Rate-Dependent Temperature Increases in Shear Bands of a Bulk-Metallic Glass

Using an infrared (IR) camera, we observed in situ the dynamical shear-banding processes of the geometrically constrained specimens of a Zr-based bulk metallic glass in a quasi-static compression at various strain rates, measured the temperature evolutions within the specimens, and calculated the temperature increases in shear bands. Strain-rate-dependent serrated plastic flow is a result of shear-banding operations. The average temperature increases in the specimens are observed during the plastic deformation and their magnitudes are strain rate dependent. The temperature increases in shear bands are related to strain rates. The higher the strain rates, the larger the temperature increases in a shear band. The shear strain in a shear band may be responsible for the strain-rate-dependent temperature increase in a shear band. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Dynamic Recrystallization: The Dynamic Deformation Regime

Severe plastic deformation (PD), especially involving high strain rates (>10³ s^–1), occurs through solid-state flow, which is accommodated by dynamic recrystallization (DRX), either in a continuous or discontinuous mode. This flow can be localized in shear instability zones (or adiabatic shear bands (ASBs)) with dimensions smaller than 5 μ, or can include large volumes with flow zone dimensions exceeding centimeters. This article illustrates these microstructural features using optical and electron metallography to examine a host of dynamic deformation examples: shaped charge jet formation, high-velocity and hypervelocity impact crater formation, rod penetration into thick targets (which includes rod and target DRX flow and mixing), large projectile-induced target plug formation and failure, explosive welding, and friction-stir welding and processing. The DRX is shown to be a universal mechanism that accommodates solid-state flow in extreme (or severe) PD regimes. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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Effects of Deformation Twinning on Energy Dissipation in High Rate Deformed Zirconium

The posit that deformation twinning can result in energy storage is examined by measuring the temperature increase of zirconium during adiabatic compression at high strain rates and correlating the response with the resulting microstructure. After examining the underlying assumptions of homogeneous deformation via microscopy and numerical modeling, it is determined that the occurrence of twinning does not correlate with significant energy storage relative to dissipation, and the appearance of storage may be caused by inhomogeneity in the deformation. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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Severe Plastic Deformation of Difficult-to-Deform Materials at Near-Ambient Temperatures

Plane-strain machining can be used to impart large plastic strains in alloys that are difficult to deform by other severe plastic deformation (SPD) processes. By cutting at low speeds, the heating caused by friction with the tool can be reduced to insignificant levels. The utility of this approach for characterizing microstructure development in SPD is demonstrated using a variety of commercial alloys that exhibit different deformation behaviors and strengthening mechanisms, including CP-titanium, aluminum alloy 6061-T6, nickel-base superalloy IN-718, and pearlitic plain-carbon steel. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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Effect of Low Temperature on Fatigue Life and Cyclic Stress-Strain Response of Ultrafine-Grained Copper

Fatigue life curves and cyclic stress-strain curves of ultrafine-grained (UFG) copper of purity 99.9 pct produced by equal-channel angular pressing (ECAP) were determined under stress control at room temperature (RT) and at a temperature of 173 K. The obtained curves were compared to the corresponding curves obtained on conventional-grain (CG) copper. At both temperatures, the lifetime of UFG copper is longer than that of CG copper. The S-N curve of UFG copper is temperature dependent, while its cyclic stress-strain curve is temperature insensitive. To explain this temperature effect, two mechanisms of cyclic plastic deformation were proposed: the temperature-independent bulk dislocation mechanism taking place in the entire loaded volume and the temperature-dependent localized mechanism consisting of cooperative grain boundary (GB) sliding along the shear plane of the last ECAP pass taking place in the surface layer and leading to formation of surface fatigue markings. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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