Nanotwinned copper, martensitic thin films, and growth mechanism of nanowire oxides

February 3, 2011

[1] Fracture toughness and fatigue crack growth characteristics of nanotwinned copper

A Singh et al

Recent studies have shown that nanotwinned copper (NT Cu) exhibits a combination of high strength and moderate ductility. However, most engineering and structural applications would also require materials to have superior fracture toughness and prolonged subcritical fatigue crack growth life. The current study investigates the effect of twin density on the crack initiation toughness and stable fatigue crack propagation characteristics of NT Cu. Specifically, we examine the effects of tailored density of nanotwins, incorporated into a fixed grain size of ultrafine-grained (UFG) copper with an average grain size of 450 nm, on the onset and progression of subcritical fracture under quasi-static and cyclic loading at room temperature. We show here that processing-induced, initially coherent nanoscale twins in UFG copper lead to a noticeable improvement in damage tolerance under conditions of plane stress. This work strongly suggests that an increase in twin density, at a fixed grain size, is beneficial not only for desirable combinations of strength and ductility but also for enhancing damage tolerance characteristics such as fracture toughness, threshold stress intensity factor range for fatigue fracture and subcritical fatigue crack growth life. Possible mechanistic origins of these trends are discussed, along with issues and challenges in the study of damage tolerance in NT Cu.

[2] Structure of martensite in deformed Ti–Ni–Cu thin films

Meng et al

Microstructural evolution in a high-Cu-content Ti50.2Ni30Cu19.8 shape-memory thin film deformed in the B19 martensite state was studied by transmission electron microscopy, and the deformation mechanisms were clarified. The thin film was prepared by magnetron sputtering deposition. In the undeformed film, the B19 martensite has mainly {0 1 1}B19 twins with a small number of {1 1 1}B19 twins. The tensile deformation involves a reorientation of the {0 1 1}B19 twin domains, de-twinning of the {0 1 1}B19 and {1 1 1}B19 twins, and a stress-induced B19–B19′ transformation with the production of (0 0 1)B19′ compound twins. The film shows a large recoverable strain of 5.5%, which is far beyond the stress plateau.

[3] Driving force and growth mechanism for spontaneous oxide nanowire formation during the thermal oxidation of metals

Yuan et al

The spontaneous formation of oxide nanowires (and whiskers) from the oxidation of metals is a well-established phenomenon that has, however, long resisted interpretation. Here we report new fundamental insights into this phenomenon by studying CuO nanowire formation during the thermal oxidation of copper. It is shown that the volume change associated with the solid-state transformation at the CuO/Cu2O interface produces compressive stresses, which stimulate CuO nanowire growth to accompany the interface reaction. A kinetic model based on the stress-driven grain-boundary diffusion followed by rapid surface diffusion of cations on the sidewall of nanowires is developed to account for CuO nanowire growth. The mechanism proposed explains our observations on CuO nanowires and other past observations.

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