Evolution of the two-phase microstructure L1₂ + DO₂₂ in near-eutectoid Ni₃(Al,V) alloy

Transmission electron microscopy and powder x-ray diffraction methods have been used to investigate the evolution of two-phase (L1₂ + DO₂₂) microstructures from the quenched fcc phase of the Ni-5Al-20V (at. %) alloy. The microstructure after annealing in a temperature range from 650 to 900 °C differs from the eutectoid structure which might be expected for the alloy according to the eutectoid-type phase diagram of the Ni₃Al-Ni₃V section. This structure results from fast kinetics of ordering in the fcc → L1₂ and fcc → DO₂₂ phase transitions. Four main stages in the microstructural evolution were observed. Stage I is the formation of spheroidal coherent L1₂ clusters in a disordered fcc matrix. During stage II the L1₂ clusters transform into cuboidal precipitates, and the fcc matrix orders into three DO₂₂ variants (which may have interfaces that are wetted by thin fcc layers). In stage III accommodation of misfit (elastic energy) between different phases and variants occurs by formation of (110) twins or a single variant of the DO₂₂ phase and tetragonally strained lamellae of the L1₂ phase. Stage IV is a discontinuous coarsening process in which a coarse incoherent two-phase structure replaces the fine coherent one. Grains of the coarse structure are nucleated on high-angle boundaries of primary fcc or other surfaces. Many of the grains are found twinned.