The microstructural evolution and thermal stability of nanocrystalline ball-milled Ni-15 at.% W powder

A mixture of pure Ni powder and pure W powder with a nominal composition Ni-15 at.% W was subjected to ball milling in a planetary mill and in a shaker mill. The microstructural evolution upon milling and upon subsequent annealing in the temperature range from 25 to 800 degrees C was investigated using ex situ and in situ x-ray diffraction integral breadth (single-line and Williamson-Hall) methods and whole powder pattern modeling as well as scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. A nano-crystalline Ni(W) solid solution phase develops upon ball milling, with a higher W content by milling in the shaker mill as compared to the planetary mill. Grain coarsening studies indicated a very high stability of the nanocrystalline state, up to almost about 500 degrees C. Results of annealing at higher temperatures showed that, while the dissolved W content increased in the Ni matrix, a high stability against grain coarsening occurs, which can be ascribed to the presence of (segregated) W atoms at the grain boundaries.