Effect of microstructural heterogeneity on the mechanical behavior of nanocrystalline metal films
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Conventionally, mean grain size is considered the most critical microstructural parameter in determining the mechanical behavior of pure metals. By systematically controlling the distribution of grain orientations in aluminum films, we show that microstructural heterogeneity alone induces large variation in the mechanical behavior of nanocrystalline metal films. Aluminum films with relatively homogeneous microstructure (all grains with identical out-of-plane orientation) show substantially less early Bauschinger effect compared to films with heterogeneous microstructure, irrespective of film thickness or grain size. On the other hand, the films with homogeneous microstructure show relatively higher yield stresses. A direct correspondence is found between the nonuniformity of plastic deformation and early Bauschinger effect, which confirms the critical role of microstructural heterogeneity.
I. INTRODUCTION
Among various microstructural parameters, mean grain size is usually considered to be the most important determinant of mechanical properties in pure polycrystalline metals. A particularly striking example of the relationship between grain size and mechanical properties is the Hall– Petch relation1,2 which has been experimentally verified in metals with mean grain sizes ranging from millimeters down to about 1 lm. However, in nanocrystalline metals, which have mean grain sizes in the order of 100 nm or less, a significant deviation from the Hall–Petch relation has been observed.3 This deviation has been mainly attributed to changes in the deformation mechanisms that occur when the grain sizes reduce to the nanometer scale. Although plasticity in coarse-grained metals is mainly mediated by the propagation of dislocations generated from intragranular dislocation sources, nanocrystalline metals primarily accommodate plastic deformation through dislocations generated at grain boundaries.4 In addition, several unconventional mechanisms such as grain boundary migration,5 grainboundary diffusion, and sliding6,7 and twinning8 also become increasingly important at smaller grain sizes. The changes in deformation mechanisms lead to high strain rate sensitivity,9,10 stress-induced grain growth,11,12 and plastic strain recovery13,14 in nanocrystalline metals. But the question as to whether other factors might be responsible for deviations from the Hall–Petch relationship and, more broadly, whether mean grain size is the major determinant of mechanical properties in nanocrys-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.316 2826
J. Mater. Res., Vol. 26, No. 22, Nov 28, 2011
http://journals.cambridge.org
Downloaded: 16 Mar 2015
talline metals has not been thoroughly investigated. In particular, despite extensive research efforts insufficient attention has been paid on how microstructural heterogeneity affects nanocrystalline metal behavior. This is surprising since nanocrystalline metals are known to deform more heterogeneously than coarse-grained metals15,16 and often have highly het
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