High temperature oxidation resistance and microstructure change of aluminized coating on copper substrate

The outermost coating with single phase Ni2Al3 was obtained on copper surface by electrodepositing nickel followed by slurry pack aluminizing at 800 °C for 12 h. The oxidation resistance and microstructure of the coating oxidized in ambient air at 1000 °C for 25-250 h were investigated using SEM, X-ray diffraction and optical microscope methods. The results show that the copper with single phase Ni2Al3 coating possesses the best high temperature oxidation resistance, and the mass gain of the coating is 1/15 that of pure copper and 1/2 that of nickel coating, respectively. The specimen surface after being oxidized for 25 h still comprises Ni2Al3 phase. However, when the time of oxidizing treatment increases to 50 h, the Ni Al phase is formed. It is also found that the Ni2Al3 phase completely turns into Ni Al phase after oxidizing treatment for 100 h and above. The Ni Al coating shows excellent high temperature oxidation resistance when oxidation time is 250 h.

High temperature oxidation behavior of Ti(Al,Si)_3 diffusion coating on γ-TiAl by cold spray

A Ti(Al,Si)3 diffusion coating was prepared on γ-TiAl alloy by cold sprayed Al?20Si alloy coating, followed by a heat-treatment. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 120 cycles to check the oxidation resistance of the coating. The microstructure and phase transformation of the coating before and after the oxidation were studied by SEM, XRD and EPMA. The results indicate that the diffusion coating shows good oxidation resistance. The mass gain of the diffusion coating is only a quarter of that of bare alloy. After oxidation, the diffusion coating is degraded into three layers: an inner TiAl2 layer, a two-phase intermediate layer composed of a Ti(Al,Si)3 matrix and Si-rich precipitates, and a porous layer because of the inter-diffusion between the coating and substrate.

High temperature oxidation behavior of directionally solidified NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho eutectic alloy

The isothermal oxidation behavior of NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho directional eutectic alloy was investigated with the help of scanning electron microscopy and X-ray diffraction.The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance in the temperature range of 900-1100°C.When the temperature was up to 1150°C,the continuous Al2O3 oxide film ruptured.Trace rare earth element Ho distributed uniformly in the alloy and relatively high level of Al in Cr（Mo）phase are beneficial to the formation of continuous and compact Al2O3 scale.During the oxidation,a phase transformation fromθ-Al2O3 toα-Al2O3 existed on the surface of oxidation film.It resulted in the abnormal oxidation mass gain happening when the alloy was oxidized at 1000°C or 1050°C.

Interfacial Behavior of Sulfur and Yttrium in Yttrium Modified Ni _3Al Based Alloy IC6 during High Temperature Oxidation Process

The interfacial behavior of sulfur and yttrium in the yttrium modified Ni 3Al based alloy IC6 during oxidation at 1100 ℃ was analyzed by X ray line scan of electron probe microstructural analysis(EPMA). The results show that the migration and segregation of sulfur to the interface between oxide scale and the substrate at high temperature is retarded owing to the presence of yttrium. This is attributed to the desulfurization by yttrium in the melt and the trapping of sulfur by yttrium rich phases during oxidation, which leads to improving the coherence between oxide scale and substrate. Another reason of increasing the high temperature oxidation resistance of alloy IC6 by the addition of yttrium is that yttrium migrates to the grain boundaries of oxides during oxidation and hence improve their strength. This results in the transformation of the oxide scale spallation cracks from intergranular cracks for alloy without yttrium to transgranular ones for yttrium modified alloy.

MITI PROGRAM ON HIGH TEMPERATURE INTERMETALLICS

An eight year national program aiming at the development of high temperature intermetallics was started by the Agency of Industrial Science and Technology of the Ministry of International Trade and Industry (AIST MITI) in 1989 and almost successfully ended in 1997. This national program with emphasis on basic aspects of the properties and processing of high temperature intermetallics, Ti Al and Nb Al, have given a considerable impetus to intermetallics research in Japan. This is an overview of the results of the program and implications of the program for the future development of high temperature intermetallics compounds for structural applications. Alloy design, evaluation of fundamental mechanical properties, fabrication processing and surface modification have been investigated using these two alloy systems. Based on the obtained results, the concept of alloy design was established, and new technologies, were developed for metal injection moding, casting, rheocasting, sheet casting and rolling, superplastic forming, heat treatment, melting and investment casting, gas atomization, direct rolling of alloyed powders, near net shaping by HIPing alloyed powders and oxidation resistant surface coating. Some of suggested intermetallics alloys are expected to be basics for ultra high temperature structural materials in the next generation because of the extremely high strengths at high temperature, 1 100 ℃ and 1 800 ℃, beyond conventional Ni base superalloys.

