In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

The structure and mechanical properties of Fe2AlMn single crystals

The microstructure and mechanical properties of off-stoichiometric single-crystal Fe₂AlMn have been investigated. After casting, the alloy contained two sets of thermal antiphase boundaries, (a/4)?111? and (a/2)?100?, which are attributed to the fact that the compound solidifies into a bcc structure and subsequently orders to a B2 structure and then a L2₁ structure respectively upon further cooling. Crystals strained under tension at room temperature in air at 1s^?1 showed 6% elongation, whereas specimens strained at 1?×?10^?5?s^?1 showed no elongation, indicating that the compound is sensitive to the testing environment. Fracture occurred on {100} in both cases. Compression tests showed that a yield anomaly was present at intermediate temperatures, with the peak yield strength occurring at about 800?K, which is slightly below the L2₁–B2 transition temperature of 898?K. The slip systems were found to be ?111?{110} at room temperature and 800?K. Transmission electron microscopy observations showed fourfold-dissociated ?111? dislocations in specimens strained at room temperature but only paired ?111? dislocations in specimens strained at the peak temperature. The room-temperature yield strength of quenched specimens increased with increasing quench temperatures from 700 to 1100?K.     

Plastic deformation of a monoclinic Al13Fe4 at 300 ∼ 873 K

The plasticity of monoclinic Al₁₃Fe₄ particles grown in a spray-formed Al–Fe alloy is examined after being submitted to two deformation modes between 300 and 873 K by specially designed semi-in situ compression tests; one is uniform and the other is by indentation. In the uniform mode, cracks propagate through the Al₁₃Fe₄ particles along the pentagonal column planes, leaving extremely thin and heavily deformed plastic zones along the cracked faces at 300 and 473 K. In contrast, the generation of isolated dislocations and their motion govern the plastic deformation at 673 and 873 K. In the indentation mode, local deformation is achieved exclusively by individual dislocations over the whole range of temperature explored. A possible mechanism of dislocation motion in monoclinic Al₁₃Fe₄ is discussed based on Burgers vector analyses coupled with the three-dimensional observation of dislocation configurations.     

Plastic deformation of polycrystals of Co3(Al,W) with the L12 structure

The plastic behaviour of Co₃(Al,W) polycrystals with the L1₂ structure has been investigated in compression from 77 to 1273?K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950?K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100?K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L1₂ compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co₃(Al,W) and other L1₂ compounds, such as Ni₃Al and Co₃Ti, are discussed.     

Observations of glide and decomposition of a⟨101⟩ dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a?111? slip to non-a?111? slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a?010? dislocations are observed above the slip transition temperature. In contrast, a a?101?{101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a?101? dislocations are observed primarily along both ?111? directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a?101? dislocations along these directions is decomposed into two a?010? dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a?101? dislocations depends upon the strain rate. At a strain rate of 1.4 210^?4 s^?1, a?101? dislocations are observed between 800 and 1000 K. Complete decomposition of a?101? dislocations into a?010? dislocations occurs beyond 1000 K, leading to a?010? climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a?101? dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a?101? dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a?101?{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.     

Misfit dislocations in gold/Permalloy multilayers

Several groups have reported the misfit dislocation structures in Au/Ni_0.8Fe_0.2 multilayers where the lattice parameter misfit is very large. To explore the factors controlling such structures, molecular dynamics simulations have been used to simulate the vapour-phase growth of (111)-oriented Au/Ni_0.8Fe_0.2 multilayers. The simulations revealed the formation of misfit dislocations at both the gold-on-Ni_0.8Fe_0.2 and the Ni_0.8Fe_0.2-on-gold interfaces. The dislocation configuration and density were found to be in good agreement with previously reported high-resolution transmission electron microscopy observations. Additional atomic-scale simulations of a model nickel–gold system indicated that dislocations are nucleated as the first nickel layer is deposited on gold. These dislocations have an (a/6)?112? Burgers vector, typical of a Shockley partial dislocation. Each dislocation creates an extra {220} plane in the smaller lattice parameter nickel layer. These misfit-type dislocations effectively relieve misfit strain. The results also indicated that the dislocation structure is insensitive to the energy of the depositing atoms. Manipulation of the deposition processes is therefore unlikely to reduce this component of the defect population.     

