Evolution of the two-phase microstructure L1 2 + DO 22 in near-eutectoid Ni 3 (Al,V) alloy
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Transmission electron microscopy and powder x-ray diffraction methods have been used to investigate the evolution of two-phase (Ll 2 + DO22) microstructures from the quenched fee phase of the Ni-5A1-2OV (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 NisAl-NisV section. This structure results from fast kinetics of ordering in the fee —* L l 2 and fee —• DO22 phase transitions. Four main stages in the microstructural evolution were observed. Stage I is the formation of spheroidal coherent LI2 clusters in a disordered fee matrix. During stage II the L l 2 clusters transform into cuboidal precipitates, and the fee matrix orders into three DO22 variants (which may have interfaces that are wetted by thin fee 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 L l 2 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 fee or other surfaces. Many of the grains are found twinned.
I. INTRODUCTION Both L l 2 (Cu3Au-type, cubic Pm3m) and DO22 (Al3Ti-type, tetragonal IA/mmm) belong to well-known fee-derived ordered structures.1 Ordering of the L l 2 structure has been studied extensively, both experimentally and theoretically, e.g., for Cu3Au and Au 3 Cu, Ni 3 Al, Ni 3 Fe, etc.2 Much less attention has been given to the DO22 structure, although microstructural formation of the DO22 structure in the Ni 3 V alloy has been investigated in early experimental works by Tanner et al.2'3 Recently, in a combined experimental and theoretical study (by in situ diffuse scattering of neutrons and the cluster-variation method) of the Ni 3 V and Pd3V DO 22 structures, parameters relevant to the ordering process were obtained.4 However, to our knowledge, no study on coexistence of the L l 2 + DO22 phases, or on decomposition of the high-temperature fee into these two phases, has yet been published. In this paper we will address some of these issues. The coexistence of the LI2 and DO22 structures is intriguing. Although both structures are fee derivatives, there is no direct ordering relationship between them. In the language of crystallographic space groups, both L l 2 and DO22 are subgroups of the fee (Fm3m) while there is no subgroup relation between them.5 This is equivalent to the fact that the L l 2 and DO 22 structures are built with different concentration waves, of wave 3068 http://journals.cambridge.org
vectors (100)" (star) and [10 y]*, respectively.6 (Indexing in the paper will be given in coordinates of the disordered fee, unless we specify.) Therefore, in the case of congruent (isochemical) ordering of one of the phases,7 the second phase
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