Papers by Keyword: Iron Aluminide

Abstract: High temperature oxidation of TiC ceramic matrix Fe40Al and Ni3Al toughened composites (Fe40Al/TiC, Ni3Al/TiC) of different TiC contents, made by pressureless melt infiltration, was investigated. Oxidation experiments were conducted in air for up to 65 hours at 700-1100°C. Weight gains of the composites showed that Fe40Al/TiC is more oxidation resistant than Ni3Al/TiC ones. SEM/EDS and X-ray diffraction showed that the dense oxidation layers of the two types of composites are mainly composed of crystalline TiO2. The growth of the oxidation layer follows parabolic passive oxidation kinetics, the calculated values of the activation energy of the oxidation mechanism being 270-280kJ.mol-1 and 240-250kJ.mol-1 for Fe40Al/TiC and Ni3Al/TiC, respectively. Al2O3 and Ni-rich phases were also detected in the oxidation layer of Fe40Al/TiC and Ni3Al/TiC, respectively.

Abstract: D03-ordered Fe3Al single crystals containing 23, 25 and 28at.%Al were cyclically deformed at [ 49 1 ] loading axis in tension-compression mode. Giant pseudoelasticity took place in Fe-23 and 25at.%Al single crystals at an early stage of cyclic deformation, while a slight strain recovery was observed in Fe-28at.%Al. In Fe-23 and 25at.%Al single crystals, superpartial dislocations with Burgers vector of 1/4[111] moved individually dragging the nearest neighbour anti-phase boundary (NNAPB). The NNAPB pulled back the superpartials during unloading, resulting in giant pseudoelasticity and low residual dislocation density. In contrast, a couplet of the superpartials was observed to bow out leaving two superpartials in Fe-28at.%Al. This means that the dislocation couplet dragged the next-nearest neighbour anti-phase boundary (NNNAPB). The surface tension of NNNAPB is lower than that of NNAPB resulting in a slight strain recovery in Fe-28at%Al. As cyclic deformation proceeded, residual dislocation density increased with an increase in the number of cycles even in Fe-23 and 25at.%Al. In particular, persistent slip bands (PSB) were formed in Fe-23at.%Al single crystals, though PSB is seldom observed in fatigued intermetallic compounds. To-and-fro motion of superpartials during loading and unloading was suppressed by dislocation bundles, resulting in a reduced shape recovery. However, large strain recovery occurred in Fe-25at.%Al single crystals at a strain amplitude of 1.0% even at 20cycles. It was also noted that Fe-23 and 25at.%Al demonstrated tension-compression asymmetry even at [ 49 1 ] orientation; the yield stress in compression was higher than that in tension. This implies that the core structure of <111> screw dislocation played an important role in the deformation behaviour of Fe3Al single crystals.

Abstract: The effects of B addition up to 0.4 wt% (i.e., 1.61 at%) to Fe-40Al alloy on microstructures and tensile properties were investigated. The vacancy-annealed specimens following casting and extrusion were examined by optical and scanning electron microscopy and thermal neutron-induced microradiography. The addition of a large amount of B resulted in grain refinement and changed the fracture mode from intergranular to transgranular to increase the tensile elongation. Especially, the alloy containing 0.3wt% B exhibited the elongation of 16.4% under the strain rate of 1x100s-1 at room temperature. The increase in elongation with increasing strain rate was discussed in terms of suppression of hydrogen embrittlement.

Abstract: Anelastic effects caused by carbon and vacancy diffusion in Fe3Al based alloys with and without strong carbide forming elements (Ti and Nb) are investigated by internal friction measurements. The decrease of the Snoek peak in Fe–26Al–2Ti and Fe–26Al–0.3Nb alloys with respect to the Fe–26Al alloy is related to a decrease in the amount of interstitially dissolved carbon (C). The so-called X peak, which is observed in the Fe–26Al alloy, also almost disappears after alloying. In order to elucidate the vacancy contribution to the origin of the X peak radiotracer measurements of 59Fe diffusion were performed. The results indicate that the change in the interstitially dissolved C concentration is the main reason of the observed changes in the X peak, although a certain influence of Ti and Nb alloying on the total vacancy concentration may be deduced from the diffusion study.

Abstract: We have investigated interdiffusion in iron-aluminium alloys using single-phase interdiffusion couples of FexAl1−x–FeyAl1−y for three combinations of x and y for Al contents between 18 and 49.5 at. % Al. Experimental diffusion profiles were obtained from electron-microprobe analysis of the diffusion zone. Interdiffusion coefficients were deduced via the Sauer-Freise method taking into account volume changes. A temperature interval between 997 and 1447 K was covered in our experiments. Thermodynamic factors were obtained from two theoretical models and judged by an analysis of the Kirkendall effect in the diffusion couples. The Darken-Manning equation was used to deduce self-diffusion coefficients of aluminium from the present interdiffusion coefficients, the thermodynamic factors, the vacancy-wind factors, and the iron tracer diffusivities obtained recently at the M¨unster laboratory. The results show that Al diffusion is always slightly faster than Fe diffusion. The difference never exceeds a factor of three. This small difference indicates that Fe and Al diffusion in B2 ordered iron-aluminides are closely coupled.

Abstract: Fe-Al-Ta alloys with the ternary Laves phase Ta(Fe0.5+x, Al0.5-x)2 are studied experimentally with the objective of clarifying the effect of Laves phase precipitation and atomic ordering on the deformation characteristics of such Fe-Al-base alloys. The present study concentrates on the hardening effect of the Laves phase in ordered and disordered Fe-Al-Ta alloys with Al contents between 16 and 45 at.% showing the A2 disorder or the DO3 or B2 order. Ta has a low solubility in Fe-Al alloys which is beneficial for slowing down precipitate coarsening. Small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way. The alloys with 1% Ta exhibit a high oxidation resistance. The continuing work aims at adjusting the Al and Ta content for an optimum relation of high-temperature strength and low-temperature ductility with maintaining a sufficient oxidation resistance.

Abstract: Atomistic modeling based on Density Functional Theory (DFT) within the framework of the Generalized Gradient Approximation (GGA) is used to show the effects of defects such as vacancy, boron, carbon, nitrogen and oxygen substituting Fe or Al atoms in the B2-FeAl structure. The site preference of each type of defect is determined from a comparison of total energycalculations using a supercell structure, consisting of 16-atoms, within which each the various defects are introduced. The changes in lattice parameter and bulk modulus associated to the presence of the defects in the FeAl matrix are also studied.

Abstract: This paper presents some preliminarily results on microstructure modifications associated with static recrystallization in an ODS alloy. The morphology of the grains issued from static recrystallization is influenced significantly by the alignment of the oxide particles in the as-extruded starting material. Grain growth modeling confirms the effect of particle alignment on the grain morphology and shows significant control of the particle distribution nature and the initial grain size on the grain anisotropy.