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Our recent experimental work demonstrated that a neutrophil-dependent inflammatory response in the lung prevented the normal up-regulation of alveolar fluid clearance by catecholamines following hemorrhagic shock. In this study, we tested the hypothesis that the release of NO within the airspaces of the lung was responsible for the shock-mediated failure of the alveolar epithelium to respond to catecholamines in rats. Hemorrhagic shock was associated with an inducible NO synthase (iNOS)-dependent increase in the lung production of NO and a failure of the alveolar epithelium to up-regulate vectorial fluid transport in response to β-adrenergic agonists. Inhibition of iNOS restored the normal catecholamine-mediated up-regulation of alveolar liquid clearance. Airspace instillation of dibutyryl cAMP, a stable analog of cAMP, restored the normal fluid transport capacity of the alveolar epithelium after prolonged hemorrhagic shock, whereas direct stimulation of adenyl cyclase by forskolin had no effect. Pretreatment with pyrrolidine dithiocarbamate or sulfasalazine attenuated the iNOS-dependent production of NO in the lung and restored the normal up-regulation of alveolar fluid clearance by catecholamines after prolonged hemorrhagic shock. Based on in vitro studies with an alveolar epithelial cell line, A549 cells, the effect of sulfasalazine appeared to be mediated in part by inhibition of NF-κB activation, and the protective effect was mediated by the inhibition of IκBα protein degradation. In summary, these results provide the first in vivo evidence that NO, released within the airspaces of the lung probably secondary to the NF-κB-dependent activation of iNOS, is a major proximal inflammatory mediator that limits the rate of alveolar epithelial transport after prolonged hemorrhagic shock by directly impairing the function of membrane proteins involved in the β-adrenergic receptor-cAMP signaling pathway in alveolar epithelium.

The effect of the microstructural state of the mechanically alloyed Fe-40Al intermetallic on quench hardening and age softening was examined, focusing on the role of grain boundaries or the influence of precipitate or dispersoid particles. The intermetallic was prepared by milling the Fe-40Al powders with 1 wt% of Y2O3 particles and subsequently extruding to bar at 1100°C. There was a lack of change of hardness of material in the as-extruded state as the quenching temperature varies over the 400 to 1100°C range. This is attributed to the loss of vacancies during the less-than-ideal quench from high temperatures to the many sinks represented by grain boundaries as well as by particle-matrix interfaces.

The hardening produced by quenching from high temperatures, and the evolution of hardness during subsequent low temperature annealing of a Fe-40Al-0.4C alloy prepared in strip form, 0.2 mm thick, by a powder rolling procedure starting with water atomized powders were examined. The composition of the material is Fe-39.2Al-0.4C-0.02B-0.19Mo-0.86O-0.05Zr (atomic %), with Zr and B are in the form of large dispersed boride particles, Mo in solid solution and O as coarse (50-500 nm) uniformly distributed oxide particles. These insoluble dispersoids are sufficiently coarse and widely spaced that they are not expected to play a major role in influencing C and vacancy solution and precipitation and neglecting the small amount of Mo solute.

The anomalous stress peak found at intermediate temperatures has been examined in Fe-25Al and Fe-28Al alloys by hardness testing over a wide range of temperatures, using a wide range of strain rates, and studying dislocation structures in samples both slowly cooled as well as rapidly quenched after deformation. The anomalous stress rise has been found to show little strain-rate dependence, while the stress peak temperature and the high-temperature range of falling stress has been shown to be highly strain-rate dependent. There is no clear change of dislocation structure as the yield stress increases, and the stress fall is sometimes associated with 〈111〉 superdislocations and other times with 〈100〉 dislocations, depending on the strain rate considered. Such 〈100〉 dislocations appear to be created as mobile 〈111〉 superdislocations of different Burgers vector react together, which happens more readily at higher temperatures and lower strain rates, and this probably affects work hardening behaviour and flow stress levels after yield, but not the yielding process itself. The absence of any clear evidence of a dislocation pinning process operating during deformation makes it difficult to determine the mechanisms responsible for the stress rise and then fall.

