SYNTHESIS AND CHARACTERIZATION OF BULK NANOCRYSTALLINE M50 STEEL BY CRYOMILLING

Nanocrystalline high speed tool steel M50 (4.5%Mo, 4.0%Cr, 1.0%V, 0.8% C, balance Fe) was synthesized by cryogenic high energy ball milling ( cryomilling) for 25 hours. Elemental Al powder is added prior to cryom illing in order to promote the formation of nanoscale Al2O3 and AlN di spersoids in an effort to improve the thermal stability of the nanocry stalline M50 steel. High resolution transmission electron microscopy ( HRTEM) reveals the formation of various carbides (V8C7, Fe3C, and FeC) , oxides (Al2O3 MoO3 and V3O7), and a nitride phase (AIN) during cryom illing. The nanocrystalline M50-5 wt.% Al powders are consolidated usi ng hot isostatic pressing (HIP) at 1223 K and 200 MPa for 2 hours. X-r ay diffraction analysis performed on the consolidated M50-5 wt.% Al co mpact yields an average calculated grain size of 23 nm. Furthermore, T EM dark field imaging indicates that an average grain size of 17 +/- 1 1 nm is retained for the bulk nanocrystalline M50-5 wt. % Al consolida ted by HIP. (C) 1997 Acta Metallurgica Inc.