspired the exploration of the vast composition space offered by multi-principal element alloys (MPEAs). Here we present a critical review of this field, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment include definition of terms; thermodynamic analysis of complex, concentrated alloys (CCAs); taxonomy of current alloy families; microstructures; mechanical properties; potential applications; and future efforts. Based on detailed analyses, the following major results emerge. Although classical thermodynamic concepts are unchanged, trends in MPEAs can be different than in simpler alloys. Common thermodynamic perceptions can be misleading and new trends are described. From a strong focus on 3d transition metal alloys, there are now seven distinct CCA families. A new theme of designing alloy families by selecting elements to achieve a specific, intended purpose is starting to emerge. A comprehensive microstructural assessment is performed using three datasets: experimental data drawn from 408 different alloys and two computational datasets generated using the CALculated PHAse Diagram (CALPHAD) method. Each dataset emphasizes different elements and shows different microstructural trends. Trends in these three datasets are all predicted by a ‘structure in – structure out’ (SISO) analysis developed here that uses the weighted fractions of the constituent element crystal structures in each dataset. A total of 13 distinct multi-principal element single-phase fields are found in this microstructural assessment. Relationships between composition, microstructure and properties are established for 3d transition metal MPEAs, including the roles of Al, Cr and Cu. Critical evaluation shows that commercial austenitic stainless steels and nickel alloys with 3 or more principal elements are MPEAs, as well as some established functional materials. Mechanical properties of 3d transition metal CCAs are equivalent to commercial austenitic stainless steels and nickel alloys, while some refractory metal CCAs show potential to extend the service strength and/or temperature of nickel superalloys. Detailed analyses of microstructures and properties allow two major HEA hypotheses to be resolved. Although the ‘entropy effect’ is not supported by the present data, it has nevertheless made an enduring contribution by inspiring a clearer understanding of the importance of configurational entropy on phase stability. The ‘sluggish diffusion’ hypothesis is also not supported by available data, but it motivates re-evaluation of a classical concept of metallic diffusion. Building on recent published work, the CCA field has expanded to include materials with metallic, ionic or covalent bonding. It also includes microstructures with any number of phases and any type of phases. Finally, the MPEA field is shown to include both structural and functional materials applications. A significant number of future efforts are recommended, with an emphasis on developing high-throughput experiments and computations for structural materials. The review concludes with a brief description of major accomplishments of the field and insights gained from the first 12 years of research. The field has lost none of its potency and continues to pose new questions and offer new possibilities. The vast range of complex compositions and microstructures remains the most compelling motivation for future studies.
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A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiation-tolerant structural alloys for energy applications.
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This paper describes some underlying principles of multicomponent and high entropy alloys, and gives some examples of these materials. Different types of multicomponent alloy and different methods of accessing multicomponent phase space are discussed. The alloys were manufactured by conventional and high speed solidification techniques, and their macroscopic, microscopic and nanoscale structures were studied by optical, X-ray and electron microscope methods. They exhibit a variety of amorphous, quasicrystalline, dendritic and eutectic structures.
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In recent years, several applications for the laser ablation of thin metal films from the glass substrate side have been studied.In this case, the laser pulse transmits through the glass substrate, the metal film absorbs the pulse, and the energy is confined at the molybdenum-glass interface. The ablation mechanism is called confined laser ablation. It was observed that the confined laser ablation of 430 nm thin molybdenum films with ultrashort laser pulses leads to the lift-off of intact disks.The ablation efficiency can be increased by a factor of 10 compared to the direct ablation from the film side.The goal of the study is to investigate, if a confined ablation,which enables this high ablation efficiency, can be achieved also at different pulse durations and film thicknesses.For this purpose, the pulse duration is varied between 20 ns and 330 ns at a constant film thickness of 430 nm, andthe film thickness is varied between 10 nm and 5000 nm at a constant pulse duration of 10 ps. The single pulse ablation threshold fluences are determinedfor the film and glass substrate side irradiation. The determined threshold fluences are finally compared to calculated melting and evaporation threshold fluences.The results suggest that the ablation mechanism for glass side ablation is based mainly on melt dynamics, if the effective penetration depth of the laser pulse is in the order of the film thickness. A high efficient confined ablation can be observed, if the effective penetration depth is smaller than the film thickness.
