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An Al-7%Si alloy was severely deformed by equal channel angular pressing (ECAP) to study the refinement of the microstructure and associated changes of mechanical properties. The initial coarse dendritic structure was broken into an elongated sub-micron grain/subgrain structure, with high dislocation density, and distributed fine Si particles. The Si particles in the composite are seen to induce a high dislocation density during deformation and lead to faster structural refinement than in a monolithic alloy with the same composition as the matrix. The additional strengthening of the composite relative to the monolithic alloy is due to both the finer grain size and the high retained dislocation density. Severe plastic deformation leads also to an improvement of the ductility of the strong material due to the refinement of both matrix microstructure and the Si particles, We should like to acknowledge financial support of the Spanish Ministry of Education and Science under project number MAT2003-01540, as well as the award of a Juan de la Cierva post-doctoral fellowship for one of the authors (I.G.)

The high-temperature strength of a Fe-25%Al-2%Nb alloy in both a solutionised state and a precipitated state has been determined for temperatures up to 900 °C and compared with that of a solution hardened Fe3Al alloy. Some comparisons of these materials with previously reported Fe 3Al-based materials are also made. The Fe-25%Al-2%Nb material is capable of retaining good strength to temperatures above 800 °C, but rapid coarsening of the Laves precipitate particles at higher temperatures leads to strength loss at such temperatures. The presence of stable dispersed particles at the intermediate temperatures means that good strength can be retained to very low strain rates. © 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The submicron microstructure developed here after heavy deformation of Al-3Mg by equal channel angular pressing (ECAP) is shown to consist of an elongated grain and cell structure of width 70-80 nm and 300-400 nm length. There is also a high dislocation density inside these grains with some tendency to dislocation arrangement as a cell structure. Many of the grain boundaries are shown to be of low-medium angle and are not the randomly misoriented, high-angle boundaries generally assumed to be present. Annealing at low temperatures leads to a reduction of dislocation density, some reduction of grain length as transverse boundaries form in the elongated grains, and grain coarsening. At higher temperatures a duplex structure forms as some regions show localized recrystallization. The relationship of grain boundary misorientation and high-temperature coarsening and recrystallization to the applied heavy strain and alloy composition is discussed. © 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The deformation behaviour of a mechanically-alloyed Fe40Al material changes significantly as high temperature heat treatments are given to coarsen grains and reinforcing particles, and as low temperature ageing treatments are given to annihilate quenched-in vacancies and clusters. Deformation at low temperatures starts with a significant yield drop in all materials of fine grain size, indicating that there are insufficient numbers of dislocation sources. The flow stress retains some sensitivity to vacancies and their clusters up to high temperatures, this depending on the grain size of the material. Dislocations of various Burgers vectors, 〈100〉, 〈110〉 and 〈111〉, are found in the undeformed materials and it is the numbers and types of these that determine the initial flow behaviour. A weak flow stress anomaly is seen for most material states and this has been related to reactions and decompositions of 〈111〉 dislocations.

Fe Al alloys containing slightly less than 25% Al have a two-phase mixture of disordered and DO 3 -ordered phases at temperatures somewhat below 550°C. The microstructure of such alloys is similar to that of Ni-base γ-γ′ superalloys, well known for their good high temperature strength and creep resistance. Analysis of the high temperature strength of the two-phase Fe Al alloys suggests that their high temperature strength is good under conditions of slow strain rate or creep. Their deformation behaviour differs from that of the γ-γ′ superalloys, however, since it is relatively easier to shear the ordered particles than to bow dislocations down channels between the particles. The relatively low ductility of these materials seems to be related to the localization of shear by the cutting of particles rather than to environmental sensitivity, as suggested for Fe 3 Al alloys.

Fe Al alloys containing slightly less than 25% Al are a two phase mixture of disordered plus DO3-ordered phases at temperatures somewhat below 550°C. The morphology and stability of microstructure has been examined in one such binary Fe Al alloy when in the two-phase state, and compared with a ternary Fe Al Si alloy showing similar phase mixtures at similar temperatures. Phase changes in these alloys can be understood on the basis of the equilibrium phase diagram, including metastable phase boundary extensions and conditional spinodal decompositions to account for order changes occurring faster than chemical diffusional changes. Such two-phase microstructures are relatively stable against coarsening at these intermediate temperatures, particualrly for the Fe Al Si alloy which is believed to be due to its lower diffusivity and smaller interface energy. Such two-phase alloys, particularly when the composition is carefully chosen for the required stability and ordered volume fraction at a given temperature, should represent an Fe-base b.c.c. equivalent to the Ni-base superalloys.