Effect of alloying on aqueous corrosion and mechanical behaviour of iron aluminide Fe3Al

Passivity-inducing elements have been added to iron aluminide, Fe₃Al, to tackle their poor room-temperature ductility problem. The effect of alloying on the aqueous corrosion and mechanical behaviour of iron aluminides has been examined. It was found that the corrosion behaviour of intermetallic Fe₃Al-5M (M=Cr, Mo, Ta and Ti) was superior compared to that of binary Fe₃Al in electrolytes of pH 4 (H₂SO₄) and pH 8 (NaOH). The relative corrosion behaviour of these intermetallics in these electrolytes was comparable. The possible reasons for passivity enhancement have been discussed. Fe₃Al-5M₁ (M₁=Mo, Ta, V, Nb and Si) intermetallics could not be processed thermomechanically at 1000 °C because they cracked during deformation processing. The Fe₃Al-5Cr and Fe₃Al-5Ti intermetallics could be processed up to 80% deformation at 1000 °C by rolling into thin strips. These intermetallics exhibited improved room-temperature ductilities but poor yield strengths. The improvement in ductility has been attributed to passivity and microstructural effects. The low yield strengths of these intermetallics are poorly understood.     

On critical hydrogen concentration for hydrogen embrittlement of Fe3Al

The critical hydrogen concentration for hydrogen embrittlement in iron aluminide, Fe₃Al has been estimated (0.42 wppm). The estimated critical hydrogen content has been correlated to structural aspects of the decohesion mechanism of hydrogen embrittlement.     

Effect of strain rate on behaviour of Fe3Al under tensile impact

The effect of strain rate on the behaviour of Fe₃Al has been investigated experimentally in the range 90–1300 s_-1 under tensile impact. The experimental results indicate that Fe₃Al is strain-rate sensitive and its critical strain in maximum stress increases with increasing strain rate. According to the test results for Fe₃Al in air and water, it is confirmed that environmental embrittlement induced the fracture of Fe₃Al under tensile impact. With testing at faster strain rate, further hydrogen embrittlement is suppressed. This is consistent with the micrography of the fracture surface in Fe₃Al specimens, which indicates that these iron aluminides are intrinsically quite ductile. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Cr and Ni on diffusion bonding of Fe3Al with steel

Microstructure at the diffusion bonding interface between Fe₃Al and steel including Q235 low carbon steel and Cr18-Ni8 stainless steel was analysed and compared by means of scanning electron microscopy and transmission electron microscopy. The effect of Cr and Ni on microstructure at the Fe₃Al/steel diffusion bonding interface was discussed. The experimental results indicate that it is favourable for the diffusion of Cr and Ni at the interface to accelerate combination of Fe₃Al and steel during bonding. Therefore, the width of Fe₃Al/Cr18-Ni8 interface transition zone is more than that of Fe₃Al/Q235. And Fe₃Al dislocation couples with different distances, even dislocation net occurs at the Fe₃Al/Cr18-Ni8 interface because of the dispersive distribution of Cr and Ni in Fe₃Al phase.     

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.     

Preparation of Fe2O3/Al composite powders by homogeneous precipitation method

The Fe₂O₃/Al composite powders were prepared by homogeneous precipitation method. The influence of the concentration of Fe²⁺ and the molar ratio of raw materials on the preparation of Fe₂O₃/Al composite powders were investigated. X-ray diffractometer, scanning electron microscope, Fourier transform infrared spectroscopy and differential thermal analysis were used to analyze the morphology and structure of the Fe₂O₃/Al composite powders. The results show that the content of iron oxide in the composite powders could be effectively controlled by adjusting the concentration of Fe²⁺ and the molar ratio of raw materials in the plating solution. The surface of Al particle was coated with a layer of thick and dense iron oxide. The core-shell Fe₂O₃/Al composite powders with Fe₂O₃ content of 14.1% were produced, the coating efficiency of Fe₂O₃ reaches more than 77%. The iron oxide, which coated on the surface of the aluminium particle is flower-like cluster structure, each flower-like cluster is constituted by nano-flaky iron oxide.     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

Effect of transition-metal substitution on electronic and mechanical properties of Fe3Al: First-principles calculations

First-principles method based on the density functional theory was introduced to investigate the effect of impurity Cr or Ni on the site preference, mechanical properties of Fe₃Al. Formation enthalpy calculations show that Cr atoms prefer to stay at Fe_I sites while Ni atoms occupy Fe_II sites. Our investigations find that Fe₂NiAl has lower B/G (bulk modulus/shear modulus) and Poisson’s ratio compared to other (Fe_3?yX_y)Al alloys, which turns out to be a brittle alloy. The analysis of density of states and charge population indicate that the plasticity of Fe₃Al can be improved by transition-metal doping.     

Division of character zones and elements distribution of Fe3Al/Cr–Ni alloy fusion bonded joint

Abstract

A Fe₃Al/Cr–Ni alloy fusion bonded joint was divided into four character zones of a homogeneous mixture zone, a partial mixture zone, a partially fused zone and a heat affected zone. The microstructures, elements distribution and phase constitutions of the various character zones were analysed via metalloscope, SEM, electron probe microanalysis and X-ray diffraction. The results indicated that the microstructures were dissimilar in the different character zones. A 0·04–0·05 mm austenite rich band existed in the partial mixture zone. The diffusion of Fe, Al, Cr, Ni and C mainly occurred in fusion zone where Cr and Ni diffused into Fe₃Al to substitute some Fe on α ₁, α ₂, and β sublattices to form substitutional solid solution. The phase constitutions of Fe₃Al/Cr–Ni joint were Fe₃Al, γ-Fe, FeAl, NiAl, an unidentified Fe–C compound and an Fe–Cr–C compound (Cr₉Fe)₇C₃.     

