High-temperature creep and structure investigation of nearly stoichiometric Fe3Si

X-ray diffraction measurements were carried out on powdered single crystals of nearly stoichiometric Fe₃Si. The experimental data obtained in the temperature range from room temperature up to 750‡ C in terms of long-range order, thermal expansion, phase transition and Debye temperature (together with values of the Curie temperature) support the existence of two modifications of the DO₃ structure for Fe-26 at% Si alloys and a phase transition in the DO₃ structure field at 595‡ C. The high-temperature modification has a smaller thermal expansion coefficient, a higher Curie temperature and a higher Debye temperature.     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

Undercooling and supersaturation of alloying elements in rapidly solidified Al-8.5% Fe-1.2% V-1.7% Si alloy

Dispersion-strengthened Al-8.5% Fe-1.2% V-1.7% Si alloy was produced by inert gas atomization and atomized melt deposition processes. Differential scanning calorimetry was used to estimate the extent of undercooling in the alloy powders as a function of powder size and in the atomized melt-deposited alloy as a function of process parameters. The estimated undercooling was found to be a strong function of powder size and processing conditions and varied from 380–200 °C. Alloy powders of diameter greater than 180 jam did not experience any undercooling during solidification. X-ray diffraction analysis was performed to study the dependence of supersaturation of alloying elements and metastable phase formation on the extent of undercooling. When the undercooled alloy was heated to about 400 dgC, formation of Al₁₂(Fe, V)₃Si phase with b c c crystal structure from the supersaturated matrix was observed.     

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.     

Nickel-free austenitic steels for cryogenic applications: The Fe-23% Mn-5% Al-0.2% C alloys

A study has been made of the influence of carbon and aluminium additions on the mechanical properties under uniaxial tensile loading and by Charpy V notch impact tests at room temperature and ?196°C on the Fe-24 % Mn alloys. It is concluded that the Fe-24 % Mn-5 % Al-0.2 % C appears as a new nickel-free iron-based alloy which is particularly interesting for cryogenic applications. In these alloys, both additions of carbon and aluminium contribute to the stability of the austenitic phase by suppressing the γ → ? martensitic transformation of the binary Fe-24 % Mn and to the solution hardening of the manganese-rich austenitic alloy.     

The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti-3Ni alloy

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti-Ni alloys because small additions of Si lower the solidus of Ti-Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti-3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti₅Si₃ form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti₅Si₃ phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Influence of Mn addition on the ordering of DO3 Fe-28%Al intermetallics

The DO₃-type ordering in Fe-28Al and Fe-28Al-1.5Mn alloys are investigated by TEM and XRD. The results show that Mn addition into DO₃-ordered Fe₃Al alloy could decrease degree of ordering. Two major factors are considered to have effect of Mn on ordering behaviour of the alloy: reducing grain size and reducing antiphase domain size. The further investigation of deformed antiphase boundaries and slip lines in the alloy with Mn addition suggests that Mn could promote slip and cross slip in DO₃ Fe₃Al alloy during deformation.     

Ordering of Fe-Si phases

The microstructure of Fe-Si alloys containing 8 and 15.5 at % Si and heat-treated between 550 and 1200°C is studied by transmission electron microscopy, and the phase composition of alloys containing 19 and 23 at % Si is determined by x-ray diffraction. The Fe-15.5 at % Si alloy heat-treated above 700°C is found to consist of a disordered solid solution and B2 phase. The B2 particles can be thought of as portions of {100} layers consisting entirely of Si atoms and sandwiched between {100} layers of Fe atoms, that is, as a two dimensional phase. At t 675°C, a compositionally modulated microstructure develops in which the Si-enriched zones have the Fe₃ Si stoichiometry and DO₃ structure. At high temperatures, the Fe-19 at % Si alloy consists of the and B2 phases, and the Fe-23 at % Si alloy consists of the and DO₃ phases. These findings are at variance with the generally accepted Fe-Si phase diagram.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 28–35.Original Russian Text Copyright © 2005 by Ustinovshchikov, Sapegina.     

