Evolution of microstructure in the stoichiometric Ni–25at.%V alloy

The disorder-to-order reaction in the stoichiometric Ni3V alloy is a typical cubic-to-tetragonal type (A1 to D022) transformation. Microstructural evolution has been studied at different temperatures below the ordering temperature in samples of this alloy having the same initial microstructure produced by solution treatment followed by water quenching. The ordered phase has been observed to evolve through a series of structural changes, starting with the impingement and alignment of fine ordered domains corresponding to all the three variants of the ordered phase, followed by the appearance of a two-variant lamellar structure which ultimately gets converted to a structure in which each Ni3V grain comprises a single variant. The lamellar structure is found to be quite stable with regard to coarsening in response to heat treatments. Evidence has been found which indicates that a discontinuous coarsening type of reaction is one of the possible mechanisms driving the two-variant lamellar to single-variant morphological transition. It has also been observed that coarsening of domains takes place in a manner similar to Ostwald ripening wherein larger domains coarsen at the expense of smaller domains.