Instabilities and ductility of nanocrystalline and ultrafine-grained metals
Introduction
Reducing the grain size (d) in a polycrystalline material is known to have beneficial effects on its mechanical properties [1]. As shown in Fig. 1 [1], a material’s strength is expected to increase in a manner described by, e.g., the well-known Hall–Petch relationship. A concurrent rise in toughness is often expected, as is well documented for many engineering alloys after grain refinement. Based on our experience with some conventional materials, Fig. 1 also suggests that ductility, the ability of a material to plastically deform without failure under tensile stresses, could improve as well. With the advent of nanocrystalline (nc, d<100 nm) and ultrafine-grained (ufg, typically d<500 nm) materials, one is naturally curious about the possibility of extrapolating the trends seen in Fig. 1 all the way to the nc/ufg regimes; such an extrapolation has the exciting prospect of achieving all-around superb mechanical properties never possible before. The promised gain in strength (typically 5–10 times) has indeed been realized in nc materials [2]. Their ductility at room temperature (RT), on the other hand, has not lived up to the promises and is often disappointingly low compared with their coarse-grained (cg) counterparts [3].
Ductility is very important for many shaping and forming operations and for avoiding catastrophic failure in load-bearing applications. A literature survey indicates that the vast majority of the nc metals have tensile elongation to failure (percent elongation, %EL) well below 5%, exhibiting or bordering “brittle” behavior [3] (an electroplated Co seems to be an exception, which showed a respectable ∼7%EL at RT when d=12 nm [4]). This is an obvious roadblock to their practical utility. The ufg metals are generally more ductile, with %EL often of the order of 10% [5]. However, for many pure metals such a %EL is still nowhere close to their characteristic high ductility in cg form (d of many micrometers and above). More importantly, the useful uniform tensile deformation, i.e., the plastic strain before localized deformation sets in at or near the peak in the engineering stress–strain curve, is close to zero for almost all nc and ufg metals [6]. In the following, we illustrate several reasons (highlighted in bold face) that can cause the low ductility of nc metals and limit the ductility of ufg metals, using pure Cu as a model system.
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Results and discussion
Upon deformation, many nc metals are found to reach their fracture stress in, or slightly beyond, the elastic regime. As full-density bulk nc metals are difficult to process, poor sample quality is an obvious reason for a low fracture strength (σf), causing premature failure under tensile stresses, sometimes even before yielding has a chance to start. This is especially true when the bulk sample is consolidated from loose nanoparticles [2], [7]. The residual porosity (of which the last 1% is
Strategies to derive good ductility from nanostructures
Obtaining a high strength from nc or ufg metals seems to be straightforward, but that alone may not be sufficient for many applications if the material suffers from ductility problems. In the following, we outline a few strategies (highlighted again in bold face) to derive sufficiently high ductility from nanostructured (including both uc and ufg) metals. To begin with, it is certainly important to improve sample quality so that premature failure due to large pre-existing flaws can be avoided.
Conclusions
There are two major factors detrimental to the ductility of nc/ufg metals. The first is a propensity for cracking instability and brittle failure, limited by the σf. The σf appears to be elevated in the ufg regime. But it may not further increase or even decrease in the nc grain regime, due to the reduced intrinsic material toughness as well as sample flaws that are difficult to get rid of while achieving extremely small nc grains. The second factor is plastic instabilities, such as a strong
Acknowledgements
The author gratefully acknowledges the contributions of his collaborators over the past three years, in particular Y.M. Wang, M.W. Chen, D. Jia, K. Wang, D. Pan, K. Lu, K.J. Hemker, K.T. Ramesh, Y.T. Zhu and R.Z. Valiev. This work was partially funded by the US National Science Foundation, CMS-9877006 and DMR-0080361.
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