Advanced Materials & Processes

FEB 2015

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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as seen in Fig. 3. In situ imaging shows that bands of localized deformation form shortly afer yield, but do not lead to failure. Post-test metallographic char- acterization reveals that the dark bands evident during in situ imaging do not correspond to cracks or microstructural damage, as demonstrated using bright- field light optical microscopy (LOM) in Fig. 4a. Instead, these bands are kink bands in which the lamellar structure re- mains continuous but has been uniform- ly sheared and rotated (Figs. 4b and 4c). While these deformation structures lack suficient contrast when imaged using bright-field LOM (Fig. 5a), polarized light microscopy clearly reveals that complex networks of kink bands can form in these specimens (Fig. 5b). In addition to forming during uniax- ial compression tests, kink bands also oc- cur during bending , Charpy impact tests, and even high strain rate ballistic tests where local compressive strains occur. Kink band formation in a wide range of both quasi-static and dynamic mechan- ical tests suggests that it is an important deformation mode, likely to occur during many forming operations and potential structural applications. While strain localization is almost universally viewed as detrimental to a metal's mechanical properties, kink band formation in Cu-Nb nanolaminates may be an exception to this rule. Unlike other forms of strain localization, such A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y 2 0 1 5 2 0 Fig. 3 — Stress strain curve from a 65 nm layer thickness Cu-Nb nanolaminate displays a perturbation at 7% engineering strain. In situ video recording of the compression specimen reveals that this point corresponds to pronounced inhomogeneous deformation due to kink band formation. Fig. 4 — Optical and scanning electron microscopy reveal that kink bands are responsible for the pronounced shape change that occurred during compression of the 65 nm specimen. Bright-field microscopy shows the absence of cracks along the kink bands (a). Sub-region of (a) imaged using circular diferential interference contrast (b). Sub-region of (b) imaged using backscatter scanning electron microscopy (c) [7] . Fig. 5 — Bright-field image of kink bands in the 65 nm specimen showing poor kink band contrast (a). Same field of view as (a) imaged using polarized light microscopy (b). Polarized light allows the complex network of kink bands to be clearly revealed and highlights many small kink bands not evident in (a).

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