Structural Evolution of Nanocrystalline Nickel-Tungsten Alloys Upon Mechanical Alloying with Subsequent Annealing

In the current study, the two alloys, Ni-20 at. pct W and Ni-35 at. pct W, were mechanically alloyed and subsequently heat treated to evaluate their structural variations using X-ray diffraction, scanning, and transmission electron microscopy, and differential thermal analysis. In addition, the effect of Fe contamination on the progress of mechanical alloying was investigated. The results showed that the Ni-20 at. pct W contained only Ni(W) solid solution even after prolonged milling times, while the Ni-35 at. pct W was amorphized after 40 hours of milling. The composition of the amorphized alloy was estimated to be Ni-31 at. pct W. Furthermore, it was demonstrated that the nanocrystalline NiW intermetallic compound was stable at temperatures greater than 1303 K (1030 degrees C) and did not completely vanish upon peritectoid reaction. Consequently, an exceptional grain coarsening resistance was observed at high temperatures near the melting points. The mechanisms involved in this outstanding thermal stability were also probed. (C) The Minerals, Metals & Materials Society and ASM International 2013