Study on microstructure and properties of high velocity arc sprayed Fe_3Al intermetallic coating

Coating structural materials with Fe 3Al based intermetallics may rapidly lead to industrial application of their environment and wear resistant features. In the present study, high velocity arc spraying (HVAS) was used to in situ synthesize Fe 3Al intermetallic coating. The microstructural characterization and properties of the coating have been investigated. The microstructure was found to consist of Fe 3Al based intermetallic (D0 3 and B2) and α Fe regions together with fine oxide (α Al 2O 3) layers. TEM images of coating show that the solidified lamellae are polycrystalline and have a grain size of the order of about 150 nm , and there also exists amorphous state in some areas. It can be concluded that a very high cooling rate has been obtained during HVAS process. Moreover, the coating has relatively higher adhesion strength and microhardness, as well as lower density and porosity.

Microstructure and mechanical properties of Fe_3Al alloys prepared by MA-PAS and MA-HP

Fe3Al alloys with nearly full density were fabricated by plasma activated sintering（PAS） and hot pressing（HP） from mechanical alloyed Fe-28%Al（mole fraction） powders,respectively.It is found that A2-type Fe3Al alloys were obtained by PAS,and they had a heterogeneous grain size distribution,most areas had a grain size smaller than 500 nm,and other areas had a grain size of about 1 μm.Different to PAS,D03-type Fe3Al alloys with a grain size of of 1-2 μm were obtained by HP.The compression testing results show that yield strength values of Fe3Al alloys fabricated by PAS and HP are almost equal at an elevated temperature,and the compression yield strength was about 100 MPa for all at 800 ℃.The room temperature compression ductility of Fe3Al alloys by PAS was about 20%,which was superior to that of Fe3Al alloys prepared by HP and casting.

铬、钼原子对FeAl/Fe_3Al相界面结合能和电子结构的影响

基于密度泛函理论的第一性原理,研究了铬、钼原子对FeAl/Fe_3Al相界面结合能及电子结构的影响。结果表明:铬、钼原子在FeAl/Fe_3Al相界面处均优先替代铝原子的位置,替代铝后均可提高相界面的结合能以及断裂强度;铬、钼原子的添加增加了FeAl/Fe_3Al相界面态密度的成键峰数量,分化了相界面处铝和铁原子的态密度形态,增加了相界面处的电荷密度。

热形变方法与Fe_3Al基合金的组织及性能关系（Ⅰ）

采用热轧、热锻和热挤压三种方法对铸态Ｆｅ_３Ａｌ基合金进行开坯，发现开坯后Ｆｅ_３Ａｌ铸态组织中粗大柱状晶被破碎，晶界网状析出物消除；热锻和热挤压后坯料晶粒大小分布均匀，而热轧后晶粒大小不一，三种方法开坯后坯料经相同的工艺轧制成板材，在室温下对板材性能进行测试，结果表明以挤压开坯方法最佳而热轧开坯最差。

EFFECT OF ELECTRODEPOSITED Cr_2O_3 OXIDE FILM ON THE OXIDATION OF Fe_3Al

A uniform, dense and defect free Cr2O3 thin film, which is amorphous at ambient temperature, was applied on the surface of intermetallic Fe3Al by electrodeposition reaction sintering, and the effect of this film on the oxidation of Fe3Al at 900 ℃ in air was studied. The films and the oxide scales were analyzed by TEM, EDAX,SEM and XRD.It is proved that, by surface applied Cr2O3 thin film,a continuous, protective,fine grained α-Al2O3 scale was formed on Fe3Al. As a result, the adherence of the scale and oxidation resistance of Fe3Al were improved.

EFFECT OF CHROMIUM AND BORON ON ENVIRONMENTAL EMBRITTLEMENT OF FORGED Fe_3Al

The effects of addition of chromium or boron on room temperature tensile properties,fracture behavior and susceptibility to test environments (air vs. vacuuwi of forged Fe3Al have been investigated. The results indicated that both chromium and boron result in increasing room temperuture ductility and fracture strength of the Fe3Al alloy whether tested in air or in vacuum. The susceptibility to test envimnment was described with the embrittlement index I: I=(δv-δA)/δv. The embrittlement indexes, for Fe-30Al, Fe-30Al-4Cr and Fe-30Al-0. 13B (at. %), are 24%, 37% and 29%,respectively. Scanning electron microscope examination of fracture surface revealed that the fracture mode of the three alloys remains unchanged, and all of them exhibited a transgranular cleavage fracture mode when tested in vacuum or air.