Evaluation of Oxidation of Ti-Al and Ti-Al-Cr Coatings Arc-ion Plated on Ti-60 High-temperature Titanium Alloy

High-temperature titanium alloy for aeroengine compressor applications suffers from high-temperature oxidation and environmental corrosion, which prohibits long-term service of this kind alloy at temperatures above 600℃. In an attempt to tackle this problem, Ti-48Al （at. pct） and Ti-48Al-12Cr （at. pct） protective coatings were plated on the substrate of alloy Ti-60 by arc ion plating （ALP） method. Isothermal and cyclic oxidation tests were performed in static air at elevated temperatures. Phase composition, morphology of the coatings and distribution of elements were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray analysis （EDX）. The results showed that the Ti-48Al coating exhibited good isothermal oxidation resistance during exposure at 800℃, but poorer resistance against oxidation at 900℃. By contrast Ti-48Al-12Cr coating demonstrated excellent isothermal oxidation resistance at both temperatures. Cyclic oxidation tests performed at 800℃ indicated that resistance and no spallation of coatings was observed. But both coatings demonstrated good cyclic oxidation at 900℃ only Ti-48Al-12Cr coating demonstrated excellent cyclic oxidation resistance.

Oxidation Behavior of Pd-Modified Aluminide Coating at High Temperature

(Ni,Pd)AI coating, prepared by low pressure pack cementation on the Ni-base superalloy M38 where Pd-20 wt pct Ni alloy was predeposited, consists of a single β-(Ni,Pd)AI phase. The initial isothermal oxidation behavior of (Ni,Pd)AI coating was investigated by TGA, XRD, SEM/EDS at 800-1100℃. Results show that oxidation kinetics accord preferably with parabolic law at 800, 900 and 1100℃, but not at 1000℃. θ-AI203 was observed at 800-1100℃. It is found that Pd plays an important role in accelerating the diffusion of Ti from the substrate to the coating surface in the aluminide coating.

Al对Ti_(70)Zr_(20)Ta_(10)合金力学性能的影响

为了研究Al对固溶处理态Ti_(70)Zr_(20)Ta_(10)合金力学性能的影响,本文利用光学显微镜、扫描电镜和万能试验机研究了Ti_(70)Zr_(20)Ta_(10)-xAlx合金的显微组织和室温拉伸性能。结果表明,Al元素固溶到马氏体基体中形成固溶体,几乎不改变固溶态Ti-Zr-Ta合金的晶粒尺寸,Ti-Zr-Ta-Al合金室温下均处于马氏体状态。Al的加入改善了Ti-Zr-Ta合金的力学性能,Ti-Zr-Ta-Al合金室温抗拉强度随Al含量增加而升高,断裂延伸率随Al含量的增加呈先增加后减少的趋势,当Al含量为1.0at%时,Ti-Zr-Ta-Al合金断裂延伸率最大(13.92%)。

制备Si-Cr-Y高温抗氧化涂层适用于低密度Nb-Ti-Al合金

针对低密度Nb-Ti-Al合金抗氧化性能需求,开展了Si-Cr-Y抗氧化涂层研究,利用SEM、EDS、电阻内热法等手段分析了制备的硅系涂层高温抗氧化性能和微观结构在氧化前后的变化。结果表明,1200℃下涂层抗氧化寿命达到133 h,室温~1200℃空冷热震大于15000次;涂层与基体之间形成一层扩散阻氧层,保证了涂层与基体之间的结合强度;在高温大气环境下,涂层表面形成一层隔绝氧气的SiO2保护层,从而表现出Si-Cr-Y涂层具有良好的抗氧化性能。