Trabajo presentado en el Materials Research Society Symposium: High Temperature Ordered Intermetallic Alloys IX, celebrado en Boston, MA (Estados Unidos), del 27 al 29 de noviembre de 2000, This study examines the influence of microstructural parameters (grain and dispersoid size, vacancy content) and some test parameters (strain rate, protective oxide coatings, air and water vapour excluding films, and surface geometrical quality) on the tensile behaviour (yield stress, work hardening rate, tensile stress, ductility) of a mechanically-alloyed, fine-grained Fe-40Al intermetallic. Major changes of strength and ductility are obtained by changing grain size (1% and 10% for grain sizes of 100μm and lμm) and by avoiding premature stress/strain concentrators (ductility increased from 5% to 10% for imperfectly machined to prepolished samples). Ductility variations are interpreted using a slow-crack-propagation-to-instability model, where the roles of environment, surface state, deformation processes, and fracture mechanisms can be distinguished.

Considerable attention has been given to the iron aluminide family of intermetallics over the past years since they offer considerable potential as engineering materials for intermediate to high temperature applications, particularly in cases where extreme oxidation or corrosion resistance is required. Despite efforts at alloy development, however, high temperature strength remains low and creep resistance poor. Reasons for the poor high-temperature strength of iron aluminides will be discussed, based on the ordered crystal structure, the dislocation structure found in the material, and the mechanisms of dislocation pinning operating. Alternative ways of improving high temperature strength by microstructural modification and the inclusion of second phase particles will also be considered.

In the 12 years from 1984 to 1995, Adelaide-based mobile intensive care teams transported 4443 critically ill patients from rural areas in South Australia and adjacent States to tertiary-level hospitals in Adelaide. The SA Ambulance Service undertook communications, support staffing and deployment of transport. Average radial distances in 819 road missions were 71 km, in 808 helicopter missions 122 km, and in 2777 fixed-wing aircraft missions 398 km. The largest groups of patients were neonates (23%) and those with trauma (25%). Rural hospitals made 96% of the requests for intensive care transport; 4% came from ambulance or other emergency service crews at accident locations. Emergency surgical or operative obstetrical procedures were performed on 2.7% of patients before transport. One hundred and thirteen patients (2.5%) died during resuscitation or transport, with one death deemed to be preventable.

In conventional metallurgy it is well accepted that preferred crystal orientations in a polycrystalline material can have significant effects in modifying properties, with notable examples found in γ-γ′ superalloy components and Fe(Si) transformer sheets. The same effects clearly should be obtained in intermetallics, but this aspect has hardly been considered until recently. The influence of textures will be examined in iron aluminide and in γtitanium aluminides. The first compound is highly elastically anisotropic and plastically relatively isotropic. The second compound is elastically more isotropic and shows severe plastic anisotropy, both as single phase γTiAl and in the more common two-phase (γ+α2) form. Control of texture and of its uniformity throughout a component can have a major effect on its response to mechanical stresses.

The role of quenched-in vacancies in FeAl intermetallics on producing considerable hardening is well known, as is the softening on annealing as vacancies are annihilated. The present study examines quench hardening and anneal softening by quenched-in vacancies and interstitial carbon solute in Fe-40Al-C. Interstitial carbon is seen to be a more potent hardening agent than the vacancy, while the co-annihilation of vacancies and carbon atoms from solution during annealing leads to dislocation loop debris, and equiaxed or plate-like carbide precipitation, according to the annealing conditions. The processes occurring have been followed by detailed TEM studies, and are discussed in terms of the relative solubilities and diffusion rates of vacancies and carbon. The relevance of such interstitial solute hardening to the behaviour of other FeAl intermetallics is also briefly considered.

Changes in hardness of Fe-40Al powders during milling and during subsequent annealing have been examined and related to the many structural changes occurring. During milling, the material becomes significantly disordered with a small domain size and many vacancies, and refines to a nano-scale grain size, while on subsequent annealing at progressively higher temperatures these structural defects are lost as the material re-orders, loses point defects and as the grains grow to large sizes. The increase in hardness during milling can be explained by the combined contributions of vacancy hardening, ordered domain/particle hardening, and by disorder hardening within the ordered regions. Softening during annealing occurs as the domain/particle hardening, disorder hardening and vacancy hardening are successively lost. Dislocation-induced work hardening and grain size hardening are believed to play only minor roles in affecting the material hardness.

