Synthesis and characterization of nanostructured Cr3C2-NiCr

Pre-alloyed Cr3C2-25 (Ni20Cr) powder was synthesized by mechanical bah mill ing in Hexane [H3C(CH2)(4)CH3] and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average po wder size drastically decreased with time during the first four hours of mi lling; then decreased slightly as milling continued up to 20 hours. For mil ling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subj ected to heavier deformation than the chromium carbide powder. During the i nitial stages of milling, the brittle chromium carbide powders are fracture d into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for rimes up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and comp osite powders. Milling times of up to 20 hours led to the formation of a po lycrystal nanocomposite powder system in which chromium carbides, with aver age size of 15 nm, were uniformly distributed in NiCr matrix. (C)1999 Acta Metallurgica Inc.