Fabrication, microstructure, and mechanical properties of tin ...

5828 M.J. Burek et al. / Materials Science and Engineering A 528 (2011) 5822–5832aFlow stress at 5% strain (MPa)100010010~ 920 nm~ 560 nm~ 350 nm10 -5 10 -4 10 -3 10 -2 10 -1Engineering strain rate (s-1 )bEngineering strain rate (s -1 )10 -110 -210 -3~ 5.5410 -4~ 920 nm~ 560 nm10 -510 100 1000Flow stress at 5% strain (MPa)Fig. 10. (a) Log–log plot of engineering flow stress measured at ∼5% strain as a function of engineering strain rate for all tin nanopillars tested in this work. (b) Log–log plotof averaged strain rate as a function of average engineering flow stress measured at ∼5% strain for tin nanopillars. The error bars indicate one standard deviation.respectively. These specimens were compressed with the sameengineering strain rate of ∼0.001 s −1 . The stress–strain curves showthat the nanopillars exhibit strain bursts or softening events afterinitial yielding. Such strain bursts are believed to be associatedwith sudden catastrophic shear failures along crystallographic slipplanes.3.3. Post compression SEM analysisTo further understand how small-scale tin structures behaveplastically, all nanopillar specimens were carefully inspected beforeand after compression using high resolution SEM. Typical electronmicrographs of tin nanopillars with average diameters of920 nm, 560 nm, and 350 nm before and after uniaxial compressionat strain rate of ∼0.001 s −1 are shown in Figs. 7–9, respectively.These figures show two distinct plastic flow features –crystallographic shear off-sets and material extrusion. Both deformationfeatures were observed for all three nanopillar sizes. Eventhough fine crystallographic slip lines are not apparent on thepost compression images, examples of gross deformation by shearalong crystallographic planes can be found in these SEM micrographs.This type of crystallographic shear behavior has beenreported in almost all deformed single-crystalline metallic nanopillars[14–25,27–31,38–41]. Another deformation characteristic thatcan be found in Figs. 7–9 is sidewall surface wrinkles and bulgesperpendicular to the loading direction. All compressed tin nanopillarsshowed this type of plastic deformation where the compressivestrain is compensated by material extrusions from the nanopillarsidewalls. The material extrusion type of deformation in tinnanopillars is not unexpected. At high homologous temperatures, alarge number of slip systems will be activated which allows deformationin a greater number of crystallographic slip planes to occursimultaneously. The consequence is that fewer discrete slip planesare observed in the post-compression nanopillar, and instead strainresembles extrusion like deformation.3.4. Influence of strain rate and nanopillar sizeEngineering flow stresses of tin nanopillars measured at 5.0%nominal strain are plotted as a function of strain rate in Fig. 10(a).The data in this plot includes results from nanopillar compressionsof three different diameters: 350 nm, 560 nm, and 920 nm. Forthe two largest nanopillar sizes of 560 nm and 920 nm, the resultsclearly show that the tin flow stresses are sensitive to the deformationrate. The data reveals that the material strength is low whenthe structure is deformed slowly. In contrast, the yield strengthincreases when the specimen is compressed at a fast strain rate. Inorder to show a clear trend of strain rate sensitivity, the averageflow stress values of 560 nm and 920 nm diameter nanopillars areplotted in Fig. 10(b), where the error bars correspond to one standarddeviation. Fig. 10(b) shows a noticeable increase in 920 nmdiameter tin nanopillar strength from ∼46 to 112 MPa when thestrain rate increases from 0.0001 s −1 to 0.01 s −1 . Similarly, for the560 nm diameter tin nanopillars, the data shows an increase from∼61 to 96 MPa for the same increase in strain rate.The general relationship between the uniaxial strain rate andcreep stress has been well documented and is displayed in Eq. (1).To compare the tin nanopillar strain rate sensitivity with the bulksamples from the previous studies, the data from Fig. 10(b) arefitted with Eq. (1). The stress exponent extracted from this curvefit is approximately 5.54 ± 1.53, where the spread corresponds toone standard deviation. This value agrees with the single-crystalimpression creep results reported from Chu and Li [6], with exponentvalues measured near room temperatures in the range of4.4–5.0. The results indicate that the strain rate sensitivity of singlecrystaltin nanostructures is similar to bulk.Fig. 11 shows the tin nanopillar engineering flow stress resultsmeasured at 5.0% nominal strain plotted as a function of featuresize. These structures were compressed at a constant strain rate ofFlow stress at 5% strain (MPa)1000100~ D -0.572σ-1strain rate ~ 0.001 s10100 1000Pillar diameter (nm)Fig. 11. Log–log plot of engineering flow stress measured at ∼5% strain as a functionof pillar diameters. These samples were compressed at a ∼0.001 s −1 strain rate. Theerror bars indicate one standard deviation.

