Advanced Scale Bridging Microstructure Analysis of Single Crystal Ni-Base Superalloys

In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known gamma/gamma' microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can show that elements such as Re, Co, and Cr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the gamma' cubes while gamma channels show higher concentrations of Co, Cr, Re, and W. We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for gamma' volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from gamma' volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140 mm x 100 mm x 20 mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the gamma' phase and the effect of cooling rates after high temperature exposure on the microstructure.