Grain Boundary Engineering Alloy 706 for Improved High Temperature Performance

Abstract

Grain boundary engineering aims to improve material performance by optimizing the structure of interfaces in polycrystalline metals. In the present work we grain boundary engineer the Ni-Fe-based superalloy 706 and measure the effect of this process on high temperature crack growth rate. The microstructure of traditionally processed material is compared to that after grain boundary engineering using electron backscatter diffraction to identify grain boundary character according to the coincidence site lattice model. The incorporation of so called special boundaries is examined in detail through grain size and triple junction distributions. It is shown that the grain boundary engineering process can effectively disrupt the connectivity of general, crack-prone grain boundaries in the microstructure. To test a proposed structure-property relationship, crack growth rate is measured for baseline and grain boundary engineered 706 under high temperature static load – conditions which typically result in an intergranular crack path. We find an order-of-magnitude improvement in crack growth rate at low stress intensities following the grain boundary engineering process. To understand the role of grain boundary engineering in more detail we analyze secondary crack paths to determine which boundary types are truly special (i.e. arrest cracks) for this test condition. We treat the problem in a continuum way, exploring the idea that grain boundaries may become “more special” as their coincident site lattice index decreases. This approach is in contrast to the majority of the literature where a binary classification is typically assumed. The work presented here highlights the potential benefit of grain boundary engineering for improved performance of superalloys and metals in general.