Synthesis and characterization of bulk nanocrystalline M50 steel by cryomilling

Abstract

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 cryomilling in order to promote the formation of nanoscale Al 2O 3 and AlN dispersoids in an effort to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V 8C 7, Fe 3C, and FeC), oxides (Al 2O 3, MoO 3, and V 3O 7), and a nitride phase (AIN) during cryomilling. The nanocrystalline M50-5 wt.% Al powders are consolidated using hot isostatic pressing (HIP) at 1223 K and 200 MPa for 2 hours. X-ray diffraction analysis performed on the consolidated M50-5 wt.% Al compact yields an average calculated grain size of 23 nm. Furthermore, TEM dark field imaging indicates that an average grain size of 17 ± 11 nm is retained for the bulk nanocrystalline M50-5 wt.% Al consolidated by HIP.