MICROSTRUCTURE OF AGED BINARY NICKEL-HAFNIUM ALLOY

Samples of conventionally cast nickel-2 at% hafnium alloy were aged at 505: 605 and 695 degrees C for different times and investigated by X- ray diffraction, optical metallography, transmission electron microsco py and microhardness testing. It was found that, for annealing at 605 and 695 degrees C, the Ni,Hf phase precipitated firstly at grain bound aries, then formed within the matrix. In contrast, only localized prec ipitation was observed at 505 degrees C. Moreover, for long ageing tim es and high temperatures (605 and 695 degrees C) a small amount of ZrO developed as a result of the reaction between zirconium impurity in t he hafnium and the residual oxygen in the ageing chamber. (C) 1998 Els evier Science S.A.