We report ultra-large room-temperature plasticity of nanocrystalline Ni subjected to uniaxial compression. Up to 200% true plastic strain is achieved with a steady flow stress of ∼2 GPa at strain rates ranging from 10-3 to 10-1 s-1. The low temperature, high strain rate, and high flow stress demonstrate that the observed ultra-large plasticity in nanocrystalline Ni is intrinsically dissimilar to that in traditional superplastic materials deformed at high temperatures. Microstructural observations reveal significant nanograin growth accompanied with the ultra-large plastic deformation, indicating the ultra-large plasticity in nanocrystalline Ni at room temperature is mainly performed by a grain-boundary-mediated process that is driven by high stresses rather than by thermal diffusion.

Ultra-large compressive plasticity at room temperature has recently been observed in electrodeposited nanocrystalline nickel (nc-Ni) under micro-scale compression (Pan, Kuwano, Fujita, Chen, Nano Letters 7, 2108). The evolution of microstructure of nc-Ni during ultra-large deformation is outlined at a variety of strain levels, with TEM observations in combination with a TEM sample preparation technique using focused ion beam (FIB).This paper demonstrates focused ion beam (FIB) technique to prepare transmission electron microscopy (TEM) samples from microcompressed specimens. There has been a demand to prepare TEM samples from a point of interest to study microstructures on atomic scales. Conventional techniques used to make TEM samples, such as chemical polishing or ion-sputtering milling, cannot provide reliable opportunity to make TEM samples from a point of interest. With this technique, the deformation mechanism on atomic scales can be fairly connected with the result of microcompression test, which is available for size-limited materials.

Xue et al. Reply: In their Comment [1] on our recent Letter [2], Ramachandran et al. argue that As is present in our molecular beam epitaxy (MBE) system since it was previously used for the As-based material growth and that the As indeed stabilized the reconstructions of the Ga-polar GaN(0001) surface we reported. Their claim is based on the facts that they did not observe the 2 3 2 and other structures in their As-free MBE system [3] and that the 2 3 2 was observed by their reflection high energy electron diffraction (RHEED) measurements, not by scanning tunneling microscopy (STM), when they intentionally introduced As during the growth of GaN [1].We have to respectfully disagree with their argument, based on our in situ Auger electron spectroscopy (AES) measurements (Fig. 1). We conclude that the 2 3 2, 4 3 4, 5 3 5, and other reconstructions we reported in the Letter are free of As (within our AES sensitivity), and that the As adatom model proposed by them [1] cannot explain our results.We performed the AES study for the 2 3 2, 4 3 4, and 5 3 5 reconstructions of the GaN(0001) formed on the Siterminated 6H-SiC and sapphire substrates using a LEED/ AES setup (ULVAC-PHI) incorporated to the STM chamber. Figure 1(a) is the result for the GaN͑0001͒-5 3 5 reconstruction. Qualitatively the identical results are obtained for the other surfaces. The spectrum is dominated with the Ga LMN peak at about 1071 eV, and the Asrelated feature is absent, whose most intense Auger emission should appear at about 1230 eV. We repeated the Auger scans from 1100 eV in order to avoid the adverse effect from possible electron-induced desorption, and the results were the same. In order to test the sensitivity and

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