Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment

The classic Cantor (FeCoCrMnNi) isoatomic high entropy alloy was modified by separate additions of Mo and Nb in an effort to optimize its mechanical properties and sliding wear response. It was found that the introduction of Mo and Nb modified the single phase FCC solid solution structure of the original alloy and led to the formation of new phases such as the BCC solid solution, σ-phase, and Laves, along with the possible existence of intermetallic phases. The overall phase formation sequence was approached by parametric model assessment and solidification considerations. Nanoindentation-based mechanical property evaluation showed that due to the introduction of Mo and Nb; the modulus of elasticity and microhardness were increased. Creep nanoindentation assessment revealed the beneficial action of Mo and Nb in increasing the creep resistance based on the stress sensitivity exponent, strain rate sensitivity, and critical volume for the dislocation nucleation considerations. The power law and power law breakdown were identified as the main creep deformation mechanisms. Finally, the sliding wear response was increased by the addition of Mo and Nb with this behavior obeying Archard’s law. A correlation between microstructure, wear track morphologies, and debris characteristics was also attempted

Development and characterization of refractory high entropy alloys - evaluation of their mechanical properties and surface degradation behavior

The present dissertation focuses on the development, characterization and evaluation of refractory high entropy alloys. These alloys are innovative materials which are targeted on applications of specialized requirements. The aim of the effort is to prepare single-phase and/or double phase structures and connect their microstructural features with their properties and degradation phenomena. More specifically, for each individual system produced, the microstructural characteristics were studied and the theoretical prediction models of solid solution formation, were checked. The corresponding solidification mechanisms were also studied, while their morphological characteristics were further analyzed. On the properties side, selected structures were studied as far as their mechanical properties (micro- and macro-hardness and compression tests) are concerned, while their strengthening mechanisms or failure modes were clarified. With regard to the alloys surface properties, the systems were examined in sliding wear tests under various experimental conditions. Through the recording of the numerical data and the study of the worn surfaces and debris, the corresponding wear mechanisms were formulated. Finally, the study of the corrosion behavior (solutions of 3.5 wt% NaCl and Hank) took place where, by analyzing the numerical data and the examination of the corroded surfaces / sections, the respective degradation mechanisms were interpreted.Η παρούσα διατριβή αφορά την ανάπτυξη, το χαρακτηρισμό και την αξιολόγηση της συμπεριφοράς πυρίμαχων κραμάτων υψηλής εντροπίας. Τα κράματα αυτά αποτελούν καινοτόμα υλικά και στοχεύουν σε εφαρμογές εξειδικευμένων απαιτήσεων. Στόχος της προσπάθειας ήταν να παρασκευαστούν μονοφασικές ή/και διφασικές δομές και να συνδεθούν τα μικροδομικά χαρακτηριστικά τους με τις ιδιότητες και τη συμπεριφορά σε φαινόμενα υποβάθμισης. Αναλυτικότερα, για κάθε επιμέρους σύστημα μελετήθηκαν τα μικροδομικά χαρακτηριστικά και έγινε έλεγχος των βιβλιογραφικών μοντέλων πρόβλεψης φάσεων στερεών διαλυμάτων. Προτάθηκαν οι αντίστοιχοι μηχανισμοί στερεοποίησης και αναλύθηκαν τα μορφολογικά χαρακτηριστικά αυτών. Στο σκέλος των ιδιοτήτων, επιλεγμένες δομές μελετήθηκαν ως προς τις μηχανικές τους ιδιότητες (μικρο- και μακρο-σκληρότητα και δοκιμές θλίψης), ενώ προτάθηκαν οι μηχανισμοί ενίσχυσης και αστοχίας τους. Σε ότι αφορά τις επιφανειακές ιδιότητες, τα συστήματα εξετάστηκαν σε δοκιμές φθοράς ολίσθησης υπό διάφορες πειραματικές συνθήκες. Μέσω της καταγραφής των αριθμητικών δεδομένων και της μελέτης των επιφανειών και ψηγμάτων της φθοράς, διατυπωθήκαν οι αντίστοιχοι μηχανισμοί υποβάθμισης. Τέλος, έλαβε χώρα η μελέτη της συμπεριφοράς τους σε διάβρωση (διαλύματα 3.5 κ.β% NaCl και Hank), όπου, μέσω της ανάλυσης των αριθμητικών δεδομένων και της εξέτασης των διαβρωμένων επιφανειών/τομών, ερμηνεύτηκαν οι αντίστοιχοι μηχανισμοί υποβάθμισης

