Mechanical Behavior at the Nanoscale: What’s in your Toolbox?

JOM, Apr 2018

Megan J. Cordill, Christopher R. Weinberger

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Mechanical Behavior at the Nanoscale: What’s in your Toolbox?

Mechanical Behavior at the Nanoscale: What's in your Toolbox? MEGAN J. CORDILL 0 CHRISTOPHER R. WEINBERGER 0 0 1.-Erich Schmid Institute for Materials Science, Austrian Academy of Sciences , 8700 Leoben , Austria. 2.-Department of Material Physics, Montanuniversita ̈t Leoben , 8700 Leoben , Austria. 3.-Department of Mechanical Engineering, Colorado State University , Fort Collins, CO 80523 , USA. 4.-School of Advanced Materials Discovery, Colorado State University , Fort Collins, CO 80523, USA. 5.- - For materials scientists studying mechanical behavior at the nanoscale, there are several tools available for use, the most notable being a nanoindenter. Since the introduction of depth-sensing nanoindentation1 in the 1980s material scientists working the field of mechanical behavior have, at some point, used a nanoindenter to measure hardness and modulus of single grains or thin films. A nanoindenter can be compared to a hammer in the toolbox of a carpenter. It is used not only to drive in nails, but has other uses as well (to remove nails). However, like material scientists, carpenters need more than a hammer. Materials scientists want to see how their nails are reacting to the wood or how the grain of the wood changes how far the nail goes. Because we are always searching for more, a nanoindenter was installed inside a transmission electron microscope (TEM).2 With this simple addition, the indenter’s reaction to dislocations, grain boundaries, and precipitates could be observed, quantified, and simulated since now we could see and thus believe what the nail was doing. Initially, the mechanical behavior of thin films and bulk materials was evaluated with nanoindentation. When smaller samples could be achieved, such as single crystalline nanoparticles, then the superhard material behavior could also be determined.3 Additionally, with the widespread availability of focused ion beam (FIB) tools, researchers could shape their own nails, or micropillars,4 of any material to evaluate the stress–strain behavior at the microscale, and eventually, the nanoscale.5 From the stress–strain curve, yield and work hardening could be determined from within single grains or of thin films, something that was lacking at the nanoscale. There have been over 20,100 papers with Megan J. Cordill and Christopher R. Weinberger are the JOM advisors for the Nanomechanical Materials Behavior Committee of the TMS Materials Processing & Manufacturing Division (MPMD), and guest editors for the topic Mechanical Behavior at the Nanoscale in this issue. the word ‘‘micropillar’’ in the title since this technique was introduced. However, researchers wanted nanoindenters to provide them with more information, more data, more insight into the behavior at the nanoscale. For that reason, indenters were additionally installed inside scanning electron microscopes, with x-ray beam lines and equipped with high- and low-temperature capabilities. New sample geometries were created to remove the testing artefacts of the pillars, such as tensile bars and various cantilevers. From all of these innovative experiments came the ‘‘smaller is stronger’’ theory,6 dislocation starvation, sourcelimited deformation, exhaustion hardening, and discussions on FIB damage and tip degradation. While there is no doubt that the nanoindenter is the tool of choice for material scientists studying the mechanical behavior at the nanoscale experimentally, modelers tend to use tools that are appropriate to the specific phenomenon in which they are interested, which is often a compromise between domain size, time, and accuracy. For example, discrete dislocation dynamics (DDD) has provided dramatic insight into the motion of a large number dislocations in microscale samples, and nanoscale mechanics has provided the fuel to further develop this tool. Specifically, DDD has demonstrated