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The Al-rich part of the Fe-Al phase diagram between 50 and 80 at.% Al including the complex intermetallic phases Fe 5 Al 8 (ε), FeAl 2 , Fe 2 Al 5 , and Fe 4 Al 13 was re-investigated in detail. A series of 19 alloys was produced and heat-treated at temperatures in the range from 600 to 1100 °C for up to 5000 h. The obtained data were further complemented by results from a number of diffusion couples, which helped to determine the homogeneity ranges of the phases FeAl 2 , Fe 2 Al 5 , and Fe 4 Al 13 . All microstructures were inspected by scanning electron microscopy (SEM), and chemical compositions of the equilibrium phases as well as of the alloys were obtained by electron probe microanalysis (EPMA). Crystal structures and the variation of the lattice parameters were studied by x-ray diffraction (XRD) and differential thermal analysis (DTA) was applied to measure all types of transition temperatures. From these results, a revised version of the Al-rich part of the phase diagram was constructed.

The lack of a principle element in high-entropy alloys (HEA) leads to unique and unexpected material properties. Tribological loading of metallic materials often results in deformed subsurface layers. As the microstructure feedbacks with friction forces, the microstructural evolution is highly dynamic and complex. The concept of HEAs promises high solid solution strengthening, which might decrease these microstructural changes. Here, we experimentally investigated the deformation behavior of CoCrFeMnNi in a dry, reciprocating tribological contact under a mild normal load. After only a single stroke, a surprisingly thick subsurface deformation layer was observed. This layer is characterized by nanocrystalline grains, twins and bands of localized dislocation motion. Twinning was found to be decisive for the overall thickness of this layer, and twin formation within the stress field of the moving sphere is analyzed. The localization of dislocation activity, caused by planar slip, results in a grain rotation. Fragmentation of twins and dislocation rearrangement lead to a nanocrystalline layer underneath the worn surface. In addition, oxide-rich layers were found after several sliding cycles. These oxides intermix with the nanocrystalline layer due to material transfer to the counter body and re-deposition to the wear track. Having revealed these fundamental mechanisms, the evolution of such deformation layers in CoCrFeMnNi under a tribological load might lead to other HEAs with compositions and properties specifically tailored to tribological applications in the future.

This contribution presents a comprehensive analysis of the low temperature deformation behavior of CoCrFeMnNi on the basis of quasistatic tensile tests at temperatures ranging from room temperature down to 4.2 K. Different deformation phenomena occur in the high-entropy alloy in this temperature range. These include (i) serrated plastic flow at certain cryogenic temperatures (4.2 K/8 K), (ii) deformation twinning (4.2 K/8 and 77 K), and (iii) dislocation slip (active from 4.2 K up to room temperature). The importance of deformation twinning for a stable work-hardening rate over an extended stress range as well as strain range has been addressed through the use of comprehensive orientation imaging microscopy studies. The proposed appearance of ε-martensite as well as a previously uninvestigated route of analysis, essentially a quantitative time-dependent, strain-dependent, and stress-dependent evaluation of the serrated plastic flow in CoCrFeMnNi is provided.

We present the fabrication of a MoS 2−x Se x thin film from a co-sputtering process using MoS 2 and MoSe 2 commercial targets with 99.9% purity. The sputtering of the MoS 2 and MoSe 2 was carried out using a straight and low-cost magnetron radio frequency sputtering recipe to achieve a MoS 2−x Se x phase with x = 1 and sharp interface formation as confirmed by Raman spectroscopy, time-of-flight secondary ion mass spectroscopy, and cross-sectional scanning electron microscopy. The sulfur and selenium atoms prefer to distribute randomly at the octahedral geometry of molybdenum inside the MoS 2−x Se x thin film, indicated by a blue shift in the A 1g and E 1 g vibrational modes at 355 cm −1 and 255 cm −1 , respectively. This work is complemented by computing the thermodynamic stability of a MoS 2−x Se x phase whereby density functional theory up to a maximum selenium concentration of 33.33 at.% in both a Janus-like and random distribution. Although the Janus-like and the random structures are in the same metastable state, the Janus-like structure is hindered by an energy barrier below selenium concentrations of 8 at.%. This research highlights the potential of transition metal dichalcogenides in mixed phases and the need for further exploration employing low-energy, large-scale methods to improve the materials’ fabrication and target latent applications of such structures.

