Plastic deformation of FeNi invar alloys

We report on the deformation behaviour of FeNi single crystals oriented for single slip in the temperature region between 77 and 610 K. Below the Curie temperature the critical shear stress of the Invar alloys increases much more rapidly than that of any f.c.c. substitutional alloy investigated till now. Simultaneously the activation volume decreases down to 10–20 b³. Below the Curie temperature the stress-strain curves of the Invar crystals differ from those of other f.c.c. alloys. All results taken together indicate a dependence of the dislocation mobility on the magnetic properties of FeNi Invar. It is shown that the experiments cannot be understood with a simple magnetic friction stress arising from the interaction between the hydrostatic stress field of the edge dislocations and the stress dependence of the magnetization [3]. However, they can be described with an empirical friction law which had been introduced originally [18,19] for the case of strongly localized thermal activation, e.g. kink motion.     

Anomalous coarsening behavior of small volume fractions of Ni3Al precipitates in binary NiAl alloys

The kinetics of coarsening of γ′ precipitates in binary NiAl alloys containing nominally 5.72, 5.74 and 5.78 wt% Al and aged at 630°C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630°C. The volume fractions of γ′ were low between 0.02 and 0.03. the pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent γ′ precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constant decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%. Additionally, the rate constants exceed the values expected from the literature by factors of 10–40. The distributions of particles sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the γ′ precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction.     

Evidence of a thin sheet toughening mechanism in Al−Fe−X alloys

A toughening mechanism, dubbed thin sheet toughening, is proposed for improving the fracture resistance (K _IC and tearing modulus) of powder-metallurgy alloys of limited ductility. The basis of this approach is the recognition that internal delamination of thick-section components or cracked specimens into thin sheet ligaments in the fracture process zone leads to a drastic reduction in triaxial stresses, with the consequence of enhancing the critical fracture strain, fracture toughness (K _IC orJ _IC ), and tearing modulus. Theoretical analyses indicate that a factor of , increase in theK _IC value, and even greater increases in the tearing modulus are possible for idealized conditions. The predicted results are compared with experimental results of tensile,K _IC , andJ tests conducted on four powder-metallurgy Al−Fe−X alloys at 25 and 316°C. The comparison reveals that thin sheet toughening is a contributor to the highK _IC value observed in a state-of-the-art Al−Fe−V−Si alloy. Increasing the critical strain to fracture is also shown to be a possible method to improve the fracture toughness of Al−Fe−X alloys, independent of the thin sheet toughening effect.     

The microstructure of rapidly solidified AlFe alloys subjected to laser surface treatment

Using traverse rates of between 0.1 and 6.0 m/s, laser-melted tracks were produced on AlFe alloy samples containing between 0.25 and 8.0 wt% Fe. The local solidification rates were measured by taking a longitudinal section through the centre of the laser trace and the corresponding microstructures were studied quantitatively using transmission electron microscopy. Three types of microstructures were observed. At low growth rates, cellular/dendritic structures were obtained. At high growth rates, a banded structure was formed which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. At the lower concentrations, precipitate-free structures were obtained at very high solidification rates. A recent model, for dendritic growth under rapid solidification conditions, was compared with the experimental results and a good correlation was found. It was shown that, at rates close to the limit of absolute stability, steady-state planar-front growth was not the preferred growth morphology; but rather a banded structure. It was only at much higher rates that a fully precipitation-free structure, probably involving plane-front growth, developed.     

Enhanced decomposition of rapidly solidified microstructures in AlFeMo and TiAlEr alloys by plastic deformation and applied stress

In this work, the effects of plastic deformation and applied stress on enhanced decomposition of rapidly solidified microstructures in Al8Fe2Mo and TiEr alloys were examined. The results indicate that there is little effect of either plastic deformation or applied stress on decomposition of the Zone A microstructure in Al8Fe2Mo. It was concluded that decomposition of the microstructure during low temperature extrusion must be a result of adiabatic heat generated during the process. The effect of plastic deformation and applied stress on microstructural degradation of erbia-strengthened TiEr alloys was found to be significant, the extent of coarsening as identified by average particle size increasing as a result of plastic deformation and with increased applied stress. Recrystallization was observed to reduce these effects. The difference in enhanced decomposition behavior between these two microstructures was concluded to be a result of the mechanism of microstructural degradation and atomic transport.     

Mechanical behavior of Al−Li−SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al−Li alloy reinforced with 15 vol pet SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.     

Deformation and fracture behavior of a directionally solidified β/γ′ Ni-30 At. pct Al alloy

The effect of a ductile γ′-Ni₃Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni₃Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately 850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties.     