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CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.
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We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model.
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Ultra short laser pulses are used, when high requirements concerning machining quality are demanded. The cost-effectiveness plays an important role for a successful transfer of this technology into industrial applications. Therefore systems with longer ps-pulses could offer an attractive alternative compared to short ps- and fs systems. Beside fiber based systems with similar to 50 ps pulse durations also small systems with sub-ns pulses are available today. In contrast to the dramatic drop of the material removal rate when the pulse duration is raised from 10 ps to 50 ps the impact is weaker for a further increase to 540 ps and the removal rate even increases when the pulse duration is raised to 1.4 ns. As no crater formation is observed and the surface quality is better than for 1.4 ns pulses, sub-ns pulses could be an attractive alternative to machine steel compared to fiber based systems with similar to 50 ps pulse duration. (C) 2013 The Authors. Published by Elsevier B.V.
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The microscopic mechanisms of femtosecond laser ablation of an Al target are investigated in large-scale massively parallel atomistic simulations performed with a computational model combining classical molecular dynamics technique with a continuum description of the laser excitation and subsequent relaxation of conduction band electrons. The relatively large lateral size of the computational systems used in the simulations enables a detailed analysis of the evolution of multiple voids generated in a sub-surface region of the irradiated target in the spallation regime, when the material ejection is driven by the relaxation of laser-induced stresses. The nucleation, growth, and coalescence of voids take place within a broad ( $\sim $ 100 nm) region of the target, leading to the formation of a transient foamy structure of interconnected liquid regions and eventual separation (or spallation) of a thin liquid layer from the bulk of the target. The thickness of the spalled layer is decreasing from the maximum of $\sim $ 50 nm while the temperature and ejection velocity are increasing with increasing fluence. At a fluence of $\sim $ 2.5 times the spallation threshold, the top part of the target reaches the conditions for an explosive decomposition into vapor and small clusters/droplets, marking the transition to the phase explosion regime of laser ablation. This transition is signified by a change in the composition of the ablation plume from large liquid droplets to a mixture of vapor-phase atoms and clusters/droplets of different sizes. The clusters of different sizes are spatially segregated in the expanding ablation plume, where small/medium size clusters present in the middle of the plume are followed by slower (velocities of less than 3 km/s) large droplets consisting of more than 10,000 atoms. The similarity of some of the characteristics of laser ablation of Al targets (e.g., evolution of voids in the spallation regime and cluster size distributions in the phase explosion regime) to the ones observed in earlier simulations performed for different target materials points to the common mechanical and thermodynamic origins of the underlying processes.
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High peak power lasers have been used for years for ablating matter. The most relevant application of this process is laser marking. Marking meets the demands of applications although the quality of ablation has potential to be further improved. However, the qualitative results of the ablation process especially for highly efficient removal of matter in the liquid phase like drilling have not met the standards of alternative processes—the latter is only the case in niches. On the other hand, the ablation by ultrafast lasers in the pulse regime of ps or below, which might meet the quality demands in terms of geometric precision, was too slow for economically feasible application because of the lack of average power. In fact, both process domains have been developed substantially and thus lead to a technological level which make them ready for industrial innovations. In a series of three articles on laser drilling—from fundamentals to application technology—the results of more than a decade of research and development are summarized with the purpose of displaying the bright application future of this laser process. This present part I deals with fundamentals, modeling, and simulation of laser drilling. Part II covers processing techniques, whereas part III is dedicated to systems and application technology. Fundamentals, modeling, and simulation: Theoretical analysis of the process from fs- to μs-pulses involves three inputs: numerical simulation, relevant analytic modeling, and as an important input for understanding, process analysis. The reduction of the models guided by experimental input leads to descriptions and knowledge of the process, which allows for strategic improvement of the applicability. As a consequence, process strategies can be derived, meeting the challenges of the application related to shape and accuracy of the surface free of recast as well as the economical demand for high speed processing. The domains of “cold ablation,” “hot ablation,” and “melt expulsion” are differentiated. Especially, the formation of recast up to closure of the drill is quantified. Tailoring the process parameters toward the individual application according to the know-how reached by the state of the art modeling and simulation leads to sound innovations and shorter innovation cycles.
