Mechanical strength and electrical conductivity are the most important properties of conducting metallic materials used in electrical engineering. Today, there is a growing need in this field for innovative conductor materials with improved properties. Meanwhile, the main issue is that high electrical conductivity and high strength are usually mutually exclusive due to physical nature of these properties. Alloying of pure metals results in significant increase of their mechanical strength, whereas electrical conductivity dramatically drops due to the scattering of electrons at solutes and precipitates. Recent studies have shown that intelligent nanostructural design in Al, Cu, and their alloys can improve combination of high mechanical strength with enhanced electrical conductivity. It was demonstrated that mechanical strength and electrical conductivity of these materials are primarily controlled by their microstructure, of which grain size, morphology of second phases, and their distribution, as well as dislocation structure, are the most important parameters. Rapid development of the state-of-the-art methods for the microstructural characterization at nano- and atomic scale has allowed a deeper insight into microstructure–properties relationship. The approach of intelligent nanostructural design of Al and Cu alloys has even enabled to increase the material strength with simultaneous improvement of its electrical conductivity. In this case, recent works on nanostructuring alloys by severe plastic deformation are of special interest, which gives rise to fundamental questions dealing with new mechanisms of strength and electrical conductivity as well as innovation potential of practical application of nanostructured materials. These issues are considered and discussed in the present progress article.

Nanostructured Al and Cu alloys with superior strength and electrical conductivity

The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels

Microstructural evolution of a 304-type austenitic stainless steel during rolling at temperatures of 773–1273 K

The ultrafine-grained microstructures, mechanical properties and electrical conductivity of a Cu–0.87%Cr–0.06%Zr alloy subjected to multiple equal channel angular pressing (ECAP) at temperatures of 473–673 K were investigated. The new ultrafine grains resulted from progressive increase in the misorientations of strain-induced low-angle boundaries during the multiple ECAP process. The development of ultrafine-grained microstructures is considered as a type of continuous dynamic recrystallization. The multiple ECAP process resulted in substantial strengthening of the alloy. The yield strength increased from 215 MPa in the original peak aged condition to 480 MPa and 535 MPa after eight ECAP passes at 673 K and 473 K, respectively. The strengthening was attributed to the grain refinement and high dislocation densities evolved by large strain deformation. Modified Hall–Petch analysis indicated that the contribution of dislocation strengthening to the overall increment of yield stress (YS) through ECAP was higher than that of grain size strengthening. The formation of ultrafine grains containing high dislocation density leads to a small reduction in electrical conductivity from 80 to 70% IACS.

Deformation microstructures, strengthening mechanisms, and electrical conductivity in a Cu–Cr–Zr alloy

Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function of the involved elements is often lacking. It is one of the two main goals of this review to summarize the knowledge for some selected respective topics with a certain focus on non-stoichiometric, i.e., non-ideal Laves phases. The second, central goal of the review is to give a systematic overview about the role of Laves phases in all kinds of materials for functional and structural applications. There is a surprisingly broad range of successful utilization of Laves phases in functional applications comprising Laves phases as hydrogen storage material (Hydraloy), as magneto-mechanical sensors and actuators (Terfenol), or for wear- and corrosion-resistant coatings in corrosive atmospheres and at high temperatures (Tribaloy), to name but a few. Regarding structural applications, there is a renewed interest in using Laves phases for creep-strengthening of high-temperature steels and new respective alloy design concepts were developed and successfully tested. Apart from steels, Laves phases also occur in various other kinds of structural materials sometimes effectively improving properties, but often also acting in a detrimental way.

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Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

The precipitation of the Laves-phase in a low-carbon 9% Cr heat resistant steel was studied under conditions of aging and creep at 923 K. The Laves-phase of Fe2(W,Mo)-type with a ratio of Mo:W of 1:5 was observed on various boundaries in a tempered martensite structure after a long period of aging/creep. The Laves-phase precipitations provided effective stabilization of the tempered martensite lath structure and therefore promoted creep resistance, although their effect on the dislocation substructure was negligibly small. The mean size of the Laves-phase particles (D) gradually increased with aging/creep time (τ) and could be expressed as D 4 − D 0 4 = K τ , were K=5×10−34 m4/s for aging and K=13×10−34 m4/s for creep. The large Laves-phase particles, which coarsened during the creep tests, served as preferential sites for pore and crack nucleation leading to fracture.

Laves-phase precipitates in a low-carbon 9% Cr martensitic steel during aging and creep at 923 K

Structural changes during plastic deformation were studied in a Cu–0.3%Cr–0.5%Zr alloy subjected to multidirectional forging up to a total strain of 4 at the temperatures of 300 K and 673 K. The deformation behavior was characterized by a rapid increase in the flow stress at an early deformation followed by a steady-state flow at large strain. The development of the new ultrafine grains resulted from the progressive increase in the misorientations of the strain-induced low-angle boundaries, which evolve into high-angle boundaries with increasing cumulative strain through a strain-induced continuous reaction that is quite similar to continuous dynamic recrystallization. The formation of ultrafine grains was closely related to the development of geometrically necessary boundaries that is attributed to deformation banding. The grain refinement kinetics increased with an increase in the deformation temperature. At 673 K, the area fractions of the ultrafine grains with a size below 2 μm were 0.36 and 0.6 in the initially solution treated samples and the aged samples, respectively. However, the area fractions of the ultrafine grains did not exceed 0.2 at 300 K.

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Grain refinement in a Cu–Cr–Zr alloy during multidirectional forging

The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering.

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Tempering behavior of a low nitrogen boron-added 9%Cr steel

Crept microstructures were examined in a 9 pct Cr martensitic steel with low carbon content. The steel was hot forged at 1323 K (1050 °C) followed by air cooling and then tempered at 1023 K (750 °C) for 3 hours. The tempered microstructure included numerous precipitates of MX-type carbonitrides and a small amount of M23C6-type carbides. Two groups of the MX-type particles were observed. The nanoscale MX-type precipitates appeared in the form of plate- and round-shaped particles. The plate-shaped MX-type particles were approximately 15 nm in the longitudinal direction and approximately 3 nm in thickness, and the round-shaped MX-type particles were 10 nm in diameter. In addition to the fine particles, the tempered martensite contained relatively coarse MX-type particles, which have a size of approximately 90 nm. The structural changes that occurred during the creep test were associated with an increase in the sizes of the lath and second-phase particles. Moreover, the creep was accompanied by appearance of Laves and Z-phase particles.

Irina Fedorova

Papers

Laves-phase precipitates in a low-carbon 9% Cr martensitic steel during aging and creep at 923 K

Grain refinement in a Cu–Cr–Zr alloy during multidirectional forging

Tempering behavior of a low nitrogen boron-added 9%Cr steel

Microstructure Evolution in an Advanced 9 pct Cr Martensitic Steel during Creep at 923 K (650 °C)

Fine (Cr,Fe)2B borides on grain boundaries in a 10Cr–0.01B martensitic steel