Dislocation motion on octahedral and cube planes in Fe 3 Ge polycrystals

An intermetallic compound of L1₂ ordered Fe₃Ge was examined by compression in bulk at various temperatures and by semi-in-situ compression of a thin foil at room temperature. This alloy is known to show monotonic decrease in yield strength being quite different from the other well-known L1₂ intermetallics. It is shown by stereomicroscopy, Burgers vector analyses and semi-in-situ experiments that ?112? types of slips along the {111} faults may contribute to generate glide dislocations on {001} planes, which eventually act as multiplication sources for the cube slips to govern the essential part of the macroscopic deformation. The yield strength is confirmed to decrease monotonically up to 500?K and rapidly upon approaching the L1₂→D0₁₉ transformation temperature with a drastic increase in the strain rate sensitivity. The strengthening mechanism in this dual phase alloy of Fe₃Ge is discussed in terms of dislocation motion on both octahedral and cube planes by analysing the results of TEM observations with the information of Schmidt factors.     

Structure and magnetic properties of FeMnAl alloys investigated by Mössbauer spectroscopy and X-ray diffraction

The system (Fe_0.88Mn_{0.12 1–x} Al _x has been investigated in a concentration range from 5 to 14 at.% Al. We applied Mössbauer spectroscopy in the temperature range from 4 up to 900 K and X-ray diffractometry at room temperature. The as-cast samples show a bcc phase for all concentrations and exhibit broadened six-line Mössbauer spectra typical for disordered alloys. The Mössbauer spectra during a high temperature treatment show dramatic changes. These are due to ordering processes appearing at temperatures above 700 K. As an example of the observed changes, we present results obtained for the alloy withx= 14 at.% Al.     

Giant strain-induced ferromagnetism in Fe59Mn17Al24

Giant strain-induced ferromagnetism (SIF) has been observed in Fe₅₉Mn₁₇Al₂₄ single crystals. The crystals in either the L2₁ or B2 ordered state are weakly magnetic, with saturation magnetization, M _s, of 9–10?emu/g. After a 60% thickness reduction, the M _s of the L2₁ crystal increased to 89?emu/g, whereas the M _s of the B2 crystal after a 53% reduction was 96?emu/g, an M _s approximately four times larger than that of the largest SIF observed in other ordered alloys. By comparison, mechanically alloyed powder of the same composition, which had a fully-disordered bcc structure, showed a similar M _s of ～90?emu/g. The increased M _s in strained Fe₅₉Mn₁₇Al₂₄ single crystals is attributed primarily to the generation of increased magnetic coupling due to chemical disordering. This large M _s is substantially reduced after annealing at 573?K.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

Mössbauer studies of Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Reaction mechanisms between Al and Fe3O4 powders in the formation of an Al-based metal matrix composite

The reaction mechanisms between Al and Fe₃O₄ powders were investigated. Differential thermal analysis revealed that a two-step displacement reaction between Al and Fe₃O₄ took place during sintering. Initially, the Fe₃O₄ was converted to amorphous FeO at ～720°C and some of the Al was oxidized to amorphous Al₂O₃. In the final stage, when the temperature reached ～840°C, crystalline Al₂O₃ particles were produced in the molten Al–Fe liquid. The effects of cooling rate on the microstructures were studied. When the Al–Fe liquid was furnace-cooled to room temperature, proeutectic Al₃Fe plates, plate-like divorced eutectic Al₃Fe and Al₂O₃ particles were in situ formed in the Al(Fe) matrix. While quenching from 700°C, nanometer-sized Al dendrites and Al–Al₆Fe eutectic lamellae were produced in the Al matrix. However, when it was rapidly quenched from 900°C, the size of the proeutectic Al₃Fe phases was further reduced and Al₆Fe nanorods were found in the Al–Al₆Fe eutectics. A model was proposed to describe the transformation of the Al–Fe intermetallics during solidification.     

Adsorption and desorption of no from Rh{111} and Rh{331} surfaces

The adsorption and desorption chemistry of NO on the clean Rh{111} and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step and/or defect sites. At saturation coverage there was ～ 10 times more dissociated species on the Rh{331} surface at 300 K than on the Rh{111} surface. On both surfaces two molecular states of NO_ads have been identified as β₁, and β₂ which possess different chemical reactivity. Under the condition of saturation coverage the β₁ and β₂ states are populated on the Rh{111} surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the Rh{111} and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below ? 400 K primarily from the β₂ state, molecularly as NO(g). Between 300 and 400 K the β₁ state dissociates as binding sites necessary to coordinate N_ads and O_ads are freed by desorption of NO(g).     

