GRAIN TEXTURE EVOLUTION DURING THE COLUMNAR GROWTH OF DENDRITIC ALLOYS

The grain selection that operates in the columnar zone of a directionally solidified (DS) INCONEL X750 superalloy has been investigated using standard metallography and an automatic indexing technique of electron backscattered diffraction patterns (EBSPs). From the crystallographic orientations measured at 90,000 points in a longitudinal section, the grain structure was reconstructed. The grain density as measured by the inverse of the mean linear intercept was found to be a decreasing function of the distance from the chill. The evolution of the [100] pole figures along the columnar zone of the casting and the distribution of the angle theta characterizing the (100) direction of the grains that is closest to the temperature gradient were then deduced from the EBSPs measurement. It was found that, near the surface of the chill, the theta distribution was close to the theoretical curve calculated for randomly oriented grains. As the distance from the chill increased, the measured theta distribution became narrower and was displaced toward smaller theta values. At 2 mm from the chill, the most probable orientation of the grains was found to be about 0.21 rad (12 deg). The information obtained with the EBSPs was then compared with the results of a three-dimensional stochastic model (3D SM) describing the formation of grain structure during solidification. This model accounts for the random location and orientation of the nuclei, for the growth kinetics and preferential [100] growth directions of the dendrites. Although this model assumes a uniform temperature within the specimen, the simulation results were found to be in good agreement with the EBSPs measurement.