In situ studies of the transmission of strain across grain boundaries

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1 Materials Science and Engineering A 462 (2007) In situ studies of the transmission of strain across grain boundaries J.W Morris Jr. a,,m.jin a, A.M. Minor b a Department of Materials Science and Engineering, University of California, 210 Hearst Mining Building, Berkeley, CA 94618, USA b National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 29 August 2005; received in revised form 19 January 2006; accepted 4 February 2006 Abstract In situ nanoindentation in a transmission electron microscope has been used to study the transmission of strain across grain boundaries in ultrafine- and nanograined metals. Several different mechanisms are revealed. In materials with mobile grain boundaries, strain is accommodated by grain boundary movement or, more interestingly, by strain-induced grain coalescence (strain-induced coarsening). In materials with fixed grain boundaries strain is transmitted by dislocation transmission across the boundary or, more commonly, by triggering deformation in adjacent grains. Dislocations are also observed to annihilate at the boundary, leading to grain boundary sliding. These mechanisms are illustrated by direct observation of the nanoindentation of nanograined Al, ultrafine grained aluminum, dislocated lath martensite and milled, ultrafine-grained Fe Elsevier B.V. All rights reserved. Keywords: In situ microscopy; Nanoindentation; Grain boundary processes; Nanograin plasticity; Strain-induced coarsening 1. Introduction The recent development of an in situ nanoindentation stage [1,2] has made it possible to observe the response of a material to nanoindentation in real time in a high-resolution transmission electron microscope. One of the most interesting problems that can be studied with this new capability is the transmission of strain across grain boundaries under indentation conditions. While the results garnered to date are qualitative in the sense that indentation loads could not be monitored (until very recently), they reveal a number of interesting phenomena. Many of the experimental results have been described elsewhere [2 9]. The purpose of the present paper is to summarize the most interesting results in a more systematic format that brings out the variety of the mechanisms that have been observed. In a number of experiments, particularly those on pure or lightly alloyed Al, the grain boundaries were mobile under the indenter. The most striking phenomenon observed in these samples was strain-induced coarsening, in which adjacent grains spontaneously coalesce into single, larger grains. This phenomenon is observed in both fine-grained and nanograined Al, and seems to provide an important mechanism of plastic response. In other cases, mobile Al grain boundaries remain Corresponding author. Tel.: address: jwmorris@berkeley.edu (J.W Morris Jr.). spatially pinned in the overall sense, but oscillate (or flap ) in response to the indenter with surprising flexibility and mobility. In other experiments, including those on materials with contaminated or decorated boundaries, the boundaries remain fixed in space. These experiments reveal interesting features regarding the transmission of strain across boundaries that separate nominally fixed grains. Three mechanisms are observed: the transmission of dislocations across the boundary, the catalysis of deformation in adjacent grains and the annihilation of dislocation clouds at the boundary, presumably accompanied by grain boundary sliding. We describe examples of these cases in turn. 2. Experimental details The experimental details of the in situ nanoindentation experiment are described elsewhere [2,9]. The nanoindenter is borondoped diamond in a three-sided Berkovich configuration with a tip radius that varies between about 50 and 100 nm, depending on preparation. The microscope stage contains a three-axis screwpositioner to control the position of the tip and a piezoelectric ceramic crystal for fine positioning and actual indentation. Electron-transparent samples of polygranular metals are made in either of two ways [9]. In one method, metal films a fraction of a micron in thickness are deposited onto a silicon substrate that has been etched so that a long, shaped wedge /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.msea

2 J.W Morris Jr. et al. / Materials Science and Engineering A 462 (2007) protrudes from its face. The portion of the film that covers the flattened tip of the wedge is transparent to the electron beam. The grain size of the film is varied from nanometer to submicron size by controlling the deposition conditions. In the second method, focused ion beams (FIB) are used to machine samples from bulk specimens so that they present a thin, electron-transparent plate to the electron beam. The indenter is then pressed into the side of the plate. The nanoindentation experiments are performed at ambient temperature. The indentation experiments are filmed to capture the indentation process in real time, and the samples are analyzed separately to study the initial and deformed states. 