M.J. Burek et al. / Materials Science and Engineering A 528 (2011) 5822–5832 5831kling/extrusion are both observed in the post-compression SEMimage. This is consistent with the deformation mechanisms observationsin Figs. 7–9. The stress–strain data collected from uniaxialcompression of this tin nanopillar is plotted in Fig. 12(c). Thestress–strain behavior includes similar strain bursts to those illustratedpreviously in Fig. 6. It is important to note that this particularpillar was only characterized by SXRD after the compression test.No information was collected in the as-fabricated state.The SXRD analysis of the deformed tin nanopillar againidentified only one unique body-centered tetragonal (BCT) Lauediffraction pattern for the entire nanopillar volume, indicating thecompressed nanopillar is single-crystalline. The Laue diffractionpattern indicates the compressed tin nanopillar is BCT with theplane normal to the vertical axis of the pillar close to the tin (2 0 4)plane. Two individual Laue diffraction spots of (2 0 4) and (1 0 5)were extracted from this pattern and plotted in Fig. 12(a) and (b),respectively. The shapes of these two Laue spots are fairly regularwith circular geometry suggesting random distribution of dislocationsdue to the nanopillar deformation. Qualitatively, the Lauediffraction peaks of this deformed specimen are similar to the asfabricatedtin nanopillar shown in Fig. 4(a) and (b).Qualitative analysis of the (2 0 4) Laue diffraction peak inFig. 13(a) were also conducted along the intensity traces at aparticular and along the 2 axis. This profile was fitted withLorentzian curves as shown in Fig. 13(c). The FWHM value of thispeak broadening is approximately 0.458 ◦ for the compressed tinnanopillar. When compared with the peak broadening between asfabricatedand post compression spots shown in Figs. 5(b) and 13(c),the results show that there is an angular width difference of∼0.019 ◦ . This variation is within the experimental error and resolutionof the SXRD technique. The angular resolution in theSXRD experiments is limited by the charge-coupled device (CCD)camera pixel size which translates to resolution limit of ∼0.03 ◦in the Laue spot width measurement. No significant change inthe peak broadening is expected here from the small crystalsize, as well as the instrumentation before or after the deformation.It is then also reasonable to propose that there is nosignificant change in the peak broadening due to random dislocationdensity associated with the nanopillar deformation. TheLaue diffraction results discussed here suggest that the defect densityin this tin nanopillar after compression is very similar to theas-fabricated tin nanopillar described in Fig. 4. This may indicatethat the nucleation and multiplication rates of dislocations duringthe uniaxial compression process are offset by the dislocationannihilation rate at the nanopillar surface. Thus, there is no significantincrease or accumulation of dislocations in these smalltin structures. Such a finding is similar to that reported earlier byBudiman et al. [47] in an ex situ study of single-crystalline goldnanopillars fabricated from by FIB milling, despite different crystalstructures.4. ConclusionsIn conclusion, we have developed fabrication and integrationtechniques to produce