The oxidation behaviour of an oxide dispersion-strengthened (ODS) FeAl intermetallic, microalloyed with Zr and B and strengthened by a fine dispersion of Y2O3, is investigated at 1100 °C for exposures of up to 200 h. The results show that a pure alumina scale is formed irrespective of the exposure time. The oxidation rate is far inferior to that found on PM 2000, a commercial alumina forming ODS ferritic superalloy. Limited scale spallation is observed in the intermetallic alloy from the early stages of oxidation. Scale failure, which is shown to occur during the cooling stage after oxidation and not at the high temperature of oxidation itself, results from the high compressive residual stresses in the scale induced by the misfit in the thermal expansion coefficients of the scale and the substrate. Failure of the scale may be suppressed by using a very low cooling rate after oxidation.

The corrosion behaviour of an Fe3Al-base intermetallic compound with different crystal structures in a chloride containing solution has been investigated. The corrosion current densities of this intermetallic were independent of the material crystal structure showing a passive state stable with time. These corrosion rates were of the same order of magnitude as for 316L stainless steel. The pitting corrosion resistance evaluated by means of cyclic anodic polarization curves was high for all different states. Amongst the different crystal structures of this intermetallic alloy, the two ordered states present the lowest pitting probability. This Fe3Al intermetallic shows higher pitting corrosion resistance than the 316L stainless steel but its capacity for repassivation is lower. A damaging factor of influence on the pitting corrosion behaviour is the presence of non-metallic inclusions on the surface which reduce the pitting corrosion resistance by almost a half. © 1999 Elsevier Science Ltd. All rights reserved., This work was carried out under the umbrella of CEASI, Iron Aluminides program. The authors thank Shell Research Laboratories, Arnhem for the kind supply of material, and CICYT (Spain) for financial support under projects MAT95-0249-CO3-01 and MAT95-0796.

Crystals of trimethylammonio-P,P-diphenylphosphinamidate hydrate, Me3NNP(=O)Ph2 ⋅ H2O, are held together by hydrogen bonds which link two Me3NNP(=O)Ph2 and two water molecules to form a centrosymmetric O4H4 ring belonging to graph set R2 4(8). Each P=O oxygen atom can thereby act as acceptor to two hydrogen bonds, rather than to only one, as in the two-fold symmetric P=O⋅⋅⋅H–O–H⋅⋅⋅O=P motif commonly found in phosphine oxide hemihydrates. The unusual P–N bond in Me3NNP(=O)Ph2 is short enough 1.614(3) Å to indicate some multiple character.

The high strains achieved during high temperature deformation of a Ti-Al rolled sheet have been evaluated by following the microstructural evolution of tensile samples tested along the transverse and rolling directions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations have confirmed that in both types of samples extensive twinning activity occurs during deformation at temperatures of 700 and 800°C but not at 900°C. Microstructural refinement occurs by subdivision of the grains either by the twin interfaces or by subgrain formation followed by recovery/recrystallization processes. The lower strains achieved in the samples deformed along the transverse direction are a result of a more inhomogeneous microstructure due to the different deformation mechanisms involved, that include activation of superdislocations at high strains.

Refining grain size to the nanocrystalline level has been suggested as a way of improving strength while enhancing ductility and toughness. In the present study, nanocrystalline bulk FeAl has been prepared by mechanical alloying and hot forging. Powders quickly reach a state of partial order during milling, and low temperature annealing is sufficient to chemically homogenize and give full order. Contamination during milling leads to the formation of carbide and oxide particles, which stabilize fine grains during heating. Bulk materials show grain sizes of 20 nm to 100 nm depending on the consolidation temperature. Hardness and compression strength show little change over this grain size range. Fracture toughness stays high down to moderately small grain sizes, falling only for consolidation at the lowest temperatures. There appears to be a reasonable range of fine grain sizes (40-100 nm) where good interparticle bonding and high densities can be achieved leading to good strength and toughness.

