The cold spray technique is expected to effectively form a metallic coating with fine crystal grains originating from the microstructure of the original powder. We previously reported that a pure iron coating with fine crystal grains can be formed by the cold spray technique by using mechanically-milled pure iron powder. In this study, the tensile strength of the pure iron coating was investigated. The as-sprayed coating showed significantly low Young' s modulus, tensile strength, and ductility owing to the low cohesion strength between particles. For tensile strength improvement, the coating was subjected to spark plasma sintering (SPS) treatment. As a result, the Young' s modulus was considerably improved by the SPS treatments at 740 and 786℃ ; moreover, the tensile strength of the SPS-treated coating was approximately four times higher than that of the bulk material. In contrast, the ductility was not improved by the SPS treatment. The low ductility was likely attributed to the presence of Fe oxides at the particle-particle interface.

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The sacrificial corrosion protection ability of Al, Al-Mg, and Al-Si thermal spray coatings and the corrosion resistance of the defect part of the coatings, which was covered by a rust layer formed during an outdoor exposure or an accelerated corrosion test were studied by electrochemical techniques. A high-purity Al (>99.98%, Hp-Al) , a normal-purity Al (>99.5%, Np-Al) and Al alloys with 5% of Mg (Al-5Mg) , 5% of Si (Al-5Si) or 12% of Si (Al-12Si) were used for the material of the thermal spray coatings. The corrosion resistance of the defect part of the coatings was evaluated using the electrochemical reaction resistance (ERR) measured by the EIS technique. The ERR for the defect part of the Al-5Mg coating was superior than that for the defect part of other coatings, when the specimens had been subjected to the accelerating corrosion test. However, for the specimen subjected to an outdoor exposure test, the ERR for the defect part of the Al-5Mg coating was roughly equal to other coatings. The sacrificial corrosion protection ability of the Hp-Al coating was superior than other coatings, however the ERR for the defect part of it was inferior to that for the Np-Al coating.

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High-entropy alloys which is a brand-new concept metallic material, have excellent mechanical properties and have attracted a lot of attention. An improvement of the mechanical properties by microstructure control is important even for high-entropy alloys. A harmonic structure has fine grain regions which are arranged in a three-dimensional network, and coarse grain regions which are arranged between them. The harmonic structure materials exhibit excellent mechanical properties with high strength and high ductility. Cantor alloy powder, which is a typical high-entropy alloy, is produced into a harmonic structure Cantor alloy by applying the mechanical milling and spark plasma sintering methods. The harmonic structure Cantor alloy showed superior mechanical properties with higher strength and higher ductility than the homogeneous grain material. In addition, it showed even better mechanical properties in a low temperature environment. It is clarified that the control of deformation twins is important for improving the mechanical properties of the harmonic structure Cantor alloy.

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