3. Mobile boundaries First consider indentation experiments on samples with mobile grain boundaries. Pure Al has this property Pinned mobile boundary pure Al The most spectacular example of grain boundary mobility in Al occurred in the experiment whose results are illustrated in Fig. 1. In this experiment the indenter was pressed into the grain to the left of the low-angle ( 6 ) grain boundary shown. Unfortunately, still photographs can only illustrate the result. As the films show, the grain boundary moves rather smoothly to the right in the field of view, but periodically returns to its original position. The boundary seems to flap back and forth under the driving force provided by the penetration of the indenter. Several positions are illustrated in the figure. Interestingly, at the end of the test the boundary has returned to its original position. While this unexpected mobility has not been fully explained, our working hypothesis attributes it to dislocation emission from the low-angle boundary. In this model the low-angle boundary bows out under the stress imposed by the indenter. At a critical configuration the boundary emits a dislocation, deforming the adjacent grain and allowing it to relax to its original configuration. The process is then repeated periodically as long as the indentation is continued. At the completion of the experiment the boundary returns to its original position Strained-induced coarsening While the flapping grain boundary is spectacular, the strain-induced coarsening that is frequently observed in ultrafine- and nanograined Al may have much greater engineering importance. One example of the phenomenon is contained in Fig. 2 [6]. This figure shows two adjacent grains with indentation near the grain boundary, which was determined to be a high-angle boundary with a rotation of about 40 in the plane of the figure. After initial deformation, which was characterized by extensive dislocation activity in the grain under the indenter, the grain boundary between the two grains swept quickly across the smaller, resulting in spontaneous growth of the larger grain as it was eliminated. An example involving the sequential coarsening of several grains is shown in Fig. 3 [8]. The initial micrograph shows a group of five numbered grains prior to indentation. As the indenter impresses the sample, grain 2, located immediately beneath Fig. 1. Flapping grain boundary observed during indentation of a fine-grained Al film. (a) Original position of the grain boundary; the nanoindenter approaches from below. (b d) Series of micrographs taken from the filmed record of the experiment showing the boundary displaced to the right (b), then back to its original position (c), then to the right again (d). At the conclusion of the experiment the boundary returned to its original position.

3 414 J.W Morris Jr. et al. / Materials Science and Engineering A 462 (2007) Fig. 2. (a) Two grains in an ultrafine-grained Al film prior to indentation. The grain boundary angle is 41. (b) During indentation the larger grain consumed the smaller, creating the single large grain shown in (b) (ref. [6]). the indenter, is gradually consumed by grain 4 and disappears. When this process is complete grain 4 sits immediately beneath the indenter, with adjacent grains 1 and 3. Indentation causes grain 4 to expand, moving its boundaries with both grains 1 and 3. The boundary between grains 1 and 4 stabilizes. However, the boundary between grains 4 and 3 continues to migrate into grain 3 until that grain is wholly consumed. Fig. 3 shows the final, coarsened state of the film. A detailed sequence of micrographs that trace the stages of coarsening is presented in ref. [8]. Note that the coarsening process is confined to the immediate vicinity of the indenter. Grain 5, which is within the film, beneath grain 4, is not noticeably affected by the indentation and coarsening. A similar process occurs in nanograined films. While it is not always possible to image individual nanograins clearly in the configuration used for these experiments, tests in which single nanograins are followed in dark field show their progressive growth during indentation [6]. In another experiment we were able to image an isolated nanograin and follow its progressive shrinkage as the indenter pressed into the material [8]. Strain-induced coarsening appears to be a common phenomenon in the nanoindentation of ultrafine- and nanograined Al. Such coarsening is less common in alloys, where surfactants or precipitates decorate and pin the grain boundaries. However, Soer et al. [7] did observe strain-induced coarsening by propagation of a low-angle boundary in an Al Mg alloy, so the phenomenon is not unknown. Strain-induced coarsening may be an important mechanism of deformation in ultrafine-grained metals with mobile boundaries. By LeChatelier s Principle, the coarsening will always happen in a pattern that promotes deformation under the indenter and is, hence, a work-softening mechanism. Hemker [10] have Fig. 3. (a) Sequence of images taken from the indentation of a polygranular Al film showing strain-induced coarsening. The grain labeled 2 shrinks and disappears in frames (b c). Grain 3 then shrinks and disappears by frame (d). Note that grain 5, just beneath the indenter, is essentially unaffected (ref. [8]).