Antiphase boundaries (APBs) in a rapidly solidified Ni3Al alloy have been examined by transmission electron microscopy in the as-solidified state and after annealing. Dark field imaging suggests that the APB thickness increases with temperature during annealing and that a dark, disordered phase appears at temperatures above about 800°C. The boundary region at high temperatures has a chemical composition close to that expected from the phase diagram for the disordered phase, namely containing about 15 al.% Al. Examination of APBs by high-resolution electron microscopy does not confirm the disorder. however, and the so-called disordered regions showing dark contrast may in fact be caused by the significantly modified chemistry of these ordered regions. It is not clear whether disorder may form at high temperature when the average alloy composition enters the two-phase ordered+disordered region, or whether simply extensive segregation takes place to the APB without the true appearance of a second, disordered phase. Similar effects occurring at grain boundaries would lead to a structure dependent on both composition and temperature, with the mechanical behaviour of the material depending on the structural state.

There is considerable interest in the development of high strength Cu-base composites with high conductivity for applications such as high field magnets 1, 2, 3, 4, 5, 6 and 7, and essentially two alloy types have been examined, where a phase distributed in the Cu matrix during solidification is transformed into fine filaments during cold working of the material to sheet or wire 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20. Some of these are Cu-Ag alloys 8, 9, 10, 11, 12, 13 and 14, with the Ag present as inter-cellular or inter-dendritic phase, or as part of a Cu-Ag eutectic. The other alloys are of the Cu-bcc element type 15, 16, 17, 18, 19 and 20, where the bcc element (Cr, Fe, Nb, etc.) is present as primary dendrites after solidification. For Cu-Ag alloys, it has been shown that the second phase Ag is transformed into fine filaments or rods during working, and it is the reduction of the distance between such matrix-Ag interfaces that is responsible for the increased hardening 8, 12 and 14.

In astronomy there is a growing need for very high performance CCDs. These CCDs require large area, high quantum efficiency and low read noise. The necessary technologies have been developed at EEV.

Strong yield point effects are observed on reloading iron aluminides after deforming, unloading for a short time at low temperatures, and reloading. Such yield point effects are shown to be essentially independent of material microstructure and occur so quickly at room temperature that it is unlikely that diffusional effects play any role. The most important factor determining the extent of yield point stress change at a given stress and temperature condition seems to be the Al content with aluminides of composition near FeAl showing very strong yield drops and aluminides of composition near Fe3Al hardly showing any yield point effects. The likely cause of the yield point effects is dislocation locking occurring during unloading, which is believed to be due to core transformation–dissociation at the a/2〈111〉 dislocation cores.

Neutron powder diffraction measurements have been carried out in situ from room temperature to about 100°C in Fe28Al (28 at.% Al), Fe32.5Al (32.5 at.% Al) and Fe28Al15Cr (28 at.% Al, 5 at.% Cr) alloys. X-ray diffraction and TEM studies provided supporting information. The data were analysed to obtain information about the temperature dependence of the DO3 and B2 long range order parameters, the location of the Cr atoms and their effect on the ordering energies, and on the vacancy formation and migration properties in Fe28Al and Fe32.5Al alloys. The location of the ternary alloying addition in DO3 and B2 ordered Al-rich Fe3Al is shown to be consistent with considerations of interatomic bond energies.

Iron aluminides with compositions from Fe3A1 to FeAl show an anomalous stress rise at temperatures about 600–800K. Many models have been suggested to explain this behaviour, with the suggestion of hardening by vacancies presently receiving much attention. An alternative model is suggested here, valid for those alloys where the simultaneous reduction of DO3order does not interfere, based on the density of constrictions produced on mobile superdislocations and their subsequent pinning by decomposition or by their cross slip to produce jogs. The exponential increase in stress with temperature can be related to the pinning point density and the absolute values of strengthening correspond well with those predicted by this model. The orientation dependence of the stress anomaly and the tension-compression asymmetry can be related to the role of shear stresses in assisting constriction of the dislocation partials by modifying the core configuration and by assisting the expansion of the new, decomposed dislocations.