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In this article, we review special features of Gwyddion—a modular, multiplatform, open-source software for scanning probe microscopy data processing, which is available at http://gwyddion.net/. We describe its architecture with emphasis on modularity and easy integration of the provided algorithms into other software. Special functionalities, such as data processing from non-rectangular areas, grain and particle analysis, and metrology support are discussed as well. It is shown that on the basis of open-source software development, a fully functional software package can be created that covers the needs of a large part of the scanning probe microscopy user community.
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Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multidimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behavior has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.
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Laser ablation using ultrashort pulsed (USP) laser sources enables contact free materials pro-cessing with very high precision at negligible thermal influence for the processed workpiece. How-ever, the achieved productivity is still too low for industrial applications in many cases. To increase the throughput of USP laser processes, three approaches are followed: high repetition rates up to sev-eral MHz using fast beam deflection techniques, pulse bursts or parallel processing with multiple laser foci. In case of high repetition rates and pulse bursts the temporal and spatial distance of consecutive pulses becomes so small that heat accumulation and shielding effects of plasma and particles affect the processing quality and achieved efficiency significantly. In order to gain a better insight of these still barely investigated effects we apply in this work in situ imaging with an ICCD camera during ablation processes of copper and stainless steel with pulse bursts to evaluate the excitation of process luminescence as well as its relation to ablation efficiency and shielding effects.
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Single-pulse microprocessing of a steel surface with a variable pulse width (0.3–12.3 ps) and a wavelength of 515 nm was performed. The morphology of the craters was visualized by a scanning electron microscope and an optical profilometer. The nonmonotonic behavior of ablation thresholds with a minimum at≈ 1.5 ps, due to achieving the electron–phonon relaxation time of the absorbed energy in the steel was revealed. It is shown that, with an increase in the pulse width in the considered width range, the efficiency of the ablation decreases by a factor of 2, which is explained by the partial transition from the phase explosion to the surface evaporation.
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Single phase concentrated solid solution alloys exhibit enhanced mechanical characteristics and radiation damage resistance, making them promising candidate materials for applications involving an exposure to rapid localized energy deposition. In this paper, we use large-scale atomistic modeling to investigate the mechanisms of the generation of vacancies, dislocations, stacking faults, and twin boundaries in Ni, Ni50Fe50, Ni80Fe20, and Ni80Cr20 targets irradiated by short laser pulses in the regime of melting and resolidification. The decrease in the thermal conductivity and strengthening of the electron-phonon coupling due to the intrinsic chemical disorder in the solid-solution alloys is found to have important implications on localization of the energy deposition and generation of thermoelastic stresses. The interaction of the laser-induced stress waves with the melting front is found to play a key role in roughening of the liquid-crystal interface and generation of dislocations upon the solidification. A common feature revealed in the structural analysis of all irradiated targets is the presence of high vacancy concentrations exceeding the equilibrium values at the melting temperature by about an order of magnitude. Based on the results of molecular dynamics simulations of solidification occurring at fixed levels of undercooling, the generation of vacancies is correlated to the velocity of the solidification front, and the processes responsible for creating the strong vacancy supersaturation are revealed. The suppression of the vacancy generation in the solid solution alloys is also revealed and related to combined effect of enhanced vacancy mobility and higher energy of the vacancy formation in the alloy systems. The analysis of the first atomic shells surrounding the vacancy sites in Ni-Fe alloys uncovers the preference for the vacancy sites to be surrounded by Fe atoms and suggests the important role that the atomic-scale chemical heterogeneities may play in defining the behavior and properties of the single-phase concentrated solid-solution alloys.
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This paper deals with a numerical investigation of the energy deposition induced by ultrafast laser interaction with nanostructures. We calculate and analyze the intensity near-field reactive and radiative patterns resulted from the interference of the incident light with light scattered by individual subwavelength holes and bumps on the surface of metallic and dielectric materials. The role of light polarization, optical material properties, collective effects and nature of the imperfections in localized energy absorption is elucidated. The results open new perspectives in precise light manipulation by surface inhomogeneities and well-controlled surface nanostructuring by ultrashort laser.
