Environmental effects in iron aluminides

Iron aluminides based on the stoichiometric compositions of Fe₃Al and FeAl exhibit poor room temperature ductilities due to hydrogen embrittlement (HE). The presence of surface passive films reduces HE. The reduction is due to the lower rate of hydrogen liberation on the surface of iron aluminides with a passive layer. Theoretical and experimental verification for this idea are provided. The effect of addition of passivity-inducing elements Ti, Zr, V, Nb, Ta, Cr, Mo, W, Si and Ni to Fe₃Al on the thermomechanical and electrochemical behaviour has been outlined. The Cr- and Ti-alloyed intermetallics exhibited significant room temperature ductilities. Microstructural studies of the alloyed intermetallics revealed that when the addition of passivity-inducing element results in the precipitation of brittle phases with Fe and Al, they crack during the processing operation. The addition of oxygen-active elements on the embrittlement behaviour is also discussed. The effect of these additions on the potentiodynamic polarization behaviour and high temperature oxidation behaviour is also briefly addressed. Methods to minimize HE by the addition of elements that irreversibly trap hydrogen and that prevent recrystallization have also been discussed.     

Corrosion of carbon-alloyed iron aluminides

The corrosion behaviour of two carbon-alloyed intermetallics of composition Fe-28.1Al-2.1C and Fe-27.5Al-3.7C has been studied and compared with that of binary intermetallics. Potentiodynamic polarization studies indicated that the intermetallics exhibited active-passive behaviour in an acidic solution of pH = 1, whereas they exhibited stable passivity in a buffer solution of pH 8.4. Corrosion rates were also obtained by immersion testing. The variation of corrosion rate as a function of time was similar for both the intermetallics. The variation in corrosion rate as a function of time has been explained based on the observed potentiodynamic polarization behaviour. Scanning electron microscopy of corroded surfaces indicated that the carbon-alloyed intermetallics were susceptible to galvanic corrosion, due to the presence of carbides.     

Oxidation microstructure of iron aluminides

Abstract

Isothermal oxidation behavior of FeAl alloy with different surface preparations was performed at 800°C in synthetic air. The oxidation products were identified by glancing angle X-ray diffraction. Characterization of the specimens after oxidation was made using scanning (SEM) and transmission electron microscopy (TEM). The oxidation kinetics of ground sample appears much faster than that of the mirror polished sample. X-ray diffraction analysis, SEM and TEM observations of the scales indicate a predominance of faster growing θ–Al₂O₃ on ground sample.     

Sliding wear resistance of iron aluminides

Room temperature dry sliding wear behaviour of iron aluminides containing 28% aluminium and various amounts of chromium has been investigated using pin on disk wear tester. The aluminides were heat treated to have orderedDO ₃structure. It was found that wear rate of the aluminides increased with the increase of applied normal load and sliding speed. Wear resistance of the aluminides increased with increase in chromium content. SEM observation of the worn surface showed that the microcutting and microploughing were the dominant sliding wear mechanisms.     

Hydrogen transport and environmental embrittlement effects in iron aluminides

Environmental embrittlement types of six iron aluminides have been systematically evaluated using electrochemical hydrogen permeation measurement, hydrogen microprint technique and tensile test in this study. The results of hydrogen permeation and microprint technique show that three - disordered solid solution structure of binary and ternary iron aluminides (Fe-10Al, Fe-18Al, and Fe-18Al-5Cr) have higher effective diffusivity and permeation rate than B2 ordered structure of binary and ternary iron aluminides (Fe-28Al, Fe-28Al-5Cr, and Fe-40Al). The tensile test of six iron aluminides in air, vacuum treatment and hydrogen precharged were analyzed and concluded. Three - disordered solid solution structure of iron aluminides suffer in hydrogen environment which are quite different embrittlement mechanisms from those B2 ordered structure of iron aluminides with serious tensile loss in air by moisture induced embrittlement.     

Deformation resistance and structure-forming processes of iron aluminides in hot rolling

We have developed simple mathematical models of mean equivalent stress dependence on temperature and strain for selected iron aluminides. Four similar melts with 16.5–19.2 wt.% of Al, 4 wt.% of Cr and with various contents of Ti and B were studied and compared. Flat specimens graded by thickness were hot rolled. Deformation resistance was calculated from the roll force values obtained using a laboratory mill Tandem. Postdynamic structure-forming processes of the tested aluminides, as well as their cracking susceptibility, were investigated by metallography. The differences in the deformation behavior and formability of the tested aluminides were described. __________ Translated from Problemy Prochnosti, No. 1, pp. 44–47, January–February, 2008.     