Frustration driven magnetic states of A-site spinels probed by <Emphasis Type="Italic">μ</Emphasis>SR

The normal A-site spinels MnAl₂O₄, FeAl₂O₄, CoAl₂O₄, as well as related mixed (Mn_0.5Fe_0.5Al₂O₄) and partially inverted (Fe_1.4Al_1.6O₄) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T _M is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl₂O₄, Fe_1.4Al_1.6O₄) or the formation of a spin liquid phase (FeAl₂O₄, CoAl₂O₄, Mn_0.5Fe_0.5Al₂O₄). The magnetic ground state of MnAl₂O₄ shows coexistence of antiferromagnetic and spin liquid phases. In FeAl₂O₄ and CoAl₂O₄ long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn_0.5Fe_0.5Al₂O₄ prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe_1.4Al_1.6O₄ strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.     

The peierls energy and kink energy in fcc metals

In fcc crystals, dislocations are dissociated on the {111} glide plane into pairs of partial dislocations. Since each partial interacts individually with the Peierls potential and is coupled to its neighbour by a stacking fault, periodic variations in the separation distance d of the partials occur when dislocations running along closed packed lattice directions are displaced. This can drastically reduce the effective Peierls stress. By using the Peierls model the structure of 0°, 30°, 60° and 90° dislocations in a typical fcc metal with the elastic properties of Cu and a stacking-fault energy γ₀ in the interval 0.04?≤?γ₀?≤?0.05?J/m² was studied, and the magnitude of the Peierls energy ΔE _P and the resulting kink energies E _K were determined. Since the energies involved are of the order of 10^?3?eV/b or less, their magnitude cannot be asserted with high confidence, considering the simplifying assumptions in the model. The difference in the changes of the core configuration during displacement of dislocations of different orientations should, however, be of physical significance. It is found that a dissociated 60° dislocation generally has a higher effective Peierls energy than a screw dislocation, but the reverse is true for the kink energy, at least in Cu.     

Effects of Al concentration and resulting long-period superstructures on the plastic properties at room temperature of Al-rich TiAl single crystals

The role of Al₅Ti₃ and h-Al₂Ti long-period superstructures on the plastic properties of TiAl at room temperature is investigated on five single crystals with aluminium content comprised between 54.7 at.%, and 62.5 at.%. After annealing at 1200°C for 1?h, the Al₅Ti₃ superstructure develops in the L1₀ (γ) matrix upon increasing Al concentration except for Ti–62.5 at.%Al where h-Al₂Ti substitutes for Al₅Ti₃. The CRSS for <110]{111} first increases abruptly with the development of the Al₅Ti₃-type ordering. Then, the CRSS reaches a plateau at which dislocations assemble in groups of four to prevent extra anti-phase boundary (APB) from being engendered during glide throughout the Al₅Ti₃ phase. In Ti–62.5 at.%Al, the CRSS for ordinary slip further increases upon the precipitation of h-Al₂Ti in the L1₀ phase, whereas it decreases when the crystal is fully transformed into single-phased Al₅Ti₃. <101] superlattice dislocations are primarily activated under both the [210] and [1?1?8.6] load orientations irrespective of the Al concentration, but the dislocation microstructure strongly depends on orientation as well as on the degree of Al₅Ti₃ ordering. In the [210] orientation, the frequency of the decomposition of <101] dislocations into 1/2<110] and 1/2<112] dislocations decreases abruptly with the development of Al₅Ti₃. This is interpreted in terms of the increased difficulty to move ordinary dislocations. Under the [1?1?8.6] orientation, the density of faulted dipoles diminishes remarkably with the development of Al₅Ti₃. This is consistent with the transformation of the low energy extrinsic stacking fault of the L1₀ phase into a higher energy complex extrinsic stacking fault.     

Interaction of Lomer–Cottrell locks with screw dislocations

In fcc crystals, dislocations are dissociated into partial dislocations and, therefore, restricted to move on {111} glide planes. By junction reactions with dislocations on two intersecting {111} planes, Lomer–Cottrell dislocations along ?110? directions can be formed which are barriers for approaching screw dislocations. Treating the interaction between a dissociated screw dislocation and a LC lock conventionally, using classical continuum theory and assuming the partials to be Volterra dislocations, leads to erroneous conclusions. A realistic result can only be obtained in the framework of the Peierls model, treating the partials as Peierls dislocations and explicitly taking account of the change in atomic misfit energy in the glide plane. At even moderate stresses (at less than 3 × 10^?3 µ in Cu), the screw will combine with the LC lock to form a Hirth lock. As a result, the nature of the repulsive force will change drastically.     