IN SITU PROCESSING OF Al_(2)O_(3) WHISKERS REINFORCED Ti-Al INTERMETALLIC COMPOSITES

In situ Al2O3 whiskers reinforced Ti-Al intermetallic composites were fabricated at ~1200℃ by reaction sintering of cold-consolidated fillets consisting mainly of Ti, Al, and different additives. The phases and microstructures of the sintered composites were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The process of synthesis was investigated using differential thermal analysis (DTA). The effects of processing parameters and additives on the microstructures of the composites and the development of whisker were examined. It is found that the morphology of the whisker is strongly influenced by the additives, the exothermal reaction process, and the processing parameters.

Structure of the Intermetallic Compound Ni_3Al Synthesized under Compression of the Powder Mixture of Pure Elements Part Ⅰ: Phase Composition and Microstructure of Main Phase

It was shown by TEM and X-ray analysis that there are four types of grains of the main Ni3Al phase in the structure of the intermetallic obtained by the self-propagation high temperature method (SHS). Every type of grains has its own domain and dislocation structure. There are mono- and polydomains with and without dislocations. The grains of the main phase of monoand polydomains without dislocations and polydomains with dislocations were formed by diffusion in the solid phase. In these conditions NiAl3 phase is located on the grain boundary of the main phase. The Ni2Al3 phase is located at the triple joints of the main phase.

XRD and TEM Analysis in the Interface of Diffusion Bonding for Fe_3Al/Q235 Dissimilar Materials

Phase structure characteristics near the interface of Fe3Al/Q235 diffusion bonding are investigated by means of X raydiffraction (XRD), transmission electronic microscope (TEM) and electron diffraction, etc. The test results indicatedthat obviously a diffusion transition zone forms near the interface of Fe3Al/Q235 under the condition of heatingtemperature 1050~1100℃, holding time 60 min and pressure 9.8 MPa, which indicated that the diffusion interfaceof Fe3Al/Q235 was combined well. The diffusion transition zone consisted of Fe3Al and a-Fe(Al) solid solution.Microhardness near the diffusion transition zone was HM 480~540. There was not brittle phase of high hardness inthe interface transition zone. This is favorable to enhance toughness of Fe3Al/Q235 diffusion joint.

INTERACTION OF H_2O AND O_2 WITH INTERMETALLIC ALLOYS

The interaction of water vapor and oxygen with Ni 3(Al, Ti) and Ni 3Fe were investigated using Auger electron spectroscopy, X ray photo emission and thermal desorption. These studies showed that water dissociates on Ni 3(Al, Ti) (001) and polycrystalline Ni 3Fe to produce atomic hydrogen above 200 K, while remaining intact when exposed to Ni 3(Al, Ti) (111). In addition, pre adsorbed or co adsorbed oxygen on these surfaces strongly suppresses hydrogen production due to water dissociation. It was shown that adsorbed oxygen reacts with water vapor to produce surface hydroxyls, thus quenching the production of atomic hydrogen. Recent studies revealed an unexpected embrittling effect of boron in Ni 3Al in dry hydrogen. Experiments were proposed to elucidate the different possible roles of boron.

ONE-CELL-STATE METHOD FOR DETERMINATION OF ELECTRONIC STRUCTURE OF INTERMETALLIC COMPOUNDS

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BEHAVIOR OF Mg AND ITS EFFECT ON MECHANICAL PROPERTIES OF CAST Fe_3Al ALLOYS

Proper addition of 0.002 wt-% Mg to cast Fe_3Al alloy may obviously improve the room and high temperature tensile properties.Auger spectra show that Mg segregates near grain boundaries as very thin layers.Observation under TEM reveals that a suitable amount of Mg enhances ability of cooperative internal deformation so that the dislocations will ease the con- tinuous slip deformation through grain boundaries.The fracture feature of Fe_3Al alloy is of transgranular cleavage at room temperature and cracks initiate at grain boundaries.

Concentration Heterogeneity in Structure of Intermetallic Synthesized under Compression

By X-ray analysis, TEM and SEM methods the phase composition, microstructure, and deviation from stoichiometric composition were investigated for the Ni3Al intermetallic compound obtained by cast and self-propagation high-temperature synthesis under compression with followed homogenization anneal, with the Al concentration being 24.0 and 25.0 at. pct with and without B (0.5 at. pct). It was discovered that in SHS intermetallic alloys an average local deviation from stoichiometric composition is significantly more than in the cast alloy. Dependence of the local deviation from the stoichiometric composition on the distance has a zig zag nature at the level of the grain groups. Deviation from the stoichiometric composition at the boundaries of the general type is less than at the boundaries of the special type. When the intensity of boundary migrations of the general type decreases, the degree of deviation from the stoichiometric composition increases. There is correlation between the solubility of B in the crystalline lattice of the Ni3Al phase and the value of the deviation from the stoichiometric composition in the grains. When the B concentration in a solid solution increases, the deviation from stoichiometric composition decreases.

