Crystallization and thermal stability of mechanically alloyed W-Ni-Fe noncrystalline materials

Tungsten-nickel noncrystalline alloys containing remnant nanocrystalline W particles have been synthesized by mechanical alloying (MA). Two initial co mpositions (50 and 75 at.% VI) milled for various times have been investiga ted. The W content of the noncrystalline matrix depends on the MA (milling) time and the overall alloy W content. For 50 at.% W alloys, three exotherm ic reactions - monitored by differential scanning calorimetry - take place on continuous heating of the alloy. Crystalline W first precipitates from t he noncrystalline matrix at about 825 K. This is followed by partial crysta llization (to an fcc phase) of the noncrystalline matrix. At even higher te mperatures, two intermetallics (NiW and, to a lesser extent, Ni,W) form fro m the remaining Ni-W noncrystalline matrix along with additional amounts of the fcc and W phases. Similar crystallization processes occur for 75 at.% W alloys milled for less than 15 h. However, when milled for times exceedin g 15 h, only one exothermic reaction occurs in these alloys. The crystalliz ation temperature is found to increase with increasing W content in the non crystalline phase. (C) 2001 Elsevier Science B.V. All rights reserved.