Porosity distribution and growth during homogenization in single crystals of a nickel-base superalloy

The presence of shrinkage porosity resulting from solidification of equiaxed and directionally solidified nickel-base superalloys is well known. However, the manner in which high temperature homogenization heat treatments affect the size distribution of these pores is unknown. Single crystals of a model nickel-base superalloy grown in the 〈001〉 direction were homogenized for various times at two temperatures above the γ′ solvus and metallographically analyzed in both qualitative and quantitative terms. Spherical pores, greater than 2.0 μm in diameter, grew in size, number and volume fraction in the interdendritic regions of these crystals by a diffusion-controlled mechanism. An analysis of the various potential origins of porosity leads to the hypothesis that vacancy coalescence is the operative mechanism of pore growth. The origin of these vacancies was attributed both to cast-in vacancies and to a Kirkendall vacancy generation mechanism driven by composition gradients formed during solidification.