Abstract
Microwaves at the ISM (Industrial, Scientific and Medical, reserved internationally) frequency of 2450 or 5800 MHz have been used to prepare FeCoNiCuAl, FeCrNiTiAl and FeCoCrNiAl2.5 high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave-assisted near-net-shape technology, using a powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). High entropy alloy compositions have been selected so as to comprise at least one ferromagnetic element and one highly reactive couple, like Ni-Al, Ti-Al, Co-Al or Fe-Al. Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the load. Microwaves have been applied both for the ignition and sustaining of such reactions, showing that by the proposed technique, it is possible to control the cooling rate of the newly-synthesized high entropy alloys. Results showed also that microwave heating in predominant magnetic field regions of the microwave applicator is more effective at controlling the cooling rate. The herein proposed microwave-assisted powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the prepared high entropy alloys in all cases was good, without the dendritic segregation typical of arc melting, even if some partially-unreacted powders were detected in the samples.
Highlights
High entropy alloys (HEA) are a class of multi-component alloys composed of five or more principal constituent elements, none of which is predominant, and each with a concentration between5 and 35 atomic % [1]
The heating curve shows that at a temperature slightly above the melting point of aluminum, a strong heating rate is encountered, and this is likely to occur due to combustion synthesis reactions taking place in the load
Results show that direct microwave heating of the powder precursors occurs, until the ignition conditions are reached
Summary
High entropy alloys (HEA) are a class of multi-component alloys composed of five or more principal constituent elements, none of which is predominant, and each with a concentration between5 and 35 atomic % [1]. These alloys have a tendency to form simple structures, like face centered cubic (FCC) and body centered cubic (BCC), instead of intermetallic compounds [2] This is because the high entropy of mixing reduces the free energy of the system and stabilizes these simple structures. Assuming an arbitrary choice of a group of 10 mutually-miscible metallic elements, this would enable the design of 7099 systems with five to 13 elements in equimolar ratios alone, provided the rules for the formation of HEA are respected. These involve atomic size difference, mixing enthalpy, mixing entropy, electronegativity and valence electron concentration among constituent elements [3]
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