CONTROLLING HIGHTEMPERATURE DEFORMATION BY VARYING MICROSTRUCTURE IN A Ti47Al2Cr2Nb ALLOY

A Ti 47Al 2Cr 2Nb alloy was made by powder extrusion methods. By varying extrusion temperature, different microstructures were produced. At an extrusion temperature of 1 400 ℃ (above α transus), a uniform, fully lamellar structure was observed. In contrast, when powders were extruded at 1 150 ℃, an inhomogeneous microstructure consisting of γ,α 2 and metastable β phases was obtained. It was demonstrated that, while alloy extruded at 1 400 ℃ exhibited an excellent creep resistance, alloy with the same composition extruded at 1 150 ℃ exhibited superplasticity. The good creep resistance was resulted from the presence of fine lamellae which restrict dislocation slip within γ grains. These fine lamellae also promote the nucleation of deformation twins which impede dislocation glide along the interfaces ( γ/γ and γ/α 2) and, thus, reduces creep rate. In the case of low temperature extrusion, an elongation value of over 300% was obtained at a strain rate of 2×10 -5 s -1 and at a temperature as low as 800 ℃, which is close to the ductile to brittle transition temperature. This is in contrast to the prior major observations of superplastic behaviors in TiAl in which typical temperatures of 1 000 ℃ have usually been required for superplasticity. It was proposed that the occurrence of superplasticity at 800 ℃ is caused by the presence of a B2 phase which, during superplastic deformation (grain boundary sliding), accommodates sliding strains to reduce the propensity for cavitation at grain triple junctions and, thus, delays the fracture process.

Application of phase diagram calculations to development of new ultra-high temperature structural materials

In-situ refractory metal intermetallic composites(RMICs) based either on (Nb, Si) or (Mo, Si, B) are candidate materials for ultra-high temperature applications (>1400 ℃). To provide a balance of mechanical and environmental properties, Nb-Si composites are typically alloyed with Ti and Cr, and Mo-Si-B composites are alloyed with Ti. Phase diagrams of Nb-Cr-Ti-Si and Mo-Si-B-Ti, as prerequisite knowledge for advanced materials design and processing development, are critically needed. The phase diagrams in the metal-rich regions of multicomponent Nb-Cr-Ti-Si and Mo-Si-B-Ti were rapidly established using the Calphad (Calculation of phase diagram) approach coupled with key experiments. The calculated isotherms, isopleths, and solidification paths were validated by experimental work. The important heterogeneous multiphase equilibria in both quaternary systems identified will offer engineers the opportunity to develop materials with a balance of properties for high-temperature applications.

Oxidation performance of Fe-Al/WC composite coatings produced by high velocity arc sprayed at 800 ℃

Fe-Al/WC intermetallic composite coatings were prepared by high velocity arc spraying (HVAS) technology on 20G steel and the oxidation performance of Fe-Al/WC composite coatings was studied. The results demonstrate that the kinetics curve of oxidation approximately follows the logarithmic law and the oxidation velocity of Fe-Al/WC composite coatings is less than that of 20G steel after 5 h. The composition of oxidized coating is mainly composed of Al2O3, Fe2O3, Fe3O4 and FeO. These phases distribute unevenly. The protective Al2O3 film firstly forms and preserves the coatings from further oxidation.

THERMAL ACTIVATION ANALYSIS ON STRAIN RATE DEPENDENCE OF HIGH TEMPERATURE TENSILE PROPERTIES IN TiAl ALLOY

Tensile properties of a two phase γ Ti 47Al 1.5Cr 0.5Mn 2.8Nb alloy with a duplex microstructure were tested under strain rates ranging from 5×10 -5 to 5×10 -3 s -1 at temperatures from 1 123 K to 1 273 K. It was found that there exists approximately linear relationship between the flow stresses and the logarithm of the strain rate at different temperatures. The strain rate dependence was analyzed by thermal activation theory, and dislocation climbing has been identified as the rate controlling mechanism.

Oxidation Performance of Fe-Al/WC Composite Coatings Produced by High Velocity Arc Sprayed at 650 ℃

The Fe-Al/WC intermetallic composite coatings have been prepared by high velocity arc spraying(HVAS) technology on 20G steel and the oxidation performance of Fe-Al/WC composite coatings has been studied by means of thermogrativmetic analyzer. The results demonstrate that the kinetics curve of oxidation approximately follows the logarithmic law. The composition of the oxidized coating surface mainly is composed of A12O3, Fe2O3, Fe3O4 and FeO which distribute unevenly. The protective A12O3 film firstly forms and preserves the coatings from further oxidation.

