The effect of stoichiometry on the microstructural evolution during ordering of Ni3V

The effects of stoichiometry on microstructural evolution during ordering of Ni3V compound that transforms from cubic disorder phase (A1(fcc)) to tetragonal order phase (D022) were investigated by scanning electron microscopy, X-ray diffraction and Vickers hardness measurement. In as-homogenized state, either Ni75V25, Ni74V26 or Ni76V24 alloys exhibited lamellar-like microstructure composed of three sets of D022 variant structures with an orthogonal orientation relation to each other, and therefore the disorder phase stable at high temperature was not retained at low temperature. Recrystallized new grains nucleated preferentially at grain boundaries and then eliminated the lamellar-like microstructure as annealing time or annealing temperature increases, like ordinary recrystallization process. The microstructural evolution from the lamellar-like microstructure to the recrystallized grains was the fastest in the Ni74V26 alloy, followed by the Ni75V25 alloy and was the slowest in the Ni76V24 alloy. Hardness of the lamellar-like microstructure was higher than that of the recrystallized grains. Boron doping retarded the microstructural evolution in the Ni75V25 alloy. The microstructural evolution was correlated with anisotropic parameter between two lattice constants, a and c, and was shown to be more significant as this parameter is larger. The results were discussed in terms of the effects of stoichiometry and boron doping on driving force and atomic diffusion by which the microstructural evolution is affected.