Discrete dislocation simulations of precipitation hardening in superalloys

The low-temperature yield stress of a nickel-based superalloy, containing up to 40% Ni₃A1 precipitates (γ′), is calculated by discrete dislocation simulations. A pair of screw or 60°(a/2) ?110? dislocation glides under external stress across a {111} plane of γ phase, intersected by a random distribution of either spherical or cubic γ′ precipitates. The stress is raised until the dislocations can cut or bow round all the obstacles. In this paper the emphasis is on the cutting regime which is prevalent when the precipitates are small and/or have low antiphase-boundary (APB) energies. From a large number of simulations in the cutting regime, the effects of size, shape, volume fraction and APB energy are found to be as follows: The yield stress is proportional to the square root of the volume fraction of γ′. The yield stress depends weakly on the precipitate size in the size range 20–400?nm, for APB energies of 150, 250 and 320?mJ?m^?2. The yield stress depends linearly on the APB energy for APB energies up to 320?mJ?m^?2 in the size range 50–200?nm. At a precipitate size of 100?nm, cubes are weaker obstacles than equivalent spheres by about 25% for an APB energy of 320?mJ?m^?2; however, the shape effect on strengthening decreases with decreasing APB energy and decreasing precipitate size. When a coherency stress (from a lattice parameter mismatch of 0.3%) is added, the yield stress increases by about 10%. When solid-solution strenthening is added, it is potent when the solute is in the γ matrix, but much less potent when the solute is in γ′. When the γ′ precipitates are larger than 400?nm across and the APB energy greater than 250?mJ?m^?2, significant Orowan looping occurs. The yield stress drops inversely as the precipitate size and becomes insensitive to the APB energy but sensitive to the shear modulus. Many of these results from the full simulations differ from the analytical models of strengthening in superalloys but they can be rationalized from the results of simulations on simple homogenized precipitate structures.     

Magnetic structure of the random system near the ferro-antiferromagnetic transition

The magnetic structure of the disordered alloy Fe₆₅Ni₂₈Mn₇ was investigated in the temperature 4.2–300 K by the methods: small angle scattering of neutrons, Mössbauer effect, magnetization, magnetic contribution to the thermal coefficient of the thermal expansion, and resistivity. All measurements show that long-range ferromagnetic order appears below T_c ? 160 K. At the same time for T ? 100 K, a dramatic change of magnetic state takes place which is interpreted as the freezing of “spin glass”. An increase of the magnetic contribution to the resistivity with decreasing temperature was also found. This increase was attributed to the existence of poor-bonded magnetic moments of the Kondo-type. A model of the magnetic ground state is proposed which includes the details of magnetic behavior such as long-range ferromagnetic order, spin glass, finite ferro-and antiferromagnetic clusters, and Kondo-type states. A magnetic phase diagram of the system Fe₆₅(Ni_1?xMn_x)₃₅ is also proposed.     

Mössbauer study of aluminum-substituted hematites

A Mössbauer spectroscopic study is reported on a series of aluminum-substituted hematites, α(Fe_1-cAl_c)₂O₃, with c up to 0.32. These samples were prepared by heating aluminum-substituted goethites, αFeOOH, at 500°C. X-ray line broadening gives particle dimensions of ?200 Å to >1000 Å. Heating the samples to 900° improves crystallinity but reduces the maximum obtainable c to ?0.15. At 77 K for c?0.04 the magnetic structure is antiferromagnetic with the spins aligning close to the (111) axis. For c?0.08 and for all compositions at 298 K the spins are perpendicular to (111) and for this weakly ferromagnetic phase the supertransferred hyperfine field is (5±1) kOe per magnetic neighbor. Samples with 0.04?c<0.08 at 77 K have both magnetic phases present in varying proportions. The magnetic field at 298 K varies with aluminum content according to the 1/3-power law for the reduced sublattice magnetization. The asymmetric shape of the spectra for c?0.17 has been accounted for by a model based on the molecular field approximation with nearest neighbor superexchange interaction strength J_nn?15 K.