Search

The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep-testing at 550$^{\circ}$C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge ($f_v$: 0.23 $\pm$ 0.02 %) and thread regions ($f_v$: 0.06 $\pm$ 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which decreased from ($f_v$: 0.16 $\pm$ 0.02 %) for the initial state to ($f_v$: 0.03 $\pm$ 0.01 %) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep-strengthening MX particles increased from ($\lambda_{3D}$: 0.55 $\pm$ 0.05 $\mu m$) in the initial state to ($\lambda_{3D}$: 1.01 $\pm$ 0.10 $\mu m$) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of ($\lambda_{3D}$: 0.60 $\pm$ 0.05 $\mu m$). The locally deformed fracture region had an increased phase fraction of modified Z-phase ($f_v$: 0.40 $\pm$ 0.20 %), which implies that localised creep-strain strongly promotes the formation of modified Z-phase. The modified Z-phase precipitates did not form only on prior-austenite grain boundaries and formed throughout the tempered martensite ferritic grain structure., Comment: 11 pages, 9 figures

Two advanced, automated crystal orientation mapping techniques suited for nanocrystalline materials—precession electron diffraction (PED) in transmission electron microscopy (TEM) and on-axis transmission Kikuchi diffraction (TKD) in scanning electron microscopy (SEM)—are evaluated by comparing the orientation maps obtained from the identical location on a 30 nm-thick nanocrystalline tungsten (W) thin film. A side-by-side comparison of the orientation maps directly showed that the large-scale orientation features are almost identical. However, there are differences in the fine details, which arise from the fundamentally different nature of the spot pattern and Kikuchi line pattern in terms of the excitation volume and the angular resolution. While TEM-PED is more reliable to characterize grains oriented along low-index zone axes, the high angular resolution of SEM-TKD allows the detection of small misorientation between grains and thus yields better quantification and statistical analysis of grain orientation. Given that both TEM-PED and SEM-TKD orientation mapping techniques are complementary tools for nanocrystalline materials, one can be favorably selected depending on the requirements of the analysis, as they have competitive performance in terms of angular resolution and texture quantification.

Dendritic microstructures are frequently observed in as-solidified refractory high-entropy alloys (RHEAs), and their homogenization typically requires a long-term heat treatment at extremely high temperatures. High-pressure torsion (HPT) has been shown to be capable of mixing immiscible systems at room temperature, and therefore represents a promising technique for homogenizing dendritic RHEAs. In this work, the as-solidified RHEA MoNbTaTiVZr was processed up to 40 revolutions by HPT. It was found that the dendritic microstructure was eliminated, resulting in a chemical homogeneity at a von Mises equivalent shear strain of about 400. The study of deformation mechanism showed an initial strain localization, followed by a co-deformation of the dendritic and interdendritic regions. In the co-deformation step, the Zr-rich interdendritic region gradually disappeared. The deformation-induced mixing also led to the formation of an ultra-fine grained (UFG) microstructure, exhibiting a grain size of approximately 50 nm. The microhardness increased from 500 HV in the as-solidified to 675 HV in the homogenized UFG state. The underlying mechanisms responsible for the microhardness enhancement, such as grain refinement and solid solution strengthening, were also discussed.

A thin-film materials library in the system V–Bi–O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO2 phase M1 was detected by X-ray diffraction over the whole library, however a second phase was observed in the microstructure of films with Bi contents > 0.29 at%. The second phase was determined by electron diffraction to be BiVO4, which suggests that the solubility limit of Bi in VO2 is only ∼0.29 at%. For Bi contents from 0.08 to 0.29 at%, the phase transformation temperatures of VO2:Bi increase from 74.7 to 76.4 °C by 8 K per at% Bi. With X-ray photoemission spectroscopy, the oxidation state of Bi was determined to be 3+. The V5+/V4+ ratio increases with increasing Bi content from 0.10 to 0.84 at%. The similarly increasing tendency of the V5+/V4+ ratio and Tc with Bi content suggests that although the ionic radius of Bi3+ is much larger than that of V4+, the charge doping effect and the resulting V5+ are more prominent in regulating the phase transformation behavior of Bi-doped VO2.

