Oxidation Performance and Interdiffusion Behavior of a Pt-Modified Aluminide Coating with Pre-deposition of Ni

To refrain the interdiffusion of elements while holding good oxidation resistance,a(Ni,Pt)Al/Ni composite coating was prepared by sequential treatments of electroplating Ni and Pt and successive gaseous aluminization.In comparison with normal(Ni,Pt)Al coating,high-temperature performance of the composite coating was evaluated in isothermal oxidation test at 1100℃.Both the two coatings exhibited good resistance against high-temperature oxidation,but the interdiffusion of elements between composite coating and single-crystal(SC)superalloy substrate was greatly relieved,in which the thickness of secondary reaction zone(SRZ)and the amount of precipitated topologically close-packed phase in the SC alloy matrix were significantly decreased.Mechanisms responsible for delaying rate of coating degradation and SRZ growth/propagation are discussed.

Microstructural stability of long term aging treated Ti-22Al-26Nb-1Zr orthorhombic titanium aluminide

The microstructure development of lamellar structure of an orthorhombic Ti2AlNb-based Ti？22Al？26Nb？1Zr alloy, includingB2 decomposition and spheroidization ofO phase, was investigated. The results show that the lamellar structure is fabricated by heating the samples in the singleB2 phase field and cooling slowly in the furnace. Aging treatments are conducted in the （O＋B2） phases field by air cooling. After aging at 700 °C for a short time within 100 h, there is no significant change of microstructures, whereas the coarsening of lamellae is observed in the long-term aged microstructure. Ti？22Al？26Nb？1Zr alloy exhibits microstructural instability including the severe dissolution ofB2 lamella, discontinuous precipitation and spheroidization of O phase during the long term aging process at 700 °C up to 800 h. In addition, a pronounced formation of branch-shapedO phase lamella is observed for the alloy aged over 100 h.

MICROSTRUCTURAL EVOLUTION AND MECHANISMS OF SUPERPLASTICITY IN LARGE GRAINED IRON ALUMINIDES

The superplastic behavior has been found in Fe 3Al and FeAl alloys with grain sizes of 100～600 μm. The large grained Fe 3Al and FeAl alloys exhibit all deformation characteristics of conventional fine grain size superplastic alloys. However, superplastic behavior was found in large grained iron aluminides without the usual prerequisites for the superplasticity of a fine grain size and grain boundary sliding. The metallographic examinations have shown that average grain size of large grained iron aluminides decreased during superplastic deformation. Transmission electron microscopy (TEM) observations have shown that there were a great number of subgrain boundaries which formed a network and among which the proportion of low and high angle boundaries increased with the increase of strain. The observed superplastic phenomenon is explained by continuous recovery and recrystallization. During superplastic deformation, an unstable subgrain network forms and these subboundaries absorb gliding dislocations and transform into low and high angle grain boundaries. A dislocation gliding and climb process accommodated by subboundary sliding, migration and rotation, allows the superplastic flow to proceed.

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.

OXIDATION RESISTANCE OF NANOCRYSTAL ODS ALUMINIDE COATINGS PRODUCED BY PACK ALUMINIZING PROCESS ASSISTED BY BALL PEENING

Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ultra-fine Y2O3 powders were mixed by ball milling. The ultra-fine Y2O3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature （below 600℃） in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM （scanning electron microscope）, AFM （atomic force microscope）, EDS （energy dispersive X-ray spectroscopy）, XRF （X-ray fluorescence spectrometer） and XRD （X-ray diffraction） methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.

Microstructure evolution and corrosion behavior of Fe-Al-based intermetallic aluminide coatings under acidic condition

Two Fe-Al-based intermetallic aluminide coatings were fabricated on 430-SS(Fe-Cr)and 304-SS(Fe-Cr-Ni)substrates by pressure-assisted solid diffusion bonding with coating on pure Fe as control.The microstructure and intermetallic phases of the coatings were characterized by SEM,EDS and EBSD.A network of Cr2Al13 with matrix of Fe4Al13 was formed by inter-diffusing of Al with the substrates.The corrosion behavior of intermetallic coatings was investigated in 0.5 mol/L HCl solution by mass-loss,OCP,Tafel plot and EIS.It was found that corrosion resistance was greatly enhanced by dozens of times after the addition of Cr and Ni compared with that on pure Fe.The presence of cracks in the coating on 430-SS provided a pathway for corrosion media to penetrate to the substrate and accelerated the corrosion rate.Moreover,the corrosion product was analyzed by XRD,demonstrating that the addition of Cr and Ni facilitated the formation of more corrosion resistant phases,and therefore improved corrosion resistance.