Sonochemical synthesis of metallic Cu and Ni nanoparticles and/or their oxides and their decoration on ceramic substrates

113 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Επιστήμη και Τεχνολογία Υλικών”Με τη χρήση υπερήχων υψηλής ενέργειας είναι δυνατή η «διακόσμηση» (decoration) νανομετάλλων Cu, Ni ή/και οξειδίων τους σε υποστρώματα κεραμικής φύσης (πχ Al2O3, TiO2 και GDC (Gadolinia Doped Ceria)). Στην περίπτωση του Cu διεξήχθησαν πειράματα ηχοβόλησης διαλύματος ασκορβικού οξέος (αναγωγικό μέσο), hexadecyltrimethylammonium bromide - CTAB (διασπορέας), H2O και κεραμικού υποστρώματος για 30min και στη συνέχεια προσθήκη διαλύματος CuCl2*2H2O (πρόδρομη ένωση), NH3 (ρυθμιστής του pH) και H2O και συνέχιση της ηχοβόλησης για επιπλέον 2h. Η ηχοβόληση πραγματοποιήθηκε με έκλυση ενέργειας υπερήχων 20 kHz από την επιφάνεια ηχοδίου της τάξης των 100 και 30W/cm2 σε θερμοστατούμενο λουτρό 62οC. Μελετήθηκε τόσο η επίδραση των διαφορετικών συνδυασμών συγκεντρώσεων πρόδρομης ένωσης και υποστρώματος, όσο και οι αλλαγές στην ένταση των υπερήχων με στόχο την εξαγωγή συμπερασμάτων σχετικά με τις βέλτιστες συνθήκες διεξαγωγής των πειραμάτων. Τα δείγματα φυγοκεντρήθηκαν (3000 rpm) και ξεπλύθηκαν με απιονισμένο νερό για την παραλαβή/διαχωρισμό του υπερκείμενου υγρού και του τελικού στερεού. Το τελικό στερεό, μετά τη διαλυτοποίησή του, ελέγχθηκε με την ατομική εκπομπή με διέγερση πλάσματος (ICP-AES). Οι στερεές σκόνες εξετάστηκαν επίσης με XRD, SEM/EDX και TEM που υπέδειξαν πως όταν στο πρόδρομο διάλυμα προστεθεί ποσότητα σκόνης κεραμικού υποστρώματος, δημιουργείται σύνθετο υλικό λόγω της προσκόλλησης νανοσωματιδίων μεταλλικού Cu στην επιφάνεια της κεραμικής φάσης. Τέλος, επιλεγμένα δείγματα κατέστησαν συμπαγή μέσω της μεθόδου πυροσυσσωμάτωσης SPS (Spark Plasma Sintering). Ακολούθησε η εξέταση τους, τόσο στο Οπτικό Μικροσκόπιο για τον προσδιορισμό του πορώδους τους, όσο και στο Ηλεκτρονικό Μικροσκόπιο Σάρωσης για περαιτέρω διερεύνηση της μικροδομής τους. Για τη σύνθεση και εναπόθεση των σωματιδίων Ni(OΗ)2 ηχοβολήθηκαν διαλύματα NaOH 0,1M και Ni(CH3COO)2 4H2O 0,2M για 2h. Η ηχοβόληση πραγματοποιήθηκε με έκλυση ενέργειας υπερήχων 20 kHz από την επιφάνεια ηχοδίου της τάξης των 100 W/cm2 και 30 W/cm2 σε θερμοκρασία περιβάλλοντος. Με τη συνηχοβόληση όλων των συστατικών του αιωρήματος, διερευνήθηκε η δυνατότητα πρόσδεσης των νανοσωματιδίων σε διαφορετικά υποστρώματα (πχ Al2O3 και GDC). Τα δείγματα φυγοκεντρήθηκαν (3000rpm) και ξεπλύθηκαν τρεις φορές με απιονισμένο νερό για την παραλαβή του στερεού προϊόντος. Οι τελικές σκόνες εξετάστηκαν με XRD και SEM/EDX ενώ μετρήθηκε και το ποσοστό του ηχοχημικού επιφανειακού εμπλουτισμού τους με τη βοήθεια των μετρήσεων ICP-AES. Τέλος, για την ηχοχημική σύνθεση νανοσωματιδίων μεταλλικού νικελίου η τυπική διαδικασία σύνθεσης περιελάμβανε τα εξής: συγκεκριμένη ποσότητα πρόδρομης ένωσης NiCl2*6H2O διαλύθηκε σε αιθυλενογλυκόλη και ύστερα προστέθηκε υδραζίνη. Εν συνεχεία, προστέθηκε κατάλληλη ποσότητα NaOH και το τελικό διάλυμα τοποθετήθηκε στη συσκευή υπέρηχων για 2h. Η ένταση των υπερήχων διατηρήθηκε στα 100W/cm2 και η θερμοκρασία στους 62°C. Οι λαμβανόμενες σκόνες απομονώθηκαν μέσω φυγοκέντρησης (3000rpm) και ξεπλύθηκαν με απιονισμένο νερό. Τα δείγματα χαρακτηρίστηκαν με χρήση περίθλασης ακτίνων-Χ (XRD), SEM/EDX, TEM και ICP-AES για τον προσδιορισμό του ποσοστού της μεταλλικής «διακόσμησης».The high energy ultrasound irradiation is used for the decoration of Cu and Ni nanoparticles on different ceramic substrates (e.g. Al2O3, TiO2 and GDC (Gadolinia Doped Ceria)). In the case of Cu decoration, the sonication was conducted with the use of ascorbic acid (as a reductant), hexadecyltrimethylammonium bromide - CTAB (as a dispersing agent), H2O and different ceramic substrates for 30min. Then, the sonication continued for 2h with the addition of CuCl2*2H2O (precursor), NH3 (pH regulator) and H2O. The slurry was exposed to high intensity ultrasound radiation by employing a direct immersion horn (100 and 30W/cm2, 20kHz). During irradiation the temperature was kept at 62οC, using a thermostatic bath. Different combinations of the substrate and the precursor’s concentration, as well as the influence of the intensity changes were studied, in order to reach a conclusion about the ideal conditions of the experimental process. The final samples were centrifuged at 3000rpm, the supernatant was decanted from the solid powder and the later washed three times with distilled water. The final solids were examined by means of Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The solid powders were also examined with XRD, SEM/EDX and TEM which indicated that the addition of the ceramic substrate in the precursor solution creates a composite material, as a result of the Cu nanoparticle adhesion to the ceramic phase surface. Selected samples were also examined using Transmission Electron Microscopy (TEM), while some of the powders became dense by means of Spark Plasma Sintering (SPS) for their further characterization. For the synthesis of Ni(OH)2 nanoparticles, solutions of NaOH [0.1M] and Ni(CH3COO)2 4H2O [0.2M] were sonicated for 2h. The slurry was exposed to high intensity ultrasound radiation by employing a direct immersion horn (100W/cm2, 20kHz) at room temperature. The possibility of nanoparticle binding to the different substrates (Al2O3 and GDC) was examined. The final samples were centrifuged at 3000rpm and washed three times with distilled water in order to receive the solid powder. The powders were examined by means of XRD and SEM/EDX while the percentage of the metallic decoration was measured with ICP-AES. For the sonochemical synthesis of nickel nanoparticles the typical procedure is as follows: A certain amount of NiCl2*6H2O was dissolved in ethylene glycol and then hydrazine was added. An appropriate amount of NaOH was added, and the final solution was placed in a round bottom flask and sonicated using a high-intensity ultrasonic horn for 2h. The ultrasonic power density was 100W/cm2, while the temperature was kept at 62°C throughout the reaction. It was found that the reaction was completed only at elevated temperatures; therefore the reaction temperature was fixed at 62oC. The powders obtained were isolated by centrifugation (3000 rpm), washed three times with distilled water and dried at 100oC overnight. The percentage of nickel decoration on the ceramic substrates was estimated by means of ICP-AES, while the further characterization of the powders continued with XRD Analysis, SEM/EDX and TEM observation.Ανθούλα Β. Πούλι