large grain BCT tin nanopillars with diametersas small as 70 nm. Tin nanopillar flow stress data measuredat different deformation rates indicate that these nanostructurespossess similar strain rate sensitivity to their bulk counterpart,where the strength of tin increases with deformation rate. Additionally,the strength of tin nanopillars was observed to increasewith reduced diameter. This flow stress size dependence appearsto have the same characteristics as other single-crystalline FCCmetals tested in uniaxial compression at the nanoscale. Microstructuralcharacterization by SXRD indicates that there is no drasticincrease of dislocation within the compressed tin nanopillars. Thissuggests that the rate of dislocation generated by the deformationprocess is offset by annihilation at the nanopillar surfaceAcknowledgementsT.Y. Tsui thanks Canadian NERSC Discovery, NERSC ResearchTools and Instruments, and the Canada Foundation for Innovation(CFI) for the financial support of this research. The authorsgratefully acknowledge critical support and infrastructure providedfor this work by the Department of Energy (DOE), Officeof Science, Office of Basic Energy Sciences. The Advanced LightSource is supported by the Director, Office of Science, Office ofBasic Energy Sciences, Materials Sciences Division, of the U.S.Department of Energy under Contract No. DE-AC02-05CH11231 atLawrence Berkeley National Laboratory and University of California,Berkeley, California. The move of the micro-diffraction programfrom ALS beamline 7.3.3 onto to the ALS superbend source 12.3.2was enabled through the NSF grant #0416243. One of the authors(ASB) is supported by the Director, Los Alamos National Laboratory(LANL), under the Director’s Postdoctoral Research Fellowshipprogram (#20090513PRD2).References[1] M. Abtew, G. Selvaduray, Materials Science and Engineering R: Reports 27(2000) 95–141.[2] J. Glazer, International Materials Reviews 40 (1995) 65–93.[3] W.J. Plumbridge, Materials at High Temperatures 17 (2000) 381–387.[4] H. Mukaibo, T. Momma, M. Mohamedi, T. Osaka, Journal of the ElectrochemicalSociety 152 (2005) A560–A565.[5] F. Yang, J.C.M. Li, Journal of Materials Science: Materials in Electronics 18 (2007)191–210.[6] S.N.G. Chu, J.C.M. Li, Materials Science and Engineering 39 (1979) 1–10.[7] O.D. Sherby, Acta Metallurgica 10 (1962) 135–147.[8] R.E. Frenkel, O.D. Sherby, J.E. Dorn, Acta Metallurgica 3 (1955) 470–472.[9] J. Weertman, J.E. Breen, Journal of Applied Physics 27 (1956) 1189–1193.[10] J.E. Breen, J. Weertman, Transactions of the American Institute of Mining andMetallurgical Engineers 203 (1955) 1230.[11] F. Mohamed, K. Murty, J. Morris, Metallurgical and Materials Transactions B 4(1973) 935–940.[12] V. Raman, R. Berriche, Journal of Materials Research 7 (1992) 627–638.[13] M.J. Mayo, W.D. Nix, Acta Metallurgica 36 (1988) 2183–2192.[14] M.D. Uchic, D.M. Dimiduk, J.N. Florando, W.D. Nix, Science 305 (2004) 986–989.[15] M.D. Uchic, D.M. Dimiduk, Materials Science and Engineering A 400–401 (2005)268–278.[16] D.M. Dimiduk, M.D. Uchic, T.A. Parthasarathy, Acta Materialia 53 (2005)4065–4077.[17] C.P. Frick, B.G. Clark, S. Orso, A.S. Schneider, E. Arzt, Materials Science andEngineering A 489 (2008) 319–329.[18] Z.W. Shan, R.K. Mishra, S.A. Syed Asif, O.L. Warren, A.M. Minor, Nature Materials7 (2008) 115–119.