The mechanical behaviour of an alloy based on Fe40Al prepared from mechanically alloyed powders was examined over a wide temperature range in the fine-grained, as-extruded state as well as after recrystallizing to a large-grained state. While the fine-grained material was strong and reasonably ductile at room temperature, in contrast with the weaker and more brittle large-grained material, at high temperature the strength fell to low values, similar for both materials. This behaviour is interpreted in terms of a contribution to strengthening due to the particles present, by Orowan hardening at low temperatures and by dislocation-particle interactions at high temperature, a contribution due to the grain size, which can harden at low temperatures and soften at high temperatures, and a contribution due to the matrix. The room temperature ductility seems to be dependent mostly on the grain size, since fine grain sizes can inhibit brittle crack formation.

Iron aluminides show an anomalous peak in yield stress at an intermediate temperature, as superdislocations of 〈111〉 Burgers vector are replaced by dislocations of Burgers vector 〈100〉. Changes in yield stress and in dislocation configurations as a function of test temperature have been examined while deforming over a wide range of strain rates in order to determine the mechanisms responsible for the stress anomaly. The mechanical testing data provide some information about the process leading to the anomalous hardening but it is difficult to deduce the mechanisms operating from this. TEM studies show 〈111〉 superdislocations in climbed configurations preceding the transition in Burgers vector, and this at lower temperatures for slower testing speeds. The evidence obtained seems to support the model of the locking of dislocation segments by a local climb process as responsible for the anomalous stress behaviour.

The mechanical properties of an Fe-28 at.% A1 alloy have been examined in detail from room temperature up to 700°C. The solute additions contained in this alloy lead to higher strength at all temperatures than is shown by the binary alloy. These high strengths are associated with dissociated imperfect superdislocations from room temperature up to a stress peak. The anomalous stress peak is found at 500°C, somewhat below the temperature range where disordering of D03, order to the B2 ordered state occurs and the Burgers vector of the dislocations present changes from 111 to 100. The reason for the anomalous peak, the locking of the 111 superdislocations and the eventual change to 100 dislocations is a local climb-locking process operating on the 111 superdislocations.

The influence of annealing treatments over a range of temperatures encompassing the critical ordering temperature for the B2-DO3 change and inducing recovery and recrystallization have been examined for a deformed alloy based on the Fe3Al composition. The initial ordered state before heavy deformation plays no role on subsequent recovery and recrystallization, essentially because the heavy deformation destroys the prior order. Recovery occurs faster relative to the rate of recrystallization at higher temperatures. The extended period at lower temperatures where poorly recovered structures remain allows grain nucleation mechanisms to remain activated, in contrast to higher temperature situations where grain nucleation slows as grain growth dominates recrystallization. This evolution is slightly enhanced at lower temperatures as DO3 order appears, and leads to finer final grains after such lower temperature anneals. There is no evidence to indicate that dislocations influence the rate of ordering of this material.

The cubic titanium trialuminide alloys studied previously show limited ductility and toughness, which can be related to the difficulties of dislocation emission, multiplication and mobility and to the excessively high energies of faults associated with the dislocations. The present study examines several new alloys chosen in an attempt to reduce these fault energies and thereby improve the mechanical properties. The region of single phase L12 material is unfortunately so limited that only minor changes in composition are possible before second phases form and precipitation hardening occurs. Over the range of compositions of the L12 phase evaluated there are generally only small changes in fault energies. These changes, as well as the changes in dislocation configurations seen, may be rationalized in terms of the instability of the matrix towards the particular second phase forming.

Considerable effort is being devoted to the development of high strength, high conductivity copper alloys both for applications in electronics as well as in aerospace, where the ability to remove heat and to retain a sufficient strength is of critical importance. Examples of such alloys are Cu-Cr-Zr and Cu-Ni-Sn alloys prepared by rapid solidification and by osprey processing (1-6), in situ fabricated fiber-reinforced Cu-Cr, Fe, or Nb composites (7) and mechanically alloyed Cu-bcc metal mixtures (8-10) . the previously examined mechanically alloyed materials showed good strength after consolidation of the milled powders but showed signs of structural instabilities at high temperatures due to coarsening of the dispersed bcc metal particles. The present study examines alloys prepared by milling and consolidating mixtures of copper with other elements which will produce stable compound phases (TiB2 and Al2O3) either during milling or during subsequent annealing. Such dispersed particles should be more resistant to coarsening than the earlier bcc metal particles and should lead to improved strength and stability after high temperature treatments.