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High entropy alloys have gained significant interest due to several unique properties including enhanced radiation resistance. In this work, radiation induced segregation, a key phenomenon observed in alloys under irradiation, is examined for the first time at high angle grain boundaries under Ni heavy ion irradiation in the CoCrFeNiMn alloy. Our experimental study indicates significant Mn depletion and Co and Ni enrichment at grain boundaries. The segregation is discussed in the context of a proposed vacancy dominated radiation induced segregation mechanism and compared to existing models in conventional single core component alloys including stainless steels.
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Ablation of bulk polycrystalline zinc in air is performed with single and multiple picosecond laser pulses at a wavelength of 1030 nm. The relationships between the characteristics of the ablated craters and the processing parameters are analyzed. Morphological changes of the ablated craters are characterized by means of scanning electron microscopy and confocal laser scanning microscopy. Chemical compositions of both the treated and untreated surfaces are quantified with X-ray photoelectron spectroscopy. A comparative analysis on the determination of the ablation threshold using three methods, based on ablated diameter, depth and volume is presented along with associated incubation coefficients. The single pulse ablation threshold value is found to equal 0.21 J/cm2. Using the calculated incubation coefficients, it is found that both the fluence threshold and energy penetration depth show lesser degree of incubation for multiple laser pulses.
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The discovery of high entropy alloys at the turn of the millennium lead to a multitude of investigations of different types, focus and aims. With an increased knowledge of the new family of materials, it was possible to make a separation into true single phase high entropy alloys (HEA) and multi-phase compositionally complex alloys (CCA), which both fulfil the initial definition criteria. This review focuses on CCA that have been investigated and developed with a mechanical application in mind. A special importance is attributed to the mechanical testing methods, and priority is given to tensile testing at both room temperature and up to 700 °C. Precise microstructural characterization techniques like transmission electron microscopy and/or atom probe tomography ensure the determination of small scale phases, which could be overlooked when using only scanning electron microscopy and/or X-ray diffraction. Comparison of the investigations that meet these criteria are summarized in several tables and figures.
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Materials processing using ultrashort pulsed laser radiation with pulse durations <10 ps is known to enable very precise processing with negligible thermal load. However, even for the application of picosecond and femtosecond laser radiation, not the full amount of the absorbed energy is converted into ablation products and a distinct fraction of the absorbed energy remains as residual heat in the processed workpiece. For low average power and power densities, this heat is usually not relevant for the processing results and dissipates into the workpiece. In contrast, when higher average powers and repetition rates are applied to increase the throughput and upscale ultrashort pulse processing, this heat input becomes relevant and significantly affects the achieved processing results. In this paper, we outline the relevance of heat input for ultrashort pulse processing, starting with the heat input of a single ultrashort laser pulse. Heat accumulation during ultrashort pulse processing with high repetition rate is discussed as well as heat accumulation for materials processing using pulse bursts. In addition, the relevance of heat accumulation with multiple scanning passes and processing with multiple laser spots is shown.
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Nowadays the relevance and the robustness of ultrafast lasers are well established for many in-dustrial applications. Indeed this laser technology combines the unique capacity to process any type of material with an outstanding processing precision and a minimal heat affected zone. The key is-sue is to combine high throughput, low residual thermal load and good processing quality. Thanks to high average power and high repetition rate it is possible to achieve high throughput providing that the operating parameters are precisely tuned to the application, otherwise heat accumulation and heat affected zone may appear, leading to detrimental effects such as burr, uncontrolled melting and metal oxidation. In this paper we report on high-throughput laser ablation of stainless steel using a 100W- and 10MHz- ultrafast laser. Operating parameters such as fluence, repetition rate and scan-ning velocity have been considered. Results are discussed in terms of ablation efficiency, surface morphology, multipass and up-scaling capabilities. We observe that pulse-to-pulse pitch and delay are key parameters that must be taken into account in order to define relevant process windows. The use of polygon scanner instead of galvo scanner enables us to reduce the thermal load along the la-ser trajectory.
