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Despite decades of intensive research iron aluminides remain characterised by relatively poor ductility at room temperature and low strength at high temperatures, especially under slow strain rate or creep conditions. A variety of strengthening particles has been tested for improving high temperature strength, but each has serious limitations: typical carbide precipitates are unable to resist dissolution or coarsening at high temperatures; as-solidified iron aluminides with sufficient amounts of transition elements such as Nb or Mo show heavy solidification segregation and are embrittled by a network of Laves phase; mechanical milling with stable oxides appears an excessively expensive processing route. A new iron-aluminium alloy has been developed with Zr and Cr additions that forms fine coherent precipitates even after extended annealing at temperatures as high as 900ºC. These precipitates have a complex Fe3Zr structure and form in a cube-on-cube orientation relationship in the bcc matrix. The low solubility and diffusivity of the solute, as well as the low energy, near-coherent interface ensures excellent stability of these intermetallic precipitates. Interesting strengthening is possible for this material under the relevant high temperature creep conditions.

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.

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.

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.

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.

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.

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.

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.

The Dynamic Compaction of powdered materials becomes a particularly attractive technique for consolidating hard materials, because of the ease with which high pressures are achieved, and for metastable materials, because of the possibility of avoiding large-scale, high temperature excursions. The shock-wave conditions necessary for achieving high densities and interparticle bonding are evaluated experimentally. The bonding and melting taking place can be identified by post-consolidation studies, and the inter-relation between compact integrity and melting is shown. An attempt is made to develop a “process window” analogous to that of explosive bonding, to describe the conditions necessary for consolidation. Finally, in view of the necessity for localisation of plastic flow during consolidation, an indication of the influence of material deformation parameters is given.