Neutron powder diffraction measurements have been carried out on cubic Al3TiX compounds (X = Cu, Cr, Mn, Fe and Ni) as a function of temperature from room temperature to 1200°C. The data were analysed by the Rietveld method in order to determine the ordering and lattice site location of the constituent atoms. All these intermetallic compounds were found to be highly ordered at room temperature, and a significant increase in disorder at higher temperatures was observed only in alloys containing Cr and Cu.

A detailed examination of the variation of yield stress with temperature in an Fe3Al alloy shows a maximum at a temperature of about 500°C, slightly below the critical temperature for loss of DO3 order. At this temperature the dislocations present in the material change from being Analysis of the forces between the dislocations which induce the local climb locking process allows an estimation of the role which will be played by variations in composition of the Fe-Al alloy considered, changes in deformation rate and orientation of the applied stress.Examination of data available in the literature shows that each of the three aspects discussed, namely the influence of variations in ordered state, in Al content over the range 25% to 50%, or additions of alloying elements such as Si, straining at very fast or at slow rates, and stressing along different crystallographic axes, is completely consistent with the model proposed.

Iron aluminide alloys based on the composition Fe3Al are receiving considerable attention as structural materials for applications at high temperatures in view of their excellent resistance to oxidation and corrosion (1) as well as reasonable mechanical properties. Recently, problems associated with poor ductility at room temperature have been alleviated by small additions of Cr and by microstructure control, as well by as the realisation that the low ductility is, in part, extrinsic behaviour due to environmental attack (2-5). These materials suffer also from a loss of their good strength at temperatures above about 600°C, and recent attention has led also to the development of creep-resistant alloys (5, 6).

Alloys based on Al3Ti with the ordered L12 structure show slight ductility when the Ti content is near 25% and about 8% of Mn or Cr is present as ternary addition. Such materials are relatively soft due to the easy movement of APB-dissociated superdislocations but remain almost completely brittle. While the precise reasons for such brittleness are not clear, it seems reasonable to consider that alloying to lower fault energies may soften the material and enhance ductility. In the present study, new alloys are selected on the basis of electronic structure calculations using the discrete variational cluster method, and the ordered state, mechanical behaviour, and dislocation and fault characteristics examined.The alloys examined were based on the Al-26%Ti-8%Mn composition, with lower Ti and Mn contents, in an attempt to maintain a single-phase matrix and weaker, less-directional bonding and lower fault energies. In addition, for some alloys, Al was partially substituted by Mg.The single-phase region of the L12 Al-Ti-Mn phase is very small and second phases such as DO22 and complex AlxMn can appear which lead to hardening and strain ageing, much as Al2Ti does in more Ti-rich alloys. Mg substitution is limited to a few percent before a Mg-rich phase appears and the alloys examined are complex mixtures of this, the L12 phase, and γTiAl. The dislocation structures observed after deformation are examined to determine fault energies, and it is shown that these values can be rationalised in terms of the structural instabilities of the matrix phases and the secondary phases produced.

Iron aluminides containing about 28 at.% Al have the D03 ordered structure in equilibrium below about 550°C and the B2 ordered structure above that, with the disordered state existing at temperatures above about 900°C. The transition from the D03 state to the B2 state has been examined for several alloys containing various alloying additions using X-ray diffraction, differential scanning calorimetry and in situ transmission electron microscopy. Some of the alloying additions investigated lead to changes in both the kinetics of ordering and disordering as well as modifying the temperature range of stability of the D03 state. While the alloys examined lose most of the D03 order at the critical transition temperature, there is clear evidence of remnant long range order for temperatures up to 100–200°C above this critical temperature, at least for some of the more complex alloys. The retention of this order may be caused solute remaining on specific lattice sites after disordering of the majority Fe and Al atoms.