Oxidation performance of Fe-Al/WC composite coatings produced by high velocity arc spraying

Fe-Al intermetallics with remarkable high-temperature intensity and excellent erosion, high-temperature oxidation and sulfuration resistance are potential low cost high-temperature structural materials. But the room temperature brittleness induces shape difficult and limits its industrial application. The Fe-Al intermetallic coatings were prepared by high velocity arc spraying technology with cored wire on 20G steel, which will not only obviate the problems faced in fabrication of these alloys into useful shapes, but also allow the effective use of their outstanding high-temperature performance. The Fe-Al/WC intermetallic composite coatings were prepared by high velocity arc spraying technology on 20G steel and the oxidation performance of Fe-Al/WC composite coatings was studied by means of thermogrativmetic analyzer at 450, 650 and 800℃. The results demonstrate that the kinetics curve of oxidation at three temperatures approximately follows the logarithmic law. The composition of the oxidized coating is mainly composed of Al2O3, Fe2O3, Fe3O4 and FeO. These phases distribute unevenly. The protective Al2O3 film firstly forms and preserves the coatings from further oxidation.

FeMoAl COATING AND ITS HIGHTEMPERATURE SULFIDATION PROPERTIES

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High temperature corrosion behavior of arc-sprayed aluminum coatings on steel

Aluminum coatings were sprayed on the substrate of steel Q235 by arc spraying. The test samples of Q235, Al coatings and Al coatings with seal coat were placed in box type electric resistance furnace at 400, 500,600, 650 ℃ when their oxidation behaviors were studied. And their oxidation kinetics curves were protracted. Microstructure, microhardness, bond strength and distribution of section elements were investigated by optical microscope(OM), Vickers microhardness instrument, electric tensile test machine and EPMA. Al coatings and Al coatings with seal coat can enhance the oxidation-resistance of substrate under 500 ℃. The latter has the best corrosion resistance. The coatings can't protect the substrate against oxidation above 600 ℃. After a long time corrosion there is enriched oxygen element at interface.

High Temperature Protection Ability of Oxide Scale Formed on Fe_3Al Enhanced by Surface-applied Y_2O_3 Thin Film

The Y_2O_3 thin film was applied on Fe-3Al intermetallic compound by electrodeposition and thermal decomposition. The cyclic oxidation of the Fe-3Al specimens with and without surfaceapplied Y_2O_3 thin film was carried out at 900 ℃ in air. The results show that the selective oxidation of Al in Fe-3Al was promoted, and both of the plasticity and the adhesion of the oxide scale formed on Fe3Al were improved and the high temperature oxidation resistance of Fe3Al was enhanced markedly.

Microstructural Evolution During Self-propagating High-temperature Synthesis of Ti-Al System

In order to investigate the microstructural evolution during self-propagating high-temperature synthesis （SHS） of Ti-Al powder mixture with an atomic ratio of Ti： Al=1：1, a combustion front quenching method （CFQM） was used for extinguishing the propagating combustion wave, and the microstructures on the quenched sample were observed with scanning electron microscope （SEM） and analyzed with energy dispersive spectrometry （EDS）. In addition, the combustion temperature of the reaction was measured, and the phase constituent of the synthesized product was inspected by X-ray diffraction （XRD）. The results showed that the combustion reaction started from melting of the Al particles, and the melting resulted in dissolving of the Ti particles and forming of Al3Ti grains. As the Al liquid was depleted, the combustion reaction proceeded through solid-state diffusion between the solid Al3Ti and the solid Ti. This led to the forming of TiAl and Ti3Al diffusing layers. In addition, the combustion reaction is incomplete besides TiAl, there are a large amount of Ti3Al and TiAl3 and a small amount of Ti in the final product. This incompleteness chiefly results from the using of coarser Ti powder.

TiAl合金电弧喷涂Al涂层的高温氧化性能

为了提升TiAl合金的高温抗氧化能力,采用电弧喷涂工艺在TiAl合金表面制备了纯铝涂层,并分别在900℃和1 000℃进行了恒温氧化试验.通过绘制氧化动力学曲线,对比了有无涂层试样的抗氧化性能.运用SEM、EDS、EPMA和XRD分析其氧化前后涂层的微观形貌、成分组成、元素分布和相组成.结果表明:纯铝涂层均匀致密、无裂纹.在900℃空气中氧化100 h后,涂层表面形成了致密的Al2O3氧化膜,次表层为TiAl3相,扩散层为TiAl2相.在1 000℃空气中氧化100 h后,涂层表面氧化膜由Al2O3和少量TiO2组成,次表层为TiAl3相,扩散层为TiAl2相.电弧喷涂铝涂层提高了TiAl合金的高温抗氧化能力.