4 J.W Morris Jr. et al. / Materials Science and Engineering A 462 (2007) Fig. 4. (a) Indentation at a high-angle grain boundary in an Al film. The indented film is shown in (b); the indenter was placed at the lower grain boundary. The two micrographs are overlaid in (c) showing displacement of the boundary and defects in the two adjacent grains (ref. [5]). recently shown that strain-induced coarsening can make important contributions to the ductility of nanograined aluminum. 4. Immobile boundaries Grain boundaries that are decorated with solutes or precipitates are relatively immobile at ambient temperature, and show this behavior in nanoindentation tests. The transmission of strain across immobile boundaries is accomplished by moving dislocations across boundaries, catalyzing deformation in adjacent grains or sliding the boundary itself. All three processes have been observed in in situ experiments Dislocation transmission across boundaries The clearest example of dislocation transmission we have seen to date is in the deformation of dislocated lath martensite in an Fe C alloy [4]. Laths within a packet in dislocated martensite are separated by boundaries that are well-defined by dense arrays of dislocations, but have almost no angular misorientation; whole packets diffract as single crystals. The response of a lath boundary to nanoindentation was observed in an experiment described in ref. [4], done on a sample machined (FIB) from a bulk sample of dislocated Fe C martensite. The boundary proved surprisingly resistant to the passage of dislocations. A dense array of dislocations formed within the indented lath before any transmission across the boundary was observed. When the density within the boundary reached a critical value, however, dislocations began to penetrate the boundary in a diffuse cloud, initiating deformation of the adjacent grain. No planar arrays of dislocations are observed in these experiments. The dislocation distribution is a dense, diffuse cloud both within the indented lath and after penetrating the lath boundary. Fig. 5. (a) Indentation of dislocated Fe C martensite near a high-angle packet boundary. The indenter creates a shower of dislocations in the right-hand grain (b). The dislocations impinge on the boundary and annihilate there, without significant effect on the adjacent grain (c) (ref. [4]).

5 416 J.W Morris Jr. et al. / Materials Science and Engineering A 462 (2007) There are isolated examples of possible dislocation transmission in other indentations we have made, but this is the only example in work done to date in which the transmission of dislocations across a boundary is, clearly, the primary mechanism by which strain is transferred from one grain to another during indentation Transmission of strain across opaque boundaries Grain boundaries in alloys are relatively immobile and opaque in indentation experiments. Even in nominally pure aluminum high-angle boundaries can be difficult to move. Fig. 4 shows an example taken from an indentation of an Al film in which the indenter was positioned at the boundary between two grains [5]. The indentation was continued to a significant fraction of the thickness of the film. While the boundary has moved appreciably in space, it does not appear to have moved with respect to the material; to a very good approximation it simply follows the deformation of the bounding grains. Note the incipient deformation of the adjacent grain, to the left in the figure. While defects are visible in the interior of the grain the boundary does not appear to have moved. Nor is there evidence of dislocation transmission across the boundary; the visible defects appear to form within the grain under the stress imposed by its neighbor. A more spectacular example of an opaque boundary was revealed in the experiment shown in Fig. 5 [4]. The boundary shown is a high-angle packet boundary in dislocated Fe C martensite, machined (FIB) to extract the test specimen. Such boundaries separate different Bain variants of the martensite [11,12], and are high-angle boundaries whose planes lie close to {110}. During the nanoindentation of the grain to the right in the figure, a dense tangle of dislocations developed and was swept into the boundary. There the dislocations were annihilated, with virtually no effect on the adjacent grain. The dislocations were, apparently, accommodated by sliding the boundary perpendicular to the plane of the photo. Boundary sliding is relatively simple in this case since the boundary plane is near a {110} slip plane and the grain is unconstrained in the direction perpendicular to the foil. Still a third distinct type of opaque grain boundary behavior was observed in nanoindentation experiments on ultrafinegrained Fe, prepared by the Takaki group at Kyushu [9,13]. The sample has a mean grain size of about 150 nm, and was thinned (FIB) from a sample made by ball-milling in the presence of yttria (Y 2 O 3 ). As a consequence the grain boundaries are deco- Fig. 6. Indentation of an ultra-fine grained Fe sample whose boundaries are pinned by yttria nanoparticles. (a and b) Bright and dark-field views of the initial state with grain boundaries outlined. (c and d) Bright and dark-field views of the indented state with the original grain boundary outline superimposed (ref. [9]).

