Large strain work hardening and textures

Cu‐Nb wire composites with 0.105, 0.148, and 0.182 volume fraction of Nb filaments were produced in situ and their mechanical properties measured as a function of filament size and interfilament spacing. The yield stress and the ultimate tensile strength increased with both niobium volume fraction and overall composite reduction. At room temperature, the ultimate tensile strength of the Cu–18.2 vol% Nb composite reduced by 99.999% in cross‐sectional area (100–200 A filament thickness) reached the value of 2230 MN/m2 (323 ksi) and further increased to 2850 MN/m2 (413 ksi) when measured at 77 °K. These values are higher by a factor of 4 than the values predicted by the rule of mixtures based on the highest reported strength of both niobium and copper. The composite strength is as high as that of the best copper whiskers and is shown to closely approach the theoretical strength of the material. The anomalous increase in strength despite the low volume fraction of reinforcing filaments suggests that the filam...

Anomalous increase in strength of in situ formed Cu‐Nb multifilamentary composites

Characterization of the strength and microstructure of heavily cold worked CuNb composites

We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. Similar phenomena occur between solids in frictional contact such as in tribology, friction stir welding, and explosive joining. The resulting compounds are characterized by very high interface and dislocation density, chemical mixing, and atomic-scale structural transitions at heterointerfaces. Upon straining, the phases form into nanoscaled filaments. This leads to enormous strengthening combined with good ductility, as in damascene steels or pearlitic wires, which are among the strongest nanostructured bulk materials available today (tensile strength above 6 GPa). Similar materials are Cu-Nb and Cu-Ag composites, which also have good electrical conductivity that qualifies them for use in high-field magnets. Beyond the engineering opportunities, there are also exciting fundamental questions. They relate to the nature of the complex dislocation, amorphization, and mechanical alloying mechanisms upon straining and their relationship to the enormous strength. Studying these mechanisms is enabled by mature atomic-scale characterization and simulation methods. A better understanding of the extreme strength in these materials also provides insight into modern alloy design based on complex solid solution phenomena.

Metallic composites processed via extreme deformation: Toward the limits of strength in bulk materials

An overview of the low-energy dislocation structure (LEDS) theory of low-temperature, that is non-creep, dislocation-based crystal plasticity is presented, as systematically developed over the past 35 years. It is ultimately based on G. I. Taylor's 1934 theory of work hardening wherein he assumed that stress application causes the essentially instantaneous athermal generation of dislocation structures which are in equilibrium with the applied stress and which on stress release and reversal are stable up to the previously highest stress magnitude. As will be shown, at least in principle even though many details are still lacking, the following phenomena of crystal plasticity are readily explained by means of the LEDS theory: (1) the four stages of work hardening; (2) the shape of the stress-strain curve; (3) the temperature dependence of work hardening; (4) the low-temperature strain rate dependence of the flow stress; (5) the difference between ‘planar’ and ‘wavy’ glide materials (as exemplified ...

G. Wassermann

Papers

Microstructure and anomalous mechanical properties of in situ-produced silver-copper composite wires