The unique structural and electronic properties of transition metal dichalcogenides (TMDs) and in particular tungsten disulphide (WS2) make them interesting for a variety of applications such as the electrocatalytic hydrogen evolution reaction (HER) for water splitting devices and chemiresistive gas sensors. The key parameter for the realisation of these devices is the controlled large-area growth of WS2 combined with tuning the surface morphology and electronic properties which is achieved by bottom-up fabrication methods such as chemical vapour deposition (CVD). In this study, 2H-WS2 films are fabricated by a new metal–organic CVD (MOCVD) process resulting in the growth of crystalline, pure, and stoichiometric films which was accomplished at temperatures as low as 600 °C as confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS)/nuclear reaction analysis (NRA), and Raman spectroscopy. The surface morphology of WS2 layers was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). Following successful process development, the WS2 layers were deposited on conducting FTO/glass substrates and their catalytic activity for the HER was evaluated in a linear sweep voltammetry (LSV) experiment. Furthermore, the temperature-dependent sensor response towards NO2, CO, and NH3 was investigated for WS2 films deposited on special sensor chips, revealing a p-type response towards NO2 and NH3 and sensitivities of around 20% for NO2 and NH3 concentrations of 1.5 ppm and 7.6 ppm, respectively. These promising results demonstrate the effectiveness of scalable CVD-grown WS2 and pave the way for practical applications by modulating the properties of materials to achieve enhanced electrocatalytic and sensing performances employing WS2 layers.

Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it.

Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys.

The Cr-Co-Ni system was studied by combining experimental and computational methods to investigate phase stability and mechanical properties. Thin-film materials libraries were prepared and quenched from high temperatures up to 700 °C using a novel quenching technique. It could be shown that a wide A1 solid solution region exists in the Cr-Co-Ni system. To validate the results obtained using thin-film materials libraries, bulk samples of selected compositions were prepared by arc melting, and the experimental data were additionally compared to results from DFT calculations. The computational results are in good agreement with the measured lattice parameters and elastic moduli. The lattice parameters increase with the addition of Co and Cr, with a more pronounced effect for the latter. The addition of ∼20 atom % Cr results in a similar hardening effect to that of the addition of ∼40 atom % Co.

Ag–V–O thin-film materials libraries, with both composition (Ag22-77V23-78Ox) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7 were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm2) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag32V68Ox) which confirmed the presence of Ag2V4O11 as the dominating phase along with the minor phases AgV6O15 and AgVO3. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting.

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

The microhardness distribution in the different zones of a friction stir welded reduced activation ferritic/martensitic steel has been investigated and correlated to the hierarchical martensitic microstructure in the respective zones, characterized by electron backscatter diffraction orientation analysis. It is found that the variation of prior austenite grain size, packet size, and block width in different subzones is influenced by the peak temperature and effective strain rate during the friction stir welding process. The distribution of the microhardness correlates directly with the geometrically necessary dislocation density observed in the different zones.

Cr-Al-N thin film materials libraries were synthesized by combinatorial reactive high power impulse magnetron sputtering (HiPIMS). Different HiPIMS repetition frequencies and peak power densities were applied altering the ion to growth flux ratio. Moreover, time-resolved ion energy distribution functions were measured with a retarding field energy analyzer (RFEA). The plasma properties were measured during the growth of films with different compositions within the materials library and correlated to the resulting film properties such as phase, grain size, texture, indentation modulus, indentation hardness, and residual stress. The influence of the ion to growth flux ratio on the film properties was most significant for films with high Al-content (xAl = 50 at. %). X-ray diffraction with a 2D detector revealed hcp-AlN precipitation starting from Al-concentration xAl ≥ 50 at. %. This precipitation might be related to the kinetically enhanced adatom mobility for a high ratio of ions per deposited atoms, leading to strong intermixing of the deposited species. A structure zone transition, induced by composition and flux ratio JI/JG, from zone T to zone Ic structure was observed which hints toward the conclusion that the combination of increasing flux ratio and Al-concentration lead to opposing trends regarding the increase in homologous temperature.

A new wrought CrCoNi-based medium-entropy superalloy (MESA) was designed by changing the composition of a commercial superalloy of type C-263, which is used for stationary components in gas turbines. ∼5 at.% Cr and 0.85 at.% Ti + Al were added at the expense of Ni while the Ti/Al ratio was decreased. Owing to these modifications, the brittle η phase, which is stable in C-263 below 900 °C is no longer observed in C-264. Besides, the solvus temperature and volume fraction of the γ′ phase in the peak-aged state are larger in C-264 (∼935 °C, 13.5%) compared to C-263 (∼890 °C, 12.8%), resulting in superior tensile and creep properties. The stress and temperature dependencies of the creep rates were described by power-law and Arrhenius relationships. The stress exponents were between 4 and 5, while the apparent activation energies were 550 and 400 kJ/mol for C-264 and C-263, respectively. During creep at 880 °C in air, internal nitridation in both MESAs resulted in the formation of TiN precipitates, with C-264 being slightly more affected due to its higher nitrogen solubility. Due to its superior creep resistance, good malleability and machinability, the C-264 MESA is currently commercially available from VDM Metals International.