A Finite Difference Method for Determining Interdiffusivity of Aluminide Coating Formed on Superalloy

A numerical method has been developed to extract the composition-dependent interdiffusivity from the concentration profiles in the aluminide coating prepared by pack cementation. The procedure is based on the classic finite difference method (FDM). In order to simplify the model, effect of some alloying elements on interdiffusivity can be negligible. Calculated results indicate the interdiffusivity in aluminide coating strongly depends on the composition and give the formulas used to calculate interdiffusivity at 850, 950 and 1050癈. The effect on interdiffusivity is briefly discussed.

Effect of Substrate Characteristics on Interdiffusion Coefficients of Ni and Al Atoms in β-NiAl Phase of Aluminide Coatings

Interdiffusion coefficients at 950℃ and 1050℃ are calculated by Wagner analysis method as a function of composition of β-NiAI phase. The β-NiAI phase is formed by pack cementation on surface of superalloy. Results of the calculation show that interdiffusion coefficients in β-NiAI phase strongly depend on the compositions and vary over several orders of magnitude. Compared with the interdiffusion coefficients in the stoichiometric β-NiAI phase, the interdiffusion coefficients in β-NiAI phase formed on superalloy is obviously small, probably due to the composition, complicated microstructure and precipitates. However, it could be seen clearly that the shapes of the diffusivity curves are very similar to each other. The similarity of the diffusion curves and the difference between interdiffusion coefficients imply that the compositions, microstructures and precipitates of superalloy have a distinctly adverse effect on the interdiffusion of Ni and Al atoms during aluminization, but do not change the essential characteristics of β-NiAI phase.

Effects of Microstructure on the Tensile, Fracture Toughness and Fatigue Behaviour of Gamma Titanium Aluminides

The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions

Retraction note to “Development and characterization of hot dip aluminide coated stainless steel 316L”(J. Cent. South Univ., 10.1007/s11771-018-3937-y)

The Editor-in-Chief has retracted this article [1] because Figures 11 and 12 appear to be identical with Figures 3 and 4respectively from a previously published article [2].Sehrish MUKHTAR,Waqas ASGHAR,Zubair BUTT,Zaheer ABBAS,Mudaser ULLAH and Rana ATTA-UR-REHMAN did not respond to correspondence about this retraction.

Performance of the Aluminide Coating of SomeLow Alloy Steels

Two low alloy steels 0.5Cr-0.5Mo-0.25V and H85 were pack-aluminized at 900°for 4 h by using Fe-Al powder mixture containing 48% Fe, 20.6% Al- 29.4% Al2O3 and 2% NH4Cl by weight. The microhardness and oxidation resistance at 900℃ of the aluminide coatings were studied. It was found that pack-aluminizing improves the microhardness of the 0.5Cro.5Mo-0.25V steel while it reduces the microhardness of the H85 steel. Pack aluminizing highly improves the oxidation resistance after 20h exposure at 900℃ in air for the investigated steels.

GAMMA TITANIUM ALUMINIDE ALLOY TECHNOLOGY:STATUS AND FUTURE

Gam matitanium aluminide ( γ TiAl) alloys are emerging as a revolutionary engineeringmaterialsfor hightemperaturestructuralapplications. Onthebasisoftheinformation avail ablein the public domain, this paper discussesthe historical background,status and future prospect of gam maalloytechnologyintheareasofalloy development/ design,processdevelop ment, and applications.

Preparation of aluminide coatings at relatively low temperatures

A method was presented to prepare aluminide coatings on metals by combining the pack aluminizing with the ball impact process. This technique applied mechanical vibration to a retort, which was loaded with pack-aluminizing powder, specimens and alloy balls. Pack aluminizing was carried out with repeated ball impact, which accelerated chemical reactions and atomic diffusion. Aluminide coatings were formed at a relatively lower temperature (below 600 ℃) and in a shorter treatment time, compared with the conventional pack aluminizing. The effects of the operation temperature and the treatment time on the formation of the coatings were analysed. The SEM, EDS and XRD analysis results show that the aluminide coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings mainly consist of Al-rich phases such as η-Fe2Al5, θ-FeAl3 and ?CrAl5. Oxidation resistance was studied by high-temperature tests. The formation mechanism of the Al-coatings was also investigated. This technique provides a new approach for industrial diffusion coatings with great energy and time savings.

Effect of Microstructure on Wear Behavior of Iron Aluminide Based Coatings

An experimental study has been carried out to investigate the effect of microstructure on sliding wear behavior of Fe-Al coatings and Fe-Al/WC composite coatings produced by high velocity arc spraying (HVAS) and cored wires. After heat treatment at 300°C, 450°C, 550°C, 650°C and 800°C, the microstructure of the coatings will be changed. The changes of microstructure have obvious effects on the microhardness of the coatings, which may be the most important factor influencing the coatings sliding wear behavior. After heat treated at 450°C-650°C, increasing of the amounts of iron aluminides (including Fe3Al and FeAl ) and dispersion strengthening of Fe2\V2C and Fe6W6C will lead to a rise of microhardness of the coatings. Increasing the microhardness through heat treatment would improve the sliding wear resistance of the iron aluminide based coatings coatings.

