Amorphization and thermal stability of mechanical alloyed W–Ni–Fe

The tungsten heavy alloys with the composition of 80.7W–13.2Ni–6.1Fe (at.%) from the elemental powders of W, Ni and Fe were mechanically alloyed (MA-ed). A W–Ni base amorphous phase imbedded remnant nano-crystalline W with the grain size of 10–20 nm was achieved by suitable MA technology. Phase transition during MA and thermal stability of mechanically alloyed 80.7W–13.2Ni–6.1Fe alloys were investigated by using a combination of XRD, TEM, DTA and DSC analysis. Ni (Fe) first dissolves in crystalline W during milling, and then amorphization of W (Ni, Fe) supersaturated solid solution occurs after milling for 20 h. The volume fraction of amorphous in the as-milled powders increases with extending milling time up to 60 h, and the average composition of the amorphous phase is proximately 63W–37Ni (Fe). Different phase transformations occur in the powders milled for 20 and 60 h during subsequent annealing. For the powders milled for 60 h, besides the relaxation of crystal defects and lattice distortion, the crystallization of amorphous phase and the precipitation of NiW compound are confirmed during subsequent annealing. When temperature increasing above 1100 °C, NiW partially decompose in Ni solid solution and crystalline W through a peritectoid reaction. The primary melting temperatures of the nano-crystalline powders obtained by mechanically milled for both 20 and 60 h decrease by nearly 200 °C compared with that of the un-milled powder mixture.