The microstructures of rapidly solidified hyper-eutectic AlBe alloys

Several alloys of hyper-eutectic composition (in the range 4.4–20.0 at% Be) have been rapidly solidified by melt-spinning in an He environment. For low solute compositions, the microstructure consists of cells elongated in the transverse direction of the ribbons. The intercellular regions consist of a refined dispersion of Be particles whereas the cells themselves (the intracellular regions) contain a small supersaturation of Be. As the solute concentration is increased the microstructure at first becomes more refined, consisting of a dispersion of Be particles (≈ 50 Å in diameter with an interparticle spacing of ≈ 200 å) in an Al matrix. At higher concentrations, a bimodal distribution of Be particles is present in the Al matrix, the finer precipitates being similar in appearance to those described above, and the coarser particles being ∼0.1–0.15 μm in diameter. These various microstructures may be most satisfactorily described on the basis of a set of metastable phase transformations involving a monotectic reaction. In this way, the refined distribution of Be particles is a product of this reaction, whereas the larger particles are thought to form during a pro-monotectic transition.     

Correlation of yield strength with internal coherency strains for age-hardened Cu−Ni−Fe alloys

The maximum yield strengths for a given aging temperature were measured for age-hardened Cu−Ni−Fe alloys. The yield strengths were found to be proportional to the difference in cubic lattice parameters of unstressed precipitating phases and independent of other factors such as precipitate particle size and precipitate volume fraction. The yield strength dependence on lattice parameter differences alone indicated coherency stresses controlled the yield strengths. An analysis of the yield strength based only on internal coherency strains and stresses subsequently led to the derivation of an equation for the yield strength,i.e., where is the Taylor factor for converting from single crystal shear stress to polycrystalline tensile stress results,C _ijare single crystal elastic stiffness constants and Δa is the difference in, anda ₀ the average of the cubic lattice parameters of the precipitating phases. The equation indicates the yield strength is dependent only on the internal coherency strains and independent of particle size and precipitate volume fraction, as observed. The correlation of the experimentally measured yield strengths with the equation was quite good.     

Mechanical behavior of Al−Li/SiC composites: Part II. Cyclic deformation

The deformation and failure mechanisms under cyclic deformation in an 8090 Al−Li alloy reinforced with 15 vol pct SiC particles were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes in low-cycle fatigue life is discussed.     

Effects of Al content on porous Fe–Al alloys

Abstract

Porous Fe–Al alloys with the nominal composition ranging from Fe–20 wt-%Al to Fe–60 wt-%Al have been fabricated by Fe and Al elemental powder reactive synthesis. The effects of the Al content on the pore properties of resultant porous Fe–Al alloys were systematically studied. It has been found that the volume expansion, the open porosity and the permeability can be manipulated by varying the Al content and that their maximum values are reached at Fe–45 wt-%Al. Their mechanical properties suggest that they are strong enough for the filtration applications.     

Aging behavior of an Al−Li−Cu−Mg−Zr alloy

The aging processes in an Al-Li-Cu-Mg-Zr alloy have been examined by means of electrical resistivity measurements and high-resolution transmission electron microscopy coupled with an image-processing system. The specimens quenched in iced brine after solution treatment, were reheated at a constant rate of 1 K/min up to 773 K. Six reactions were clearly separated in the temperature derivative of the resistivity/temperature curve, i.e., there was a slight increase at temperatures around 333 K, a large decrease at around 368 K, a significant decrease at around 448 K, a large increase at around 538 K, a remarkable decrease at around 568 K, and a final broad increase at around 623 K. Each reaction observed by the electrical resistivity measurement was examined metallographically. In the as-quenched specimen, spherical undissolved β′ (AlZr₃ L1₂ structure) particles dispersed, but the matrix was already ordered congruently into an L1₂ structure. The first reaction at around 333 K is probably due to the increase of the degree in the congruent ordering, but the second one, at around 368 K, is thought to rise from the rearrangements of antiphase domain, boundaries (APDBs) such as the partition of Li atoms between an APDB and the matrix, the APDBs lying parallel to the {100} and {110} planes. Reheating to temperatures around 448 K induces the phase separation, with well-defined interfaces into Li-rich, ordered δ′ (L1₂) and Li-poor, less-ordered regions, and the Ost-wald ripening of the ordered regions follows. The reactions at 538, 568, and 633 K were identified as the dissolution of δ′ particles into the matrix, the precipitation of δ (AlLi B32) and S′ (Al₂CuMg orthorhombic) particles, and the dissolution of both δ and S compounds into the matrix, respectively. SADAYOSHIITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University     

Phase equilibria in Ti−Fe−Nb alloys

Isothermal sections for 750 and 600°C have been constructed for the Ti?Fe?Nb ternary phase diagram in the region of titanium-rich alloys by the use of metallography, x-ray diffraction, and electron-probe microanalysis. At 750–600°C, 3.0–3.5 mass % niobium raises the solubility of iron in α-titanium from 0.05–0.06 mass % to 1.0–1.5 mass %.