Journal Club August 2010: Deformation Mechanisms in Nanotwinned FCC Metals
Although the nineties witnessed an intensity of research on nanocrystalline metals owing to their ultra-high strength, the early enthusiasm was met with severe disappointment due to their brittle nature and loss of structural stability. In contrast, research over the past few years has provided compelling evidence that nanotwinned structures may be the optimal motifs for the design of both high-strength high-ductility materials. Indeed, nanotwinned Cu containing twin lamella of 20-100 nm thickness has been shown to possess a yield strength in excess of 1 GPa with elongation to failure as high as 14%, which is in sharp contrast to nanocrystalline Cu having a yield strength of about 400 MPa and elongation to failure of about 2-3% for comparable grain sizes . These discoveries have raised several basic materials science questions. In this journal-club issue, I hope to highlight some of those questions and some of the key papers that have partially spurred research in the area of nanotwinned metals.
Nanocrystalline metals are defined as those with a range of grain sizes finer than 100 nm. The many benefits of nanocrystallinity include, inter alia, ultra-high yield strength, superior fatigue and wear resistance, and possibly superplastic formabilty at low temperatures and/or high strain rates. Some of these superior properties are attributed to the fact that the grain boundaries (GBs) arrest the lattice dislocation motion, thereby making plastic deformation difficult at smaller grain sizes. Thus, nanocrystalline metals are also known to exhibit the Hall-Petch strengthening characteristic of their coarse-grained counterparts. However, as advances led to further insight into the governing deformation mechanisms, they also revealed a number of issues that seriously restrict the utility of the enhanced properties achievable by nanostructuring. As already emphasized, one of the major issues that severely impacted the practical application of nanocrystalline metals is that their ductility is typically limited to a few percent in uniaxial elongation, and reduces further with decreasing grain size. Furthermore, it is well established that when grain sizes fall below 100 nm, there is a radical transition in the deformation mechanisms as GB mediated processes, such as defect nucleation from GBs and GB sliding, become dominant. Among other consequences, this leads to undesirable grain instability as indicated by stress-induced grain growth, which ultimately results in the loss of strength, and possibly other benefits of nanocrystallinity.
A particularly intriguing development began a few years ago, with the experimental studies of Lu et al.  aimed at designing a novel technique for nanostructuring in face-centered-cubic (fcc) metals by introduction of coherent twin boundaries (CTBs) within ultra-fine crystalline metals having a grain size of a few hundred nanometers (Figure 1). The twin lamella thickness within each grain ranges between 20-100 nm. These are known as nanotwinned structures and have shown extraordinary properties of ultra-high yield strength as well as ductility, high strain rate sensitivity, and electrical conductivity, excelling their nanocrystalline counterparts, as shown in Figures 2 and 3. This has stimulated significant interest in nanotwinned fcc metals, and has made them the subject of extensive experimental and theoretical investigations. Since the ultimate goal is to gain insight into the deformation mechanisms that govern the behavior of nanotwinned fcc metals and suggest specific pathways for their optimal design, the key questions that drive these investigations are the following:
- Are nanotwinned structures optimal motifs for strength, ductility and stability in fcc metals?
- Is there a critical twin lamella thickness for the enhanced performance of nanotwinned metals?
The numerous studies performed till date reveal that the CTBs have a very high shear strength compared to most GBs and are also effective barriers to dislocation motion. This leads to a strengthening mechanism similar to that of the GBs. However, a unique feature of the CTBs is that the twin planes are also slip planes for fcc metals which enables them to accommodate large plastic strains by absorption of dislocations thus enhancing ductility. In addition, recent studies also show that nanotwinned metals are quite stable under deformation.
Figure 1: TEM image of the microstructure of an as-deposited Cu sample with nanoscale twins in randomly oriented grains ;
Figure 2: Tensile stress-strain curve for nano-twinned Cu with 20-100 nm twin spacing, a nanocrystalline Cu (mean grain size ~ 30 nm) and a coarse-grained polycrystalline Cu (mean grain size > 100 μm) ;
Figure 3: Plots comparing the yield strength, elongation-to-failure, and rate sensitivity of nanocrystalline Cu and nano-twinned Cu ;
I would like to conclude by introducing the following three papers that seek to address some of the critical issues mentioned above:
 L. Lu, Y. Shen, X. Chen, L. Qian, K. Lu, "Ultrahigh strength and high electrical
conductivity in copper", Science 304 (2004) p. 422.
 K. Lu, L. Lu, S. Suresh, "Strengthening materials by engineering coherent internal boundaries at the nanoscale", Science 324 (2009) p. 349.
 X. Li, Y.Wei, L. Lu, K Lu, H. Gao, "Dislocation nucleation governed softening and maximum strength in nano-twinned metals", Nature 464 (2010) p. 877.
As mentioned earlier, the paper by Lu et. al  presents the experimental study which examines the strengthening properties of twin boundaries in fcc metals. The paper by Li et. al  is a very recent work which provides an interesting insight, by way of simulations, into the onset of softening below a critical twin lamella thickness also observed in recent experiments. At this critical spacing of about 15 nm, the studies indicate a transition from the Hall-Petch type strengthening to a mechanism governed by nucleation and motion of dislocations parallel to the twin planes. Finally, the paper by Lu et. al  is a recent concise review of the deformation mechanisms governing the strength, ductility and failure tolerance in nanotwinned metals and also discusses some of the challenges and open questions.