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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.

The effect of Zr addition on the melting temperature of the CoCrFeMnNi High Entropy Alloy (HEA), known as the “Cantor’s Alloy”, is investigated, together with its micro-structure, mechanical properties and thermomechanical recrystallization process. The base and Zr-modified alloys are obtained by vacuum induction melting of mechanically pre-alloyed powders. Raw materials are then cold rolled and annealed. recrystallization occurred during the heat treatment of the cold-rolled HEA. The alloys are characterized by X-ray diffraction, electron microscopy, thermal analyses, mechanical spectroscopy and indentation measures. The main advantages of Zr addition are: (1) a fast vacuum induction melting process, (2) the lower melting temperature, due to Zr eutectics formation with all the Cantor’s alloy elements, (3) the good chemical alloy homogeneity, and (4) the mechanical properties improvement of re-crystallized grains with a coherent structure. The crystallographic lattice of both alloys results in FCC. The Zr-modified HEA presents a higher recrystallization temperature and smaller grain size after recrystallization with respect to the Cantor’s alloy, with precipitation of a coherent second phase, which enhances the alloy hardness and strength.

New composite ZrO2/SiO2 coatings on grade 4 titanium have been produced by the sol-gel synthesis and dip coating, in order to improve durability of the substrate. Mechanical testing by scratch test and nanoindentation allowed to identify the coatings with better adhesion, lower tendency to crack, stiffer, harder and with lower friction against diamond in dry sliding. Potentiodynamic polarization tests in Dulbecco’s phosphate-buffered saline solution were used to evaluate a possible extra corrosion protection imparted by the coatings to the already excellent corrosion resistance of the passivated grade 4 titanium substrate. The silica-containing coatings appeared to be the best tradeoff between the improvement of mechanical and corrosion resistance properties.New composite ZrO2/SiO2 coatings on grade 4 titanium have been produced by the sol-gel synthesis and dip coating, in order to improve durability of the substrate. Mechanical testing by scratch test and nanoindentation allowed to identify the coatings with better adhesion, lower tendency to crack, stiffer, harder and with lower friction against diamond in dry sliding. Potentiodynamic polarization tests in Dulbecco’s phosphate-buffered saline solution were used to evaluate a possible extra corrosion protection imparted by the coatings to the already excellent corrosion resistance of the passivated grade 4 titanium substrate. The silica-containing coatings appeared to be the best tradeoff between the improvement of mechanical and corrosion resistance properties.

Materials comprising carbon nanotube (CNT) aligned nanowire (NW) polymer nanocomposites (A-PNCs) are emerging as next-generation materials for use in aerospace structures. Enhanced operating regimes, such as operating temperatures, motivate the study of CNT aligned NW ceramic matrix nanocomposites (A-CMNCs). Here we report the synthesis of CNT A-CMNCs through the pyrolysis of CNT A-PNC precursors, thereby creating carbon matrix CNT A-CMNCs. Characterization reveals that the fabrication of high strength, high temperature, lightweight next-generation aerospace materials is possible using this method. Additional characterization and modeling are planned., Comment: 7 pages, 5 figures, conference proceeding