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Owing to good physical compatibility with high-temperature metallic structural materials, Cr2O3- or Al2O3-forming MCrAlY coatings have been extensively used to protect hot section components in aerospace and other fields from high-temperature corrosion. Currently, MCrAlY coatings are normally manufactured by means of physical methods, including electron beam-physical vapor deposition, magnetron sputtering, ion plating, thermal spray, etc. This review paper introduces novel nanocomposite-type MCrAl(Y) coatings prepared by electrochemical methods of nanocomposite electrodeposition and further modified "electrophoretic deposition + electrodeposition." The novel MCrAl(Y) coatings exhibit apparently enhanced ability in protective chromia or alumina scale formation, which can be attributed to the two-fold "nanoscale effect" of the dispersed particles and metal matrix grains. Based on a large amount of oxidation data of the nanocomposite-type NiCrAl coatings under different oxidation conditions, oxide maps showing the relationship between coatings' composition and type of the oxides are constructed to offer guidance for designing the novel coatings. Moreover, the "electrophoretic deposition + electrodeposition" method has been further applied to conveniently integrate a metallic-ceramic diffusion barrier into a MCrAl(Y) coating/Ni-based superalloy system which can significantly block the interdiffusion between the coating and the matrix. [ABSTRACT FROM AUTHOR]

Using non-aqueous solvents for metal electrodeposition can suppress hydrogen evolution reactions, thereby preventing hydrogen embrittlement in metallic structural materials. Ethylene glycol, which has a large solubility for metal acetates, might be a good non-aqueous solvent candidate. However, its relatively high viscosity and poor wettability lead to forming loose metal deposits during the electrodeposition, which is not suitable for anti-corrosion applications. In this work, we modify the ethylene glycol solution by adding ethanol and systematically investigate the roles of ethanol concentration, deposition potential, and temperature on the physical properties of the solution, electrochemical behavior of cadmium deposition, and microstructure of the deposits. Our results show that adding ethanol significantly reduces the viscosity of the electrolyte and improves its interfacial wettability, which promotes the formation of compact and thick deposits. [ABSTRACT FROM AUTHOR]

Austenitic stainless steels with Cr-containing passive films are susceptible to pitting corrosion with the presence of aggressive anions. Repeated cold rolling and subsequent annealing were adopted to enhance the pitting resistance of a 316LN austenite stainless steel by balancing the deformed and the recrystallized microstructure. The highly ultra-fine grained structure formed during cold rolling gradually evolved to partially or fully recrystallized structures depending on the annealing parameters. Their microstructures were characterized by EBSD and the corrosion behaviors were evaluated through electrochemical techniques in a typical 3.5 wt% NaCl solution. The results show that the short-time annealed sample with a partially recrystallized structure exhibits extraordinary corrosion resistance compared with the cold rolled or the fully recrystallized one. This might be attributed to the elimination of the corrosion sensitive sites introduced during cold rolling by a proper extent of recrystallization without too many annealing twins, which are beneficial for pit nucleation. • Cold rolled stainless steels with different ratio of recrystallized grains were prepared. • Pitting resistance of the steel is significantly enhances by partial recrystallization. • The influence of recrystallization and twins on pitting corrosion are discussed. [ABSTRACT FROM AUTHOR]

The recrystallization behavior below the γ′ solvus of a single crystal nickel base superalloy was investigated. Both tension and surface grinding were adopted to introduce the pre-deformation before annealing. The results showed that surface recrystallization of the tensile samples were mainly controlled by the nucleation process which was closely associated with the dislocation density in the matrix channels. Full recrystallization occurring in the deformation bands of fractured 〈011〉 sample indicated that the pinning effect of the second phase could be significantly weakened by sufficient shearing of dislocations. The tendency of recrystallization was reversed in annealed samples processed by surface grinding. This was attributed to the high ability of nucleation on the grinded surface and the change of controlling mechanism to the pinning effect of the γ′ phase in the deformation layer. [ABSTRACT FROM AUTHOR]

The orientational dependence of microstructure evolution and the relevant deformation mechanism of a single crystal superalloy were investigated. It showed that the annealing microstructures of below the γ’ solvus were strongly associated with the deformation distribution between the γ’ phase and the matrix. Several influence factors concerning the anisotropic deformation distribution were discussed in <001><011> and <111> orientations in this paper. It was concluded that orientations which were favorable to dislocation development in the matrix channels and suppressed the occurrence of particle shearing would enhance the driving force for recrystallization or rafting at high temperatures. [ABSTRACT FROM AUTHOR]

The deformation and fracture in tension of a single crystal nickel-based superalloy were studied with relation to the crystallographic orientations. The [011] oriented sample exhibited significant heterogeneous deformation by asymmetric necking and deformation bands. On the contrary, the [111] one deformed in a macroscopically homogeneous way. Microstructure observation and EBSD analysis showed that those deformation bands in the [011] oriented sample were formed by concentration of particle shearing and led to lattice rotation of about 15°. Slip systems operating in the matrix channels caused the huge difference in the deformation behavior of the two orientations. Some related studies by other researchers carried out at different temperatures or stress states were also discussed. [ABSTRACT FROM AUTHOR]