[19] J.R. Greer, W.C. Oliver, W.D. Nix, Acta Materialia 53 (2005) 1821–1830.[20] C.A. Volkert, E.T. Lilleodden, Philosophical Magazine 86 (2006) 5567–5579.[21] J.R. Greer, W.D. Nix, Applied Physics A: Materials Science and Processing 80(2005) 1625–1629.[22] D. Kiener, C. Motz, T. Schöberl, M. Jenko, G. Dehm, Advanced Engineering Materials8 (2006) 1119–1125.[23] D. Kiener, W. Grosinger, G. Dehm, R. Pippan, Acta Materialia 56 (2008) 580–592.[24] A.T. Jennings, M.J. Burek, J.R. Greer, Physical Review Letters 104 (2010) 135503.[25] K.S. Ng, A.H.W. Ngan, Acta Materialia 56 (2008) 1712–1720.[26] H. Bei, S. Shim, E.P. George, M.K. Miller, E.G. Herbert, G.M. Pharr, Scripta Materialia57 (2007) 397–400.[27] H. Bei, S. Shim, G.M. Pharr, E.P. George, Acta Materialia 56 (2008) 4762–4770.[28] S. Brinckmann, J.-Y. Kim, J.R. Greer, Physical Review Letters 100 (2008)155502–155504.[29] J.-Y. Kim, J.R. Greer, Applied Physics Letters 93 (2008) 101913–101916.[30] J.-Y. Kim, J.R. Greer, Acta Materialia 57 (2009) 5245–5253.[31] J.-Y. Kim, D. Jang, J.R. Greer, Scripta Materialia 61 (2009) 300–303.[32] J.-Y. Kim, D. Jang, J.R. Greer, Acta Materialia (2009).[33] G. Lee, J.-Y. Kim, A.S. Budiman, N. Tamura, M. Kunz, K. Chen, M.J. Burek, J.R.Greer, T.Y. Tsui, Acta Materialia 58 (2010) 1361–1368.[34] S.M. Han, T. Bozorg-Grayeli, J.R. Groves, W.D. Nix, Scripta Materialia 63 (2010)1153–1156.[35] J. Greer, J.-Y. Kim, M.J. Burek, JOM 61 (2009) 19–25.[36] J.R. Greer, D. Jang, J.-Y. Kim, M.J. Burek, Advanced Functional Materials 19 (2009)2880–2886.[37] A.S. Schneider, B.G. Clark, C.P. Frick, P.A. Gruber, E. Arzt, Materials Science andEngineering A 508 (2009) 241–246.

5832 M.J. Burek et al. / Materials Science and Engineering A 528 (2011) 5822–5832[38] A.S. Schneider, D. Kaufmann, B.G. Clark, C.P. Frick, P.A. Gruber, R. Mönig, O. Kraft,E. Arzt, Physical Review Letters 103 (2009) 105501.[39] S.-W. Lee, S.M. Han, W.D. Nix, Acta Materialia 57 (2009) 4404–4415.[40] J.-Y. Kim, D. Jang, J.R. Greer, Acta Materialia 58 (2010) 2355–2363.[41] M.D. Uchic, P.A. Shade, D.M. Dimiduk, Annual Review of Materials Research 39(2009) 361–386.[42] M.J. Burek, J.R. Greer, Nano Letters 10 (2010) 69–76.[43] M. Tian, J. Wang, J. Snyder, J. Kurtz, Y. Liu, P. Schiffer, T.E. Mallouk, M.H.W. Chan,Applied Physics Letters 83 (2003) 1620–1622.[44] N. Tamura, A.A. MacDowell, R. Spolenak, B.C. Valek, J.C. Bravman, W.L. Brown,R.S. Celestre, H.A. Padmore, B.W. Batterman, J.R. Patel, Journal of SynchrotronRadiation 10 (2003) 137–143.[45] A.S. Budiman, Department of Materials Science and Engineering, Stanford,2008.[46] B.C. Valek, Stanford University, 2003.[47] A.S. Budiman, S.M. Han, J.R. Greer, N. Tamura, J.R. Patel, W.D. Nix, Acta Materialia56 (2008) 602–608.[48] G. Feng, A.S. Budiman, W.D. Nix, N. Tamura, J.R. Patel, Journal of Applied Physics104 (2008) 043501–043512.[49] A.S. Budiman, N. Li, Q. Wei, J.K. Baldwin, J. Xiong, H. Luo, D. Trugman, Q.X.Jia, N. Tamura, M. Kunz, K. Chen, A. Misra, Thin Solid Films 519 (2011)4137–4143.[50] Z. Budrovic, H. Van Swygenhoven, P.M. Derlet, S. Van Petegem, B. Schmitt,Science 304 (2004) 273–276.[51] J.R. Greer, C.R. Weinberger, W. Cai, Materials Science and Engineering A 493(2008) 21–25.[52] M. Nagasaka, Japanese Journal of Applied Physics 28 (1989) 446–452.