The direct liquid injection chemical vapor deposition (DLI-CVD) of uniform and dense zirconium oxide (ZrO2) thin films applicable as corrosion protection coatings (CPCs) is reported. We present the entire development chain from the rational choice and thermal evaluation of the suitable heteroleptic precursor [Zr(OiPr)2(tbaoac)2] over the detailed DLI-CVD process design and finally benchmarking the CPC behavior using electrochemical impedance spectroscopy (EIS). For a thorough development of the growth process, the deposition temperature (Tdep) is varied in the range of 400 – 700 °C on Si(100) and stainless steel (AISI 304) substrates. Resulting thin films are thoroughly analyzed in terms of structure, composition, and morphology. Grazing incidence X-ray diffractometry (GIXRD) reveals an onset of crystallization at Tdep ≥ 500 °C yielding monoclinic and even cubic phase at low temperatures. At Tdep = 400 °C, isotropic growth of XRD amorphous material is shown to feature cubic crystalline domains at the interfacial region as revealed by electron diffraction. Corrosion results obtained through EIS measurements and further immersion tests revealed improved CPC characteristic for the 400 °C processed ZrO2 coatings compared to the ones deposited at Tdep ≥ 500 °C, yielding valuable insights into the correlation between growth parameter and CPC performance which are of high relevance for future exploration of CPCs.

International audience; We have investigated the partitioning of solutes at crystal defects in intergranular Cr-rich M2B borides after creep at 850°C/185MPa and annealing at 850°C for approximately 3000 hours in a polycrystalline nickel-based superalloy. Borides were found to coarsen in both cases, with the borides after creep to be the thickest (800-1100nm), compared to borides annealed in the absence of external applied load (400-600nm). Transmission electron microscopy revealed that the coarsened borides have either a tetragonal I4/mcm structure, or an orthorhombic Fddd, with those two structures coexisting in a single particle. The presence of a very high density of planar faults is systematically 2 observed within the coarsened borides. The faults were correlated with chemical fluctuations of B and Cr, revealed by atom probe tomography. In addition, partitioning of Ni and Co was observed at dislocations within the borides after creep providing insights into the deformation of borides.

The structure and chemistry of the orthorhombic O′ phase after quenching of a Ti-23 at.%Nb-2 at.%O was measured using aberration-corrected transmission electron microscopy and atom probe tomography. The nanosized O′ phase, formed in the vicinity of ω, is enriched with oxygen and slightly depleted in Nb. Upon annealing, ω dissolves and the O′ phase develops in β up to 350 °C, above which temperature it transforms to colonies of fine α phase. Another needle-like form of α with lower Nb content is thought to nucleate from Nb-lean regions related to spinodal decomposition of β.

A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization...

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states.

Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system.

Nanoindentation has become a rather mature tool to characterize and measure mechanical properties of materials on micro- and nanometer length scales. However, the different types of indentation size effects (ISE) hamper the assessment of true, i.e., scale independent, material properties by nanoindentation. In the present study, the mechanisms of the ISE in spherical indentation have been investigated and quantified. Furthermore, it has been shown that the determination of non-local crystal plasticity parameters from nanoindentation results is possible. The thus-parameterized non-local crystal plasticity describes the higher-order size effect occurring in spherical indentation correctly and offers the possibility to understand the mutual contributions of geometrically necessary dislocations (GND) and statistically stored dislocations (SSD) to the material response. Nanoindentation experiments have been performed into a single grain of an ARMCO iron specimen, where the load displacement curves have been recorded, and the post-indentation surface topologies have been characterized. Furthermore, transmission electron microscopy has been performed to analyse the plastically deformed volume beneath the indentation. By comparing experimental and numerical nanoindentation results with respect to the plastic zone and dislocation structures, we validate the non-local crystal plasticity model of indentation and increase our understanding of the underlying mechanisms of the ISE in spherical indentation.