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Single-phase concentrated solid solution alloys have attracted wide interest due to their superior mechanical properties and enhanced radiation tolerance, which make them promising candidates for the structural applications in next-generation nuclear reactors. However, little has been understood about the intrinsic stability of their as-synthesized, high-entropy configurations against radiation damage. Here we report the element segregation in CrFeCoNi, CrFeCoNiMn, and CrFeCoNiPd equiatomic alloys when subjected to 1250 kV electron irradiations at 400 °C up to a damage level of 1 displacement per atom. Cr/Fe/Mn/Pd can deplete and Co/Ni can accumulate at radiation-induced dislocation loops, while the actively segregating elements are alloy-specific. Moreover, electron-irradiated matrix of CrFeCoNiMn and CrFeCoNiPd shows L10 (NiMn)-type ordering decomposition and <001>-oriented spinodal decomposition between Co/Ni and Pd, respectively. These findings are rationalized based on the atomic size difference and enthalpy of mixing between the alloying elements, and identify a new important requirement to the design of radiation-tolerant alloys through modification of the composition.
Article
Temperature dependent thermophysical properties, including specific heat capacity, lattice thermal expansion, thermal diffusivity and conductivity, have been systematically studied in Ni and eight Ni-containing single-phase face-centered-cubic concentrated solid solution alloys, at elevated temperatures up to 1273 K. The alloys have similar specific heat values of 0.4–0.5 J·g− 1·K− 1 at room temperature, but their temperature dependence varies greatly due to Curie and K-state transitions. The lattice, electronic, and magnetic contributions to the specific heat have been separated based on first-principles methods in NiCo, NiFe, Ni20Cr and NiCoFeCr. The alloys have similar thermal expansion behavior, with the exception that NiFe and NiCoFe have much lower thermal expansion coefficient in their ferromagnetic state due to magnetostriction effects. Calculations based on the quasi-harmonic approximation accurately predict the temperature dependent lattice parameter of NiCo and NiFe with < 0.2% error, but underestimated that of Ni20Cr by 1%, compared to the values determined from neutron diffraction. All the alloys containing Cr have very similar thermal conductivity, which is much lower than that of Ni and the alloys without Cr, due to the large magnetic disorder.
Article
Laser spot size and pulse number are two major parameters influencing the ablation of solids. The extended defect model describes the dependence of the threshold fluence on the basis of high and low density defects. This model was successfully applied to silicon and stainless steel. It is demonstrated that heat accumulation cannot describe the experimental results.
Article
Equimolar CrMnFeCoNi high entropy alloy (HEA) is one of the most notable single phase multi-component alloys up-to-date with promising mechanical properties at cryogenic temperatures. However, the study on the corrosion behavior of CrMnFeCoNi HEA coating has still been lacking. In this paper, HEA coating with a nominal composition of CrMnFeCoNi is fabricated by laser surface alloying and studied in detail. Microstructure and chemical composition are determined by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are used to investigate the corrosion behavior. The coating forms a simple FCC phase with an identical dendritic structure composed of Fe/Co/Ni-rich dendrites and Mn/Ni-rich interdendrites. Both in 3.5 wt.% NaCl solution and 0.5 M sulfuric acid the coating exhibits nobler corrosion resistance than A36 steel substrate and even lower icorr than 304 stainless steel (304SS). EIS plots coupled with fitted parameters reveal that a spontaneous protective film is formed and developed during immersion in 0.5 M sulfuric acid. The fitted Rt value reaches its maximum at 24 h during a 48 hours’ immersion test, indicating the passive film starts to break down after that. EDS analysis conducted on a corroded surface immersed in 0.5 M H2SO4 reveals that corrosion starts from Cr-depleted interdendrites.
Article
Defect production and growth in CrFeCoNi, a single-phase concentrated solid solution alloy, is characterized using in situ electron irradiation inside a transmission electron microscope operated at 400–1250 kV and 400 °C. All observed defects are interstitial-type, either elliptical Frank loops or polygonal (mostly rhombus) perfect loops. Both forms of loops in CrFeCoNi exhibit a sublinear power law of growth that is > 40 times slower than the linear defect growth in pure Ni. This result shows how compositional complexity impacts the production of Frenkel pairs and the agglomeration of interstitials into loops, and, thus, enhances the radiation tolerance.
Article
The irradiation behavior of AlxCoCrFeNi (x = 0.1, 0.75, and 1.5) high entropy alloys was studied under 3 MeV Au-ions irradiation. The microstructural change and volume swelling due to irradiation were investigated using transmission electron microscopy and atomic force microscopy. The results showed that, with increasing the Al contents, the phase crystal structures of the as-cast samples evolved from face-centered cubic (FCC), to FCC + body-centered cubic (BCC), and BCC and irradiation-induced volume swelling increased. All alloys showed exceptional structural stability when irradiated up to over 50 displacement per atom at 298 K, and the irradiation-induced volume swellings in AlxCoCrFeNi HEAs were significantly lower than conventional nuclear materials under similar irradiation dosages.
