Evolution and thermal stability of Ni 3 V and Ni 2 V phases in a Ni-29 at. pct V alloy
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TRODUCTION
Extensive investigations on the disorder to order transformation occurring in stoichiometric Ni3V and Ni2V alloys have been carried out earlier by many workers.[1–4] Tanner[1,3] has confirmed that the ordering reaction in the former alloy is associated with extremely fast kinetics, and that it cannot be suppressed by rapidly quenching from the disordered phase field. The formation of the ordered Ni2V phase in the latter alloy occurs at a relatively slower rate. The main observations made by Tanner[1,3] and by Tanner and Ashby[2] regarding ordering in these alloys are summarized as follows. The ordering reaction in either alloy is of the first order and, depending on the reaction temperature and time, it may proceed toward completion through the operation of several mechanisms, singly or in combination. In the stoichiometric Ni3V alloy, three variants of ordered Ni3V form—the [001] direction of each variant being parallel to one of the ^100& directions of the parent fcc (A1) structure. At low aging temperatures, the ordered Ni3V domains exhibit a mosaic assembly of prism-shaped transformation twins bounded by {110}A1-type twin planes, whereas, at high aging temperatures, a lamellar morphology develops.[1] In the stoichiometric Ni2V alloy, the ordering reaction leads to the formation of a microstructure comprising a mosaic assembly of transformation twins within the interior of the original disordered grains surrounded by ordered regions that outline the boundaries of the grains. This duplex microstructure results from the nucleation, which occurs by homogeneous as well as heterogeneous modes, of ordered crystals, and their subsequent growth. Homogeneous nucleation occurs in the grain body while heterogeneous nucleation occurs at the grain boundaries, with the two processes being competitive. At lower temJ.B. SINGH, M. SUNDARARAMAN, and P. MUKHOPADHYAY, Scientific Officers, are with Materials Science Division, Bhabha Atomic Research Centre, Mumbai-400 085, India. Manuscript submitted July 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
peratures, the homogeneous transformation is replaced by a continuous ordering process. There is considerable contemporaneous interest in developing microstructures consisting of two ordered phases in view of their potential for exhibiting superior properties.[5] A well-known example is that of the duplex TiAl/Ti3Al microstructure that has been reported to be associated with improved toughness, creep resistance, and fatigue resistance.[6–9] A two-phase field containing the Ni3V and the Ni2V phases is present in the binary nickel-vanadium phase diagram (Figure 1). The two ordered phases emerge over a composition range with the vanadium content ranging from 25 to 33.3 at. pct through a eutectoid decomposition of the disordered fcc phase at temperatures below 908 7C.[10] It may be pointed out in this context that it would be interesting to develop duplex microstructures comprising the Ni3V and the Ni2V phases and to examine the stability of and the properties associated wi
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