how dislocation source truncation can give rise to the smaller-is-stronger behavior as well as how dislocation starvation can give rise to exhaustion hardening.7 However, to understand fundamental defect mechanisms, such as dislocation nucleation, classical atomistic simulations are more often used. Atomistic simulation methods have further clarified the role dislocation nucleation plays in contributing to mechanical response at these scales.8 The inequity of length scales associated with the fundamentals of mechanical behavior, notably defects, has been a great challenge and often prevents direct comparisons between experiments and modeling. This problem has been partially alleviated at the nanoscale since the sample sizes are reduced below 1 lm, allowing a direct size-scale comparison. This has led to more in-depth discussions of the mismatch in time scales. However, necessity is the mother of invention, and advancements in both experiments and modeling in efforts to bridge this gap have come about. This includes the development of high-speed in situ TEM to study high strain rate behavior,9 as well as accelerated time scale sampling methods for both molecular dynamics simulations10 and discrete dislocation dynamics.11 In the first paper of this issue focusing on mechanical behavior at the nanoscale, nanoindentation and ultrasonic velocity measurements were used to independently confirm that hydrogen exposure can reduce the shear modulus and the c44 stiffness constant by approximately 20%. This reduction of the elastic properties of hydrogencharged nickel was also determined to reduce the necessary stress needed for dislocation nucleation and motion compared to uncharged nickel. The details of these findings can be found in ‘‘Probing the Effect of Hydrogen on Elastic Properties and Plastic Deformation in Nickel Using Nanoindentation and Ultrasonic Methods’’ by Samantha Lawrence and co-workers. Nanoindentation is also an ideal technique to study super-elastic and shape memory effects in materials. In ‘‘A Nanoindentation Study of the Plastic Deformation and Fracture Mechanisms in Single-Crystal CaFe2As2’’ by Seok-Woo Lee and coworkers, it was found that indenting along the [001] generated strain bursts (pop-ins), radial cracking, and lateral cracking. Examination of the indent imprints correlated to the first pop-ins being related to dislocation nucleation in the single crystal and the radial crack formation to the second pop-ins in the load–displacement curves. The lateral cracking under the indent was examined with density functional theory calculations to reveal that the atomic layered structure of the CaFe2As2 has a weak (001) plane compared to the (100) plane. Since an indenter was incorporated into a TEM, our understanding of dislocation nucleation and plastic deformation of single crystals has improved. However, our experimental understanding about how grain boundaries interact is still in its early stages. Qianying Guo and Gregory Thompson demonstrate how nanocrystalline grains plastically deform in their paper ‘‘In-situ Indentation and Correlated Precession Electron Diffraction Analysis of Polycrystalline Cu Thin Films.’’ With precession electron diffraction, the diffraction contrast and microstructure phenomena can be better deciphered to provide more insight into grain boundary preservation, strain distribution and dislocation nucleation at large-angle grain boundaries. Dislocation velocities in single-crystal body-centered cubic (BCC) Nb, face-centered cubic (FCC) Au, and an Al0.3CoCrFeNi high-entropy alloy (HEA), also FCC, were evaluated in the paper ‘‘Fast Slip Velocity in a High-Entropy Alloy’’ by Rizzardi, Sparks, and Maaß. Here, clear statistically significant differences were observed between the BCC and FCC materials, but not much difference between the Au and HEA. The results indicate that the slip in the FCC examples proceeds statistically with the same peak velocity and could suggest that the mobility of dislocations is dominated by a weakest-link situation. In order to provide models of mechanical behavior, the behavior