Laser metal deposition (LMD) was used to in-situ alloy a crack-free Al0.6CoCrFeNi compositionally complex alloy (CCA) with 3 at.% Ti and 0.25 at.% C additions on an initially ferritic H10 tool steel from an elemental powder blend. After LMD, the material was annealed at 900 °C for 30 min to induce martensitic hardening in the substrate. The CCA in both as-deposited and annealed states exhibited a lamellar microstructure consisting of four phases: a matrix of interwoven disordered and ordered body-centered cubic phases, titanium carbides distributed randomly within the microstructure, and disordered face-centered cubic (FCC) plates that precipitated at the grain boundaries and grew towards the center of the grains. Chemical analyses along the build-up direction of the coating revealed a compositional gradient, similar in both as-deposited and annealed states, due to the intermixing between the substrate and the CCA. Despite a strong variation of the Fe-content, the hardness and the microstructure remain roughly constant in the major part of the as-deposited coating, which contains a large fraction of FCC plates that are beneficial to increase ductility and ensure a good compatibility with the substrate. In contrast, the upper part of the as-deposited coating, corresponding to the last solidified melt pool after LMD, has a much lower FCC fraction with an enhanced hardness. After annealing, the hardness of the tool steel substrate significantly increased and the FCC volume fraction in the coating increased from ~16% (as-deposited) to ~58%. Overall the microstructure of the coating became more homogeneous while its hardness decreased only by 10–15%. These results demonstrate that the CCA can be employed as a protective coating on a less expensive tool steel to improve its lifetime during service.

A thin-film materials library in the system V-Bi-O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by the thermo-mechanical treatments during processing. Here, the influence of a thermal exposure alone to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated gamma only; from this, the evolution of gamma-prime volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent gamma to gamma-prime phase transformations. Subsolvus heat treatments yielded a unimodal gamma-prime distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same gamma-prime population during heating, but dissolves as the temperature increases further. The gamma-prime then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the gamma-prime formed during heating or cooling.

Atom probe tomography was combined with transmission electron microscopy to characterize in atomic detail the structure of nanocrystalline high entropy alloy CrMnFeCoNi thin films before and after exposure to air at 500 °C for 5 min. Mn and Cr oxide scales were observed on the surface of the sample. These results on the nanoscale for short experimental time agree with literature reports on bulk counterparts after oxidation at 900 °C for 100 h, which means that the oxidation performance of complex materials can be tested in an accelerated way. Moreover, oxidation-related sub-surface depletion of Mn and Cr together with partial decomposition of the initial Cantor phase yield a FeCo-B2 sub-surface phase. In comparison to coarse-grained bulk material, the faster oxidation of the nanocrystalline material was attributed to an enhanced outwards diffusion of Cr and Mn along grain boundaries and within the newly formed B2 phase.

A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking.

The cast microstructure of the Al0.6CoCrFeNi compositionally complex alloy was successfully refined with small additions of Al, Ti and C and its mechanical properties were optimized. In the as-cast state, this alloy has a Widmanstätten microstructure with coarse grains (∼110 μm) of a strong BCC/B2 matrix and soft FCC plates (∼65 vol.%) with large widths (∼1.3 μm). The addition of 0.25 at.% C to Al0.6CoCrFeNi stabilizes the FCC phase and favors the formation of a coarse dendritic microstructure making this alloy unsuitable for structural applications. In contrast, alloying of either 3 at.% Al, Ti, or 3% Ti and 0.25% C to Al0.6CoCrFeNi refined its Widmanstätten microstructure, i.e. the thickness of the FCC plates and/or the size of the prior BCC/B2 grains were significantly reduced. As a result of these microstructural changes, Al and Ti containing alloys show an outstanding strength (twice higher than that of Al0.6CoCrFeNi) and ductilities ≤5% at 20 °C. These properties are retained at 400 °C but at 700 °C, the strength and ductility of almost all alloys decrease. However, Ti containing alloys exhibit much larger ductilities (∼50%) at 700 °C due to their high density of grain boundaries which accommodate plastic deformation through grain boundary sliding. Keywords: High-entropy alloys, Microalloying, Microstructural refinement, Grain boundary sliding, Tensile properties, Fracture surfaces

Cr-Al-N thin film materials libraries were synthesized by combinatorial reactive high power impulse magnetron sputtering (HiPIMS). Different HiPIMS repetition frequencies and peak power densities were applied altering the ion to growth flux ratio. Moreover, time-resolved ion energy distribution functions were measured with a retarding field energy analyzer (RFEA). The plasma properties were measured during the growth of films with different compositions within the materials library and correlated to the resulting film properties such as phase, grain size, texture, indentation modulus, indentation hardness, and residual stress. The influence of the ion to growth flux ratio on the film properties was most significant for films with high Al-content (

Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime.

The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading.

A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library.