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.

Modeling the effect of thermal–mechanical processing parameters on the density and length fraction of twin boundaries in Ni-base superalloy RR1000

“…Grain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by modifying the character of the grain boundaries in the microstructure of the alloy [7]. Introducing special low-Σ boundaries, such as Σ3 twin boundaries, is key to GBE and the improvement of properties such as resistance to corrosion [8], creep deformation [9], and fatigue crack propagation/initiation [10,11]. In polycrystalline materials, each grain boundary can be characterized by its orientation defined by the two adjacent grains.…”

Grain boundary engineering of powder processed Ni-base superalloy RR1000: Influence of the deformation parameters

“…Breaking up the connectivity of the random grain boundary network by populating the grain boundary network with a high proportion of low energy Σ3 boundaries has been key to achieving these improved properties [32][33][34][35][36][37][38]. Currently, one of the limiting factors associated with widespread commercialization of grain boundary engineering is that conventional approaches rely on multiple iterations of cold work followed by annealing to exploit strain annealing or strain induced boundary migration mechanisms for both the formation of Σ3 boundaries and their migration into the random boundary network.…”

Characterization of abnormal grain coarsening in Alloy 718

“…Usually, the only way to obtain good quantitative information of twin volume fraction is through EBSD. Papers on grain boundary engineering do offer some characterisation techniques, but these usually only focus on the proportion of twin boundaries to other high angled grain boundaries and the amount of triple points that contain two or more twin boundaries to reduce crack propagation, often failing to characterise grain morphology [8,9]. Some efforts to characterise and predict twinned microstructure have been made in Nickel.…”