Phase separation into an A2+B2 two-phase microstructure in refractory compositionally complex alloys (RCCA) has been speculated as being spinodal in nature with continuous chemical distribution during the separation. However, these reactions might instead occur as precipitation by nucleation and growth. In order to unequivocally elucidate the distinct nature of phase separation sequence in RCCA from the system Ta-Mo-Ti-Cr-Al, atom probe tomography and electron microscopy techniques were utilized on samples that were annealed over multiple orders of magnitude in time. The composition 82(TaMoTi)-8Cr-10Al (at.%) was chosen, as it exhibits a two-phase microstructure, with a desired A2 matrix and embedded B2 phase. Quenching the samples from 1200°C resulted in a microstructure consisting of ordered clusters (2 nm) of distinct chemical composition. Subsequent annealing at 800°C to 1000°C leads to an increase in the volume fraction of the precipitating phase, which saturates after 10 h. Further annealing leads to the ripening of the microstructure; however, the absolute size of the precipitates stays <100 nm even after 1000 h. For the investigated conditions, the interface between matrix and precipitate can be considered sharp within the resolution of the applied techniques and no significant change in the transition of chemical composition across the interface is observed. Therefore, the phase separation mechanism is confirmed to be phase nucleation and growth in contrast to the possible spinodal decomposition, as hypothesized for other RCCA systems. The impact of precipitation and coarsening on the hardness of the alloy is discussed.

The directionally solidified eutectic alloy NiAl-(Cr,Mo) is a promising candidate for structural applications at high temperatures, due to its increased creep resistance compared to its single phase B2ordered NiAl counterpart. This system yields an eutectic trough connecting the invariant reactions of the ternary alloys NiAl-Cr and NiAl-Mo. During directional solidification (DS) along this trough the evolved microstructures of the two-phase eutectic is changing from fibrous to lamellar and back to fibrous morphology while increasing and decreasing the amounts of Mo and Cr, respectively. To investigate these effects in the morphology, the phase-field method has proven to be predestined in the last decades. However, as the modeling of quaternary systems is challenging for the simulation with a grand potential based phase-field model, the focus of this work is on the generation of a material model for one defined compound namely NiAl-31Cr-3Mo. The modeling is validated by investigating the microstructure evolution in two- and three-dimensional simulations of the DS process for two different growth velocities and by investigating their undercooling spacing relationships. The evolving microstructures obtained from three-dimensional large-scale simulations are presented and validated with corresponding micrographs from scanning electron microscopy (SEM) of directionally solidified samples with the same growth velocities. The simulation results show the theoretically expected behaviors and are in qualitative and quantitative accordance with DS experiments. The study of NiAl-31Cr-3Mo serves as the basis for a comprehensive data-driven analysis of microstructure properties and system quantities of the entire quaternary material NiAl-(Cr,Mo). With this, an accelerated design of advanced materials is promoted.

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types.

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to 1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)ss reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trend predicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diffraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientation relationship between NiAl matrix and (Cr,Mo)ss reinforcing phase was ( 100 ) NiAl|| ( 100 ) Cr,Mo and ⟨ 100 ⟩ NiAl|| ⟨ 100 ⟩ Cr,Mo. An orientation relationship described by a rotation of almost 60° about ⟨ 111 ⟩ was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed.

Mo-Ti alloys form solid solutions over a wide range of compositions, with lattice misfit parameters increasing significantly with titanium content. This indicates a strong increase in the critical stress for edge dislocation motion. Here, we probe the transition from screw to edge dislocation-dominated strengthening in Mo-Ti solid solutions with titanium content up to 80 at%. The alloys were scale-bridging characterized to isolate the impact of substitutional solid solution strengthening. Mechanical testing yielded no significant influence of grain boundaries or grain orientation. The results were corrected for the strengthening by unavoidable interstitial oxygen. Modelling of screw and edge dislocation-controlled solid solution strengthening was applied to the results to evaluate the contributions of both dislocation types. The analysis reveals that screw dislocation motion controls the strength in allows with less than 40 at% titanium, while edge dislocation motion provides comparable strength for 60–80 at% titanium. These results in a system of reduced chemical complexity support the recent investigations of edge dislocation-controlled strengthening found in high-entropy alloys. In body-centered cubic alloys, screw dislocations are considered to be strength-controlling. Here, a systematic investigation of Mo-Ti alloys with varying lattice misfit reveals a transition from screw to edge dislocation-controlled strength.

Due to its outstanding ductility over a large temperature range, equiatomic HfNbTaTiZr is well-suited for investigating the influence of temperature and plastic strain on deformation mechanisms in concentrated, body centered cubic solid solutions. For this purpose, compression tests in a temperature range from 77 up to 1073 K were performed and terminated at varying plastic strains for comparison of plastic deformation behavior. The microstructure and chemical homogeneity of a homogenized HfNbTaTiZr ingot were evaluated on different length scales. The compression tests reveal that test temperature significantly influences yield strength as well as work hardening behavior. Electron backscatter diffraction aids in shedding light on the acting deformation mechanisms at various temperatures and strains. It is revealed that kink band formation contributes to plastic deformation only in a certain temperature range. Additionally, the kink band misorientation angle distribution significantly differs at varying plastic strains.