ELECTRON-MICROSCOPY AND DIFFRACTION OF ORDERING IN NI-W ALLOYS

Electron microscopy and diffraction studies of ordering in stoichiomet ric Ni-20%W and off-stoichiometric Ni-15%W alloys have been carried ou t. The specimens of Ni-20%W were first disordered at 1398 K for 4 h an d then quenched rapidly into water. Short range order (SRO) spots were observed at {1 1/2 0} positions. Two hitherto unknown metastable pha ses: D-2h(25)-Ni2W and DO22-Ni3W were observed in the diffraction patt erns. Long range order (LRO) transformations were studied at 1103 and 1213 K. Kinetics and mechanism of transformations have been identified . Ni-15%W specimens were solution treated at 1523 K for 1 h followed b y quenching in water. SRO spots similar to those found in Ni-20%W were observed in this alloy as well. The transition to LRO was studied at 1093 K. Distinct Ni4W precipitates could be observed after 5 h of anne aling at this temperature. After 100 h of annealing precipitates were found to grow into faceted shape coherent with the disordered matrix. After prolonged annealing for over 150 h the Ni4W precipitates began t o lose coherency by the generation of misfit dislocations. The microst ructural observations have been compared for the stoichiometric and of f-stoichiometric alloys.