In Situ TEM Investigation of Abnormal Grain Growth in Nanocrystalline Nickel
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In Situ TEM Investigation of Abnormal Grain Growth in Nanocrystalline Nickel David M. Follstaedt1, Khalid Hattar2, James A. Knapp1 and Ian M. Robertson2 1 Physics, Chemistry, and Nano Science Center, Sandia National Laboratories Albuquerque, New Mexico 87185-1056 2 Materials Science and Engineering, University of Illinois at Urbana-Champaign Urbana, Illinois 61801 ABSTRACT In situ transmission electron microscopy was used to show that nanocrystalline nickel produced by pulsed-laser deposition undergoes abnormal grain growth at moderate temperatures (225-400°C). The growth rate was found to increase with thickness for the three film thicknesses examined, 50, 80 and 150 nm. The abnormal growth proceeded in an irregular manner: initiation sites and growth direction were unpredictable, and the grains exhibited no preferred orientation. Some abnormal grains show internal boundaries between sections with different orientations, including twins. The grains contain many defects, including dislocations, stacking faults and surprisingly, stacking fault tetrahedra. The stacking fault tetrahedra are not a result of quenching nor of electron irradiation-induced lattice displacements; instead, they are thought to form from vacancies trapped in the growing grain as it incorporates lower-density material at the high-angle grain boundaries in the nanocrystalline matrix. INTRODUCTION Metals with nanometer-scale grain sizes (~1 nm) [2-4]. These characteristics of PLD Ni films make them an ideal material to examine the intrinsic mechanical properties of this metal [2] and to identify the fundamental deformation modes [3,4] operating in nanograined metals. However, the high density of grain boundaries and their inherently non-equilibrium energetics raise the issue of the stability of nanocrystalline metals over extended periods or at elevated temperatures. The stabilities of the mechanical properties of nanograined films are directly dependent on the stability of their internal structures. Nanometer-size grain structures are inherently unstable, and grain growth depends on both time and temperature. Growth in these systems is generally observed to occur via abnormal grain growth, in which a limited number of grains grow at a rate much higher than the surrounding grains. This growth has been characterized as erratic and
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catastrophic [5], and results in a bimodal distribution of grain sizes. The factors controlling abnormal growth are not well understood, but a variety of possible ones have been proposed: substrate texture [6,7], variations in grain boundary energy [8], applied load [9], deposition conditions [10], and contamination [11-13]. For example, electrodeposited nanograined nickel [11] and nickel alloys [14] are prone to sulphur contamination that results in abnormally large rectangular grains after annealing. The present in situ TEM study investigates the grain growth mechanisms and the resulting microstructures in nanograined PLD Ni, and also examines PLD Cu to a limited extent. The PLD Ni film
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