In situ TEM Observations of Grain Growth in Nanograined Thin Films
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In situ TEM Observations of Grain Growth in Nanograined Thin Films K. Hattar1, J. Gregg1, J. Han2,T.Saif2 and I. M. Robertson1 Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801 2 Department of Mechanical and Industrial Engineering, University of Illinois, Urbana IL 61801 1
ABSTRACT In situ transmission electron microscopy analysis is used to study the stability of nanograined and ultra-fine grained thin films at elevated temperatures. In the free-standing Au and Cu films, grain growth was dependent on annealing temperature and time with growth observed in both materials at temperatures greater than 373K. Both materials exhibited abnormal grain growth although it was more prevalent in Au than in Cu, which may be a consequence of pinning of the Cu grain boundaries by impurities. The formation and destruction of twins was observed to play a critical role in the grain growth, with the twins retarding the growth in gold, but not in Cu. In constrained Au films no grain growth was observed on annealing at temperatures below 636 K. At 636 K, the eutectic temperature, the microstructure transformed to the eutectic structure with the first stage being the annihilation of the grain structure. INTRODUCTION Structures with average grain sizes extending from a few tenths of a micrometer to tens of nanometers can exhibit very different optical, magnetic, electrical, and mechanical properties from their large grained counterparts. This is a consequence of the critical length-scale for a particular physical property being surpassed as grain size decreases. As more structures and devices are created with an ultra-fine or nanograined microstructure it becomes necessary to ensure that the grain size remains constant with time and temperature as any significant grain growth or agglomeration results in a change in properties [1]. This becomes even more important with nanograined thin films due to the non-equilibrium state of the microstructure and strain-induced effects. Strategies for improving the stability of nanograined materials involve reducing the mobility of the grain boundaries through impurities: second phase additions or adding impurities that segregate to the grain boundary and lower its energy. As noted by Liu and Kirchheim, the latter approach may be better because of the lower temperature sensitivity of the boundary energy compared to the boundary mobility [2]. The emphasis of this paper is on the comparison of grain growth mechanisms in nanograined FCC Au and Cu both as freestanding films and films constrained by a substrate. Grain growth, in general, is affected by many factors including grain boundary type and size, surface morphology, structure of grain boundary triple points, as well as chemistry, contamination, and chemical gradients [1,3]. In thin films, grain growth can be influenced by surface diffusion because of the proximity of the free surfaces and grain boundary diffusion due to the volume per unit area of grain boundary. If the film is supported on a substrate
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