Effect of cerium addition on microstructures of carbon-alloyed iron aluminides

The effect of Ce addition on the microstructure of carbon-alloyed Fe₃Al-based intermetallic has been studied. Three different alloys of composition, Fe-18.5Al-3.6C, Fe-20.0Al-20C and Fe-19.2Al-3.3C-0.07Ce (in at%), were prepared by electroslag remelting process. Their microstructures were characterized using optical and scanning electron microscopies. Stereological methods were utilized to understand the observed microstructures. All the alloys exhibited a typical two-phase microstructure consisting of Fe₃AlC carbides in an iron aluminide matrix. In the alloy without Ce addition, large bulky carbides were equally distributed throughout the matrix with many smaller precipitates interspersed in between. In the alloy with Ce addition, the carbide grain sizes were finer and uniformly distributed throughout the matrix. The effect of Ce addition on the carbide morphology has been explained based on the known effect of Ce in modifying carbide morphology in cast irons.     

Role of surface passive films in the hydrogen embrittlement of iron aluminides

The room temperature hydrogen embrittlement problem in iron aluminides has restricted their use as high temperature structural materials. Previous studies have established that surface films affect hydrogen embrittlement (HE). The effect of surface passive layer on the hydrogen embrittlement behaviour of iron aluminides has been critically reviewed in this presentation. The role of thermomechanical treatments in affecting the mechanical properties has been discussed from a processing-structure-properties correlation view point. The alloy development philosophy to yield ductile iron aluminides has been outlined based on this review. Novel iron aluminide intermetallics that are being currently synthesized and characterized along these lines at IIT Kanpur are finally introduced.     

Effect of chromium on the corrosion behavior of iron aluminides in thiosulfate-chloride solution

Aqueous corrosion characteristics of iron aluminides in thiosulfate-chloride solution were studied as a function of chromium addition. Four kinds of iron aluminides, namely, FA-61, FA-77, FA-72 and FA-78, were prepared by arc melting followed by thermomechanical treatment. The corrosion behavior in thiosulfate-chloride solution for the prepared alloys were investigated by electrochemical tests (potentiodynamic test, potentiostatic test and electrochemical impedance spectroscopy (EIS) measurement) and surface analyses. The results of the potentiodynamic test indicated that the breakdown potential increased with increasing Cr content. Cr additions were found to prevent passive film from undergoing pitting corrosion. In EIS measurement, the depression angle was inversely related to pitting resistance, and decreased with increasing chromium content. The SEM observations of the sample surfaces reveal the different forms of pit as a function of chromium content. The AES results give evidence that the thiosulfate ions are reduced on the metallic surface, which inhibits the repassivation process.     

Effects of alloying elements on the electrochemical characteristics of iron aluminides

Effects of alloying elements on the electrochemical characteristics of iron aluminides in the H2SO4, H2SO4+KSCN and HCl solutions were investigated using electrochemical tests. The corrosion morphologies in iron aluminides were analysed by utilising optical microscopy. It was found that the addition of Cr and Mo to iron aluminides increased the corrosion potential, pitting potential and repassivation potential. The active current density, passive current density and reactivation current density decreased as Cr and Mo were added. In the case of Mo addition, the passive current density was slightly higher in the H2SO4 solution than in solutions containing SCN- and Cl-. When B was added to samples, the corrosion potential and repassivation potential decreased, whereas the active current density, passive current density, reactivation current density and pitting potential increased. Iron aluminides containing Mo and Cr showed remarkably improved intergranular and pitting corrosion resistance to SCN- and Cl- solution. On the other hand, B addition accelerated granular and intergranular corrosion by precipitation of borides. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hot corrosion of atomized iron aluminides doped with boron and reinforced with alumina

The hot corrosion resistance of sprayed and atomized Fe–40 at.% Al, Fe40Al+0.1B and Fe40Al+0.1B+10Al₂O₃ intermetallic materials have been evaluated in NaVO₃ at 625 and 700°C using polarization curves and linear polarization resistance measurements. Also, the results were supported by X-ray diffractometry and electron microscopy studies. The tests lasted 10 days. At 625°C, the Fe40Al+0.1B+10Al₂O₃ material exhibited the lowest corrosion rate, whereas the Fe40Al had the highest corrosion rate. At 700°C the three materials exhibited erratic behavior during the first 100 h, and after this all the intermetallics had the same corrosion rate. However, the corrosion rate was higher at 625 than at 700°C. The results are discussed in terms of an electrochemical mechanism, the establishment of an Al₂O₃ layer, which is more protective in the Al₂O₃-containing aluminide and seems to increase its protectiveness as the temperature increases from 625 to 700°C.     