This paper concerned mmc in some important way and contained information about, utilized, or discussed these subjects: basic topics:, data, experimental, mmc, mmc data materials and material types:, al2o3/cu, billets, cu matrix material characteristics and data fields:, mechanical properties, recrystallizations, strain rates, strains, yield strength data testing and test conditions:, optical microscopy, sem, tensile tests, manufacturing:, annealing, cold drawings, spray depositions miscellaneous and special technical terms:, electronic applications, engineering applications, microstructures.

The recrystalization process of a copper aloy containing different distributions of fine and coarse particles has been studied by following the softenin that occurs during isothermal anneals between 500 an 700º C. The mechanisms involved in the formation of the new grains have been studied by >in situ> heating experiments in the TEM and by observation from the bulk samples at different stages after annealing. The mechanism of granin formation seems independent of particle distribution an involves the reorientation of the deforamtion cells into subgrains and into well defined grains by dislocation rearrangement en the cell walls.

Recently there have been conflicting reports concerning the dissociation scheme of superdislocations in Al3Ti alloyed to the cubic structure. This discrepancy can perhaps be explained by the difficulty of applying invisibility criteria for dislocations imaged under weak beam electron microscope conditions. The present study examines dissociated dislocations under a wide range of imaging conditions to deduce that dissociation by the creation of an antiphase boundary (APB) fault is the scheme operating, at room temperature as well as at high temperature. At the triple-point nodes of intersecting dislocations there are localized regions of superlattice intrinsic stacking faults (SISFs), which allow an estimation of SISF energy for comparison with the APB fault energy. This comparison confirms that dissociation by the creation of an APB fault should be the preferred scheme. After high-temperature deformation many of the superdislocations have their APB on cube planes and remain mobile with no evidence of dislocation locking. Such observations, some made on materials showing anomalous strengthening and others on materials not showing this anomaly, confirm that Kear-Wilsdorf locking is not the cause of the anomalous strengthening sometimes found for these materials.

The development of Fe 3 Al aluminides has been restricted in the past by poor ductility at ambient temperatures, and it is only recently that possible solutions to this problem have been found. It was shown a few years ago (1) that the addition of 2-6% Cr to a Fe 3 Al base alloy led to good ductility and this improvement was explained by a reduction of the APB energy, increasing the separation of the superpartial dislocations and thereby allowing easier dissociation of these dislocations, easier cross slip and a reduced tendency to stress and strain concentrations. However, at a later stage, an alternative explanation was proposed (2,3) based on examinations of both FeAl and Fe 3 Al alloys under different environments, and the ductility change was explained in terms of chemical attack at the tip of a crack leading to local hydrogen embrittlement. The present study re-examines the behaviour of a Fe 3 Al alloy both with and without the addition of Cr. Strength, work hardening behaviour and failure ductility are examined under conditions where environmental effects should not be important, and the mechanical behaviour is interpreted in terms of significant variations in the type of order, the ordered domain structure and the resulting dislocation structures. It is seen that the addition of Cr can lead to a better ordered material and the differences in ordered state between the two materials can significantly affect dislocation behaviour and mechanical properties.

Titanium trialuminides have interesting strengths at high temperatures and may become useful engineering materials providing their toughnesses can be improved. One possible way to achieve this is by structural refinement. In the present study, powders of two titanium trialuminide alloys with L12 crystal structures have been milled to refine their microstructure and then consolidated by hot isostatic pressing. The behaviour of the powders during milling has been examined, and the mechanical properties of the consolidated materials evaluated and compared with the properties of bulk materials. During milling the microstructure was refined to about the micrometre level, but there were no signs of significant disordering or other changes of crystalline structure as often found during intense ball milling. Following consolidation, the materials retained significant work hardening but, despite their refined microstructures, were somewhat more brittle than the reference states. The materials with fine grain sizes failed by intergranular failure rather than by transgranular cleavage as seen for materials of larger grain size. The fracture behaviour is discussed in terms of the failure mechanisms in titanium trialuminides.