NiAl-Cr-Mo Medium Entropy Alloys: Microstructural Verification, Solidification Considerations, and Sliding Wear Response

A series of NiAl-Cr-Mo systems were produced and assessed as far as their microstructure and their sliding wear resistance is concerned. The NiAl content was kept constant and seven compositions of Cr-Mo were tested, namely, 40Cr-0Mo, 30Cr-10Mo, 25Cr-15Mo, 20Cr-20Mo, 15Cr-25Mo, 10Cr-30Mo, and 0Cr-40Mo. It was observed that most of the systems contained primary phases, eutectic microconstituents, and, occasionally, intermetallic phases as the outcome of peritectic reactions. The extent and the nature of all these microstructural features was proved to be affected by the Cr/Mo relative ratio, and an attempt was conducted in order to explain the microstructural features based on solidification and other related phenomena. It was observed that the increase of the relative Mo/Cr ratio led to a significant restriction/elimination of the eutectic microconstituent. The sliding wear response of the produced system seems to diverge from the classical sliding wear laws of Archard and is based on multiple factors such as the nature of the oxide phases being formed upon sliding, the nature and the extend of the intermetallic phases being formed upon solidification, and the integrity and rigidity of the primary phases—last to solidify areas interfacial region and the factors that may influence this integrity

Nanoscale Magnetic Properties of Additively Manufactured FeCoNiAlxMnx High-Entropy Alloys

Magnetic properties of High-Entropy Alloys based on the Fe-Co-Ni-Al-Mn system are reported. High-Entropy Alloys are cutting-edge technological materials containing five or more elements in relatively high concentrations (5–35 at.%) within one or several solid-state solutions. These solutions are stabilized at the nanometer scale due to the high contribution of the mixing entropy to the Gibbs free energy, which can overcome the enthalpic contribution. Two magnetic alloys are found in FeCoNiAlxMnx (1.6 at.% x 7.8 at.%) samples processed by laser metal deposition. The magnetic techniques used to screen the materials were magneto-optical imaging and magnetic force microscopy. The former allows characterizing magnetic properties within the mm-μm scale, while the latter is efficient down to the nanometer scale. Magnetic screening confirms the importance of the nanostructure in defining magnetic properties of the alloys, and the trends in the magnetic behavior as a function of the alloy composition are revealed. The experimental results suggest that it is possible to form unique alloys, which may outperform conventional magnetic materials used in a variety of applications such as transformers, screening shields and wind power generators

Process–Structure–Property Relationship in FeCoNiAlxMnx Complex Concentrated Alloys Processed by Additive Manufacturing

Fourteen alloys of the FeCoNiAlxMnx system were processed by laser metal deposition (LMD). The feedstock was a weighted and proportional blend of the containing elemental powders, targeting the nominal alloy compositions. Prior to processing, the composition and particle characteristics of the feedstock were assessed. The microstructural features and crystal structures of all LMD processed materials were characterized with scanning electron microscopy/energy dispersive spectroscopy and x-ray diffraction, in both as-received and heat-treated conditions. Selected samples were investigated via scanning transmission electron microscopy and electron backscattered diffraction for further structural understanding. Hardness tests, under various indentation loads and dwelling times, were performed to assess the mechanical properties of the processed samples. The results showed a rise in hardness as Al and Mn contents increase. The variation of hardness with composition follows a reverse sigma-type curve, reflecting the microstructural evolution and grain size variations in the alloys. Based on the hardness data, we suggest a trained and validated predictive model, which can be used in alloy design for future developments

Nanoscale Magnetic Properties of Additively Manufactured FeCoNiAlxMnx High-Entropy Alloys

Magnetic properties of High-Entropy Alloys based on the Fe-Co-Ni-Al-Mn system are reported. High-Entropy Alloys are cutting-edge technological materials containing five or more elements in relatively high concentrations (5–35 at.%) within one or several solid-state solutions. These solutions are stabilized at the nanometer scale due to the high contribution of the mixing entropy to the Gibbs free energy, which can overcome the enthalpic contribution. Two magnetic alloys are found in FeCoNiAlxMnx (1.6 at.% x 7.8 at.%) samples processed by laser metal deposition. The magnetic techniques used to screen the materials were magneto-optical imaging and magnetic force microscopy. The former allows characterizing magnetic properties within the mm-μm scale, while the latter is efficient down to the nanometer scale. Magnetic screening confirms the importance of the nanostructure in defining magnetic properties of the alloys, and the trends in the magnetic behavior as a function of the alloy composition are revealed. The experimental results suggest that it is possible to form unique alloys, which may outperform conventional magnetic materials used in a variety of applications such as transformers, screening shields and wind power generators

Process–Structure–Property Relationship in FeCoNiAlxMnx Complex Concentrated Alloys Processed by Additive Manufacturing

