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

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Tungsten-nickel noncrystalline alloys containing remnant nanocrystalline W particles have been synthesized by mechanical alloying (MA). Two initial compositions (50 and 75 at.% W) milled for various times have been investigated. 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 exothermic reactions - monitored by differential scanning calorimetry - take place on continuous heating of the alloy. Crystalline W first precipitates from the noncrystalline matrix at about 825 K. This is followed by partial crystallization (to an fcc phase) of the noncrystalline matrix. At even higher temperatures, two intermetallics (NiW and, to a lesser extent, Ni4W) form from 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 exceeding 15 h, only one exothermic reaction occurs in these alloys. The crystallization temperature is found to increase with increasing W content in the noncrystalline phase. © 2001 Elsevier Science B.V. All rights reserved.

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Materials Science and Engineering A