Development of nanocrystalline structure during cryomilling of Inconel 625
- PDF / 936,591 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 11 Downloads / 182 Views
Nanocrystalline Inconel 625 alloy, with a uniform distribution of grains, was synthesized using cryogenic mechanical milling. Microstructures of the powder, cryomilled for different times, were investigated using transmission electron microscopy (TEM), scanning electron microscopy, and x-ray diffraction. The results indicated that both the average powder particle size and average grain size approached constant values as cryomilling time increased to 8 h. The TEM observations indicated that grains in the cryomilled powder were deformed into elongated grains with a high density of deformation faults and then fractured via cyclic impact loading in random directions. The fractured fragments from the elongated coarse grains formed nanoscale grains. The occurrence of the elongated grains, from development to disappearance during intermediate stages of milling, suggested that repeated strain fatigue and fracture, caused by the cyclic impact loading in random directions, and cold welding were responsible for the formation of a nanocrystalline structure. A high density of mechanical nanotwins on {111} planes was observed in as-cryomilled Inconel 625 powders cryomilled, as well as in Inconel 625 powder milled at room temperature, Ni20Cr powder milled at room temperature, and cryomilled pure Al.
I. INTRODUCTION
The mechanical milling/alloying process was originally developed by the International Nickel Company for the production of oxide dispersion strengthened superalloys.1 Benjamin, the developer of mechanical milling/ alloying, and his colleagues stated that the process involves repeated welding, fracturing, and rewelding of powder particles under a highly energetic ball charge.2 In terms of milling mechanisms, however, Benjamin and his coworkers primarily studied interparticle behavior, whereas the issue of grain size evolution was not addressed.1–3 More recently, the mechanical milling process has attracted considerable interest, primarily as a result of its potential to generate nanocrystalline and other nonequilibrium structures in large quantities.4 –16 On the basis of a study on microstructures of ball-milled Ru and AlRu, the formation of a nanocrystalline structure is thought to evolve from the development of dislocation cell structures within shear bands.6 Plastic deformation leads to the formation of dislocation cell within shear bands; then dislocation cells transform into low-angle grain boundaries and finally form nanocrystalline grains surrounded by high-angle grain boundaries via grain rotation.6–8 In such a dislocation cell mechanism, the contribution of the fracture and welding processes in the powder particles, as described by Benjamin and his 2724
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 12 Mar 2015
co-workers,1–3 to the formation of a nanocrystalline structure has heretofore never been studied. It is anticipated that the welded fragments of the original coarse grains should form new grains, although Benjamin and his co-workers did not explicitly demonstrate
Data Loading...