Microstructure and internal friction of spray deposited Al-3.3Fe-10.7Si alloy

Al-3.3Fe-10.7Si alloy has been experimentally made with spray deposition technology. The internal friction of the alloy which was directly associated with the microstructures under spray deposited, extruted and heat treated conditions has been investigated using a low frequency inverted torsion pendulum over the temperature region of 10–300 °C. An internal friction peak was observed in the temperature range 50–250 °C in the present alloy. The Q^-1 peak decreased after extruted and in subsequent to the earliness of isothermal annealing, which was found to be directly attributed to the precipitation of FeAl₂ and Al– Fe– Si intermetallics from the supersaturated aluminium alloy matrix. We suggest that the internal friction peak in the alloy originates from grain boundary relaxation, but the grain boundary relaxation can also be affected by FeAl₂ and Al– Fe– Si intermetallics at the grain boundaries, which will impede grain boundary sliding.     

The effect of minor additions of titanium on the fracture toughness of Fe-12 at% Ni alloys at−196° C

The reasons for the improvement in the fracture toughness of an Fe-12 at% Ni base alloy at –196° C by the addition of small amounts of Ti were investigated employing transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Auger electron spectroscopy. Ti additions ranging from 0.18 to 0.99 at% and heat treatments of 2 h at 550, 685 and 820° C respectively followed by a water quench were considered, since previous work by Witzke and Stephens had shown that maximumK _IC occurred for an Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 685° C. It was found here thatK _IC at –196° C for all the alloys and heat treatments correlated with the fraction of ductile fracture compared to intergranular and cleavage fracture, the latter modes being predominant in the Fe-12 at% Ni base alloy without Ti additions. Cubic and rectangular shaped inclusions were noted in the SEM fractographs of the alloys with the Ti additions. A fine precipitate was observed by TEM for the Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 550° C; this precipitate was not observed for the 685 and 820° C heat treatments of the same alloy. Auger mappings of the fracture surfaces indicated a weak to moderate association of the interstitials C, N and O with Ti, the degree of which depended on the particular interstitial and the heat treatment temperature. It was concluded that the increase inK _IC due to the initial 0.18 at% addition of Ti was due to a scavenging of interstitials which normally segregate at the grain boundaries and to the refinement of the microstructure. The subsequent decrease inK _IC with further Ti additions was attributed to the increase in flow stress and slight lowering of the fracture stress resulting from these additions. It was further inferred that the Ti additions and heat treatment on the flow and fracture stresses may be due in a large part to their influence on the amount, size and distribution of the precipitate which was observed.     

Characterization of dispersed intermetallic phases in an Al-8.32wt%Fe-3.4wt%Ce alloy

A rapidly solidified Al-8.32Fe-3.4Ce (wt%) alloy was prepared by gas atomization and extrusion. The intermetallic phases present and their thermal stability, at temperatures up to 400°C, have been investigated by means of transmission electron microscopy (TEM). The metastable Al_mFe, Al₈Ce and equilibrium Al₁₃Fe₄ phases were detected in the as-extruded sample and the sample heat-exposed at 230°C, whereas the equilibrium Al₁₃Fe₄ and Al₁₃Fe₃Ce phases existed in the samples heat-exposed at temperatures above 315°C. The Al_mFe and the Al₈Ce phases were firstly observed in this alloy. The Al₁₀Fe₂Ce and Al₂₀Fe₅Ce phases, which were reported by the others in the similar alloys, do not exist in our samples. In addition, various domain structures in Al₁₃Fe₃Ce were also studied.     