High-temperature oxidation of Fe3Al containing yttrium

The effect of yttrium addition on the oxidation behavior of Fe₃Al alloys was investigated in terms of oxidation rate and oxide adhesion in the temperature range of 800 to 1100 °C. The oxidation rate of the alloys, Fe-14.3 wt% Al and Fe-14.1 wt% Al-0.3 wt% Y, was nearly identical, and the parabolic rate constant as a function of temperature is found to be K _p = 5128 exp[–39506 (cal/mol)/RT] mg²/cm⁴ hr. While the alumina scale formed on the Y-free Fe₃Al alloy was observed to be fragile and spalled easily, the oxide layer formed on the Fe₃Al-Y was protective, dense, and adhesive. Based on the microstructural, morphological, and compositional studies, the adhesion improvement due to the yttrium addition was discussed in terms of growth stress, the formation of pegs and scale growth mechanism.     

Hot working of Fe3Al based alloy

Abstract

For many years Fe₃Al based alloys have been of interest because of their excellent oxidation and sulphidation resistance. However, there are still some obstacles to industrial application of these materials, including the poor hot workability of Fe₃Al based alloys. Four types ofhot working (hotrolling, hotforging, hotpressing, and hotpressing plus hot forging) for the processing of Fe₃Al ingots have been studied in the present work, and the results indicate that a combination of hot pressing and hot forging might be an optimal method for breakdown of the Fe₃Al ingot.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Fe3Al-Fe3AlC intermetallics for high temperature applications: An assessment

The advantage of carbon containing dual-phase iron aluminides is that they exhibit relative improvement in high temperature strength as well as reduction in probability of hydrogen embrittlement (HE) over the single-phase iron aluminides, which were the main impediment for the commercialisation of these materials. Owing to its practical and scientific importance a study on the dual-phase Fe₃Al-Fe₃AlC intermetallics is presented, which deals with the current status of research and developments, understanding of physical metallurgy, HE, oxidation resistance and thermal stability; and its limitation and future challenges.     

Diffusivity of Al and Fe near the diffusion bonding interface of Fe3Al with low carbon steel

The distribution of elements near the Fe₃Al/Q235 diffusion bonding interface was computed by the diffusion equation as well as measured by means of EPMA. The results indicated close agreement between the two for iron and aluminium. Diffusion coefficient in the interface transition zone is larger than that in the Fe₃Al and Q235 steel at the same temperature, which is favourable to elemental diffusion. The diffusion distance near the Fe₃Al/Q235 interface increased with increasing heating temperature,T , and the holding time, t. The relation between the width of the interface transition zone,x , and the holding time,t , conformed to parabolic growth law: x bd² = 4.8 × 10⁴ exp(-133/RT) bdt -t ₀bd. The width of the interface transition zone does not increase significantly for holding times beyond 60 min.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.     

合金元素对块体纳米晶Fe3Al材料磁学性能的影响

为了研究合金元素对块体纳米晶Fe3Al材料磁学性能的影响,通过铝热反应熔化法制备了纳米晶Fe3Al以及分别含Ni质量分数10%、Cr质量分数10%、Mn质量分数10%和含Ni质量分数10%-Cu质量分数2%的块体纳米晶Fe3Al.在振动样品磁强计(VSM)上测得合金的磁滞回线,分析其磁性能,采用X射线衍射仪进行结构分析和平均晶粒尺寸计算.结果表明:各样品的磁滞回线呈倾斜状且狭长,磁滞损耗很小;含Ni质量分数10%的样品饱和磁化强度Ms较大,剩余磁化强度Mr和矫顽力Hc较其他样品最小,具有较好的软磁性能;添加合金元素后几种材料的晶粒尺寸变小,磁性能有较大变化,合金元素对纳米晶Fe3Al块体材料的磁性能影响明显.     

Water vapor effect on high-temperature oxidation behavior of Fe3Al intermetallics

Abstract

Fe₃Al intermetallics (Fe₃Al, Fe₃Al-Zr, Fe₃Al-Zr,Mo and Fe₃Al-Zr, Mo, Nb) were oxidized at 950 °C in dry and humid (11 vol% water) synthetic air. Thermogravimetric measurements showed that the oxidation rates of the tested intermetallics were lower in humid air than in dry air (especially for Fe₃Al-Zr, Mo and Fe₃Al-Zr, Mo, Nb). The addition of small amounts of Zr, Mo or Nb improved the kinetics compared with that of the undoped Fe₃Al. Fe₃Al showed massive spallation, whereas Fe₃Al-Zr, Fe₃Al-Zr, Mo and Fe₃Al-Zr, Mo, Nb produced a flat, adherent oxide layer. The rapid transformation of transient alumina into alpha alumina may explain the decrease in the oxidation rate in humid air.     

Water vapor effect on high-temperature oxidation behavior of Fe3Al intermetallics

Fe₃Al intermetallics (Fe₃Al, Fe₃Al-Zr, Fe₃Al-Zr,Mo and Fe₃Al-Zr, Mo, Nb) were oxidized at 950 °C in dry and humid (11 vol% water) synthetic air. Thermogravimetric measurements showed that the oxidation rates of the tested intermetallics were lower in humid air than in dry air (especially for Fe₃Al-Zr, Mo and Fe₃Al-Zr, Mo, Nb). The addition of small amounts of Zr, Mo or Nb improved the kinetics compared with that of the undoped Fe₃Al. Fe₃Al showed massive spallation, whereas Fe₃Al-Zr, Fe₃Al-Zr, Mo and Fe₃Al-Zr, Mo, Nb produced a flat, adherent oxide layer. The rapid transformation of transient alumina into alpha alumina may explain the decrease in the oxidation rate in humid air.