Evolution of the microstructure of Ni75V25 and Ni75V20Al5 alloys at changing the temperature of their heat treatment

• The three-variant D022 structure is formed in the temperature range liquidus–solidus.
• Disordered solid solution crystallizes at the temperature corresponding to the solidus point.
• The three-variant D022 structure transforms at 1200 °C into two- and at 800 °C into one-variant.
• Solid solution transforms into one-variant D022 through intermediate D1a below 1045 °C.
• Al lowers heat resistance of Ni–V alloys due to quasi-liquid clusters formation above 1200 °C.

Experimental proofs are presented, which indicate that the three-variant structure of the D022 phase is formed in the Ni75V25 alloy in the liquidus–solidus temperature range. The rest of the liquid crystallizes into the A1 solid solution at the solidus temperature. At lowering the temperature of heat treatment, relaxation of these types of structures proceeds according to two different scripts: the transition of the three-variant D022 structure into the two-variant one takes place at 1200 °C and the transition into the one-variant structure occurs at 800 °C. The A1 solid solution transforms into the one-variant D022 microstructure through the intermediate body-centered tetragonal Ni4V(D1a) phase at temperatures below 1045 °C. Introduction of aluminum into alloys of the Ni–V system reduces the maximum possible heat resistance of these alloys, since at temperatures slightly above 1200 °C, in the Ni–A1 diffusion couple, there takes place the phase transition ordering – phase separation with formation of aluminum clusters, which at such temperatures are present in a quasi-liquid state.