M.A. Muñoz-Morris

An analysis of strengthening mechanisms in a mechanically alloyed, oxide dispersion strengthened iron aluminide intermetallic

Abstract An iron aluminide alloy of base composition Fe-40Al has been prepared by mechanical alloying and processed using a variety of powder consolidation methods and heat treatments to produce a range of grain sizes and oxide dispersoid sizes. The strengths of these materials have been determined at room temperature and related to the various aspects of microstructure. Fine dispersoid particles may pin grain boundaries and help determine the fine grain size and contribute very significantly to the material strength. Grain size strengthening is shown to be a rather small component of the material strength, with the matrix strength being rather high for this intermetallic. The influence of other factors such as texture and the direction of application of stress (tension or compression) are also briefly discussed.

The effect of heat treatments on the microstructural stability of the intermetallic Ti–46.5Al–2W–0.5Si

Abstract The effect of heat treatment on the microstructure of a γ-based TiAl alloy containing W and Si has been studied. The starting microstructure of a cast and heat-treated commercial γ-based TiAl alloy component has been interpreted in terms of the phase changes between an α-Ti 3 Al structure at high temperatures, an α 3 +γ decomposition at intermediate temperatures, and the change to a B2+γ microstructure at lower temperatures. The fine lamellar microstructure produced after fairly rapid cooling from high temperatures consists mostly of α 2 +γ lamellae but transforms surprisingly rapidly at lower temperatures (950–1000°C) into a globular structure composed of a γ matrix and equiaxed particles of B2 phase, occasionally of remnant α 2 phase or of Ti 5 Si 3 silicide. Material strength, as assessed by microhardness, falls rapidly as the initial α 2 +γ lamellar microstructure transforms to the globular one becoming an essentially γ+B2 mixture.

Possibilities for improving the corrosion resistance of Fe–40Al intermetallic strip by prior oxide protection

Abstract Iron aluminides do not generally show good aqueous corrosion resistance. The present study attempts to improve their corrosion resistance by prior surface oxidation. While good corrosion resistance can indeed be obtained, it is found to be difficult to ensure that the oxide layer is continuous and defect free.

Effect of Equal Channel Angular Pressing (ECAP) on microstructure and properties of Al-FeAlCr intermetallic phase composites

An aluminium matrix composite was prepared by mixing commercial aluminium powders and 15 vol % of FeAlCr powders and consolidation by hot extrusion. The extruded composite was subjected to severe plastic deformation by equal channel angular pressing (ECAP) at room temperature and at 150°C. The extruded composite presents a uniform distribution of particles although some defects are observed such as residual pores and particle agglomerates. The particle distribution does not show a significant change due to ECAP. The extruded composite exhibits a relatively fine grain size of the order of 1-2 μm that was refined to 550 nm after three ECAP passes at room temperature by route A and to 636 nm after four passes at 150°C by route Bc. The yield stress of the composites was increased by 140 to 180% after ECAP as compared with the extruded condition.

The Role of Carbon and Vacancies in Determining the Hardness of FeAl Intermetallic in the Quenched and the Aged States

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.

An examination of strain ageing in a Mg–Y–Zn alloy containing Gd

Dynamic strain ageing has been observed on straining a Mg–Y–Zn–Gd alloy in both the quenched and the annealed states, where long-period-structural-order phase and solution or precipitation of Gd is found. Deformation mechanisms are analysed in terms of solute diffusion to mobile dislocations, the role of excess vacancies and the presence of a Taylor dislocation network. In quenched material, the operation of multiple deformation modes leads to the creation of a three-dimensional (Taylor) dislocation network such that pipe diffusion along forest dislocations can readily lead to pinning of a moving dislocation. The many planar precipitates present in annealed material inhibit non-basal flow such that the Taylor dislocation network does not form and bulk diffusion of solute is required for flow serrations to appear.

The influence of work hardening, internal stresses, and stress relaxation on ductility of ultrafine grained materials prepared by severe plastic deformation

Ultrafine grained materials prepared by methods of severe plastic deformation appear to show good ductility for their high strength. To a large extent this ductility enhancement, for the given strength, is shown to correspond to the fracture ductility and not the uniform ductility at maximum stress. The improved fracture ductility is often due to the refinement or removal of the coarse defects that act as sites for failure nucleation. The low work hardening rate inherent to the very fine microstructures produced by severe plastic deformation essentially condemns such materials to very low uniform ductility. Stress relaxation occurring during unloading after processing, and changes of internal stresses during reloading for mechanical testing, appear to play a significant role in determining deformation behaviour near the onset of plastic flow, and this can affect the measured uniform strain.

Strengthening at High Temperatures in an Iron-Aluminium Alloy by the Precipitation of Stable and Coherent Intermetallic Particles

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.