Effect of milling temperature and additive elements on an Fe-C system alloy prepared by mechanical alloying

Mechanical alloying (MA) of elemental powder mixtures of Fe-C and Fe-C-X (X = Si and Cr) was performed in an argon atmosphere at various temperatures using a planetary ball mill. MAed powders were examined by X-ray diffractometry, differential thermal analysis and scanning electron microscopy. MAed powders were hot isostatic pressed (HIPed) at both lower and higher temperatures than that of the eutectoid transformation point A(1). The Vickers hardness and microstructure of HIPed compacts were examined. In the first stage of milling, amorphization of graphite occurs. As the milling time increases, super-saturated iron solid solution is formed by repeating fracture and cold-welding of iron powders. The iron/carbon amorphization and the solution of carbon atom into iron solid solution progress parallel with each other. Finally the iron/carbon amorphous is formed entirely. This behavior depends on the milling temperature. The Vickers hardness of the HIPed compact is maximal at a carbon concentration of 6 mass% for HIP at 853 K and 7 mass% for HIP at 1053 K, respectively. The hardness depends on the density of voids and the amount of Fe3C and graphite. Graphitization of Fe3C is promoted by silicon addition but retarded by chromium addition. (C) 1999 Elsevier Science S.A. All rights reserved.