Microstructural evolution during the heat treatment of nanocrystalline alloys
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Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.
I. INTRODUCTION
Nanocrystalline materials reside in a nonequilibrium state as a consequence of their extremely fine structurallength-scales and large volume fraction of grain boundaries. This energetically unfavorable condition has motivated a number of studies in recent years aimed at characterizing the stability of these materials and the likelihood for grain growth. Not surprisingly, it has generally been found that pure nanocrystalline materials are highly unstable with rapid grain growth occurring at relatively low temperatures. Nanocrystalline Ni and Co, for example, have onset temperatures for grain growth in the range of 220–310 °C,1–4 while Al, Sn, Pb, and Mg have all shown significant grain growth at room temperature.5,6 Several nanocrystalline materials have also been found to undergo grain growth due to mechanical deformation at low temperatures. 7–9 The instability of nanocrystalline structures generally limits the finest achievable grain size to ∼10 nm in pure systems, and stands as a barrier to application and testing at elevated temperatures. In addition to grain growth, increasing attention has been focused on the evolution of grain-boundary structure during annealing of nanocrystalline materials, largely due to the dominant role these regions play in determining properties.10–13 In particular, it has been
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0403 J. Mater. Res., Vol. 22, No. 11, Nov 2007
suggested that the driven techniques typically used to produce nanocrystalline specimens result in nonequilibrium grain boundaries containing a large number of excess dislocations.14–17 Upon heating, kinetic processes allow these defects to annihilate in a process termed “grain-boundary relaxation.” This phenomenon has been directly observed in coarse-grained18,19 and nanocrystalline structures20 through high-resolution microscopy experiments, and has also been suggested as an explanation for the broad exothermic reaction that has been found to precede grain growth in a number of nanocrystalline materials.1–3,21–23 Recent mechanistic models have further connected this kind of relaxation with strengthening,24–26 providing insight on disparate pro
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