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Based on the NiAl-Cr pseudo-binary phase diagram, ranging from hypo-to hyper-eutectic areas, alloys with various compositions were produced by arc melting. The microstructures were compared with the theoretical predictions of Tang et al. (Chem Chem Phys 18(29):19773-19786, 2016) and they were proved to be in agreement. The partial substitution of Cr by Mo in three different compositions revealed eutectic and hypo-eutectic microstructures with the eutectic micro-constituent modified from fibrous to lamellar. The importance of T0-A2 and T0-B2 temperatures [the maximum temperatures under which, partitionless solidification of A2 and B2 phases respectively happens (Laughlin and Hono in Physical metallurgy, Elsevier, Amsterdam, 2014)] and their relation to Te (eutectic temperature) proved to be the dominant factor that shifts the microstructures to off equilibrium morphologies in the case of NiAl-Cr alloys.

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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.

MoTaNbV x Ti refractory high entropy alloys were tested with dynamic indentation technique in order to assess their creep behavior. The loading rate was kept constant and three indentation depths were applied. It was found that, by increasing the indentation depth and the V content of the alloys, the stress exponent was reduced. This reduction was associated with the maximum applied stress at the peak load before the holding period and the increase of lattice distortion parameter with increasing the V content.

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