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S Nag,
A Devaraj,
R Srinivasan,
R E A Williams,
N Gupta, G B Viswanathan,
J S Tiley, S Banerjee,
S G Srinivasan,
H L Fraser,
R Banerjee
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ABSTRACT: Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.
Physical Review Letters 06/2011; 106(24):245701. · 7.37 Impact Factor
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ABSTRACT: Reliable communication over the discrete-input/continuous-output noncoherent multiple-input multiple-output (MIMO) Rayleigh block-fading channel is considered when the signal-to-noise ratio (SNR) per degree of freedom is low. Two key problems are posed and solved to obtain the optimum discrete input. In both problems, the average and peak power per space-time slot of the input constellation are constrained. In the first one, the peak power to average power ratio (PPAPR) of the input constellation is held fixed, while in the second problem, the peak power is fixed independently of the average power. In the first PPAPR-constrained problem, the mutual information, which grows as O (SNR<sup>2</sup>), is maximized up to second order in SNR. In the second peak-constrained problem, where the mutual information behaves as O (SNR), the structure of constellations that are optimal up to first order, or equivalently, that minimize energy per bit, are explicitly characterized. Furthermore, among constellations that are first-order optimal, those that maximize the mutual information up to second order, or equivalently, the wideband slope, are characterized. In both PPAPR-constrained and peak-constrained problems, the optimal constellations are obtained in closed form as solutions to nonconvex optimizations, and interestingly, they are found to be identical. Due to its special structure, the common solution is referred to as space-time orthogonal rank one modulation, or STORM. In both problems, it is seen that STORM provides a sharp characterization of the behavior of noncoherent MIMO capacity.
IEEE Transactions on Information Theory 03/2009; · 3.01 Impact Factor
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ABSTRACT: Code design for the low SNR MIMO noncoherent correlated Rayleigh fading channel is considered. Design rules which exploit the correlations in the transmit antennas in the MIMO case, to provide gains over the corresponding SIMO case are presented. The Chernoff bound on the average pairwise error probability (APEP) is used to study the effect of the receive correlation matrix on system performance at different SNR regimes. Based on a lower bound on the APEP, which is related to the Bhattacharya coefficient, a technique is proposed to design codes for use with transmit beamforming, with codewords having unequal prior probabilities. The motivation for such codes with unequal priors arises from recent information theoretic results on the low SNR channel. Such constellations are shown to perform substantially better than constellations designed assuming equal priors, at low SNRs
Information Theory, 2005. ISIT 2005. Proceedings. International Symposium on; 10/2005
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ABSTRACT: Nanocrystalline (nc) materials are known to deform via mechanisms not accessible to their coarse-grained counterparts. For example, deformation twins and partial dislocations emitted from grain boundaries have been observed in nc Al and Cu synthesized by severe plastic deformation (SPD). This paper further develops an earlier dislocation-based model on the nucleation of deformation twins in nc face-centered-cubic (fcc) metals. It is found that there exists an optimum grain-size range in which deformation twins nucleate most readily. The critical twinning stress is found determined primarily by the stacking fault energy while the optimum grain size is largely determined by ratio of shear modulus to stacking fault energy. This model formulated herein is applicable to fcc nanomaterials synthesized by SPD techniques and provide a lower bound to the critical twining stress.
Journal of Applied Physics 08/2005; 98(3):034319-034319-8. · 2.17 Impact Factor
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ABSTRACT: Deformation twins (DTs) in nanocrystalline (nc) Al were both predicted by atomic simulations, and observed experimentally. However, despite encouraging preliminary results, their formation mechanism remains poorly understood. Here we present an analytical model, based on classical dislocation theory, to explain the nucleation and growth of DTs in nc Al . A 60° dislocation system consisting of a 90° leading partial and a 30° trailing partial is found to most readily nucleate and grow a DT. The model suggests that the stress for twin growth is much smaller than that for its nucleation. It also predicts an optimal grain size for twin nucleation. The model successfully explains DTs observed experimentally in nc Al and is also applicable to other nc metals.
Applied Physics Letters 12/2004; · 3.84 Impact Factor
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ABSTRACT: A full dislocation often dissociates into two partial dislocations enclosing a stacking fault (SF) ribbon. The SF width significantly affects the mechanical behavior of metals. Al has very high stacking fault energy and, consequently, very narrow SF width in its coarse-grained state. We have found that some SFs in nanocrystalline Al are surprisingly 1.4–6.8 nm wide, which is 1.5–11 times higher than the reported experimental value in single crystal Al. Our analytical model shows that such wide SFs are formed due to the small grain size and possibly also to the interaction of SF ribbons with high density of dislocations. © 2004 American Institute of Physics.
Applied Physics Letters 04/2004; 84(18):3564-3566. · 3.84 Impact Factor
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ABSTRACT: We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy. © 2003 American Institute of Physics.
Applied Physics Letters 08/2003; · 3.84 Impact Factor
