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

Microstructural evaluation of thermal-sprayed CoCrFeMnNi0.8V high-entropy alloy coatings

The aim of this work is to improve the understanding of the effect of the cooling rate on the microstructure of high-entropy alloys, with a focus on high-entropy alloy coatings, by using a combined computational and experimental validation approach. CoCrFeMnNi0.8V coatings were deposited on a steel substrate with high velocity oxy-air-fuel spray with the employment of three different deposition temperatures. The microstructures of the coatings were studied and compared with the microstructure of the equivalent bulk high-entropy alloy fabricated by suction casting and powder fabricated by gas atomization. According to the results, the powder and the coatings deposited by low and medium temperatures consisted of a BCC microstructure. On the other hand, the microstructure of the coating deposited by high temperature was more complex, consisting of different phases, including BCC, FCC and oxides. The phase constitution of the bulk high-entropy alloy included an FCC phase and sigma. This variation in the microstructural outcome was assessed in terms of solidification rate, and the results were compared with Thermo-Calc modelling. The microstructure can be tuned by the employment of rapid solidification techniques such as gas atomization, as well as subsequent processing such as high velocity oxy-air-fuel spray with the use of different spray parameters, leading to a variety of microstructural outcomes. This approach is of high interest for the field of high-entropy alloy coatings

(FeMnNi)84(AlTi)16 high‑entropy alloy: correlation of microstructure, strengthening mechanisms and hardness at various conditions (As‑Cast, solution treated, aged)

A (FeMnNi)84(AlTi)16 high-entropy alloy was produced by vacuum arc melting successfully. The microstructure of the as-cast state showed the existence of two FCC phases along with potential precipitates. The solution treatment response of the alloy for 2 h at 1150 °C and the effect of aging time at 750 °C in the microstructure and microhardness were also evaluated. It was observed that the solution treatment parameters were insufficiently low to dissolve the as-cast precipitates into the matrix. The double FCC matrix identified may be correlated with a solidification range and insufficient diffusion during the solidification process. The maximum hardness at 90 min aging time can be mainly attributed to the precipitation shearing mechanism in both matrix areas. The lower hardness value reported at 160 h aging time was estimated that it is derived by the change of the main strengthening mechanism from shearing to Orowan. The island-like precipitates that depleted Ti element from the Ni-rich intergranular area may be identified as a Ni2AlTi Heusler phase

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

A New Cooling-Rate-Dependent Machine Learning Feature for the Design of Thermally Sprayed High-Entropy Alloys

Highly accurate machine learning (ML) approaches rely heavily on the quality of data and the design features that are used as inputs to the model. The applicability of these methods for phase formation predictions is questionable when it comes to the design of thermally sprayed high-entropy alloy (HEA) coatings using gas or water atomized powders as feedstock material. Phase formation from liquid state depends on the cooling rate during atomization which is several orders of magnitude higher when compared to arc-melted as-cast HEAs. In addition, during plasma spray the powder melts in the flame and re-solidifies under different cooling rates during deposition. To our knowledge, almost all ML algorithms are based on available datasets constructed from relatively low cooling rate processes such as arc melting and suction casting. A new approach is needed to broaden the applicability of ML algorithms to rapid solidification manufacturing processes similar to gas and water atomization by making use of existing data and theoretical models. In this study the authors introduce a cooling-rate-dependent design feature that can lead to accurate predictions of the HEA powder phase formation and the subsequent phases found in the spray coated materials. The model is validated experimentally and also by comparing the predictions with existing coating related data in the literature