Preparation of Fe3AI based iron aluminides from elemental and prealloyed povvders

Abstract

Iron aluminides were prepared by a powder metallurgy process from elemental powders, mixtures of prealloyed and elemental powders, and prealloyed powder. The sintering behaviour of various powders was studied using scanning electron microscopy, optical microscopy, and density measurement. It was found that sintering of elemental powder involved two distinct processes, i.e. alloying and densification, but sintering of prealloyed powder involved densification alone. The addition of prealloyed powder to elemental powders was helpful in restraining the swelling of sintered samples, the degree of swelling of sintered samples being reduced as the amount of prealloyed powder increased. For samples made from Fe-25 at.-%Al prealloyed powder, remarkable shrinkage was measured after sintering at 1250°C for 1 h. Within the correct range, their density increased with sintering temperature and time, but prolonged sintering at high temperature resulted in the loss of aluminium and a two phase microstructure. The difference in sintering behaviour between the various powders was discussed on the basis of thermodynamics.     

球磨合成Fe3Al金属间化合物及其固相反应机理

Fe3Al金属间化合物是一种新型耐高温材料,并因其电热和磁性能受到重视。本文研究由铁、铝元素混合粉末利用高能球磨工艺合成Fe3Al。通过对冷焊现象的分析,用适量的有机物有效地控制了机械合金化过程。利用X射线衍射研究了反应物在球磨过程中的结构演变。通过对Fe／Al固相反应热力学的分析,认为Fe／Al原子比相等成分附近,固相反应最容易进行。     

FeAl系合金抗高温氧化性能研究进展

FeAl金属间化合物具有优异的力学性能和抗高温氧化、硫化腐蚀性能,被公认为是高温结构材料、高温气体除尘领域内具有重要应用价值的新材料,因而成了人们研究的重点课题之一。系统地讨论了铁铝金属间化合物的氧化过程、机理以及研究的进展情况。     

Effect of hydrogen on the mechanical behaviour of carbon-alloyed Fe3Al-based iron aluminides

The effect of hydrogen on the mechanical behaviour of two carbon-alloyed iron aluminides was studied. Weakening of some carbide-metal interfaces in the presence of hydrogen was indicated. The effect of cathodic hydrogen charging on the microstructure has also been addressed.     

Enhancement of high temperature strength and room temperature ductility of iron aluminides by alloying

Abstract

The chemical ordering in intermetallics results in reduced atomic mobility and therefore increased resistance to plastic deformation at elevated temperatures. This intrinsic source of high temperature strength leads to the inherent brittleness of polycrystalline ordered intermetallics at room temperature. The requirements for optimum high temperature strength and ductility at ambient temperature are often incompatible. Iron aluminides possess high strength up to 873 K. There is an anomalous (positive) temperature dependence of yield and flow strengths. Iron aluminides have yet to achieve satisfactory elevated load bearing capability. Alloy additions have the potential for improving elevated temperature strength and room temperature ductility; whichever is more critical for the application. Elements such as Cr, Ti, Mn, Co, and Mo produce higher flow stress due to solid solution strengthening. Elements such as Zr, Ta, Nb, Re, and Hf go into solution partly, reprecipitate, effectively pin dislocations and thereby cause strengthening. Mo, Zr, and Hf produce good tensile strength at elevated temperatures but ductility decreases. Element B strengthens by grain boundary cohesion. The improvement in room temperature ductility can be achieved through modification of the crystal structure by changes in stoichiometry, macroalloying, microalloying, and control of the environment. B, TiB₂, and Cr are notable for enhancing ductility. The paper is an overview of the present status of iron aluminides in this respect.     

Effect of process variables on tensile properties of ingot processed versus strip cast iron aluminides

Abstract

The effects of cold rolling and subsequent heat treatment are reported for ingot cast-hot rolled and strip cast iron aluminides (23–29 at.-%Al). Partially recrystallised microstructures obtained by hot rolling at 800°C gave higher strength and elongation to fracture than hot rolling at 1000° C. Elongation increased and proof strength decreased in proceeding from DO₃ to mixed B2 + DO₃ and to B2 order as a result of heat treatment following cold rolling. Variability in the properties of strip cast material was associated with casting defects such as laps, but the best properties were comparable with those of ingot processed material. The detrimental effects of tensile specimen preparation by spark erosion are discussed.