Mª Antonia Muñoz-Morris

Microstructure of severely deformed Al–3Mg and its evolution during annealing

Abstract 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 70–80nm and 300–400nm 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.

Microstructure and room temperature strength of Fe-40Al containing nanocrystalline oxide particles

Abstract The microstructure and room temperature strength of a mechanically alloyed ODS FeAl alloy are examined in a large number of differently processed and heat-treated materials and the strengthening contributions due to nanocrystalline oxide particles, to fine grain size, and to matrix effects are analysed. Submicron grain materials with small oxide particles show significant hardening due to the fine particles and a similar, significant hardening due to the fine grain size. The texture created during processing influences strengthening by determining the barrier efficiency of the lower-angle or higher-angle grain boundaries present, and thus modifies the Hall–Petch slope describing grain-size strengthening.

Mechanical behaviour of dilute Al-Mg alloy processed by equal channel angular pressing

Abstract Heavy deformation by ECAP of Al–3Mg leads to a high level of strengthening due to the fine microstructure, with perhaps additional dislocation hardening. Analysis of grain boundary hardening is difficult due to the different numbers of dislocations and the various types of grain boundaries present.

The influence of some microstructural and test parameters on the tensile behaviour and the ductility of a mechanically-alloyed Fe–40Al alloy

Abstract The present study examines the influence of a wide range of microstructural parameters and tensile test conditions on the tensile behaviour of a mechanically-alloyed, fine grained Fe–40Al intermetallic. Major changes of tensile strength and ductility are obtained by reducing the grain size (with the ductility increasing from 1 to 10% for grain sizes of 100 and 1 μm), by avoiding environmental attack during the test, and by avoiding premature stress/strain concentrators (with the ductility increasing from 5 to 10% as imperfectly machined samples have their sample surfaces polished). Ductility variations are interpreted using a model based on the slow propagation of an initial crack which eventually reaches a condition of instability, and where the respective roles of environment, plastic deformation processes, and fracture mechanisms can be distinguished. The tensile ductility is highly sensitive to the surface state, meaning the degree of exposure to the environment, the extent of geometrical stress raisers, and the microstructure made up of the grain and particle size and distribution.

The influence of composition and low temperature annealing on hardness and ductility of rapidly solidified Al-Ni-Ce alloys

Abstract The crystalline state and mechanical properties of several rapidly solidified Al–Ni–Ce alloys have been examined in both as-solidified materials and after low temperature annealing to crystallise initially amorphous materials. Significant changes in mechanical properties occur due to changes of both chemical composition and of crystalline structure. Further detailed examinations of deformation mechanisms are necessary.

Hardening and softening of FeAl during milling and annealing

Abstract 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.

Crystallization of a Al-4Ni-6Ce glass and its influence on mechanical properties

Abstract The crystallization behaviour of a rapidly quenched, amorphous Al–4Ni–6Ce material has been examined during low temperature annealing, and shown to commence by the nucleation of nanoscale Al crystals, which stop growing once they reach a size of about 10 nm. In a subsequent, second phase of crystallization the dendritic Al crystals grow rapidly with the formation of a crystal–glass eutectic-like mixture. The nanoscale Al crystals lead to some hardening of the glass, but do not modify the good ductility of the material. Subsequent hardening and embrittlement of the glass–nanocrystal composite may depend on slight changes of the extent of crystallization or instead on changes occurring in the glass itself.

High temperature structural coarsening of an ODS FeAl intermetallic

Abstract The coarsening of oxide particles and grains in an ODS FeAl intermetallic has been examined during annealing at high temperatures under conditions where recrystallization does not yet occur. Coarsening of the yttria particles is shown to be controlled by the diffusion of the metallic species through the intermetallic matrix, while the increases of grain size by grain growth are determined by the pinning action of the large number of yttria particles. Hardness falls during the annealing and is seen to depend on both particle size increases and grain size increases.

Oxidation behaviour of Fe40Al alloy strip

Abstract The oxidation behaviour of FeAl foils of 200 μm thickness was investigated in air at 900 and 1100°C for up to 1000 h exposure. Microstructural examination reveals the formation of an even alumina scale with small islands of (Fe,Al)-oxides which may be associated with chemical heterogeneities reaching the surface. No cracking or spallation was found. At the lowest temperature the scale is decorated with small platelets associated with metastable θ-alumina which transforms to the more stable α-alumina with progressing oxidation. At the highest temperature, metastable phases were not observed. For the longest exposure, however, whiskers of alumina on the top of the scale were found to develop with progressing oxidation. The observation of significant differences between the oxidation behaviour of thin foils and thick materials regarding scale spallation is related to the relaxation of growth stresses in the scale by spontaneous deformation of the substrate by creep.

Obtaining good ductility in an FeAl intermetallic

Abstract Deformation and fracture mechanisms leading to tensile failure have been examined in a mechanically-alloyed and a recrystallized Fe–40Al intermetallic. It is shown that tensile failure begins by the initiation of a small crack near the sample surface at a given level of strain, and crack propagation occurs rather quickly thereafter such that the total strain at failure is only slightly larger than the crack initiation strain. Crack propagation is initially slow, and affected by the test environment, and later it is faster and unaffected by test environment. Deformation localizes into slip bands and crack nucleation takes place easily, at a small strain, in the recrystallized material of large grain size. In the as-extruded material, deformation remains homogeneous under the influence of the small distributed particles, and crack initiation over a scale of many grain dimensions is slow. Possibilities for further ductility improvements by microstructure control are discussed.