TEM Study on Hydrogen-Induced Cracking of Fe_3Al Alloy

Hydrogen-induced cracking (HIC) of Fe3Al alloy was studied by in situ transmission electron microscope (TEM). Electron transparent specimens were mounted onto a constant displacement device. Stress was applied to the specimen by using a bolt through the device. The results showed that hydrogen enhanced the dislocation emission and motion in Fe3Al alloy. A dislocation free zone (DFZ) was formed following the dislocation emission. Microcrack initiated in the DFZ or at the main crack tip when the emission reached a critical extension. Hydrogen played an important role in the process of brittle fracture of Fe3Al alloy.

ADVANCE IN Ti_3 Al AND TiAl INTERMETALLICS RESEARCH IN CISRI

Recent work on Ti_3Al base alloy,TAC-1, developed in CISRI, including investigations of effects of modification in alloying and TMP on microstructural characteristics and mechanical behaviour,indicates that alloy TAC -1 is very promising structural material for a number of elevated temperature applications. Stuody dealing with a reverse relation between RT ductility and toughness found on TiA base alloy, TAC-2, led to development of a special heat treatment technique for producing a fine filly lamellar microstructure (FFL), which gives a high and balancing ductility and toughness. This is also of significance for promoting engineering applications of this alloy.

Structure of the Intermetallic Compound Ni_3Al Synthesized under Compression of the Powder Mixture of Pure Elements Part Ⅱ: Influence of Alloying by Boron on the Phase Composition and the Microstructure of Grains of the Main Phase

The Ni_3B phase was formed when boron (0.5 at. pct B) was added to the intermetallic of sto- ichiometric and off-stoichiometric (Ni-24 at. pct Al) compounds. In the alloy of stoichiometric composition the particles of Ni_3B phase has the size around 0.1μm and is located on the grain boundary of the main phase. The decreasing of concentrations of Al in the ofF-stoichiometric alloy leads to increase in the degree of the long-range order parameter, increasing the concen- trations of boron in the solid solution and decreasing its localization on the grain boundary. Microalloying of boron leads to increasing in the fraction of grain monodomains with disloca- tions up to 0.7 in the alloy of the off-stoichiometric composition and up to 1 in the alloy of the stoichiometric composition. It was established the correlation between the degree of the concentration inhomogeneity, average density of the dislocations and the average long range-order parameter.

TENSILE PROPERTIES AND CREEP RESISTANCE OF Fe_3Al－BASED ALLOYS CONTAINING NIOBIUM

Small amount(0.5-0.8at.%) of niobium addition to the multicomponent alloys based on Fe3Al(Fe-28Al-5Cr-0.5Mo-0.05Zr-0.05B-0.1Ce) results in the microstructural refinement and the increase in the yield strength at temperatures up to 600℃,but is not beneficial to room temperature ductility.Results of microanalysis indicate that the addition of niobium causes the formation of precipitates with complex compositions which strengthen both matrix and grain boundaries and enhance the creep resistance of the alloys.

Fe_3Al在H_2S-CO_2-Cl^-共存腐蚀介质中的腐蚀行为研究

通过腐蚀模拟现场工况,研究了Fe_3Al在H_2S-CO_2-Cl^-共存腐蚀介质中的腐蚀行为,研究发现:在试验压强4.0 MPa(P_H_2s=1.5 MPa,P_(co_2)=2.5 MPa),温度为60℃,试验周期为72 h的情况下,Fe_3Al的腐蚀速率为0.2106 mm/a,腐蚀形貌出现局部点蚀和裂纹,腐蚀产物为FeS;通过极化实验表明,腐蚀电流先减小后增大,前期由于腐蚀产物膜的作用,腐蚀减慢,后期由于局部发生腐蚀,腐蚀加剧,这与高压釜实验相吻合。

EFFECTS OF Cr AND Nb ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF Fe_3Al ALLOYS

The effects of Cr and Nb on tensile properties at room and elevated temperatures and also on the microstructure have been investigated.An improvement in ductiliy plasicity and the cleavage strength of Fe_3AI may result from adding Cr;the improvement is more pronounced at over 400℃.The formation of Fe_2Nb containing A1 precipitates in the Fe_3AI+Nb alloy provide significant strengthening effect.Additions of Cr and Nb in Fe_3AI can cause a signifi- cant improvement in the yield strength at 550℃.

EFFECT OF Ce AND Cr ON ROOM TEMPERATURE MECHANICAL PROPERTIES OF Fe_3Al

Small amount addition of Ce to FeAl results in a significant increase of room temperaturestrength of the alloy,and the combined addition of Ce and Cr improves both strength andductility.The fracture mode of FeAl containing Ce is a mixed one of approximately 70%transgranular cleavage and 30% intergranular failure.In the deformed specimens of binaryFeAl the dislocations observed are mainly single dislocations with =(1/4)a~’〈111〉.However,a number of superlattice dislocations can be observed in the deformed specimens ofthe alloys with Cr addition.