EVOLUTION AND THERMAL-STABILITY OF NI3V AND NI2V PHASES IN A NI 29 AT. PCT V ALLOY

Eutectoid decomposition of the disordered fee Ni-V solid solution in t he composition range of 25 to 33.3 at, pct V gives rise to a mixture o f the ordered Ni3V and Ni2V phases. In the present work, the evolution and thermal stability of these phases were studied in a Ni-29 at. pct V alloy. Solution-treated and water-quenched specimens, when aged at 850 degrees C, were found to exhibit two types of microstructure. In t he first, the Ni2V phase precipitated in a lamellar Ni3V matrix where a pair of conjugate lamellae corresponded to two variants of the Ni3V phase. In the second morphology, the Ni3V phase precipitated within a Ni3V matrix comprising a single variant of the Ni3V phase. The Ni2V ph ase was observed to precipitate in a plate-shaped morphology, exhibiti ng {120}(fcc)-type, habit planes. The precipitation of the Ni2V plates in the Ni3V lamellae resulted in zigzag interfaces between adjacent N i3V domains. Both the microstructures were found to be thermally quite stable and did not coarsen appreciably on prolonged aging. However, t he prolonged aging caused the renucleation of the Ni3V and the Ni2V ph ases in the vicinity of the grain boundaries in a manner similar to '' recrystallization.'' The stability of the aged microstructure could be attributed to the nature of the interfaces between different domains of the Ni3V and Ni2V phases.