David G. 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.

The influence of microstructure on the ductility of iron aluminides

Abstract Considerable effort has been devoted over the last decade to the development of iron aluminides as materials for high temperature applications, where their good oxidation and corrosion resistance, combined with reasonable strength, may be utilised. Poor formability and ductility, however, particularly at room temperature, has hampered the exploitation of these materials. The present review examines the present state of understanding of the factors which influence the ductility. Recent research has made clear the important influence of testing environment, the role of Al content and minor additions of B, as well as the effect of quenched-in vacancies. The extent to which other factors, such as alloying additions and microstructural features, affect the ductility has not received the same attention, and is examined in the present study. Alloy strengthening, by almost any mechanism, is seen to lead to a dramatic loss of ductility. The only parameter allowing both strength increase and ductility improvement for a given set of Al/B/vacancy/environment conditions is the grain size. The best ductility for a given alloy, which should have as low an Al content as compatible with other requirements, is obtained by refining the grain size and by maintaining the alloy in the softest possible state. For the most part these conclusions are drawn from analysis of the behaviour of B2 ordered FeAl alloys, although similar trends seem also to apply to alloys of slightly lower Al content where DO3 ordering can occur. The observations drawn can be understood in terms of the mechanisms leading to the nucleation and propagation of brittle fracture, either as transgranular cleavage cracks or as grain boundary cracks. The possible role of additional factors, such as the texture, or grain and grain boundary distribution, surface layers producing protective stress effects, and strain homogenising or crack arresting dispersions, has not been sufficiently evaluated to determine whether any further improvements of ductility are possible.

Strength and Ductility of Fe-40Al alloy prepared by mechanical alloying

Abstract 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.

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 high temperature oxidation behaviour of an ODS FeAl alloy

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 supressed by using a very low cooling rate after oxidation.

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.

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.

Long range order and vacancy properties in Al-rich Fe3Al and Fe3Al(Cr) alloys

Abstract 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 DO 3 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 DO 3 and B2 ordered Al-rich Fe 3 Al is shown to be consistent with considerations of interatomic bond energies.

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.

Hardness and toughness of Mosi2 and MoSi2-SiC composite prepared by reactive sintering of powders

Abstract Samples of monolithic MoSi2 and a MoSi2–SiC composite have been prepared by milling elemental powders followed by reactive hot pressing to dense materials with fine microstructures. Mechanical properties (hardness and toughness) have been deduced from hardness imprints and related to the scale of the microstructure. Toughness is increased somewhat in the monolithic intermetallic at fine grain size as well as by the addition of the SiC second phase. Cracking begins always by intergranular decohesion, which seems little affected by the grain size or the presence of the second phase, and subsequently becomes transgranular in MoSi2 and mixed transgranular–intergranular in the MoSi2–SiC composite. The propagation of such longer cracks depends on the microstructure, with both finer matrix grain size and the presence of SiC leading to more difficult crack growth.