Phase transitions in alloys of the Ni–Mo system

The structure of Ni–20 at.% Mo and Ni–25 at.% Mo alloys heat treated at different temperatures was studied by the method of transmission electron microscopy. X-ray photoelectron spectroscopy was used to detect the sign of the chemical interaction between Ni and Mo atoms at different temperatures. It is shown that at high temperatures the tendency toward phase separation takes place. The system of additional reflections at positions {1 ½ 0} on the electron diffraction patterns testifies that the precipitation of crystalline bcc Mo particles begins in the liquid solution. At 900 °C and below, the tendency toward ordering leads to the precipitation of the particles of the chemical compounds. A body-centered tetragonal phase Ni4Mo (D1a) is formed in the Ni–20 at.% Mo alloy. In the Ni–25 at.% Mo alloy, the formation of the Ni3Mo (D022) chemical compound from the A1 solid solution has gone through the intervening stage of the Ni4Mo (D1a) and Ni2Mo (Pt2Mo) formation.

Graphical abstractThe structure of Ni–20 at.% Mo and Ni–25 at.% Mo alloys was studied by methods of TEM and XPS. It is shown that at high temperatures the tendency toward phase separation takes place in the alloys and crystalline bcc Mo particles precipitate in the liquid solution. At 900 °C and below, the tendency toward ordering leads to the dissolution of Mo particles and precipitation of the particles of Ni3Mo, Ni2Mo or Ni4Mo chemical compounds.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► “Chemical” phase transition “ordering-phase separation” is first discovered in alloys of the Ni–Mo system. ► It is first shown that the phase separation in the alloys studied begins at temperatures above the liquidus one. ► The formation of Ni3Mo from A1 has gone through the intervening stage of the Ni4Mo and Ni2Mo coexistence.