Oxidation behavior of FeAl alloys with and without titanium

The influence of Ti addition on the high temperature oxidation behavior of FeAl intermetallic alloys in air at 1000°C and 1100°C has been investigated. The oxidation kinetics of FeAl alloys was examined by the weight gain method and oxide products were examined by XRD, SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5Al alloys could be described as different parabolas that followed the formula: (W/S)² = K _p t + C. The parabolic rate constant, K _p values are approximately 2.4 and 3.3 mg² cm^–4 h^–1 for Fe-36.5Al alloy and about 1.3 and 2.0 mg² cm^–4 h^–1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only -Al₂O₃ oxide for Fe-36.5Al alloy while there are -Al₂O₃ and TiO oxides for Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were addressed based on the microstructural evidence.     

Properties and Structural Changes of Al-3Cu-2Li Alloy after Thermal Treatment

A quantitative evaluation of microstructure and some mechanical properties of Al-3Cu-2Li (wt.%) alloy submitted to solution annealing at 503°C with succeeded age-hardening at three selected temperatures 163°C, 180°C and 190°C for 17 hours were investigated. With increasing the temperature of artificial ageing increases the strength of the alloy and reaches the maximum at 190°C, what is connected with decreasing of the deformation characteristics. Besides precipitates of Al₂Cu and Al₂CuLi the Cu₃Al particles were found.     

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.     

TEM observation of DO3 structure in Fe3 Al alloy with Mn addition

The structures of DO₃ Fe-28Al-1.5Mn alloy, including ordering degree, superdislocation, APD and APB, were investigated by TEM. The results showed that addition of manganese into DO₃ Fe₃Al could not change the ordered type of the alloy, but could reduce APD size and then reduce ordering degree of the alloy. The fourfold superdislocations were retarded in DO₃ Fe₃Al alloy after Mn addition. Undeformed alloy with Mn has mainly twofold superdislocations. As deformation increases, the twofold superdislocations slip and decompose into unit dislocations, and unit dislocations slip and slip cross, leading to better ductility. The deformation mechanism of DO₃ Fe-28Al-1.5Mn alloy was controlled at first by twofold superdislocation and at last by unit superdislocation.     

Influence of substrate orientation on low-temperature epitaxial growth of ferromagnetic silicide Fe3Si on Si

Influence of substrate orientation on epitaxial growth of the ferromagnetic silicide Fe₃Si on Si was investigated using low temperature (60-300 °C) molecular beam epitaxy. Transmission electron microscopy (TEM) measurements revealed that Fe₃Si layers were epitaxially grown on Si(110) and Si(111), while random poly-crystal Fe₃Si layers were formed on Si(100). From the Rutherford backscattering spectroscopy measurements, the values of the χ_min of the Fe₃Si layers grown at 60 °C on Si(100), Si(110), and Si(111) were evaluated to be 100%, 97%, and 41%, respectively. This dependence on the substrate orientation was explained on the basis of the atomic alignments at the Fe₃Si/Si interfaces.     

The load transfer effect in the true threshold creep behaviour of an Al-8.5Fe-1.3V-1.7Si alloy reinforced with alumina short fibres

The creep in an Al-8.5Fe-1.3V-1.7Si alloy dispersion strengthened with fine Al₁₂(Fe,V)₃Si phase particles and reinforced with alumina short fibres—Composite in the following—is investigated at temperatures 648, 698 and 748 K. The results are compared with those obtained for the composite matrix Al-8.5Fe-1.3V-1.7Si alloy (Alloy in the following) at the same temperatures. Both, the Alloy and the Composite exhibit true threshold creep behaviour; the true threshold stress decreases rather strongly with increasing temperature. However, independently of temperature, it is about twice as high in the Composite than in the Alloy. This is explained employing the concept of the load transfer effect in the true threshold creep behaviour. The results strongly suggest that rather dramatic enhancement of creep resistance of an Al-8.5Fe-1.3V-1.7Si alloy can be reached introducing into it mechanically strong short fibres of micrometer dimensions, provided the aspect ratio of the fibres and their volume fraction are large enough.     

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.