THE INFLUENCE OF ENVIRONMENT ON FATIGUE AND FRACTURE OF IRON ALUMINIDES

This paper reviews recent research on embrittlement of iron aluminides brought about by exposure to moisture or hydrogen. The tensile and fatigue crack growth behavior of several Fe-Al alloys, ranging in aluminum content from 16 to 35a%, is described. It will be shown that tensile ductility and fatigue crack growth behavior are dependent on type and degree of long range order, grain structure, temperature and environment. Environments studied include vacuum,oxygen, hydrogen gas, electrolytically charged hydrogen and moist air. All cases of embrittlement are ultimately traceable to the interaction of hydrogen with the lattice.

HEAT TREATMENT BEHAVIOR OF CAN-FORGED TITANIUM ALUMINIDE ALLOY IN (α+γ) REGION

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Laser cladding of titanium alloy coating on titanium aluminide alloy substrate

A new diffusion bonding technique combined with laser cladding process was developed to join TiAl alloy to itself and Ti alloys. In order to enhance the weldability of TiAl alloys, Ti alloy coatings were fabricated by laser cladding on the TiAl alloy. Ti powder and shaped Ti alloy were respectively used as laser cladding materials. The materials characterization was carried out by OM, SEM, EDS and XRD analysis. The results show that the laser cladding process with shaped Ti alloy remedy the problems present in the conventional process with powder, such as impurities, cracks and pores. The diffusion bonding of TiAl alloy with Ti alloy coating to itself and Ti alloy was carried out with a Gleeble 1500 thermal simulator. The sound bonds of TiAl/TiAl, TiAl/Ti were obtained at a lower temperature and with shorter time.

DEPOSITION OF ALUMINIDE AND CHROMIUM-MODIFIED ALUMINIDE COATINGS ON TiAl ALLOYS USING THE HALIDE-ACTIVATED PACK CEMENTATION METHOD

The halide activated pack cementation method is utilized to deposit aluminide and chromium modified aluminide coatings on TiAl alloys. Emphasis is placed on the effect of alloying elements on the aluminizing kinetics as well as on the study of formation of chromium modified aluminide coating. The addition of a small amount of Nb and Cr in the TiAl improves significantly the aluminizing kinetics of TiAl alloys by increasing the reaction rate of active Al atoms with substrate through grain refinement as well as by increasing the solid state diffusion of Al through the formation of stable TiAl 3 layer. Cr and Al are simultaneously co deposited by diffusion into K 5 alloy, by a single step, pack cementation process. The morphologies of the coating formed on K 5 substrate, i.e., an external layer with L1 2 structure and an underlying interdiffusion zone, are presented. The mechanism for the formation of Cr modified aluminide coating is discussed.

Microstructure and crack sensitivity of electron beam welded titanium aluminide joints

Microstructural characterization and crack formed mechanism during electron beam welding of titanium aluminide Ti-45Al-1.7Cr-1.7Nb were investigated. The results show that the welded microstructure exhibits columnar and dendritic structure. Microstructural constituents in the fusion zone are a massive gamma structure and some lamellar structure consists of alternating platelets of α2 and γ. The major contributing factor of the susceptibility to solidstate cracking is thermally induced stress. The import role of the suppression of the α phase decomposition, the difference of α2/γ phases thermal expansion coefficient and the lamellar spacing changes of lamellar structure with cooling rate all play effect on the crack forming mechanism.

A New Process for Titanium Aluminides Production from TiO_2

This paper describes a new process for producing titanium aluminides, in particular TiAl, from TiO2 raw material. On the basis of obtained results, the non-completed reaction of TiO2 with Al and Ca in a special reaction vessel results in the production of granulates of titanium aluminides especially Ti3Al and other Ti- Al phases as the metallic product and Ca12Al14O33 as the non-metallic product. By adding KClO4 in the mixture, a nearly completed reaction can be carried out. The products of this reaction are titanium aluminide particularly TiAl as the metallic part and CaAl4O7 （grossite） as the non-metallic slag part. Both product and slag are produced in a separated form. This process, called KRH-method is described in this article. The scanning electron microscopic microstructure of metallic part of the product shows different phases： the matrix phase is TiAI, where the needle form precipitation is TiAl2 and the plate form precipitation includes TiAI and Ti3Al phases. The microstructure of the remelted metallic part indicates dendritic phase with a lamellar structure comprising of TiAl and Ti3Al phases. The interdendritic phase of TiAI is also seen.