Investigation of Microporosity in Single-crystal Nickel-base Superalloys by Different Experimental Techniques

T. Link (Sp), Technische Universität Berlin (Germany); A. Epishin, Federal Institute for Materials Research and Testing, Berlin (Germany); H. Haibel, Hahn-Meitner-Institute, Berlin (Germany) 
 
Modern single-crystal nickel-base superalloys have an increased level of microporosity. This is an undesirable side effect of the high content of refractory elements. During solidification these elements show a strong dendritic segregation which has to be equalized by an extensive homogenisation, resulting in numerous micropores. A further increase of microporosity takes place during high temperature service.
The aim of present investigation was to quantify this microporosity in superalloys in different conditions. This task is not trivial, because the pores are small, about 10 micrometer or smaller, their volume fraction low (0.1-0.5 vol. %) and the distribution inhomogeneous. Different experimental techniques were applied to characterise the microsporosity. They were compared to find the most accurate method.
Quantitative light microscopy gave satisfactory results for the pore size, but the scattering of the volume fraction is above 0.1 vol. %.
Density measurements for the characterisation of the microporosity growth during creep were performed by gas pycnometry and by the Archimedes method. Pycnometry gave a scatter somewhat below 0.1 vol. %. The accuracy of the Archimedes method depends on the liquid used: With dodecan we reached about 0.05 vol. %, with water 0.01 vol. %. Thus density measurements in water were used to characterise microporosity growth during creep, which does not exceed 0.1-0.2 vol. % under the testing conditions applied.
High resolution X-ray tomography was used to visualize the spatial arrangement of the micropores. In the undeformed superalloys the micropores are concentrated in the interdendritic region. During high temperature creep under high stresses new smaller micropores are generate everywhere. The shapes of these two pore types were found to be different. By means of 3D image analysis pore formation and size distribution were investigated quantitatively.
The results of the microporosity measurements are discussed under the aspect of the formation mechanism.