Formation of a thick interior interdiffusion zone (IDZ) dispersing brittle topologically close packed (TCP) precipitates is a key factor of unacceptably damaging the high temperature creep resistance of advanced single crystal (SX) Ni-base superalloy, when it is coated with conventional NiAl coating and exposed to high temperature environment. In this work, a NiRe-Al 2 O 3 layer between the NiAl coating and the SX Ni-base superalloy was developed by electrophoretic deposition of α-Al 2 O 3 nanoparticles followed by NiRe alloy electrodepostion. The 100 h oxidation at 1050 °C indicates that the intermediate layer worked well as a diffusion barrier (DB), not only decreasing the thickening rate of deterioration zone in the superalloy, but also totally preventing the coating from being penetrated by refractory elements from the underlying alloy and DB, which is always observed for typical metallic DBs. The beneficial effect of the new DB on interdiffusion is discussed. • A new cermet type diffusion barrier was developed by combination of electrophoretic deposition and electrodeposition. • This DB significantly prevented the interdiffusion between a NiAl coating and a single crystal Ni-base superalloy. • The barrier effect on the formation of interdiffusion zone in coating and superalloy is interpreted. [ABSTRACT FROM AUTHOR]

Fully recrystallized ultra-fine grained 316L steel with different grain sizes were prepared by high-energy ball milling followed by hot isostatic sintering. The relationship between grain sizes and the corrosion behaviors in salt water was studied. The results showed that the re-passive ability was always stronger for finer grained samples by formation of much denser and shallower pits, however, the influence of grain size on the pitting potential was insignificant and the passive film even seemed to become susceptible to cathodic polarization when the grains were refined to sub-micron scale. It was notable that there existed many contradictory conclusions in relevant studies. The ambiguity in the relationship between grain size and corrosion behaviors of stainless steels was discussed in detail. • Fully recrystallized ultra-fine grained 316L steel is prepared by high energy ball milling and hot isostatic sintering. • The effect of fully recrystallized grain refinement to sub-micron scale on the corrosion behavior of the 316L steel is shown • The reason which causes contradictory results of grain size effect on corrosion for stainless steels is discussed [ABSTRACT FROM AUTHOR]

A dense uniform distribution of nano-scaled Si precipitates has been achieved in Al-7Si alloy by high pressure solution treatment (HPST) and aging treatment. Precipitation behavior of Si phase was investigated using transmission electron microscopy (TEM). The results reveal that Si clusters initially precipitate from the Al(Si) solid solution, providing preferable nucleation sites for the equilibrium Si phase. The Si crystals exhibit spherical shape at first, and then grow rapidly parallel to {111} Si planes. The shortage of growth ledges on {111} interfaces leads to the inhibition of growth perpendicular to these planes, which results in Si triangles and platelets with high aspect ratio. An equation about the relationship between the relative free energy and the equilibrium ledge densities on Al-Si interfaces was proposed. [ABSTRACT FROM AUTHOR]

The 9 wt% Cr containing steel T91 was deposited with a Ni-16Cr-14Al nanocomposite coating by combining electrophoretic deposition and electrodeposition. After 500 h oxidation at 800 °C in 10 % H 2 O containing air, the bare T91 steel grew a triple-layered oxide scale consisting of an outer MnCr 2 O 4 layer, an intermediate Cr 2 O 3 layer and an inner SiO 2 layer. However, the nanocomposite-coated T91 grew an oxidation resistant oxide scale in the wet air, which was double-layered and consisted of outer NiAl 2 O 4 and inner α-Al 2 O 3. Pre-oxidation at 1000 °C in dry air further reduced the oxidation rate of the coated T91, due to the formation of an exclusive α-Al 2 O 3 scale. The oxidation-resistant mechanism of the nanocomposite-coated steel without and with the pre-oxidation treatment was discussed. • An Al 2 O 3 -forming NiCrAl nanocomposite was coated onto T91. • The nanocomposite grew α-Al 2 O 3 at 800 °C in 10 % H 2 O containing air. • The nanocomposite coating increased the resistance of T91 to wet air oxidation. • Pre-oxidation at 1000 °C further helped the coating to resist the wet air oxidation. [ABSTRACT FROM AUTHOR]

Formation of external scale rather than internal particles of chromia and alumina is of great importance for Cr- and Al-containing metallic materials to resist oxidation at high temperatures. Here we report a new strategy to apply metallic nanoparticles rich in Cr and Al to manufacture oxidation-resistant NiCrAl coatings with a well-designed structure, which can not only make the external oxide scale formation a quick and easy process but also be manufactured by simple electroplating and electrophorestic deposition following an established oxide map, so as to allow the composite coatings to smartly develop the best oxide scale against specific oxidizing environments. The method can be used to economically and systematically manufacture the MCrAlY (M = Ni, Co, or their combinations) coatings on superalloys and the metal-ceramic diffusion barriers between them. [ABSTRACT FROM AUTHOR]