of the fundamental carriers of plastic deformation, i.e. dislocations, must not only be characterized but this information must also be suitably transferred to larger scales. For example, the work by Khanh Dang and Douglas Spearot titled ‘‘Pressure Dependence of the Peierls Stress in Aluminum’’ provides an atomic level study of how stress states, which can be quite large in small volumes, alter the properties of dislocations in aluminum. This type of atomic-level characterization is important for developing physically informed DDD simulations. Similarly, it is important to develop methods that are able to simulate dislocations without interatomic potentials such that more rigorous data, such as density functional theory, can be used as inputs. In the paper ‘‘Comparing Modeling Predictions of Aluminum Edge Dislocations: Semidiscrete Variational Peierls–Nabarro Versus Atomistics,’’ Lucas Hale examines the robustness of Peierls–Nabarro models in representing dislocation structures. This collection of papers is a well-balanced representation of ongoing work in the area of nanoscale mechanics. In all of the experimental papers, the nanoindenter is the main tool of choice and acts as the primary method of investigation, supplemented by additional experimental techniques providing important advancements in our field. The modeling papers show a balance of both explaining experimental observations as well as further efforts in developing comprehensive physically-based models. However, we do not expect that these methods signify the pinnacle of nanoscale mechanics, as new methods, enhancements, and models are sure to be developed to further our understanding of mechanics at the smallest and most fundamental level. Access the papers published under the topic ‘‘Mechanical Behavior at the Nanoscale’’ in the July 2018 issue (vol. 70, no. 7) of JOM via the JOM page at ‘‘Probing the Effect of Hydrogen on Elastic Properties in Nickel Using Nanoindentation and Ultrasonic Methods’’ by S.K. Lawrence, B.P. Somerday, M.D. Ingraham, and D.F. Bahr. ‘‘A Nanoindentation Study of the Plastic Deformation and Fracture Mechanisms in SingleCrystalline CaFe2As2’’ by Keara G. Frawley, Ian Bakst, John T. Sypek, Sriram Vijayan, Christopher R. Weinberger, Paul C. Canfield, Mark Aindow, and Seok-Woo Lee. ‘‘In-situ Indentation and Correlated Precession Electron Diffraction Analysis of Polycrystalline Cu Thin Films’’ by Qianying Guo and Gregory B. Thompson. ‘‘Fast Slip Velocity in a High-Entropy Alloy’’ by Q. Rizzardi, G. Sparks, and R. Maaß. ‘‘Pressure Dependence of the Peierls Stress in Aluminum’’ by Khanh Dang and Douglas Spearot. ‘‘Comparing Modeling Predictions of Aluminum Edge Dislocations: Semidiscrete Variational Peierls–Nabarro versus Atomistics’’ by Lucas M. Hale. 1. W.D. Nix , Metall. Trans. A 20 , 2217 ( 1989 ). 2. A.M. Minor , J.W. Morris , and E.A. Stach , Appl. Phys. Lett . 79 , 1625 ( 2001 ). 3. W.W. Gerberich , W.M. Mook , C.R. Perrey , C.B. Carter , M.I. Baskes , R. Mukherjee , A. Gidwani , J. Heberlein , P.H. McMurry , and S.L. Girshick , J. Mech . Phys. Solids 51 , 979 ( 2003 ). 4. M.D. Uchic , D.M. Dimiduk , J.N. Florando , and W.D. Nix , Science 305 ( 5686 ), 986 ( 2004 ). 5. Z.W. Shan , J. Li , Y.Q. Cheng , A.M. Minor , S.A. Syed Asif , O.L. Warren , and E. Ma, Phys. Rev. B Condens. Matter Mater. Phys. 77 , 1 ( 2008 ). 6. A. Misra , J.P. Hirth , and R.G. Hoagland, Acta Mater. 53 , 4817 ( 2005 ). 7. A.A. Benzerga , Int. J. Plast 24 , 1128 ( 2008 ). 8. T. Zhu , J. Li , A. Samanta , A. Leach , and K. Gall , Phys. Rev. Lett . 100 , 25502 ( 2008 ). 9. E.A. Stach , Mater. Today 11 , 50 ( 2008 ). 10. P. Tiwary and A. van de Walle, Multiscale Materials Modelling for Nanomechanics , vol. 245 , ed. C. Weinberger and G. Tucker (Cham: Springer, 2016 ), pp. 195 - 221 . 11. R.B. Sills , A. Aghaei , and W. Cai , Model. Simul. Mater. Sci. Eng . 24 , 45019 ( 2016 ).

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Megan J. Cordill, Christopher R. Weinberger. Mechanical Behavior at the Nanoscale: What’s in your Toolbox?, JOM, 2018, 1-3, DOI: 10.1007/s11837-018-2849-5