6 J.W Morris Jr. et al. / Materials Science and Engineering A 462 (2007) rated with, and stabilized by nanoparticles of yttria. The results of an indentation experiment are illustrated in Fig. 6, in which the original positions of the grain boundaries are drawn in dashed lines. Note that only the grains immediately beneath the indenter are noticeably affected. All other boundaries are fixed, and, as illustrated by the grains lit up in dark field, there is little evidence of defect generation or internal deformation. 5. Conclusion This brief review illustrates the variety of mechanisms that are available for the transmission of strain from grain to grain during the nanoindentation of ultrafine-grained metals. Mobile boundaries are displaced under the indenter, often leading to straininduced coarsening of the microstructure. Relatively immobile boundaries transmit strain by transmitting dislocations (lowangle boundaries), by catalyzing strain in adjacent grains or by grain boundary sliding. In all cases, the boundary resists strain, and the grains being indented are severely deformed before their neighbors are significantly affected. This intense localization of the indentation strain is surprising, it is certainly exaggerated to some extent by the small lateral dimension of the sample, which allows deformation perpendicular to the plane of the foil. Acknowledgements The work of M. Jin and J.W. Morris Jr. was supported by the National Science Foundation under grant DMR A.M. Minor, and the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, were supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC03-76SF References [1] E.A. Stach, T. Freeman, A.M. Minor, D.K. Owen, J. Cumings, M.A. Wall, T. Chraska, R. Hull, J.W. Morris Jr., A. Zettl, U. Dahmen, Microsc. Microanal. 7 (2001) 507. [2] A.M. Minor, E.A. Stach, J.W. Morris Jr., Appl. Phys. Lett. 79 (2001) [3] W.A. Soer, J.T.H.M. De Hosson, A.M. Minor, E.A. Stach, J.W. Morris Jr., Thin Films-Stresses and Mechanical Properties. Mater. Res. Soc. Symposium Proceedings, vol. 795, Mater. Res. Soc., Warrendale, PA, USA, 2004, pp [4] T. Ohmura, A.M. Minor, E.A. Stach, J.W. Morris Jr., J. Mater. Res. 19 (2004) [5] A.M. Minor, E.T. Lilleodden, E.A. Stach, J.W. Morris Jr., J. Mater. Res. 19 (2004) [6] M. Jin, A.M. Minor, E.A. Stach, J.W. Morris Jr., Acta Mater. 52 (2004) [7] W.A. Soer, J.Th.M. De Hosson, A.M. Minor, J.W. Morris Jr., E.A. Stach, Acta Mater. 52 (2004) [8] M. Jin, A.M. Minor, J.W. Morris Jr., Thin Solid Films, in press. [9] M. Jin, D. Ge, A.M. Minor, J.W. Morris Jr., J. Mater. Res. 20 (2005) [10] K.J. Hemker, Johns Hopkins University, private communication, February [11] T. Maki, K. Tsuzaki, I. Tamura, Trans. ISIJ 20 (1980) 209. [12] Z. Guo, C.S. Lee, J.W. Morris Jr., Acta Mater. 52 (2004) [13] S. Takaki, K. Kawasaki, Y. Kimura, in: R.S. Mishra, et al. (Eds.), Ultrafine Grained Materials, TMS, Warrendale, PA, 2000, p. 247.