Development of nanocrystalline structure during cryomilling of Inconel 625

Nanocrystalline Inconel 625 alloy, with a uniform distribution of grains, w as synthesized using cryogenic mechanical milling. Microstructures of the p owder, cryomilled for different times, were investigated using transmission electron microscopy (TEM), scanning electron microscopy, and x-ray diffrac tion. 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 powde r were deformed into elongated grains with a high density of deformation fa ults and then fractured via cyclic impact loading in random directions. The fractured fragments from the elongated coarse grains formed nanoscale grai ns. The occurrence of the elongated grains, from development to disappearan ce during intermediate stages of milling, suggested that repeated strain fa tigue and fracture, caused by the cyclic impact loading in random direction s, and cold welding were responsible for the formation of a nanocrystalline structure. A high density of mechanical nanotwins on {111} planes was obse rved in as-cryomilled Inconel 625 powders cryomilled, as well as in Inconel 625 powder milled at room temperature, Ni20Cr powder milled at room temper ature, and cryomilled pure Al.