Fourteen alloys of the FeCoNiAlxMnx system were processed by laser metal deposition (LMD). The feedstock was a weighted and proportional blend of the containing elemental powders, targeting the nominal alloy compositions. Prior to processing, the composition and particle characteristics of the feedstock were assessed. The microstructural features and crystal structures of all LMD processed materials were characterized with scanning electron microscopy/energy dispersive spectroscopy and x-ray diffraction, in both as-received and heat-treated conditions. Selected samples were investigated via scanning transmission electron microscopy and electron backscattered diffraction for further structural understanding. Hardness tests, under various indentation loads and dwelling times, were performed to assess the mechanical properties of the processed samples. The results showed a rise in hardness as Al and Mn contents increase. The variation of hardness with composition follows a reverse sigma-type curve, reflecting the microstructural evolution and grain size variations in the alloys. Based on the hardness data, we suggest a trained and validated predictive model, which can be used in alloy design for future developments.publishedVersio

Probing the structural evolution and its impact on magnetic properties of FeCoNi(AlMn)x high-entropy alloy at the nanoscale

We report the first nanoscale investigation of FeCoNi(AlMn)x high-entropy alloys (HEAs) processed by laser metal deposition. The structural evolution of the alloy upon chemical composition variation (0.2 ≤ x ≤ 1.5) was investigated by combining imaging and spectroscopies in (scanning) transmission electron microscopy (S)TEM with density functional theory (DFT). A gradual change from a face-centered cubic (FCC) towards an ordered full-Heusler (L21) phase by increasing the Al and Mn contents was observed. Direct imaging and atomic-scale calculations revealed a nanoscale interplay between B2 and L21 ordered structures for x = 1.5, wherein the latter, Al and Mn occupy two different Wyckoff sites. By decreasing x, the FCC phase dominates exhibiting intense phase separation tendency, ordering phenomena, and nano-precipitation. Although not chemically discriminated, plasmon-peak splitting in low-loss electron energy loss spectra revealed the presence of two valence electron densities within the FCC phase. Lorentz TEM showed that the ordered nano-precipitates and nano-sized grains with a structure based on a tripled FCC unit cell are pinning-sites for magnetic domain walls and dislocations. All alloy compositions exhibited soft-magnetic behavior with coercivity (Hc) values< 1000 A/m. The FeCoNi(AlMn)1.5 alloy with L21/B2 nanostructure showed the highest magnetization (Ms) with relatively low Hc, attributed to the large magnetic moment of Mn and the synergistic effect of Mn-Al according to DFT, whilst ordering does not impose a negative effect. Phase separation trends within the FCC phase seem to decrease the Ms however, the overall impact on the magnetic behavior is not intense, opening up for new avenues for tuning FeCoNiAlMn properties through chemically-designed phase decomposition regimes.publishedVersio

Probing the structural evolution and its impact on magnetic properties of FeCoNi(AlMn)x high-entropy alloy at the nanoscale

We report the first nanoscale investigation of FeCoNi(AlMn)x high-entropy alloys (HEAs) processed by laser metal deposition. The structural evolution of the alloy upon chemical composition variation (0.2 ≤ x ≤ 1.5) was investigated by combining imaging and spectroscopies in (scanning) transmission electron microscopy (S)TEM with density functional theory (DFT). A gradual change from a face-centered cubic (FCC) towards an ordered full-Heusler (L21) phase by increasing the Al and Mn contents was observed. Direct imaging and atomic-scale calculations revealed a nanoscale interplay between B2 and L21 ordered structures for x = 1.5, wherein the latter, Al and Mn occupy two different Wyckoff sites. By decreasing x, the FCC phase dominates exhibiting intense phase separation tendency, ordering phenomena, and nano-precipitation. Although not chemically discriminated, plasmon-peak splitting in low-loss electron energy loss spectra revealed the presence of two valence electron densities within the FCC phase. Lorentz TEM showed that the ordered nano-precipitates and nano-sized grains with a structure based on a tripled FCC unit cell are pinning-sites for magnetic domain walls and dislocations. All alloy compositions exhibited soft-magnetic behavior with coercivity (Hc) values< 1000 A/m. The FeCoNi(AlMn)1.5 alloy with L21/B2 nanostructure showed the highest magnetization (Ms) with relatively low Hc, attributed to the large magnetic moment of Mn and the synergistic effect of Mn-Al according to DFT, whilst ordering does not impose a negative effect. Phase separation trends within the FCC phase seem to decrease the Ms however, the overall impact on the magnetic behavior is not intense, opening up for new avenues for tuning FeCoNiAlMn properties through chemically-designed phase decomposition regimes