Cubic trialuminides deform by the movement of dislocations which are clearly dissociated as APB superdislocations at high temperatures, with debate about whether these are dissociated as APB or SISF superdislocations at low temperatures. These materials are characterized by the following mechanical behaviour: (i) strength variable with ternary element addition or titanium content; (ii) mild strength anomaly at high temperature, sometimes; (iii) serrations in stress-strain curve at intermediate temperatures; (iv) low tensile ductility (≈0) at room temperature, increasing at higher temperatures, with an intermediate temperature minimum. These properties are explained by the fine microstructure and its variation locally and with temperature: (a) a tetragonal component of order in ternary alloys in addition to the basic L12 order, varying with ternary element and content; (b) precipitation (sometimes on dislocations) during high temperature testing - accounting for the strength anomaly; (c) extra-ordinarily rapid solute collection at dislocations and APB’s - accounting for strain aging and minimum ductility phenomena; (d) strain relaxation at crack tips restricted to single slip planes by the structural modification produced by shear - making a major contribution to brittleness. All these processes are particularly acute in the titanium trialuminides because of the structural instabilities involved, and the fast kinetics of atom rearrangement; thus low temperatures during testing or during cooling after prior heat treatment play a major role. Similar effects may be of importance in other intermetallics such as FeAl, TiAl.

Molecules of 13H-dibenzo­[a,i]­fluoren-13-one, C21H12O, strad­dle a crystallographic mirror plane and are essentially planar, with a dihedral angle of only 1.9 (1)° between the two naphtha­lene ring systems. Repulsive intramolecular C=O⋯H interactions therefore do not explain the larger distortions found in isomeric ketones.

Titanium trialuminide alloys suffer problems of intense segregation during solidification and extreme brittleness when subjected to tensile stresses. In this study, spray forming techniques have been used to obtain homogeneous materials of fine microstructure, containing various second phase particles, and the deformation behaviour of these materials examined over a range of temperatures.The microstructure and deformation behaviour of an iron-modified titanium trialuminide with the L12 structure is reported here.Deformation mechanisms are interpreted by analysis of dislocation structures and further confirmation is obtained by activation volume measurements: the importance of Peierls effects at low temperatures and cross slip at high temperatures is shown. The material remains brittle in tension both at room temperature and at high temperatures: the second phase particles in the material are yet not present in sufficient volume fraction to greatly affect plastic behaviour.

Crystallographic and chemical analysis of the microstructures obtained by annealing a heavily deformed Ti-24Al-11 Nb alloy have been performed. The influence of the niobium distribution within the different phases present and the variation of degree of long range order caused by this distribution have been related to the deformation mechanisms leading to strain hardening at the early stages of deformation. In particular, slip band formation and twinning have been observed to occur within the (α2 phase according to the different degree of order measured.

Fe-35Al alloys containing various amounts of ZrB2 have been prepared by melt spinning and the microstructure and its stability examined. The mechanical properties are evaluated both on as cast materials as well as after high temperature heat treatments. The ZrB2 additions lead to a dramatic increase in hardness and strength. In addition, small amounts of ZrB2 lead to significant increases in ductility. Because the microstructure is fairly stable, these improvements in properties are maintained even after high temperature exposures.Alloys with up to about 1%ZrB2 have the dispersoid particles arranged in an imperfect cellular microstructure after rapid solidification. The cell walls contain many fine particles and are efficient barriers against dislocation propagation. Strain occurs as dislocations escape through gaps in the imperfect cell walls at a stress level that is controlled by the size of these gaps. Ductility is improved since the deformation that results from such controlled strain progression through the material is much more uniform and large stress and strain concentrations are avoided.

Crstal nucleation during rapid solidification generally occurs rather slowly except at active heterogeneities or unless very large undercoolings are achieved. Many typical microstructures are then essentially of columnar morphology, for example from the bottom surface of a melt-spun ribbon or from a heterogeneity on the surface of a powder particle. In such cases the microstructure may be considerably refined by the presence of many active nucleants for heterogeneous nucleation distributed throughout the melt. Such microstructural refinement is analysed here during rapid solidification of a laser-melted surface containing fine TiB2 particles. Simulation of the cooling and solidification conditions confirms these particles to be highly effective nucleating substrates, capable of greatly increasing nucleating rates. As a result it is possible to obtain materials possessing greatly refined grain sizes.