Influence of Nitrogen Content on Thermal Stability and Grain Growth Kinetics of Cryomilled Al Nanocomposites

Abstract

Nanocomposite powders of Al 5083/B 4 C were produced via cryogenic milling (cryomilling) of boron carbide (B 4 C) particles in Al 5083 matrix. The effect of milling time (up to 24 hours), and consequential nitrogen content, on grain growth in the nanocrystalline Al 5083 matrix was investigated. Thermal stability was studied at temperatures as high as~0.96 T m and annealing times of up to 24 hours. Average grain sizes increased with time and temperature and tended to stabilize after longer annealing times, regardless of nitrogen content. Higher thermal stability was observed in samples with higher nitrogen content, with the average grain size remaining in the range of 30 nm, even after exposure to the most extreme annealing conditions. This behavior was attributed to the retarding effect that nitrides have on grain growth, as a result of pinning grain boundaries. Kinetic studies based on the Burke equation showed two thermally activated grain growth regimes-a low-temperature regime with an activation energy of 15 kJ/mol and a high-temperature regime with an activation energy of 58 kJ/mol.

Thermal stability of the nanostructure of mechanically milled Cu–5 vol% Al2O3 nanocomposite powder particles

“…The kinetic strategy which utilizes small second phase particles to retard the movement of grain boundaries has also been shown to be highly effective in stabilizing the microstructure of nanocrystalline materials at elevated temperatures up to 0.65 T m . [16][17][18][19][20][21][22] Although many nanostructured materials stabilized by fine second phase particles have been reported, systematic investigation of the grain growth behavior and kinetics of such kind of nanostructure is still missing. The present study is to investigate the grain growth of the nanocrystalline Cu matrix and other microstructural changes of the nanostructured Cu-5 vol% Al 2 O 3 nanocomposite powder particles during isothermal annealing at different temperatures in the range of 300-600°C.…”

The Influence of Grain Size Determination Method on Grain Growth Kinetics Analysis

“…It is well established that grain size is a primary determinant of mechanical behavior, as illustrated by the Hall–Petch relationship between grain size and yield strength . Extensive research related to reducing grain size into the nanometric regime, partly motivated by reports of novel phenomena and unusual combinations of physical and mechanical properties, further highlights the importance of this structural parameter . The driving force for grain growth, however, increases with decreasing grain size and hence the ability to retain these finer grains despite potential exposure of a material to elevated temperatures during processing, in use, or both is critically important .…”

“…Extensive research related to reducing grain size into the nanometric regime, partly motivated by reports of novel phenomena and unusual combinations of physical and mechanical properties, further highlights the importance of this structural parameter . The driving force for grain growth, however, increases with decreasing grain size and hence the ability to retain these finer grains despite potential exposure of a material to elevated temperatures during processing, in use, or both is critically important . In order to understand the role of grain size and grain size stability on physical and mechanical behavior, it is imperative that grain size can be measured accurately and reproducibly, and can, therefore, be used as the basis for investigating thermal stability and kinetic grain growth mechanisms.…”

“…Furthermore, XRD‐based methods are generally only valid for average grain sizes below 100 nm . Despite these limitations, the simplicity and ease of XRD analyses, relative to the intricate sample preparation required for TEM, renders the former technique more attractive especially when a large dataset is being evaluated, or when an average grain size is more relevant. These limitations of XRD‐based methods are fairly widely acknowledged in the published literature.…”

“…As an order of magnitude estimate, such variation is not significant. Consequently, it is common in the literature to use only one XRD‐based method for analysis …”