THE GROWTH AND ELEVATED-TEMPERATURE STABILITY OF HIGH REFRACTORY NICKEL-BASE SINGLE-CRYSTALS

Additions of refractory alloying elements to single-crystal and polycrystalline nickel-base alloys are known to provide potent high temperature strengthening. However, high levels of elements such as Re, W and Ta can result in unexpected modes of degradation during solidification and subsequent long-term exposures at high temperatures. Here the contributions of refractory alloying additions to various mechanisms of high temperature strengthening are discussed. In addition, the results of experiments which have identified two new modes of degradation associated with the presence of high levels of refractory alloying elements in nickel-base single crystals will be presented. Nucleation and growth of isolated columnar grains during solidification and discontinuous reactions at defects in the single-crystal structure will be shown to be extremely sensitive to the level of refractory alloying additions in a number of nickel-base single-crystal alloys.