Chapter
The creep behavior of thin-walled specimens cannot be assumed to be as uniform as for thick-walled specimens. The difference is attributed to the influence of oxidation. Oxidation tests have been carried out on single crystal nickel-based superalloy M247LC SX specimens with different thicknesses down to 0.1 mm. The results show that the γ′-volume fraction is strongly affected by oxidation in near surface regions. Depletion of the aluminium reservoir required to establish a protective alumina scale causes the change. Based on these results five single crystal nickel-based superalloys with different compositions have been cast. These alloys represent different near surface regions within an oxidation affected specimen. Creep tests on single crystals of these alloys have been performed to quantify the influence of different γ′-volume fractions on secondary creep. The stress dependence was fit to the Norton creep law and the exponent n, exhibited sigmoidal behavior as a function of the γ′-fraction, almost doubling at 50% volume fraction. This behavior suggests a change in the dominant creep mechanism. These results are essential for modeling and predicting the creep properties of thin-walled specimens in turbine blade applications.
Article
High average laser powers can have a serious adverse impact on the ablation quality in ultra-short pulsed laser material processing of metals. With respect to the scanning speed, a sharp transition between a smooth, reflective and an uneven, dark ablated surface is observed. Investigating the influence of the sample temperature, it is experimentally shown that this effect stems from heat accumulation. In a numerical heat flow simulation, the critical scanning speed indicating the change in ablation quality is determined in good agreement with the experimental data.
Article
The solid-solution hardening potential of the refractory elements rhenium, tungsten and molybdenum in the matrix of single-crystal nickel-based superalloys was experimentally quantified. Single-phase alloys with the composition of the nickel solid-solution matrix of superalloys were cast as single crystals, and tested in creep at 980 °C and 30–75 MPa. The use of single-phase single-crystalline material ensures very clean data because no grain boundary or particle strengthening effects interfere with the solid-solution hardening. This makes it possible to quantify the amount of rhenium, tungsten and molybdenum necessary to reduce the creep rate by a factor of 10. Rhenium is more than two times more effective for matrix strengthening than either tungsten or molybdenum. The existence of rhenium clusters as a possible reason for the strong strengthening effect is excluded as a result of atom probe tomography measurements. If the partitioning coefficient of rhenium, tungsten and molybdenum between the γ matrix and the γ′ precipitates is taken into account, the effectiveness of the alloying elements in two-phase superalloys can be calculated and the rhenium effect can be explained.
Article
High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m1/2. Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.
Article
Ultrashort pulsed lasers can accurately ablate materials which are refractory, transparent, or are otherwise difficult to machine by other methods. The typical method of machining surfaces with ultrashort laser pulses is by raster scanning, or the machining of sequentially overlapping linear trenches. Experiments in which linear trenches were machined in alumina at various pulse overlaps and incident fluences are presented, and the dependence of groove depth on these parameters established. A model for the machining of trenches based on experimental data in alumina is presented, which predicts and matches observed trench geometry. This model is then used to predict optimal process parameters for the machining of trenches for maximal material removal rate for a given laser.
Article
Investment casting molds with different numbers of shells and pre-heating temperatures were investigated in this study. The primary layer consists of colloidal silica bound ZrSiO4 with additions of CoAl2O4 to achieve fine grains and to reach a good surface quality, whereas the following layers consist of mullite bound by colloidal silica. Interface temperatures (alloy/mold) that are necessary to determine heat transfer coefficients were obtained by linear extrapolation. Heat transfer coefficients in the range of 300–660W/(m2K) were obtained. The castings were examined with regard to grain size and secondary dendrite arm spacing. Physical properties of the investment casting mold were examined by differential scanning calorimetry (DSC) and Laserflash methods for temperatures up to 1300°C. The specific heat capacity was determined to 1.13J/(gK), thermal diffusivity was found to be in the range of (4–5)×10−7m2/s and the thermal conductivity to be 1±0.1W/(mK).