It is well-established that stressing nanocrystalline metals can lead to grain growth which can result from tensile or compressive stress, high pressure torsion or the result of microhardness testing with an indenter [1][2][3]. The effect can be large, especially in the case of high-purity single-element nanocrystalline metals. The grain growth in the vicinity of a Vickers indenter during hardness testing of nanocrystalline copper has shown that an initial average grain size of about 20 nm doubles after 10 seconds of indenter dwell time [2]. The grain growth persists over hours of dwell time. At liquid nitrogen temperature grain growth is even more striking, with the rapid appearance of some large grains approaching micrometer size. The fast growth at the cryogenic temperature suggests that the grain growth is stress driven. It is not clear what the mechanism is for grain growth during indentation.To shed some light on these questions, we have carried out electron microscopy studies of the internal structure of nanocrystalline Cu near microhardness indentations. Foils taken from the vicinity of indents have been examined for changes in the internal structure and for chemical information using high resolution (scanning) transmission electron microscopy (HR(S)TEM) operated at 200 kV and equipped with an energy dispersive X-ray (EDX) spectrometer for chemical analyses. The nanocrystalline samples used in this study are made by inert gas condensation and in-situ compaction. In the present study, foils have been examined from indents made at room temperature with 10 sec indenter dwell time and 30 min. dwell time, and from indents made at 77 K with 1 min. dwell time and 30 min. dwell time. A large number of grains and grain boundaries have been examined which has made it possible to draw some general, qualitative conclusions about the effect of indenter dwell time and temperature of indentation on the internal structure of the nanocrystalline Cu.General observations about the effect of indenter dwell time and temperature show grains are smaller away from the indents. At both room temperature and 77 K, the grain sizes in the vicinity of the indenter become larger as the dwell times increase. At both the temperatures, the number of low angle grain boundaries is greater at the longer dwell time. Arrays of parallel dislocation lines are seen at room temperature (Fig.1). The arrays are present especially at 30 min dwell time when the grains have grown considerably; fewer such arrays are seen at 77 K. Twins have been observed with a (111) twinning plane in the <110> direction. But, less frequently, twins are also seen with a (110) twinning plane in the <111> direction (Fig. 2). The number of twins appear to increase with increasing indenter dwell time, indicating that they are deformation twins. They are more frequent at 77 K than at room temperature.The results indicate that besides observations of grain growth, the number of low angle grain boundaries increase with indenter dwell time and decreasing temperature, suggesting...
Nanocrystalline metals are important because they exhibit exceptionally high strengths and therefore have generated considerable interest [1,2]. Nanocrystalline copper samples were made by inert gas condensation (IGC) and in situ compaction in equipment at Argonne National Laboratory. They were made from 99.999% pure Cu stock. The average grain size of each sample was determined from transmission electron microscopy observations of ~ 500 grains. The grain size distribution was quite narrow. The samples had a density of 98-99% of the coarse-grain value [1]. Vacuum conditions and helium purity have recently been improved further by an order of magnitude or greater. Sample hardness was measured with a microhardness tester, as a function of dwell time of the hardness indenter in the sample over dwell times from 5 s to several hours and temperatures ranging from room temperature (RT) liquid nitrogen temperature (LN2) (figure 1).We have obtained structural and chemical data of grains and grain boundaries at the atomic resolution and on the nanoscale, respectively, using FEI FE(S)TEM Tecnai and CM30 Environmental-TEM (ETEM) [3] instruments. Grain boundary compositions were determined using the electron nanoprobe in the STEM mode. The data are quite informative. Our studies of the RT sample have shown grain sizes of ~ 15-70 nm in areas away from the indented regions and 80nm-180 nm near the indented regions, suggesting grain growth near the indents. High concentrations of dislocations were observed (figure 2). The high internal strains seen in figure 2 come from the built-in strains in the nanoclusters before compaction caused by cluster coalescence and/or from the deformation produced by the indenter. In the LN2 sample, we have observed much larger grains varying from 0.1 microns to about 0.5 microns. We have found fewer grain boundaries and fewer dislocations than in the RT sample. Coherent boundaries between two (110) grains and atomically flat boundaries between (110), (111) and (010) grains were observed in both the samples. Atomic resolution imaging has revealed clean grain boundaries (figure 3, between (111) grains showing 110 lattice planes). Grains exhibit Moire fringes due to overlapping grains. Atomic scale twinning along <111> in (110) grains were observed (shown in figure 4 in LN2 sample) and along <110> in (111). The atomic structure and nano-probe analyses using X-ray spectroscopy (EDX) are consistent with Cu, with no impurity segregation at the grain boundaries (figure 5).References: [1]
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.