DENDRITIC GROWTH AND CRYSTALLINE QUALITY OF NICKEL-BASE SINGLE GRAINS

It is a usual observation that subgrains exist in nickel-base single g rain components solidified by the lost wax process. The associated mis orientations are generally small, but they can eventually lead to cast ing defects in the case.of highly complex mold shapes. This work prese nts an attempt to relate the formation of subgrain boundaries with the development of the dendritic solidification microstructure. Experimen tal investigations have been undertaken on cast components made of AM1 nickel-base superalloy designed for high temperature turbine blades. Single grains were obtained by means of a grain selector at the bottom of each part. Metallographic observations have been made to character ize the dendritic array, together with gamma diffraction to measure th e crystalline quality of the material and X-ray topography for mapping of misorientations on a dendritic scale. Small misorientations betwee n dendrite stems have been found at the upper end of the selector whic h lead to the formation of subgrains. Moreover, during the growth proc ess, the total mosaicity of the material increases, firstly as a conse quence of an increase in the misorientations between subgrains, and se condly because of a decrease of the internal quality of each subgrain. It is proposed that misorientations are due to thermomechanical stres ses which build up during gamma' precipitation at temperatures slightl y below the solidus temperature of the alloy.